Setting the scene for success: Designing a demo system -Chapter 4

Designing a demo system 4

LOUDSPEAKERS

Loudspeaker design is really my favourite domain, and it has been the core of my professional activity for 18 years.

I have been designing speakers for Hi-Fi, recording studios and mainly professional sound reinforcement.

Therefore, what will be difficult for me will be not to go too far into details, in order to make this text more accessible than the reference 4 volumes book edited by the JAES on this topic.

I will start by defining loudspeakers categories, by technology and by shape (a more determining factor than one may think at first glance).

• Loudspeaker categories

There is one criterion that is the most critical to intelligibility, dynamic performance and overall quality: The mid- and high-frequencies range. There are 4 main technologies, so I will describe them as 4 categories.

Why focusing on mid & highs ? Just because not only it is the most critical range to sound quality, the most sensitive range to the ear, but also the one which is the most challenging to speakers designers.

In comparison, the low frequency range is ‘easy’ if you put together sensible means.

Dome tweeter speakers

In the residential domain, these are in vast majority. In recording studios, they are popular as well, but more near-field than in main monitors systems. In Public address, they do just not exist at all.

The main reason is the low available Sound Pressure Level (SPL) with this kind of tweeter. Some of the best performing ones can reach a sensitivity of 96 dB /1 w /1m (very few of them, and generally horn-loaded) whilst handling a maximum power of 20 W. Most of them have a sensitivity of 90 to 91 dB/1W/1m. So, doing the maths, we can reach a maximum SPL at 4m (typical listening distance for a Home Cinema) of 92 dB in theory: at a 4m distance, you have 12dB of attenuation versus SPL at 1m. Now this is without counting the thermal compression loss, which on such tweeters at full power can be 5 dB or even beyond. So, we only have 87 dB left!*

Of course, and fortunately, the SPL requirement in the high frequency range is less than elsewhere, the maximum required energy being located in the 100-500 Hz or even 100-3000 Hz ranges (see curves below).  

The above curves represent the energy distribution statistics of audio signals. The red curve corresponds to classical music programs, the blue one to rock music, and the green one to miscellaneous signals including music, speech and noises. Classical music is quite gentle to dome tweeters when compared to other programs.

However, such tweeters are quite interesting in regard of some ergonomic criteria:

         ° Low cost

         ° Small size, for some of them

         ° Good damping and linearity

         ° Ease of system design

The downside is,  in view of their SPL limitations I do think that most are not suitable for Home Cinema applications: They are just not capable of reproducing correctly the large SPL variations that can be found on many movies soundtracks.

Still, I must admit I came across a couple of exceptions that most likely compensate the low efficiency by a larger than usual power handling (actually, I did not have the possibility to open and disassemble these speakers, so I don’t know why they achieve well)

Above is one of the very few non-loaded dome-tweeters designs I can think of that left me a highly favourable impression (I discard all the designs I have been personally involved in for deontological reasons)

Horn loaded dome tweeters are typically providing significantly better SPL figures as they simply offer a better sensitivity (95 to 98 dB/1W/1m)

HF Compression driver loudspeakers

Affording acoustic efficiency comprised between 20% and 25%, and directional gains of typically 15.5 dB (this is for a 90° x 40° coverage horn, which is a standard in the industry), the sensitivity reaches 117.5 dB /1W /1m (1 acoustical Watt provides an approximate SPL of 109 dB at 1m in 360° spherical radiation).

This is better…
So, at 4m we still have 105 dB for 1 W and 115dB for 10 W (a power at which the compression driver is still running cool, so the thermal compression is negligible).

Another interesting aspect is the technical breakthrough that occurred in horn design by the late 70’ies: The directivity control.

Controlling HF and MF directivity has allowed an accurate determination of the audience coverage and in predicting how the loudspeaker would match the room geometry. We will look deeper into this in a next chapter about room acoustics.

    JBL 9360, a typical component in THX cinema loudspeakers

From a subjective standpoint, compression drivers provide an unrivalled clarity, precision, intelligibility and a great feel of immense headroom.

Modern horn & compression driver assemblies do not have the flaws that are often mentioned as inherent to the technology, i.e. distortion, roughness, coloration, etc.

These are biased comments due to old-time train stations P.A. with a single low-cost compression driver loaded by a folded steel horn trying desperately to reproduce the whole audio spectrum (practically limited to 500 Hz-5000 Hz by the horn itself).

Some Hi-Fi audiophiles invented the notion of ‘projected sound’ that, I admit, I have difficulties in understanding.  It is a general criticism against horn/compression designs. Maybe this is what the shape of the horn visually suggests, in the same way as floor-scratching spikes below loudspeakers stands suggest ‘mechanical diodes’ that would drain vibrations out to the ground (BTW, by ‘ground’, do they mean the floor?)

Fortunately, some other audiophiles are horn-loaded speakers addicts, so I’m happy to let them debate….

There is another sub-category of compression drivers designs: the coaxial (or ‘dual concentric’ speakers.

A compression driver is installed at the rear of the LF driver magnet, the exit of which goes through its axis. It is loaded partly by a gently expanding throat inside the LF driver magnet, which exit is followed by the LF diaphragm that in fact builds the last part of the horn.

There are a few advantages:

-       At the crossover frequency, the directivity of the LF and the HF is the same.

