Now while we solve seemingly more complicated problems in large auditoria, integrated commercial and residential complexes, and critical listening spaces such as audiometric rooms and studios, home theaters deserve a mention because of the challenges presented by small rooms.
There are two aspects to acoustical treatment for home theaters.
- Noise Isolation: To ensure there’re acceptable levels of sound going in and out of the HT room.
- Sound Colouration: The other is to treat the room to reduce colouration of sound due to the room response, reverberation, standing waves, etc.
In this article, we will deal with only the latter. Noise Isolation can only be dealt with on a case by case basis, and some aspects of it will be covered in an article on Residential Noise treatment, coming up soon. For now, let’s look at some problems that small rooms share with larger halls.
- Reverberation Treatment : For music, and movies, a reverberation time of 0.8-1.0 second is ideal. This can be controlled to a great extent by room furnishings – but there’s a scientific basis to that. The number of myths surrounding this is so large, that I’ll have to write another post to quell those.
- Flutter Noise: This happens between two parallel surfaces.
- Ringing : The room’s response amplifies some frequencies and suppresses others. This is unique to each room.
Measurements and calculations will tell us exactly where the problem spots are, and precisely which frequencies sound sore. There are many thumb rules going around, but the scientific principles on the basis of which some of these originated must not be forgotten. There’re enough myths to bust on this, but I’ll do that in another post. For now, it is important to remember that despite thumb rules that seemingly work, there are always exceptions, and that’s why each case is a unique experience.
There are challenges unique to small rooms. The dimensions of most of these rooms are equivalant to the wavelength of low frequency waves – 8ft to 20 feet.
- Room Modes: The foremost of them is to even out the bass frequencies in the room. The dimensions of a small room can sometimes be exact multiples of the wave whose wavelength is an integral multiple of any one side of the room. Room modes are an important part of the colouration of sound in a room, and calculations can help in this area. While many modes are possible in theory, not all of them will have an effect on the sound. Designing bass traps can help for some of the problem frequencies.
- Diffraction: Bass frequencies have wavelengths long enough for the wave to bend around life-size objects such as sofas, or chairs. This leads to irregular shadow zones, and amplified sound in some areas. The reason for mentioning this is that the peaks and dips are very clearly audible.
- Harsh, unnatural colouration, with amplified sound. Human ears can hear speech and music very clearly upto 10s of meters. Small rooms should ideally not have amplified sound, and especially not amplified speech. Too much of this in a day will temporarily shift your hearing threshold, making you lose sensitivity to softer, subtle sounds. Unless a room is acoustically treated, amplified sounds in small rooms tends to sound harsh and unnatural, and lead to auditory fatigue – over time. This is especially an issue when we deal with practice rooms for drummers. Fatigue and annoyance have been widely studied, and good advice should make sure you don’t experience them too early into your movie. The final commissioning of a room must always be done after testing it for amplified sound.
For acousticians, home theaters are ripe grounds for understanding the behaviour of low frequencies in small rooms. Each case can be a challenge depending on the site constraints.
How much treatment is really needed?
Once the frequency response of the room has been “smoothened” out after treatment, the real issue we see around us is an over treated room. Small rooms can sound very dead if treatment is not accurate. I will need to write another post on various aspects of sound – tonality, warmth, brilliance, etc. These are measurable quantities, to some extent, but these are more critical for large auditoria. For home theaters, these are generally looked at only if the room doesn’t sound right. So, for home theaters, the right amount of reverberation must be present in the room. For a room this small, it is vital to first ascertain the interior decoration of the room – the kind of upholstery, the number of sofas, the type of carpeting, etc. These can unfavourably tilt the balance post the treatment.
However there are no hard rules on what a room should sound like. People enjoy a wide amount of variations, and at least in the Indian context, people contact acoustical consultants only when there is a “problem”, which is to say, that enough times there isn’t a “problem”. :). Thankfully, factors such as individual preferences play an important role beyond fancy room treatments. The key is to be aware of what your home theater consultant is doing with your room’s response. Treatment is expensive, and much of it is sometimes unnecessary.
The Commercials
I know of enough home theater vendors here, who also undertake to do acoustical treatment, mostly as a business perspective here. I’ve been asked to provide consultation by vendors who then proceed to bill the client 4 times what I charge them. I’m not saying that’s wrong – no one’s here for charity, and vendors have overheads, unlike consultants. But the choice of hiring a qualified consultant rests with the buyer.
In the Indian context, with not enough places teaching acoustics as a dedicated subject, most AV vendors provide generalized, thumb-rule-based acoustical treatment for each project. In fact, many of them offer it ‘merely’ as an added service. That surprises me because many of them started out as audiophiles themselves, and they talk about precise soundstage, and terms like that. But when it comes to precise acoustical treatment, a lot of them think thumb rules-based acoustical treatment is good enough. Sure. So is watching a movie on a laptop. You won’t miss the story, I promise. Next, I also know DIY enthusiasts who like to “EQ out” a room’s faults. Now I do resort to parametric equalization to kill the odd bass boom when all other things fail, but depending on equalization to substitute for accurate acoustics is not just foolish, it is cumbersome. You’ll have to change settings for each song you play.
Thankfully, there is a growing set of people who appreciate the science that acoustics is, and give it due importance in their scheme of things. There’s a growing crowd of AV vendors who are not just trying to make a sale, they’re trying to provide a good overall experience – which is significantly influenced by the acoustics of the room. Also, there’s a growing crowd of people who can tell good sound from bad. There’s just so much happening in the audio scene in Bangalore alone!
A fraction of the money spent on one such project will get you correct technical advice from a qualified acoustical consultant, on what’s really necessary, and even get you unbiased options for installation, keeping in mind your budget and aesthetic preferences.
To conclude, home theaters are vital spaces for quiet time, noisy time, music time, game time, movie time, family-bonding time, and the quality of your time spent in this room can be significantly enhanced by precise treatment that makes your room sound clear, intimate and warm. And I’m not reeling adjectives here. The last three are measurable quantities. 
