Recording Magazine’s Acoustics Series Part 4
Recording Magazine sends out a newsletter to its subscribers every few weeks. The newsletter is (coincidentally) titled “Sound Advice” and this month it features the fourth in a series about room acoustics. Room acoustics is one of the biggest concerns for Recording Magazine readers. I know that this is also a big issue for those of you in the voiceover world. I asked permission to reprint this newsletter (and will ask to reprint the others in the series as well) so that those of you with home studios can also benefit from the information. I want to personally thank Brent Heintz, VP/Associate Publisher for granting permission, allowing me to share this great information with you.
Here is the fourth newsletter in the series on Room Acoustics:
Welcome back to Sound Advice on Acoustics! Last month, we talked about how the relative dimensions of a room can make standing waves (modes) better or worse, and introduced special ratios called Golden Means that will help produce the smoothest bass behavior… but all that just tells us where the modes are, it doesn’t make them go away!
This naturally brings us to the question of what to do about these modal issues, once the room has been analyzed. In the case of new room or room-in-a-room construction, following the Golden Mean ratios (we printed a table of them last time) is an excellent place to start.
If space permits, it’s also recommended that room volume (L x W x H) be at least 1500 cubic feet (for example 16′ x 12′ x 8’=1536 cu’)—this will push the lower-order modes down in frequency and provide greater density and evenness in the useful bass range.
In the case of an existing space, however, adjusting room dimensions is, of course, not usually an option. Sometimes, after mapping out the locations of the nodes and antinodes, simply rearranging the location of speakers and listening position in the room can help to avoid being in the path of the most severe artifacts. Examples: Not establishing a critical listening position right up against a wall; placing speakers in a more neutral location.
But while this can help, even good modal spacing and careful room layout isn’t really an adequate alternative to a truly balanced room response for serious use, so in a professional situation the preferred option is to address problematic standing waves with room treatments.
One approach, frequently used in studio construction, is the splaying (angling) of the walls/floor/ceiling, to avoid having the parallel surfaces which give rise to standing waves. However, this will not really eliminate standing waves, but will simply shift their distribution in the room slightly. This may help to reduce the severity of some modes a little, but it will also make it more difficult to calculate and map them out. Unless you have access to computer software to help you map out the room’s response, it’s probably better to stick with a rectangular shape with predictable modal behavior, and turn to other solutions.
Another way to deal with standing waves might be to absorb them. Unfortunately the absorption of low frequencies (by typical porous absorbers that you could easily apply to walls and surfaces) would require depths that are comparable to the wavelengths of the frequencies to be absorbed, which would be impractical.
In practice, the commonly available sheets and tiles you often see in studios are only effective at mid and higher frequencies, well above those at which standing waves form. To absorb low frequencies, with their room-sized wavelengths, the best approach is to “trap” the low frequency waves via the use of cavities at or behind the walls. This approach has given rise to the term “Bass Traps” for some such cavity-based solutions.
A bass trap is a tuned cavity with a depth of a quarter-wavelength of the frequency at which maximum absorption is desired. Such a cavity could be built into one of the parallel walls that are contributing to the formation of a standing wave. The wave would enter the cavity at its point of maximum pressure buildup, at the wall. Inside the cavity, maximum pressure would develop at the back, resulting in zero pressure at the mouth (opening), countering the normal pressure buildup there.
A bass trap will be effective at 1/4 wavelength and odd-numbered multiples of 1/4 wavelength. This technique can be effective, but of course requires some extra depth to be available behind the walls if built in.
Another approach is based on a design called a Helmholz resonator (an example of this is a long-necked bottle). For our purposes this would consist of an opening or series of openings (like the neck of the bottle) into a cavity (or connected series of cavities). The resonant frequency of the cavity is determined by the length of the “neck” and the volume of air in the cavity. Sound is absorbed at and around the resonant frequency (matched to the frequency of a standing wave).
It’s possible to construct your own resonators, usually consisting of a panel with holes or slots in it in front of a cavity (corner placement is a good choice; these can even be free-standing). The exact size and spacing of the holes is the key to “tuning” such a perforated resonator to a particular mode—the formulas involved are not too complex, but there isn’t enough space to really get into the specifics here.
There are quite a few other types of low-frequency absorbers, and if building your own is not feasible, many commercial standalone solutions are available that work very well.
Wow, all that discussion just to deal with the low end! There’s more to deal with next time; see you then…