-       The source coherence is perfect, whatever the cutoff frequency. This allows raising the cutoff frequency beyond the typical driver-size vs. wavelength limit.

-       The size of the whole assembly.

There is still a tradeoff:

-       The LF diaphragm induces Doppler distortion in the HF, as this is also a moving horn. Of course this depends on the amplitude of the LF diaphragm displacement.

Tannoy is the first brand that one will name when it comes to dual-concentric loudspeakers. In the professional domain, L-Acoustics and APG are currently using this technology.

Ribbon loudspeakers

Ribbons have very low inertia diaphragm, as the driving force is more or less uniform on the whole emitting surface. For this reason, they can provide the same level of clarity and intelligibility as compression drivers.

This is only a qualitative criterion, not a quantitative one. In practice, few ribbons are able to provide very high SPL levels, but still they exist.

The most common use of ribbon transducers is for HF reproduction, in combination with classical voice-coil loudspeakers for the lows and mids. In such an application, the largest the ribbon, the largest the available SPL is. The main problem is that 8” to 12” high ribbons suffer from excessive directivity in the far field, and are not large enough to provide substantial near-field coverage.

An alternative consists in using small-sized horn-loaded ribbons in order to increase the sensitivity, and hence the available SPL, and provide directivity control.
Some ribbon HF drivers have a serious flaw: Their diaphragm is under-damped, generating strange resonances. This might sometimes be perceived as pleasant, like adding a little bit of reverberation. However, it is departing from reproducing correctly the signal, so I cannot recommend such HF drivers.

Finally, there are some quite interesting designs using very high ribbons handling the majority of the audio spectrum. In that case, the far-field directivity is extremely high in the mid and high-frequencies, but the near-field sound coverage is sufficiently extended as its height is the one of the ribbon (see drawing below)

Lateral view of a ribbon transducer sound field. The height of the sound field is constant in the near-field zone, and its shape is a portion of a cylinder. The spherical portion of the sound field (in green) is actually a portion of a cylinder having a horizontal angle identical to the one of the cylindrical sound field.

Two things must be noted :

            ° The directivity of the far field (spherical) increases with the reproduced frequency and the height of the source, so, as the frequency increases, the vertical angle of the far-field coverage is reduced, and the Dv distance between the junction of the two sound fields and the source increases.

            ° The attenuation of SPL with distance is only -3 dB per doubling distance in the cylindrical field, as the wavefront expands only in one direction, whereas in the spherical field the attenuation is -6 dB per doubling distance and the wavefront expands in 2 directions.

Side effect: The further you are from the source, the more this attenuation law increases the relative level of HF versus mi and low frequencies. In normal sized rooms, this effect is not of enough amplitude to seriously affect the tonal balance

In this type of large-bandwidth ribbon transducers, we find BG group and Wisdom audio.

Wisdom audio ribbon loudspeakers

The wide-band drive units

Wide-band loudspeakers used to be very popular a few decades ago, especially in their whizzer-cone configuration. This is the only configuration that is compatible with a dimension capable of providing a sufficient sensitivity.

Often their sensitivity is quite good (94 dB to 99 dB /1W/1m, typically for an 8” driver), but they only offer a low power handling.

This type of transducers has been out of fashion by the late 60’s, when they have been replaced by low-efficiency 2-way designs.

For at least a generation, these designs have been only used for low-cost industrial P.A., typically in-ceiling fed by a 70V or 100 V line.

It is only at the beginning of this century that some audiophiles re-discovered the liveliness and high dynamic capability of this type of drivers.

A handful of manufacturers have focused on improving these designs, and particularly on the linearity of their frequency response, quite uneven in most cases.

Example of a 8’’dual-cone driver (mounted on a plane baffle) frequency response

Most audiophile loudspeakers designs based on this type of driver use very large rear folded horns to increase the low-frequency response, which is typically weaker than the mid-highs. The main problems of this approach are the size of the final loudspeaker, the cost of the cabinet, and the low power handling (typically 30W to 50 W). This low power handling is due to the need of a very low moving mass, implying a small voice-coil.  

To be honest with you, although I would never endorse this approach for the drawbacks I have mentioned, it has been one of my most enjoyable listening experiences. The loudspeaker was using a 8’’Lowther driver loaded with an unfinished front horn loading of the ‘Theater Voice’ type in a huge ported cabinet.

I think a more sensible design could be a 2-way type, with a classical high-power bass driver for the bass up to somewhere like 250  to 500 Hz, crossed over with a dual cone high-efficiency wide band driver reproducing frequencies above the cutoff.

A digital crossover with DSP could also be used to smooth out the HF response of the whizzer cone.

This type of design would keep the dynamic capability and the clarity of the wideband driver, provide a sufficient SPL, and would be of a decent size, unlike the loudspeaker of the above picture.

Electrostatic loudspeakers

These speakers have proven excellent in reproducing a realistic vocal sound. The reference is still the very old Quad ESL 57, although its limitations make it totally unsuitable for Home Cinema:

-       Low SPL output

-       Erratic directivity

-       Room placement sensitivity

Advertisement for the Quad ESL 57. The model must be as old as my grand’ma now!

More recent designs in electrostatic panels seem to have overcome these limitations, but I haven’ t heard anything right for Home Cinema in this technology so far.

