I've been encountering some problems with standing waves in the room were I do my mixing. The bass response is very uneven and I'm considering investing in some acoustic treatment, but I don't really know were to start. What are my options?
Technical Editor Hugh Robjohns replies: Standing waves are a very common problem where low-frequency sounds reflect between walls and ceilings to interfere with the direct sound from the monitors. Where the reflections and direct sound meet in phase the level is increased by up to 6dB, but in areas where they arrive out of phase the level is reduced often as much as 20 or 30dB — this is what causes the lumpy and uneven bass response you describe.
This problem is an acoustic one, and electronic equalisation of the monitoring signal can't help because the in-room frequency response is different in every part of the room. The only way to deal with this issue is, as you say, to treat the room's acoustics properly to reduce the amount of reflected low-frequency sound, but since low-frequency sounds have long wavelengths, the acoustic treatment required tends to be physically large as well.
The solution is to install bass absorbers, generally referred to as 'bass traps', to soak up low-frequency energy and stop it from reflecting back into the room from the walls and ceiling. Although these absorbers would appear to reduce the amount of low frequencies in the room, the perceived level of bass normally increases when bass traps are installed, because the cancellations of bass frequencies in the room no longer occur! Another point worth mentioning is that while it is easy to install too much absorption in the mid- and high-frequency areas, resulting in a boxy-sounding room, there is almost no restriction to the amount of low-frequency absorption other than cost and aesthetics. Most rooms benefit from more rather than less! It is also possible to combine bass trapping with mid- and/or high-frequency absorption in the same physical unit.
There are many commercially available acoustic treatment products from companies like Auralex, Primacoustic, Max Wall and Real Traps, amongst others. Most of these companies offer free planning services too, where they estimate the required amounts and placements of their products from a scale plan you supply of the room you want to treat. However, these solutions can quickly become expensive, and the efficacy of the typical corner-mounting foam bass traps is not particularly good. More efficient (if bulky) designs are available which are also fairly easy to build yourself using paper-backed rigid fibreglass panels, mounted across the room corners, built into the ceiling, or hung spaced from walls. A Google web search on bass traps will produce many useful sources of information and construction details.
However, most of us make use of relatively small spare rooms for out music studio and so less bulky alternatives are required. These include Helmholtz resonators, which tend to operate over fairly narrow frequency ranges (so multiple units of varying sizes are required), and membrane absorbers or 'panel traps.' The latter are far more space efficient and can be constructed to work over a pretty wide frequency range. An excellent source of information on panel traps is available at www.ethanwiner.com/basstrap.html [Ethan Winer, by happy coincidence, is also the author of this month's Sounding Off — Ed]. So, although few home studios have installed much in the way of acoustic treatment, this really is the secret to creating accurate monitoring and, by extension, transportable mixes, particularly in terms of the bottom end of the mix. It seems odd that people will invest thousands on computers, mic preamps, mixers and monitors, but baulk at spending a few hundred on decent acoustic treatment! But good bass trapping will more than recoup its cost in the improvements to your mixes when you can hear exactly what is going on!