Acoustics 101 Course by John Dahl of THX - page 3

Standing Waves (Room Modes)

Below ~150Hz wavelengths become much longer and are thus able to interact strongly with most rooms. A 150Hz wavelength is 1130/150 = 7.53 feet so it is capable of reacting fairly strongly in a room with an 8 foot ceiling. Wavelengths below 150Hz get progressively longer and can now begin to interact with higher ceilings and other room dimensions. Potential problem frequencies within a rectangular room can be approximated by using several freeware programs which are readily available. John had suggestions for more sophisticated room modeling programs as well:

The bottom line on standing waves, though, is that they can be very difficult to get rid of. Classic solutions as outlined by John include:

• Changing one of the room dimensions
• Moving seating locations
• Moving subwoofer locations
• The use of diaphragmatic absorption, and
• Equalization ( a minimum of 1/12 octave, parametric is required; 1/3 octave EQs with fixed frequency points are virtually useless)

Rattles

One of the final steps to perform on an all-but-complete home theater install is to do the rattle check. Rattles are usually very prominent for specific low frequencies. Usually they're caused by acoustical or mechanical coupling with loose fixtures, lights, furniture and doors. Rattles can be insidious in that they can sound like a blown or distorted speaker or an amplifier that's being overdriven. So the best time to ferret them out is before you play your first movie blockbuster that evening.

Rattles can be pinpointed by a couple of methods. If you have access to an audio oscillator you can hook it up through a spare input to your pre-pro or receiver and slowly sweep from 20Hz up to 1000Hz. Another alternative is to use single test tones especially in the low frequency ranges as can be found on numerous test CDs on the market. Examples of the excellent test tone discs available are: the 5.1 Audio Tool Kit mentioned in Tony Grimani's calibration course, any of Joe Kane's Video Essentials series or the Avia software from Ovation.

In the case of my home theater/living room "rattle test" I already had the R.A.B.O.S. test tone disc that came with my Infinity Intermezzo 1.2 subwoofer. When I had used it to set up the sub I had already been pre-warned of my living room/home theater's rattles. So I set about tracking down the rattle sources and figuring out how to tame them. John recommends tightening loose fixtures or isolating them with rubber pads, caulk or insulation. I was fortunate in that there was only one extraneous rattler which had previously made its presence known.

In the small bar alcove in my home listening area we have a couple of very nice display plates collected from a European vacation. We display the plates are on the bar-wall, supported by a decorative twisted wire frame. When I was calibrating the Infinity sub the first time and the CD hit the 56Hz band, those two plates started rattling like mad against the wire frame. A simple application of clear "Quakehold! Gel" at the three points where the plate touched the wire frame solved the problem. I had found this product at our local True Value hardware store. But for those of you not living in California earthquake country I can also recommend Dap Fun-Tak which is available at most Home Depots.

Noise Control

Attenuating background noise is the easiest way to increase the dynamic range capability of your system. In our increasingly noisy society many of us have become almost immune to the racket that goes on all around. Most of our vehicles, quiet though they may seem, commonly have 65 mile-per-hour "cruise" noise levels in the high 60dB to low 70dB level. Similarly, a quiet neighborhood is at best down around 40dB. So to try to get our listening environment down to a target of, say, 20dB can become a formidable task.

Background noise interferes with our perception of loudness. It masks low level signals and subtle details in music and movie soundtracks. And it makes speech intelligibility more difficult. Unlike our local movie theater whose quietness we usually take for granted, our homes are filled with all the mechanical devices we need to live comfortably; air conditioning, plumbing and refrigerators with their compressors which cycle on and off all day long. We seem not to pay attention to these devices until we've "completed" installing our home theater electronics and plopped down to watch our first movie.

Now the "second phase" of our home theater installation can begin. The thrill of getting all that cool electronic gear needs to be replaced with the reality that we don't necessarily have to buy that last 20 or 30dB of dynamic range with a more powerful amplifier. We can work for it by isolating refrigerator or air conditioning motors a bit better on their suspensions. We can also insulate doors and windows so we don't hear whistles or rattles. Lining air conditioning ductwork can also help immensely in keeping you theater cool but also quiet.

Do all these "fixes" sound like a lot less fun than buying and playing with all the cool electronics? You're right. They are less fun. But the room as was repeated by every CEDIA instructor is 50% of your home theater system. And this is the message that John Dahl and his fellow CEDIA instructors managed to drive home so convincingly.

By Patrick Hart