• Loudspeaker shapes

There are mainly 4 types of loudspeaker shape:

‘’Bookshelf loudspeakers’’

These are mostly rectangular loudspeakers of a small size. They are supposed to be installed on a bookshelf. Typically used for near distance 2 channel Hi-Fi, they need to provide a decent low-frequency extension in spite of their small volume. This is at the expanse of the sensitivity and of course of the available SPL. For this reason, such loudspeakers cannot be used for true Home Cinema installations, except maybe as surround speakers ought to their small size, provided they do not have a rear-firing vent.

‘Classical’ loudspeakers

These are loudspeakers that have similar proportions to the ‘bookshelf’ type, but with larger dimensions. Actually, they are way too big to be placed on a bookshelf. They were the most popular ones when the WAF was not around. Today, they have a quite pertinent application in dedicated Home Cinema rooms, hidden behind a screen.

The can be stand mounted, on a support structure, or even preferably integrated in a baffle wall to avoid rear wall reflections (see in the Acoustics chapter that will come soon).

Floor-standing loudspeakers

Typically, they are of the same designs as bookshelf speakers, but instead of being installed on a stand (actually, most bookshelf speakers cannot be installed on a bookshelf, as they have a rear-firing port!) they are significantly higher so as the tweeter is at about ear-height when the speaker is just standing on the floor. Therefore, for a same footprint there is a significantly increased loading volume allowing a decent bass extension together with a nearly acceptable efficiency.

However, in most cases, the available dynamic range is still limited by a direct-radiating dome tweeter.

There are a few ‘floor standing’ loudspeakers that are designed for a good dynamic range. However, their shapes and sizes make them a bit difficult to install behind a screen…

                 

 “In-Wall”loudspeakers

These are to be installed in a hole cut into a drywall partition. This type of speakers has long been considered as cheap, entry-level designs only because they were meant to blend in the decor. For this mere reason, everyone assumed that the appearance and the ergonomics were the only designing factor, and that it was accepted that they would sound awful.

Bias!

These loudspeakers, of course, have to be quite shallow (3” to 4” max.), which implies:

-       Quite shallow drive units

-       Small acoustic load (a few litres max.)

The first criterion is problematic for horns and compression drivers, but still allows the use of ribbon drivers or dual-cone wideband drivers.
Actually, as the size of a horn is to be proportional to its lower limit frequency, one can use very short horns provided the HF is cut off at > 2 kHz. This typically implies a 3 way-design with a midrange driver.

The second criterion is not a problem in Home Cinema, as you can always use a separate subwoofer to reproduce the lowest frequencies (<100 Hz)

Finally, in-wall loudspeakers do provide some inherent advantages:

-       The rear wall reflection is not existing, as they are mounted flush in the wall

-       They ease the installation of the screen on the wall, not being protruding.

-       The WAF criterion is fully optimized, allowing their use in multimedia rooms

Recently, well-respected manufacturers have started producing high-profile and thoroughly designed in-wall speakers. However, as they do not use compression drivers, the SPL levels are generally limited below what we would expect.

‘On The Wall’ loudspeakers

There is very little to say about these : Their designs are similar to In-wall loudspeakers, except that the rear wall reflection is to be dealt with. When it has not been overlooked by the design team, results can be quite good, and they are easir to install than In-Wall speakers.

On – Wall loudspeaker by Artcoustics

• Other criteria

-Thermal compression
Thermal compression is a natural, inherent self-protection of the loudspeakers (against excessive power) and a plague for a good sound reproduction.

What is it?
When the voice coil temperature raises, its resistance increases significantly. It can double or even more in some instances.
Applying the Ohm law, when a resistance increases, the power fed into the drive unit decreases, as the signal is a voltage. This translates into a reduced output.

Now, as the more power you feed into a loudspeaker, the higher its voice-coil temperature is, this creates a compression effect.

It is a dynamic non-linearity due to a loss of sensitivity, which on a multi-way system might well end up into a tonal unbalance at high levels: There is no reason why an HF driver should have the same level of thermal compression as the LF driver at the same time.

SPL and thermal compression curves (here called ‘Power Compression’ or PC)

Example of thermal compression as provided by a professional driver manufacturer (Beyma document) Please note the exceptional performance of this driver : the compression is only 2.2 dB at 700 W (this driver accepts 1.6kW)

But there is worse…

Temperature changes the impedance of the driver, as seen. If the loudspeaker comprises a passive crossover of an order higher than 1, the shape of the crossover function will be changed, as it is determined by the ratio of capacitances and inductances vs. the driver impedance. These changes, of course, will not be the coherent in the low-pass section and in the high-pass section.

This is most likely to incur an erratic phase response at the crossover frequency, with a crossover alignment that varies with input level.

There is nothing you can do about it, except using an active crossover instead of a passive one, or a series crossover instead of a parallel layout.

-Loads

The most common acoustic loads are the bass-reflex and the sealed box.

The bass-reflex load

When you need bass extension, it is the best compromise to get it naturally in a reasonable volume. By ‘naturally’, I mean without electronic eq.

However, it has its inherent flaws:

• The loudspeaker is often underdamped, with a sound continuing after the signal has been cut.
• Vents exit noise, due to the speed of air in the vent and the sudden change of acoustic impedance at the exit.
• Irregular and phase response and reactive impedance in the 80Hz-100 Hz-zone making a proper crossover alignment most unlikely in this region.
  
