Acoustics: Home Theater Sound

How to tame a room's acoustics!

Important design essentials!

Where to sit:

Rooms resonate.

At bass frequencies, the wavelength is so long you can get areas of high or low pressure. In a high you’ll hear too much bass, and a certain frequency - the 'one note' boom. In a low you won’t hear much bass at all.

Optimum Room Dimension for great acoustics

If you think of your room as a pipe organ PIPE, you can imagine that openings into or out of the room can change its resonance.

Blowing across the mouth of an empty bottle is similar - the more volume in the bottle, the resulting resonant frequency is lower.

Wide open doors or hallways, or open windows can make a big difference in the bass acoustics as the volume of air and placement of standing waves within it gets more complex.

Bass Control

Turning the bass up or down will make it louder or softer where it is already better or worse. Turning the crossover point too high will make it more directional.

Ideally, your sub will come on where your main speakers don't have much oomph. Turn your sub to where you can just hear it, then back it off a touch.

A self-calibrating home theater receiver might be able to cope somewhat with bass high and low pressure zones by altering the bass to the speakers individually.

What about "bass traps?" You may have seen those expensive padded cylinders, wedges, and thick screens to absorb problematic standing waves in the low bass? For the most part they don't work, because the wavelengths of low bass frequencies are so large that any "trap" would have to be annoyingly large to become effective. In my listening room, the bass traps – designed by an acoustics architect – are each about 9 feet high by 2 feet wide in a trapezoidal wall slot about 2 feet deep. Think of a bass trap as a backwards bass drum. Backwards in that it has a surface that vibrates turning sound into heat inside it. Our advice – try bass traps before you buy. You might get lucky.

Many people turn their subwoofer up too loud. Believe it or not, it should just be there and not generally be humming away. Also, if it has a frequency control on it, turn it down some. It should seamlessly pick up where your other speakers start to crap out. Many people make the mistake of wanting to get BANG FOR THEIR BUCK so they set the volume of their sub too high. It's not supposed to be THERE all the time unless the acoustics of your main speakers are somewhat wimpy.

Midrange and high frequency absorption.

You can buy or make absorptive panels which will help alleviate some of the reflections. Here’s a trick: If your speakers are in place, then have someone gently slide a mirror along the wall in question. When you can see the speaker in the mirror, there’s your spot to place the absorption.

I just want to mention something I’ll cover later in accessories. There are MOTION CONTROL chairs which not only will kick you in the butt on the low notes, but are being programmed to lean and tilt and bump when called for on certain films. Talk about immersive!

If you build from scratch:

You are probably going to spend some moolah on this. Even if you do it yourself.

Acoustic isolation (from the rooms adjacent, above or below.)

Be sure to discuss this with your significant other and your acoustics architect. The pounding of the subwoofer can turn neighborly into ugly, not to mention domestic tranquility into hostility. Bass frequencies are easily transmitted through walls, ceilings, or floors.

The best construction technique for eliminating sound transmission is MASS - thick, concrete walls. Even using this technique on some of the walls, such as in a daylight basement situation, will improve the Sound Transmission Coefficient of the room.

Note! Interior acoustic panels won’t reduce sound transmission. And the ceiling is a great leakage point for your bass acoustics.

One of the easiest, and most common sound transmission reduction techniques is to mount the drywall on resilient channel, also known as “c-channel”, “hat-channel” and “R/C channel” or “R/C”. This thin metal is affixed to the wall studs and ceiling joists. The drywall is then mounted to the resilient channel, effectively decoupling it from the room’s structure. Sound is then not transmitted as effectively from the drywall to the room’s structure or vice versa.

Staggered stud wall construction is an extremely effective technique to reduce sound transmission as well. This technique keeps opposite walls from sharing studs. Dedicated studs will reduce sound transfer from the home theater room’s drywall, into the wall studs and into the adjoining room’s drywall. Drywall will re-radiate the sound from the studs into an adjoining room. When constructing the wall, use top and bottom plates at least one size larger than your studs. For example, if you are using 2x6 wall studs, use a 2x8 for the top & bottom plates. Use at least R-19 batt insulation inside the wall. Lay it horizontally and weave it between the studs.

Mo’ betta:

Even mo’ mo’:

When locating outlets or any other wall penetration, be sure two penetrations on opposite sides of the wall do not share a common stud bay. A penetration on opposite sides of the wall, in a common bay, gives sound an unobstructed path from your media room to the adjoining room or vice versa. Seal the outlets with insulating foam to further reduce sound transmission.

HOW MUCH SOUND VARIES IN THE ROOM

Watch the 7 minute video titled: NON-MODAL PEAKS AND NULLS IN SMALL ROOMS As the microphone is moved only a short distance, you can read on the meter the difference in sound at that frequency. You should be amazed. This isn’t trickery. But it explains why you can’t tell in a store what something will sound like at home beyond generally, why moving your gear into a new house (or room) changes everything, and even how much difference you can get from seat to seat in the same room! Be sure to see our sections on surround sound, speakers for home theater, receivers, stereo, and audio in general.