The Case Against (Most) Acoustic Room Treatment

Installers are familiar with using acoustic treatment in home cinemas. But is it always required? Rob Sinden, founder of distributor Gecko Home Cinema, presents the case against (some) treatment. 

When you install a good audio system, the weakest link in sound quality is typically the way the room changes the sound of the speakers. There are two ways to try and correct these errors: room treatment and room correction. Well over half of what you hear will be created by the room so understanding the pros and cons of both methods is well worth understanding.

The Size of the Problem

Assuming you bought good speakers, they will provide even frequency response so each note on a piano say, will be heard as loud as the next. 

The problem is we don’t listen to speakers from 1m away. In a home cinema, the ‘money seat’ is typically 4m from the speakers. From this position you may only hear 20% of the sound coming from the speakers while the rest has been reflected on the walls, ceiling and furniture in the room.

If you measure the sound of the same speakers from 4m away, and from 1m away – the difference you will see will be something like the graph above.

The green line shows the measurement in the room while the smoother yellow line shows the measurement taken from 1m away.

These days most speaker manufacturers don’t provide useful information about their products, but the industry standard for expressing the response of a speaker, is that it must not vary by more than 6db across its usable range. Place the same speaker in a room and the variations are now typically over 15db. This means that some notes will be heard two or three times louder than others. 

So, Let’s Fix the Room

As the room has created the changes to our speakers, treating the room seems like an obvious place to start. But first you have to understand a little about sound waves and the differences high and low frequencies.

The rule of thumb with absorbing frequencies is that material you use has to be half the depth of the wavelength we want to treat. 

A high frequency at 10,000Hz is just 3.4cm long; a low frequency at 40Hz is 8,600cm long. Putting treatment on a wall to reduce a 3.4cm long wave is no problem, but the 4,300cm depth of material required to absorb a peak at 40Hz is totally impractical.

Room treatment is a very low-resolution approach to a very high-resolution problem. Trying to smooth out 20,000 frequencies with a few different thicknesses of foam simply can’t give the smooth speaker response that our speakers were designed to produce.

Limitations of Room Treatment 

Because of the length of bass waves, room treatment has almost no effect on these the most obvious errors created by the room. At higher frequencies they are much more effective, but they simply don’t have the resolution to restore the smooth even response our speakers should deliver.

Room treatments cannot restore the even frequency response of our speakers, so the benefits they focus on is reducing reverberation time. This is the time it takes for a sound produced in your room to die away so it can’t be heard. 

Room treatment is very effective at this, but so are curtains, carpets and soft furnishings. The reverb time in your bedroom will be lower than your kitchen for example.

Most UK rooms have carpet, sofas and other features in the room that will naturally absorb and diffuse the sound from your speakers. If your room is carpeted but has now absorption or diffusion, a little treatment on the area of the side wall where most sound is reflected from, can work wonders. 

The problem with room treatment is that it’s recommended by people selling it. Unsurprisingly, their recommendations are that the more treatment you add, the better the sound will get, which simply isn’t true. A room with too much treatment is an unpleasant space to spend time in or to listen to music or film in.

Look at any concert venue or space designed for the enjoyment of music and you’ll see hard, reflective sidewalls, not absorption. Music-lovers will visit a particular venue because the rooms acoustics will add to the sound of the orchestra or band. Reflections from the room provide a positive addition to the performance and shouldn’t be regarded as the enemy and removed with treatment. 

The vogue for selling treatment often creates dead spaces that are unnatural to sit in and that make the enjoyment of music and film at their best impossible.

Room Correction

Some room treatment is essential in some rooms, but even the most extensive design and treatment cannot restore the even response our speakers were designed to give.

Fortunately, room correction does have the potential to fix room errors and to restore the sound of our speakers. It is such a powerful tool that it is found in almost every surround sound receiver. 

Most room correction systems use a microphone that you place in the seating positions, firing towards the ceiling. When you have taken your measurements, it will adjust the sound sent to your speakers to match the target curve the correction system sound best. These systems vary in quality but all will help reduce the biggest errors created in the room. 

Caveat Emptor

The consumer looking to buy a great Hi-Fi or home cinema needs to weigh up the cost v performance of the systems available, including any design and treatment. When you do this, the price/performance of the total solutions with extensive treatment rarely stack up

In the Hi-Fi world, the reflections from the room are recognised as a positive addition to the direct sound from the speakers. Just like spaces where music is performed, rooms for listening to Hi-Fi are understood as a positive addition to the performance, not something that needs to be removed.

Room correction is a much powerful, higher resolution tool than treatment. It works with the positive additions your room creates rather than trying to remove them and can be used with Hi-Fi and home cinema systems. 

Commercial Reality?

Foam and fabric are cheap to produce and when treated as ‘acoustic devices’, can be sold at crazy prices. The profit in selling treatments seems attractive, but do you really want to tell clients you can only deliver the best sound in one room? Unless you can demonstrate great sound in regular rooms, we are preventing the sale of great audio systems throughout a home.

If your ‘solution’ to great audio is room treatment, perhaps it’s worth looking for other alternatives.


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