  

The sealed-box load
It provides the best dynamic response and the shortest group delay. Its relatively simple impedance and phase responses make it reasonably easy to crossover in the 80 Hz-100 Hz region, provided its natural resonance frequency is not located in this area. However, if the loudspeaker is supposed to provide an extended bass response it will have to be bulky and/or to use the assistance of electronic eq.

• Ideal response for crossing over with a subwoofer around 80 -100 Hz
• Very simple cabinetry and hence reduced manufacturing cost.
• Thermal problem: The rear of the bass drive unit is not in contact with ambient air. When hard driven, the temperature raises inside the cabinet, not allowing a proper cooling of the voice coil, increasing thermal compression. Very few drivers manufacturers have addressed this issue by placing the driver motor in front of the cone (Volt, Peerless. See the Volt driver in the speaker on the picture below)
  

 

The dual chamber ‘symmetrical’ load

Quite uncommon, this type of load is sometimes used for subwoofers

Its bandwidth is too limited for any another application

The transmission line

Quite unusual, although it generally provides a quite good bass response. It requires a bulky and expensive cabinet.

The open baffle

Quite pleasant to listen to when properly setup, it is very sensitive to room acoustics and to its placements with respect to the nearest walls. This is due to its 8-shaped directivity (dipole). For this reason, it is not really usable in Home Cinema installations.

Well, all this being said, I’ll leave the ‘politically correct ‘ region and give my raw opinion…

>   The Hi-Fi ball and chain
I’m not able to count the number of Hi-Fi loudspeakers manufacturers, but sometimes I feel they are more than their customers. It looks like speakers design is an ‘open’ sport contest.

>   The history
From studio facilities, the design of loudspeakers has evolved in the late sixties to residential applications. From a specification which was to reproduce the sound as realistically as possible (wasn’t it a good idea?) it has changed to reproducing recorded music as pleasantly as possible. From then, loudspeakers started to become specialized. Strangely enough, the ‘High-Fidelity’ label was applied not on the loudspeakers that were meant to sound ‘realistic’, but to those which were designed to sound ‘pleasant’.

>   The MP3
Worse, worser, worsest (is it time for my English grammar lesson?). Now music is recorded and mixed so as to sound ‘not too bad’ on a mobile phone. Or at least one has to hear a little bit of it. This of course cancels the possibility to maintain a sort of dynamic range in the musical content.
When you play one of these tracks on a normal sound system, it sounds pretty boring (surprised?). Here comes the Hi-Fi smart designer: The loudspeakers are designed to add a little bit of pleasantness on these boring soundtracks. What’s wrong with it? By the way, this is called ‘musicality’.

>   The brute
Is the movie soundtrack. With its full dynamic range and its devastating sound effects, it was never intended to be played back on a mobile phone. The recording studio has a sound system that is generally the same as the playback one, a proper professional sound reinforcement system of a commercial cinema. The recorded track is sometimes directly copied on a blu-ray, sometimes remixed (we’ll never know unless we are there).

>   The gap
Is between the design criteria of Hi-Fi loudspeakers and the requirement of movie soundtracks playback, even in a residential environment. So, if you really need to playback both recorded music and movie soundtracks through the same system, you need to put together the dynamic capability of a concert P.A. system and the sweet distortion-free, coloration-free, natural sounding of near-field monitors, but with a slightly less ‘midrange forward’ tonal balance.

>   The way to do it
Is anything but easy. I cannot provide a magic 5 minutes design course on loudspeakers that will solve it all. I can just list some tings I would definitely avoid:

•  Low- power bass loudspeakers
•   Low- sensitivity  midrange drivers
•  Unloaded Dome  (direct-radiating is another wording) tweeters 
•   Parallel Passive crossovers
•  Expansive and bulky acoustic loads

And there is one thing I would definitely want:

•   A digital crossover with DSP eq and delay functions.

>   The way to evaluate it
Do not play your favourite CD on it. It is always pleasant to hear what you like, but this can fool you.
Instead, take a microphone (either a high grade one, or a “vocals” one like the Shure SM 58), a mike preamp or a small mixing desk, and feed this signal in mono to one loudspeaker (never two at the same time). Now, speak in the microphone and listen to your own voice.
You may argue that you don’t recognize your voice when it is recorded. Well, this is not surprising if you only listen to check the recorded message of your voice mail, but with a decent microphone and a good loudspeaker, it should sound right. Also, you should have a reference loudspeaker to make AB comparisons, and means to switch instantly from A to B.
It will take only a couple of seconds to identify which of the two speakers sound right and which one is definitely wrong. But the “right” one might not be 100% right: It is only better than the other one…

Now, if you are in doubt on how the voice should sound, try to get an old Quad ESL 57. This is hard to beat in terms of natural voice, although, as mentioned before it is not suited to Home Cinema.

