In part one of this series on acoustics, we discussed why acoustics is so important. The room is responsible for (more?) than half of the playback quality. So reason enough to pay close attention to it. In this episode, we look at the concept of room-modes.
Anyone working with acoustics cannot ignore the term room-mode. Simply explained, a room-mode is a problem area in the room. Sound (pressure) can accumulate there, causing things to be amplified, or the pressure can actually disappear, causing things to be attenuated or not audible at all.
What is important to note is that the room needs to be closed. If the door is open, most of the room modes are already gone, because then the energy can drain away.
White noise
You can hear room modes well if you walk around the room while white noise is playing. As you walk around, you can hear the energy in the white noise shifting. This is true in any room: including good rooms. The difference is that in a good room you will hear that it is very gradual and also not very drastic, while in a bad room it will shift a lot and sometimes very suddenly.
Room modes will in most cases express themselves in the lower frequencies. Not only is the most energy in the bass: the wavelengths are also the longest, so they fit nicely in the room. A 20 Hz tone is about 17 meters long. Now that tone already fits at half the wavelength, so 8.5 meters approximately. If the room has that length, then you will suffer from room modes at 20 Hz, but also multiples of that. So 40, 60, 80, etc….
A 30 Hz tone is 11.4 meters, at 40 Hz it is 8.6 meters long and 50 Hz tone comes in at 6.9 meters approximately. Be aware that you may divide these lenghts by two, in terms of creation of room modes and release of energy. So if a room falls within the wavelength, room mode occurs there, plus at frequencies that are multiples of the base frequency.
So measuring?
We have already shown some math. Room modes can be calculated. You can measure the length, width and height of the room to determine at what frequencies problems may arise.
The formula is quite simple: room mode (f) = n x v /2 x L.
L is the length of the room, v is the speed of sound and n is the ‘order’. For a 5-meter room, it then comes to:
f1 = 1 x 343 / 2 x 5 = 34.3 Hz. The second harmonic is then: f2 = 2 x 343 / 2 x 5 = 68.6.
Sweep
Is math not your strong suit and you just want to hear where the room mode is? You can do that, too. A good way to find out what area the problems are in is to work with a sweep. This way you can first roughly determine in which frequency range the problem is located and then see exactly where it is per tone.
We use an audio test CD with sweeps, white noise, pink noise and test tones between 20 Hz and 20 KHz. The CD is on Tidal and Qobuz and is called Audio Check – Audio Line Up Test Tones. This is a handy CD / album to do these tests with.
What to do with it
Ok: fine… you now know what frequency the problem areas are at. Most likely somewhere between 30 and 50 Hz (and multiples). Exactly in that area where most speakers start playing: Great! Uhh…Not…
Fortunately, it can be solved. Both with cheaper means, like thick packs of glass wool, and pricey, but compact, active solutions, like the PSI AVAA.
In a living room, a thick pack of glass wool is probably not really a realistic solution. An AVAA is, but they cost quite a bit of money (but work really well!). A golden mean could be a hybrid solution where parts are passively addressed with stylish diffusers and absorbers and the heavy work is done with a DSP. Think of the area below 150 Hz, which is very difficult to fix with passive solutions.
We will go into DSPs and active and passive room correction in future articles.