Audio Excellence

How to place a system in the room

Installing a sound system for a Home Theater is quite often problematic.

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

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

The projector axis must be in the vertical 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 can never be emphasised enough!

The three front speakers must be of the same type (forget about the dedicated “central channel” speaker designs. These are made for television, not proper cinema at home).

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

The surround speakers must be higher than the audience if you do not have height channels. If there are only 2, they should be placed behind the audience. If there are 4 or more, they should surround the audience. If you have height channels (3D audio), then you can place the surround speakers at about ear level.

Any mirror or reflective 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 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 are not suitable for the application.

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

Fortunately, although not ideal, typical living room acoustics are acceptable 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 an acoustic 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 are involved, such as loudspeaker position 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 studio control rooms, and tend to maintain a certain level of reverberation whilst rejecting early discrete reflections.
http://www.audiosonica.com/en/course/post/196/Acoustics-LEDE_Control_Room
http://realtraps.com/rfz.htm

Now, there is another aspect among acoustic problems: The sound nuisance due to 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 difficult it is to prevent it from going through the walls. The sound proofing in the low frequencies is dependent 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 on the top floor of a skyscraper, I will let you imagine the discussion with the architect!

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

These are only generalities. I will go into more detail 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 was 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, expensive and requiring at least a full day of skilled labour for proper 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 a synonym for stunning thrills and sheer luxury.

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

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

A solution came from Hi-Fi speaker 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 a reduction in the size of the satellites, operating in “small” mode. The centre loudspeaker was to be located below the screen (a TV screen cannot be A.T.) so to make it practical, it was designed to 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.

Serious marketing made small 5.1 Home Cinema packs 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 specialised in the field. The most recent technologies had 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 loudspeaker manufacturers are now designing for the TV “Home Cinema” market, just because there are many more loudspeakers to sell there than in the 2-channel installations.

And this is a closed market: Most dealers have never heard of any professional sound reinforcement brands (apart from the JBL brand which is on both markets), and even 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 quiet residential environment, the noise floor is about 25 dB in the best cases. So, if you add 110 dB, the SPL is 135 dB. This is not lethal, but the threshold of pain for a very short time (expressed in milliseconds, not even seconds) is 130 dB.

Over a longer period the threshold of pain is about 120 dB.

However, a Home Cinema is in no way supposed to provide pain or even discomfort. A norm has been set by THX (again!) specifying the maximum level to be reached at any seat in the audience at 105 dB. With a noise floor of 25 dB, this allows an 80 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 110 dB (for a very short time), and this is actually what movie sound engineers do.

When comparing this figure of 80 dB with the maximum dynamic range of live music, which is 60 dB, the difference is 20 dB.

Note, in the case of recorded music, the dynamic range rarely exceeds 30 dB.

So, the conclusion is: Home Cinema is supposed to deliver up to 20 dB more than a music playback system. Now, 20 dB is 100 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 100 =2, 10 x log 100 = 20 dB

Would this mean that swapping from a Hi-Fi system to a Home Cinema one I would need 100 times more power? This can’t be true!

Well, it is actually…Sorry!

Maths is ruthless with loudspeaker manufacturers!

Practically, there are solutions to avoid using 5,000 Watt power amplifiers, don’t worry. The answer lies in speaker efficiency. Remember, we are looking for 20 dB.

Now, if we replace a loudspeaker with a sensitivity of 84 dB /1W /1m with another one having a sensitivity of 95 dB /1W / 1m, we have already found 11 dB. 
We are left with only 9 dB to find.

By increasing the power by 8 fold (log 8 = 0.9, so 10 x log 8 = 9 dB), we get the desired level. So, if our Hi-Fi loudspeaker was driven by a 100 Watt amplifier, we only need 800 Watts…

Let’s do it again in a different way:

If we have a speaker with 95 dB / 1W /1m sensitivity, we need 15 dB more to reach the desired maximum level of 110 dB.

As log 33 = 1.5, 10 x log 330 = 15 dB

So we only need 33 Watts, right?

No, it is wrong!

The listening distance is not 1m, it is more. The equation for SPL (Sound Pressure Level) is not the same as for WL (Power Level), 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

In this calculation, we have disregarded an essential phenomenon: thermal compression. This is quite a problematic phenomenon: when a loudspeaker is fed with a signal that is near to its maximum power capability, it’s resistance dramatically increases, resulting in a loss of sensitivity, typically between 4 to 8 dB. So, our nice figure of 95 dB/1W/1m is gone!

Well, combining distance and thermal compression, 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 power handling of about 1kW

Not that big though!

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.

For height channels, the sound level and the bandwidth are even further reduced, so these speakers do not need to be powerful.

To be followed soon!

Patrice Congard