>   Finally,
There is no need for garden-hose sized loudspeaker cables which provide an extremely low insertion loss and oddities if you have a passive crossover with an inductor having a parasitic resistance of 1 or 2 Ohms in series with the driver voice coil, the amplifier damping factor is gone anyway. Still, these expansive, bulky and rigid cables do have a real consumer advantage: It is very difficult to hang yourself with it…

Technical appendix

• When calculating the acoustic pressure, the formula is: SPL = 20 Log P/P₀,
  P0 being the reference pressure
• When calculating the power, the formula is: WL = 10 Log W/W₀
  
  

This just makes sense as W is proportional to P²  

• Let’s look deeper into the first calculation:
   96 dB /1W/1m are 96+10Log 20 for 20Watts = 109 dB
  at 4m, the attenuation is 12 dB, that is 97 dB (spherical waves are attenuated by 6 dB per doubling the distance).
  Now, if we lose 6dB because of the thermal compression, we are left with 91 dB
• Let’s look deeper into the second calculation:
  1W in 360° spherical radiation provides 109 dB at 1m. If we reduce the radiating surface to a 90° sector in one axis and 40° in the other axis, the sphere area is reduced by a factor of 36. The power being the same, the Intensity Level is increased by a factor of 36.

As the intensity is proportional to power, the formula is :

IL= 10 Log I/I₀
In our case, 10 Log 36 = 15.5 dB
The 20%  efficiency gives an attenuation of :
10 Log 0.2 =-7dB
So the sum is 109 dB + 15.5 dB -7 dB, that is 117.5 dB. At 4m, we still have 105.5 dB for  1W  power. At such a low power, the compression driver that is typically handling 50 Watts will not even warm. There is no thermal compression.

Now if we raise the power to 50 Watts, the level will increase by 10 Log 50 = +17dB, but we if we lose 6 dB of thermal compression, the level will be 116.5 dB @ 4m. This is sufficient!

This might help now

TO BE FOLLOWED!

Setting the scene for success: Designing a demo system - Chapter 3

In the last chapter, we made an overview of the SPL needs. Now we can have a look at the Home Cinema sound system configurations.

A professional approach to sound is often disturbing the typical Hi-Fi salesman’s or consumer audio professionals.

It only takes to read a so-called « technical » brochure about a loudspeaker to understand how deep is the problem: You will find supposedly meaningful words like “rhythm”, “pace”, “precision”, “fast”, but nothing that will really help the installer to design a system, like:

• Directivity response vs. Frequency either polar or Matlab
• Directional gain
• Maximum SPL @1m without cheating (like when forgetting the thermal compression)
• Frequency response at maximum level (generally different from what it looks like at 1W)
• Distortion curves at various levels (at least 10V)
• Waterfall response diagrams
• A CAD drawing
  
  

Without this information, the system designer is left to either do an approximate job, or to perform all these measurements himself. This is particularly critical when matching the loudspeaker to the room acoustical environment and the audience geometry.

Such information is compulsory and most commonly provided with professional  (I mean here: non-residential) loudspeakers, as sound engineers simply do not accept working without it.
I have to say, strangely enough, I never found a directivity curve provided with any loudspeaker aiming at the residential market.

Now, if you start believing that such a concept as directivity control has never been considered in the design of residential loudspeakers, I cannot really say that it is 100% true.

Maybe only 99%... (I'm kidding ;-) )

I am not suggesting here that most residential loudspeakers are ill designed; there are good choices around. But in many cases, you will have to pay for a nice veneer or a glossy lacquer when your speakers will end up hidden behind a screen!

And the sound system designer is left alone without the proper information…

• Unbalances

A Home-Cinema sound system comprises much more components than a Hi-Fi system, needless to say.

So it is easy to understand why the most demanding customer, willing to get only the best available components for each function of his home cinema, will end up postponing the purchase of his new Bugatti Veyron.

For the vast majority of even affluent customers, there is a limit to the budget, and hence compromises.

 To illustrate this, let me first describe a typical Hi-Fi system. It comprises:

• 1 CD player
• 1 vinyl turntable (optionally)
• 2 analog line-level cables
• 1 integrated amplifier
• 2 loudspeaker cables
• 2 loudspeakers
  
  

Diagram 1

Now if we describe the audio inventory of a basic Home Cinema sound system, there must be at least:

• 1 Blu-ray player
• 1 HDMI cable
• 1 digital audio cable or a 2^nd HDMI cable 
• 1 AV receiver
• 7 loudspeaker cables, at least 100m total
• 1 line-level audio cable
• 3 front loudspeakers
• 4 surround loudspeakers
• 1 amplified Subwoofer

This, of course, is not including the video part of the installation.

Diagram 2

And for a more sophisticated installation, we will find:

• 1 Blu-ray player
• 1 AV server
• 1 switcher
• 2 HDMI cables
• 1 power conditioner
• 1 preamplifier-AV processor
• 8 audio line-level cables
• 2 DSP crossovers
• 12 XLR balanced audio cables
• 12 channels of power amplifiers
• 2 Subwoofers (passive)
• 3 bi-amped front loudspeakers
• 4 passive surround loudspeakers
• 12 loudspeaker cables, total at least 150m

Diagram 3

It is now easy to understand how important the cables budget could be, and that esoteric audiophile cables are to be discarded (in addition, their stiffness and diameter can become a nightmare for the installer).

It is also easy to understand that mono-block valve amplifiers weighing 50 kg each and rated 12 Watts are not appropriate.

So you will have to forget about spending days doing A/B comparisons between single components or cables.

An HC system needs to be optimized in the perspective of its total budget, even if large.

So, in this case, optimization means balance. Let me get to the point:

It is known that what you hear in a system is its weakest link. Balancing a system means changing the weakest link to a better one, until there is no weakest link at all.

Now, where do you start?

Well, you must have an opinion about components quality.

