How I would setup a demo system: Chapter 2

HOW I WOULD SETUP A DEMO _ Chapter 2

How to place a system in the room

Installing a sound system for a Home Cinema (Theater) is problematic most of the times.

A proper location of all the elements is essential for a good result. It has to follow basic, common sense rules, but these do not suffer compromises, and here lies the difficulty.

These rules can be deducted simply by observing how the commercial cinemas (theaters) that are all designed according to a similar layout:

-       The projector axis must be in the median plane orthonormal to the projection surface

-       The centre channel speaker must be in this same plane

-       It is most desirable that the screen is Acoustically Transparent, that the centre speaker is near to the middle of the screen height and that the 3 front speakers are at the same height behind the screen. This will never be enough emphasized!

All explanations about why and how the screen must be Acoustically Transparent are available from our website www.screenexcellence.com. However, if you need further comments or explanations, you can always ring me or send me an e-mail.

-       The three front speakers must of the same type

-       The left and right speakers must not be too near to the lateral walls

-       The surround speakers must be higher than the audience. If there are only 2, they should be placed behind the audience. If there are 4 or more, they should surround the audience.

-       Any mirror or reflecting surface is most unwanted near to the screen

-       The projector must be fixed in a secure and stable way

-       The screen has to be installed at the right height. Although THX recommends that they are kept between +/- 15°, I think the sightlines at the centre row should not deviate more than 5° from the horizontal

-       The projection axis has to be either parallel or perpendicular to the main room axis, never in diagonal

Whenever it is not possible to meet these basic rules, the very choice of the room has to be questioned.

Is an acoustic treatment needed?

The answer lies in the question: A treatment is needed when the acoustics is not suitable for the application.

In the case of a fully decorated, cosy living room, the acoustics is often suitable and a specific treatment is not needed. However, there is often a problem with the symmetry rule, which can imply a partial or total refurbishing.

Fortunately, although not ideal, typical living room acoustics are suitable for Home Cinema, with reverberation times (RT60) comprised between 0.4 and 0.6 seconds

In dedicated cinema rooms, the situation is quite different: At the start, there is nothing in the room, but walls, a ceiling and a floor. In that situation, it is not a treatment that is needed, but an acoustic design.

This cannot be simply performed by adding lots of absorbent materials, but involves a thorough knowledge of acoustics and of Home Cinema specific criteria (which are quite similar to LeDe and RFZ, by the way)*.

Other criteria have to be implied, such as loudspeakers positions and directivity.

The basic rule here is that Acoustics is an exact science, and if someone does not have the required education he should not get involved in it.

* LeDe stands for “Live end-Dead end” and RFZ for “Reflection Free Zone”. These two techniques are now common in recording studios control rooms, and tend to maintain a certain level of reverberation whilst rejecting early discrete reflections

Now, there is another aspect in acoustic problems: The sound nuisance due to the transmission through the walls, ceiling and floor to the neighbourhood. Typically, it is not really an issue in fully detached houses, but in any type of dwelling it can become a nightmare. The main problem is that a Home Cinema generates most of its acoustic energy in the low frequencies, unlike most acoustic nuisances.

The lower the frequency, the more it is difficult to prevent it from going through the walls. The sound proofing in the low frequencies is depending only on the mass per unit area of the partition. So, if you want to reduce the nuisance by 6 dB, you will need to double the mass of the walls / floor / ceiling. If your flat is located at the last floor of a skyscraper, I let you imagine the discussion with the architect!

Again, soundproofing is to be designed by a professional (some acoustic engineers are specialized in soundproofing whereas others are specialized in room acoustics. If you are facing nuisance problems, you may need two different consultants).

These are only generalities. I will get more into details in subsequent chapters.

Now, let’s get into the core of Home Cinema design!

Introducsound

From history, sound is what has defined Home Cinema: In the late eighties (yes, this was last century…already!), there has been a move from super-8 souvenir movies with a single tiny wideband speaker firing from a noisy projector to “Home Theater” when the Dolby Pro-Logic format was released.  With the laser disc, and later with the DVD, people could enjoy real movie sound at home.

The installations were really high-end, generally made by AV contractors usually involved in commercial installations. The sound equipment and the whole technical approach were directly derived from the sound reinforcement industry.

Similarly, the video projectors were 3-CRT types, quite bulky, heavy, expansive and requiring at least a full day of skilled labour for being properly setup.

The installation budgets, needless to say, were quite serious and early adopters were only affluent people. But it was worth it, as it could provide really impressive results that could never have been imagined in a home only a few years before.

These installations gained prestige, and the term “Home Cinema “or “Home Theater” became synonym of stunning thrills and sheer luxury.

This was really tempting for the marketing managers of TV manufacturers, which were struggling to sell their new 16/9 format sets. They have been quite fast to cannibalize the term “Home Cinema”, which by 1997-98 was already synonym of 16/9 TV.

However, it was not practical to sell a sound system worth $ 50,000 and cupboard-sized speakers to pair with a TV set!

A solution came from Hi-Fi speakers manufacturers. In search of a new market (2 channel stereo had already been a declining market for a decade), they created small 5.1 Home Cinema packs, sized and priced to match TV sets.

