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!