This is where the image becomes plain white and the sound an unbearable silence…

How to evaluate the respective quality of the various elements in a complex system as pictured above in diagram 3?

I’m sure my answer will disappoint all fanatic readers of AV magazines: You need professional experience.

If you read tests in the medias, you will never find anything that justifies the 1 to 20 price ratio between components which are all rated as “excellent”. So, how will you select your components?

A tiny minority only selects world-renowned brands, top class ranges, and very, very expansive stuff. Their customers are the like who will not even have to postpone the Bugatti Veyron purchase…

Others (they are more) spend their time on forums. Actually, they don’t need to get involved in HC installation, they do not have time for this as they can spend all days on the internet.

Some others rely on 3 letters products certifications which prove that their suppliers have paid the flat fee plus the royalties (it’s funny, all certifications have 3 letters labels! none is for free)

What I consider as the best approach is to rely on your own experience with the equipment, as you don’t have the time to spend your life testing and comparing components. As a professional, you have installation jobs on your schedule.

However, you need to get a new component under test from time to time, otherwise the risk is to become outdated.

When designing video systems, things are crystal clear: You choose a screen (and Excellent one, preferably) and a projector after checking their compatibility (lumens vs. dimension) and then you calibrate the projector, play an image and evaluate the result.

Simple!

When designing a sound system, it is not so simple

An HC sound system is involving quite a few components, and the complexity of the system is an exponential function that has the number of components as exponent. Then, the results are audible instead of visible.  This makes a real difference, as the sound is by definition vanishing immediately, whereas an image can be made steady.

There are some measurement methods which are rigorous and reliable, fortunately. Again, they are better known in the professional audio industry than in the residential one.

• The mains supply

Considering the number and diversity of devices connected to the mains, it is quite difficult if not impossible to have a thorough evaluation of the power supplies of each element.

A well designed power supply will not incur any problem even with a relatively unsteady or polluted mains supply.

However, many electronic devices, even in the “high-end” ranges, suffer from dependence with respect to the mains supply quality.

This quality varies with the location of the installation (it is generally better in cities than in the countryside) and with the time of the day.

Therefore, it is wise to be careful about the electrical part on the installation, not only for safety reasons, but also for sound quality.

A few things to do:

1)    Draw a direct line from the mains connecting board. Only audio devices must be connected to this line. Any other electrical appliance, especially light control devices and/or machines comprising an electrical engine have to be discarded.

2)    Check the grounding quality (< 10 Ohms preferably)

3)    The cross sectional area of the conductors must be sufficient to handle at least twice the total maximum power consumption of all connected devices.

4)    Use a power conditioner providing sufficient available power for at least all devices that are operating at line level. When estimating the necessary power. Provide a large headroom.

5)    Install a surge protection device.

As far as 4) is concerned, typical « audiophile »  sockets strips are not providing sufficient available power and are sold at a price that can be justified by the amount of silver, gold or complex engineering, but not by the insulation they provide.

Ideally, the power amplifiers should also be connected to a power conditioner. However, their power consumption requires a very powerful, bulky and expansive power conditioner (see image below). For this reason, such devices are not common in Home Cinema installations.

 22 KVA mains conditioner

• The sources

Many audio/video sources provide a decent quality at very affordable prices. This does not mean that you should use a Blu-ray player sold at £99.99 to feed a £100,000 installation !

There are typically 3 types of A/V sources :

Ø  Blu-ray players

These devices are the simplest to use and to install. They are also the most affordable ones. Still you need to checK

• HDMI standard (which generation ?)

• The nerve-wrecking duration of the starting process

• The mechanical sturdiness

• The image and sound quality, by comparing it to the competition 

• At the moment, one brands seems to have a leading edge (check our stand at the past 2014 ISE)

 

-      

Ø  The HTPC

One advantage of  HTPC  devices is the  « all in one » aspect. You can record blu-rays, download video, shortcut lousy legal announcements, get a fast access to recorded material, in short, happiness!

But…

Either you are a true Geek, and configuration problems, compatibility issues etc. are peanuts for you,

And

You know how to feel like Mr.Everybody and design interfaces that even your client’s grandma will find funny to use

Or

You do not have both of these skills, and you are heading to a real nightmare.

Ø  Audio/Video servers

Audio / Video servers are a bit like HTPC, but they are pre-configurated and (nearly) ready to use. They are seemingly the ideal solution, combining a user-friendly interface and a huge storage capacity (count in Terabits).

In fact, most still suffer from two limitations:

-       Very high prices

-       Or outdated performances (Full HD was not available on some servers for quite a while, for instance, or no lossless sound format)

-       Or sometimes both problems

So, the remarquable comfort of use of these AV servers has a price   

The choice is yours…

• Decoder-preamplifiers

For once, this is easy: Since lossless audio formats are available (DTS Master and Dolby True HD), you don’t really feel like listening to anything else (in Home Cinema, I mean).

So you first need to check that decoders support both these formats.

Other things you will need to check are:

•   HDMI quality and stability. It sometimes “bugs”

• Menus clarity and ease of use

• Clarity of the various functions and configuration, especially the ambiguous “Bass Management” and “LFE Mix”. You definitely need to understand what the device is doing, otherwise you will not be able to achieve what you want. 

•  In the case of integrated “receivers”, the availability of pre-out connectors is essential. It will allow you to insert control devices, like a digital crossover or an equalizer.