The need for a subwoofer allowed reducing the size of the satellites, operating in “small” mode. The centre loudspeaker was to be located below the screen, so to make it practical, it was designed operate in a horizontal position.

For the sake of symmetry, 2 LF drivers were installed in parallel on each side of a tweeter, and the inherent horizontal directivity issues were just swept under the carpet.

typical 5.1 "home cinema" package

Serious marketing made it a mass market, but admittedly, the systems and the results had (still have) very little in common with the original Home Cinema.

Most fortunately, at the same time some high-end installers continued installing real Home Cinema rooms and specialized in the field. The most recent technologies have been incorporated, and the whole concept progressed dramatically, particularly in terms of imaging.

Today, there is still one single name for “Home Cinema” TVs and proper Home Cinemas, although it is widely perceived that they are not the same thing.

Some Hi-Fi dealers have tried to fill the gap, jumping into the installation business whilst keeping their Hi-Fi culture of tiny low-efficiency speakers connected with garden-hose sized cables. However, most of their sound systems still sound like Hi-Fi, not like professional theatres. Guess why?

Well, residential loudspeakers manufacturers are now designing for the TV “Home Cinema” market, just because there are much more loudspeakers to sell there than in the serious installations.

And this is a closed market: Most dealers never heard of any professional sound reinforcement brand (apart from JBL which is on both markets), and even some manufacturers ignore them.

Still, there is a small technical problem…

The sound pressure level (SPL)

Here we are in deep trouble.

Digital formats allow dynamic ranges of typically 110 dB. The dynamic range is the difference between the background noise (“noise floor” in jargon) and the maximum available sound level (SPL).

In a typical quiet residential environment, the noise floor is about 30 dB. So, if you add 110 dB, the SPL is 140 dB. I am not sure it is lethal, but the threshold of pain for a very short time (expressed in milliseconds, not even seconds) is 130 dB.

Over a larger period (in minutes), the threshold of pain is about 120 dB, however, this level of 120dB is acceptable for a short transient sound. With a noise floor of 30 dB, this allows a 90 dB dynamic range.

Now, the real noise of a blast (like there is supposed to be in some action movies) is really deafening. I heard it once in my life, and it is definitely the loudest sound I can remember. Although not totally realistic, it is not stupid to reproduce it at 120 dB (for a very short time), and this is actually what movie sound engineers do.

When comparing this figure of 90 dB with the maximum dynamic range of live music, which is 60 dB, the difference is 30 dB

Yes, the conclusion is, Home Cinema is supposed to deliver up to 30 dB more than a music playback system. Now, 30 dB is 1,000 times the power.

Short explanation:

WL (power level expressed in dB) = 10 x log (W/W0)

W0 = reference level for comparison

W = measured power

As log 1000 =3, 10 x  log 1000 = 30 dB

Would this mean that swapping from a Hi-Fi system to a Home Cinema one I would need 1000 times more power? This can’t be true!

Well, it is actually…Sorry!

Maths is ruthless with speakers manufacturers!

Practically, there are solutions not to use 50,000 watts power amplifiers, don’t worry. The answer lies in speakers efficiency. Remember, we are looking for 30 dB

Now, if we replace a loudspeaker with a sensitivity of 84 dB /1W /1m by another one having a sensitivity of 95 dB /1W / 1m, we already found 11 dB.
We are left with only 19 dB to find.

By increasing the power by 80 fold (log 80 = 1.9, so 10 x log 80 = 19 dB), we get the desired level. So, if our Hi-Fi loudspeaker was driven by a 100 Watts amplifier, we only need 8,000 watts…

I’m a bit joking here, the comparison is not done the way it should.

Let’s do it again the right way:

If we have a speaker with 95 dB / 1W /1m sensitivity, we need 25 dB more to reach the desired maximum level of 120 dB.

As (log 320 = 2.5), 10 x log 320 = 25 dB

So we only need 320 Watts, right?

No, it is wrong!

a)     The listening distance is not 1m, it is more. The equation for SPL is not the same as for WL, it is
SPL = 20 log P/P0
with P0 = standard air pressure without sound
and P = instantaneous sound pressure
The important number here is the factor 20, which explains why when the distance doubles, the SPL decreases by 6 dB
For instance, if the distance is 3m, the loss in SPL is -20 log (3) = -9.5 dB

And if the distance is 4m, -20 log (4) = -12 dB

b)     In this calculation, we have disregarded an essential phenomenon: The thermal compression.
This is a very nasty effect: when a loudspeaker is fed with a signal that is near to its maximum power capability, its resistance dramatically increases, resulting in a loss of sensitivity, typically between 5 to 9 dB. So, our nice figure 95 dB/1W/1m is gone!

Well, we have lost something like 15 dB, what can we do?

We have only two solutions here:

- Accept a lower SPL and a reduced dynamic range

- Use serious sound reinforcement loudspeakers with a sensitivity of about 100 dB/1W/1m and a power handling of about 1kW

Maybe not that big!

The good news is that surround speakers are not concerned here: In mixing movie soundtracks, sound engineers use the same systems as in commercial cinemas where the surround speakers are never the same as front speakers. They are lower power, lower sensitivity designs and have a different directivity.

As a result, they never pan very loud sounds to the rear channels. So you do not need really powerful / sensitive speakers as surrounds.

To be followed soon!