Acoustic designs fall into two areas – soundproofing and room acoustics – and unfortunately clients usually confuse the two.

What acoustic treatments add to a room is the ability to hear the sound the way the creator intended it.

Soundproofing is about how much sound gets in or out of the room. It’s next to impossible to have this conversation on a technical basis because very few people appreciate the difference in a design that was so many dB more soundproof than another.

For this reason it is important to have a series of questions to act as a guide – who else is in the house, what the neighbours are like, what equipment will be in the room and what listening levels are used.

Acoustic treatments work by absorbing most sonic reflections, so that the original sonic image as designed by the creator is not interfered with. At the basic level, acoustic treatments are materials with a physical structure that allows sound to pass through them in a way that uses up lots of the sound energy, thereby reducing the sound that comes back out of them.

Green Glue is a cost effective product for achieving high levels of soundproofing, particularly at low frequencies. A visco-elastic damping compound, Green Glue addresses low frequency isolation problems, including bass soundproofing which is questionable with existing soundproofing methods. Green Glue is quickly and easily applied between two sheets of plasterboard or sub-flooring and can be used in either new or existing walls, ceilings or floors

Soundproofing designs are fairly obvious: they stop sound passing through structures with the purpose of leaving those 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. However, most of what we do is only a computer optimisation, which is far less demanding than a full simulation. Most of the time is taken up by the initial interviews to establish exactly what is expected and what would be appropriate.

The next big time consumer is making sure builders and other contractors follow the design, because much of it is not what they are used to, and attention to detail makes a huge difference.

The actual design time, provided all necessary input data is available, is probably only a couple of days. 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.

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.

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 level and phase between ears. 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.

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) 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. Inherently more expensive treatments are not necessarily better than low-cost ones.