• Availability of a manual “lip-sync” adjustment.

Now, if your preamp is provided with an automatic equalization function using a measurement, do not worry. What you only need to do is to bypass this automatic function.

Anyway, never use it !
We will see why in a further chapter about EQ.

Now that the first elementary requirements are met, what makes a difference between AV pre/pro is the sound quality.

I do not mention here the video quality, as I strongly believe that the video signal of the main source should be connected directly to the video projector. You will need 2 HDMI outputs from your source, as one will be sending the audio signal to the decoder.

Checking the sound quality is something, I believe, you do not need to be advised on. You’ll make your own evaluation.

• Amplifiers

The amplification of a Home Cinema system can be complex, as there are quite a few amplifier channels involved.
The above diagrams #2 and #3 illustrate this. According to the setup on diagram #2, 8 channels are needed, and on diagram #3, 12 channels.

People used to 2 channel audio may think it is too much, but this is only because they are so used to traditional Hi-Fi that they are reluctant to get into the logic of real AV sound systems.

So, rule number one is like when scuba diving: never panic!
Looking closely at diagram #3, you will see that the loudspeakers are bi-amped. This means that the HF amplifiers do not necessarily need to be as powerful as the LF ones.

It is your choice: If you are budget-oriented, you will use less powerful amps for feeding the treble channels and the surround channels, whilst using the most powerful channels for L,C,R bass channels and LFE.

If you are simplicity –oriented, then you will use large power amplifiers throughout.

In the setup pictured at #2, we can choose between 3 types of solutions. The simplest (and most common) one is to use an amplified subwoofer and a 7.1 integrated receiver. However, this does not deliver the best…

In the vast manjority of the AV receiers, the 7 channels (usually identical) are supplied by one single power supply, to minimize manufacturing costs.  If some of these receivers can offer 7 x 200w, most are limited to 7 x 175 W and oddly enough cost twice the price of entry level devices rated 7 x 155 W.

Ain’t that strange?
Looking more closely, you will find that this power rating is for a non-standard 6 Ohms load. Even more interesting…
Then, you will discover that the rated 155 W per channel is only possible when only 2 channels are in use, but that all channels driven simultaneously, you will only have 7 x 112.63 Watts!

This of course is written in extremely small fonts.

This image is not displaying the virtual AV receiver I am describing above.
 It only shown that there is a single power supply for all channels.

There is an easy explanation to this: The power supply is designed for providing a cost-optimized juice for the 2 so-called “main” channels (this is irritating: there is only one main channel in Home Theater, the centre one). When more channels need power simultaneously, the power supply just gives up.

Pursuing our investigations, we will discover that this magic do-it-all device affords a distortion level which is less than 0.5% and an s/n ratio > 90 dBA.

Not bad for a 60 years old single-ended valve amplifier!

Well, it is not, actually.

I am not trying to discourage the thorough and smart attempts of the marketing departments to dissimulate the fact that the receiver is an entry-level, low cost and low performance product by presenting it as a high-end one. The marketing manager job is difficult these days, and I really appreciate their creditable efforts.

In the pro-audio industry the use is to nickname an amplifier “black box with gain”. This really gives the right idea of what it is. Or should be.

(I call a single channel an amplifier, for the sake of simplicity)

Its function is to use the electrical energy from the mains to transform an input signal of X Volts in an output signal of X * G Volts, G being the gain.

Professional amplifiers have a normalized value for G, either 26 dB or 32 dB (G=20 and G=40, respectively, as 20 Log 20 = 26 and 20 Log 40 = 32)

This is an ideal world situation.

Please note that X*G is a voltage, not a power.

In the real world, the output signal is: (X Volts * G = X.G + e₁ + e₂ + e₃ +….. e_x)

where e_1, e_2, e_3…are harmonic distortion, non-harmonic distortion, noise level, etc.

Here comes the maximum output power…

The X*G product is limited to a definite value which is inherent to the amplifier. When X increases so that the output is becoming near to X.G _max, G is reduced and e₁ is seriously increased, especially in odd harmonics (the most unpleasant to the ear).

So it seems straightforward to select an amplifier: Maximize X. G _max, minimize e₁ + e₂ + e_3.

The larger X. G _max, the less you are likely to get near to this limit, which is called clipping level.

In some literature it is stated that some valve amplifiers can sound “louder” than what can be expected from their maximum output. This is supposed to be because their clipping is more “musical” than the one of other amplifiers, so you can drive the amplifier harder. Personally, I am very suspicious about the “musicality” of clipping.

This does not mean that amplifiers all sound about the same when they are below clipping level. My point is: I strongly believe that an amplifier that does not clip sounds better than one clipping.
Of course it suggests selecting large power (voltage) amplifiers, which is not the cheapest solution.

Now, let’s look at the power figures, which depend on the load impedance. The impedance of an actual loudspeaker is not a constant, like a resistance, but a function that varies with frequency.

The power figure is given by the Ohm formula: P = U²/R

Therefore, the power P is inversely proportional to the impedance (here expressed as the resistance R). So, if you divide R by 2, you double the power.

The problem here is that X*G _max is, in theory, constant for a given amplifier. In reality, it is only constant within a certain impedance range.

Having a look at the figures in the literature, we will see for instance and amplifier capable of delivering 200 W in an 8 Ohms load can only provide, say,  320 W in a 4 Ohms load. Applying the Ohm law, we will find that the X*G _max value is 40V into 8 Ohms, and only 36 V into 4 Ohms. This shows that 4 Ohms is outside of the linear range of X*G _max.

