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Microphone Month 4

Poor man’s acoustics

A beginning acoustician asks:

“There are a host of products available on the market for acoustic treatment. For us home studio folks though, the stuff can get expensive and we often end up using something like old carpeting we found for free! It would be helpful to have some sort of guide as to the sound absorbing and reflection characteristics of various materials, and in particular, what frequency range they are good or not so good at absorbing/reflecting. With some basic guidelines, we (the less professional) might then have a better idea of what we can skimp on and what products are really worth springing some money for.”

One of the reasons commercial acoustic materials cost what they cost is because it takes some real expertise to design materials that are linear in their absorption. That is, they don’t have big peaks and dips in their absorption coefficients at various frequencies. To some extent common sense is applicable for picking materials. Things like carpet, curtains, fiberglass insulation, foam, and other types of fabric are known to absorb sound. As a general rule they are usually much more absorbent at higher frequencies. A room in which we put carpet on all surfaces will not sound noticeably ambient or reverberant to us (a desired result), but at low frequencies nothing has changed. Consequently we have changed the tonal characteristics of the room. In addition the room may actually be too dead now. Mixes done in such a room can turn out too bright or weak in the low end. Knowing how much to treat a room is half the battle. Commercial acoustic materials sold for such purposes are designed with these factors in mind. Similarly if you are mixing on the second floor of an old house your speakers may be loosing low frequency energy through the floor, so the carpeted surfaces may work in your favor. What I am getting to here is that the science of room treatment is always an educated guess unless the specific characteristics of the room in question are measured. This even holds true for the companies selling expensive acoustic products.

We know that materials made of fabric tend to absorb high frequencies. Bass traps are designed to absorb low frequencies. They are manufactured in a variety of ways, but in general they are semi-rigid enclosures of some significant volume that can work on the longer wavelengths of low frequencies. Sometimes affixing carpet to sections of plywood and hanging them about six inches in front of walls can create effective bass traps and also serve to absorb and diffuse higher frequency sounds. I have even seen egg cartons used this way (they make decent diffusers). Paneling (especially the kind with holes for hanging things) can make great bass trap material. There have been many articles written on how to make and use your own acoustic treatment over the years, but at the end of the day they are all just clever variations on common sense. Professionally manufactured materials are based on scientific measurements and constructed in very specific ways to produce excellent results. It’s not all about absorption. Diffusion, and just reflecting sounds away from the listening position (usually to the rear of it) contribute to making a room really work well.

Helpful hints on where to place acoustic foam can be found in the 1/6/98 issue of inSync. Suffice to say that placing foam (or carpet) in just a few strategic locations can really make a big difference in the usability of a room (especially a control room).

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