Absorption and diffusion

We are already up to part four in the series around acoustics. In the earlier parts we talked about the importance of good acoustics, room modes and active versus passive correction. Now we will look at the difference between absorption and diffusion: when do you actually apply absorption or diffusion?

Basically, there are two methods to “tune” a room: absorbing energy and diffusing energy in order to, so to speak, break room modes. Diffusion really only works with higher frequencies (mids and highs), because low frequencies require huge diffusers with lots of depth. That is simply not practical.

Absorbing – dampening – energy is done with so-called absorbers. You remove energy which reduces reverberation in particular. Absorbing lower frequencies will reduce ‘hum’ in a room. Sometimes we also experience it as a kind of ‘buzz’ when it’s a little higher in the frequency spectrum.

This “buzz,” hum is often audible in somewhat harsher rooms with lots of hard surfaces. There the energy cannot get away, so you get clearly audible room modes that also express themselves in multiples of the base frequency. So suppose the first room mode is at 35 Hz, then there is also a peak at 70 Hz, 105 Hz, 140 Hz, etc. By addressing the first room mode using absorption, the other room modes also disappear.

Diffusion you apply to improve energy distribution throughout the room. This gives air and space in the reproduction. It can also break up problematic reflections, which also restores calmness to the reproduction (as in our living room).

So…?

Mostly you can take as a rule of thumb: use absorption to get rid of reverberation due to excessive energy (room becomes dryer in sound) and apply diffusion for better dispersion of energy to improve the sound image without taking away any energy. This can also reduce direct reflections and reduce an oppressive feeling in the spectrum.

Measure first!

With the Illusonic software you can perfectly take a measurement of the room. Other “room correction” software can also do this, of course. REW is free software that also works perfectly.

As stated earlier in this series, it is crucial that you do a proper measurement in the room you want to treat. Without a measurement, you are blind and basically have no idea what you are doing. With a bit of luck it can still turn out well, but we really recommend taking a measurement first and then getting started.

What we measure is reverberation time and frequency response. If the reverberation time is very long – there is a lot of echo or the room sounds very ‘restless’ – then absorption is needed to absorb energy within those areas.

If the room does not sound very ‘restless’ and there is no excessive reverberation (reverb), then it is usually wiser to work with diffusion, otherwise the room may sound too dead or lifeless. A reverb time between 150ms and 300ms is usually fine. Above 500ms is quite lively. Below 100ms is pretty “dead” for a hi-fi system. In a cinema, 60ms (RT60) is maintained, but different rules apply there than with stereo hi-fi.

Now the frequency response will certainly not be straight in an ordinary living room. Certainly not at the listening position. However, it is wise to critically examine huge peaks or dips. Take a look at the pattern (are there clear multiples of those frequencie? Are they related to the room size?).

Your author noticed that in his living room the back wall was “audible”. The sofa is two meters in front of the back wall, but the wall is quite flat and reflected mostly parts of the midrange, which is disturbing. By taking a measurement and determining in which part of the frequency spectrum this falls, you can build your own diffusers… check this article (Dutch)

Primarily check the primary reflections!

The biggest “problems” occur at the primary reflections. These are the points where the sound first hits the side walls, floor, ceiling and back wall. You can determine these points well with a mirror. If there are two of you, one person can sit in the listening position and the second person can go along the side walls and floor with a mirror. The point where the speaker is visible is an initial reflection. The ceiling will be a little trickier, but in fact that’s almost the same spot as the floor. The back wall you can draw a straight line from speaker to back wall.

By treating these first reflections – for the most part – with either absorption (in case of too much energy; reverb, reverb), or diffusion, you get rid of a lot of the problems. This is because they are the first points where the sound comes into contact with the room. You either absorb the energy so the reverb is reduced, or gone, or you diffuse the energy evenly throughout the room.

Living room-friendly approach

We have found from experience that reverberation can be treated just fine with (heavy) curtains and, for example, absorbent material in the “back” corners (i.e., the wall the speakers are playing to, not the wall behind the speakers). A thick rug in front of the speakers also helps treat the initial reflection.

The other issues can often be treated with diffusion; the side walls and the wall behind the listening position. Diffusion need not be achieved with “real” diffusers. Even a bookcase, CD rack or a cabinet with knickknacks works fine in many cases.

If there really is a heavy bass problem, but there is no possibility for large bass absorbers (which is often the case), then PSI ‘s AVAAs work particularly well. However, these are not cheap.