Bad news!

Looking at various amplifiers figures, we will see that for, say, 200 W into 8 Ohms, some amplifiers will deliver 400 W into 4 Ohms, whilst others will only deliver 280 W, for instance.

There are even amplifiers delivering less power into 4 Ohms than into 8 Ohms. This sometimes explains why the power is expressed into a 6 Ohms load: It is the load into which the power is maximum, this being used as a (poor) selling argument.

In some instances, this can be due to the circuit design, but in most cases it is a limitation from the power supply.

The technology of a good power supply is straightforward and well known, so it is not a design issue.

The real issue is cost: In most power amplifiers, the power supply is the main part of the cost (and weight in traditional analogue designs).

So, the trade-off in limiting the cost is limiting the ability of the amplifier to drive low impedance loads.

And the more linear is the power vs.impedance relation, the better the amplifier.

Well, this does not explain it all. There is more...

Nearly everyone knows, amplifiers have different sound qualities. We are in the real world, actually very far from the ideal “black box with gain”.

We can look in detail at e₁ + e₂ + e_3, etc; beyond a certain level of “decent quality” it will not give us any significant information. If distortion is below 0.01% (-80 dB below signal level) it will not be perceived as it is masked by the sound of the signal itself. Now the noise floor (which does not depend on the signal) will not be audible if the s/n ratio is higher than 110 dB (even with high-efficiency horns). By the way, the s/n ratio is a very interesting indicator of the built quality of the amplifier.

Stranger even is the significant variation of tonal balance between one amplifier and another, which is never suggested by any frequency response measurement (it is always ruler-flat).

About this, I would like to mention a strange experience I have made.

I was working at the R&D of professional sound reinforcement loudspeakers. Some products in our range still had passive crossovers (cost, again!), and we were setting up a QC measurement of the crossovers.

We used to do it a rather high levels (circa 50 Volts), as they were heavy-duty loudspeakers.

We measured the frequency response of the voltage across the crossover load. Also this is not meaningful, what we were looking for is the identical reproduction from one crossover to the other. It was only QC.

Once we found results that significantly differed from the standard. Strangely enough, all crossovers in the batch had an identical response, but significantly differing from the previous batch. The implementation was correct, and the components were not different. The only different element was the power amplifier that had been replaced by another one.

So we checked the frequency response of the power amplifier itself, driving the complex load presented by the crossovers under test.

We had the surprise to discover that it was very far from ruler-flat, so we thought it was faulty.

Then we re-installed the former amplifier. It had a quite different response, but still far from flat!

Then, we reduced the voltage by steps, and discovered that the curves became nearer and nearer to the ideal straight line when the voltage was reduced. About 10 dB below clipping, the response of both amplifiers was the same straight line.

The most concerning part was the serious unbalance of the response at high levels, with deviations of up to 6 dB for the “weakest” of the 2 amps.

I found reassuring that the “weakest” amp was the one which was not so good sounding…

It is unfortunate that I did not have the time or later the opportunity to further investigate this experiment, but I did think about it and found a possible explanation, although it is only a not-verified hypothesis.

Its is therefore to be taken with great caution

Looking back at the available power from, for instance, amplifier A:

• 200 W into 8 Ohms
• 300 W into 4Ohms

Whereas amplifier B delivers:

• 200 W into 8 Ohms
• 400 W into 4 Ohms

It is easy to understand that at frequencies where the loudspeaker impedance is at its minimum, 4 Ohms for instance, amplifier A will have its output voltage limited at a lower value than at frequencies where the impedance is higher. This of course only occurs at high levels when the voltage is near to its maximum.

Still, the result is affecting the frequency response of the amplifier, as the voltage will be higher at some frequencies than at some others.

In the same conditions, amplifier B will not meet this issue, as it is capable to deliver the same voltage into 4 Ohms and into 8 Ohms. Its frequency response will remain unchanged.

The serious variation in amplifier frequency response I described above suggests that the limitation phenomenon starts occurring at levels that are far below the clipping level: There would be a progressive modification from the straight ideal response to a load-limited clipping response as the level increases.

It looks like a “soft-knee” clipping, which would occur at different levels according to the load impedance and the power supply ability to handle it.

I insist that this is only a hypothesis, I did not investigate this phenomenon deeply enough to derive a definitive conclusion. However, this could explain why the tonal balance seems to significantly differ from one power amplifier to another one.

And there is no risk to deduct the following preferences when choosing an amplifier:

1)    When the budget allows it, it is better to use one channel per loudspeaker driver (multi-amplification) rather than passive wide-band loudspeaker systems: The impedance of individual drive units is typically easier to drive for an amplifier than the wide band load of a passive loudspeaker with a reactive crossover.

2)    When the budget allows it, it is better to use very powerful amplifiers. They are more likely to work in their linear range anyway. Further, their (unpleasant) clipping threshold is rejected at higher levels.

3)    Choose amplifiers provided with a properly dimensioned power supply and a significant headroom, so that the maximum voltage does not vary too much with respect to the load impedance.

4)    Select preferably amplifiers with a high s/n ratio (>105 dB)

Well, this has been a bit long, and I just hope you did not fell asleep.

To be followed soon…