Acoustic treatments and soundproofing

Acoustic design  
Sound proofing and acoustic treatments are often given too little attention in home theatre design.

Not everyone appreciates the importance of acoustic design in home theatres - including installers - but acoustic designs are a critical part of good home theatre designs. High quality hardware will go part of the way towards excellent sound, but it's just as important to take into account how that sound travels, reflects and absorbs.


What are acoustic designs?

Acoustic designs fall into two areas – soundproofing and room acoustics. Unfortunately, most people confuse the two.

What acoustic treatments add to a room is the ability to hear the sound the way the creator intended it. Soundproofing, on the other hand, is about how much sound gets in or out of the room. The kinds of things that will affect both of these aspects of acoustic design include who else is in the house, what the neighbours are like, what equipment will be in the room and what listening levels are used.


How do acoustic treatments work?


Acoustic treatments work by absorbing most sonic reflections, so that the original 'sonic image', as designed by the person who created the original sound, is not interfered with. At the basic level, acoustic treatments are materials that are designed to allow sound to pass through them in a way that uses up lots of the sound energy, which helps to reduce the sound that comes back out of them.


How does soundproofing work?

Soundproofing designs are fairly obvious: they stop sound passing through structures like walls, windows and doors, with the purpose of leaving the people on one side undisturbed and those on the other free to listen to whatever they please. It’s a crucial consideration for home theatre. The design itself isn’t too time-consuming, unless a full computer simulation is done.


What does an acoustic designer do?

If you consult an acoustic designer, they're likely to interview you about your needs and situation, and then possibly model and optimise your setup on a computer. Acoustic designers don’t provide architect-style plans because that’s not where they add value. They create layout-style plans for shape, size and placement of speakers, people and treatments. Then they detail anything specific to the design, like door construction and sealing, wall construction/isolation, and ventilation and/or air-conditioning requirements.

It is the acoustic designer’s job to advise people to moderate their soundproofing plans to keep them in line with other things influencing the level of soundproofing. For example, there is no need to spend a fortune on highly soundproof walls if the doors are not suitable.


Aesthetics and acoustics

Of course some of the aesthetics, like chairs and floor coverings, will affect the acoustics. Acoustic designers tend to stay out of aesthetic design, but they do need to know where they can put things and what they can hide things behind.

It is then a process of minimising early reflection points and making the low-frequency response as tight and smooth as possible. Once these aspects are as good as they can be, the dialogue is clear, sound effects and imaging work, and the bass is exciting and powerful. It also fixes the very common problem of needing to turn up the volume in quiet dialogue scenes only to be deafened when an explosion happens.


The fundamentals of acoustic design

Essentially the sound reproduction chain goes from the source to your brain via electronics, cables and speakers at one end and from your ears via nerves to your brain at the other end. In between there is the obvious link – the air and the room it is in.

The simplest way to explain how acoustic designs work is with the analogy of vision – if you can imagine watching a movie on a screen in a glass-walled, sunny room with bright lights on, then closing the curtains and turning the lights off, that’s analogous to what happens when acoustic treatment is applied to an otherwise untreated room. Our hearing is remarkably sensitive to differences in volume and space. It means we can position things in front of us very accurately. Sound reflected from walls, ceilings and other surfaces near us can interfere with the direct sound from the speakers and trick us into thinking the sound came from somewhere else.


Cost and performance

When it comes to treatments, assuming that the more expensive options are better than the cheaper alternatives is not always correct. When you realise that a doubling (or 6dB) increase in soundproofing requires double the mass, this starts to make sense. If you add one layer of plasterboard to another to get a 6dB improvement, the next 6dB will need two more layers, and the next 6dB will need four layers, and so on.

Similarly, mass loaded vinyl (MLV) - another sound proofing option - just adds mass to a wall and thus suffers the same fate, albeit at much greater cost and installation time. These problems are exacerbated at low frequency, where the addition of mass has even less effect because we are in what is known as the resonance-dominated region of the wall. Here, adding mass is just like going to a heavier guitar string – the resonance frequency simply gets lower but the magnitude of the resonance is unchanged and the problem simply shifts.

Just as you add shock absorbers to reduce bouncing in a car’s suspension, rather than adding weight to the body, damping is the only way to reduce resonances in a wall. More expensive treatments are not necessarily better than low-cost ones.