Q. Is flutter echo a problem in a well-treated room?

Sound Advice : Recording

My daughter managed to play a tough piece she’s been practising on the keyboard this weekend. She played it so well that we clapped our hands... then we noticed how strange the clapping sounded. It rang on but died very quickly, and for the time it rang on, it sounded very metallic and almost robotic.That was close to the middle of the room. The room is partially treated at the moment, with panels at the side-wall reflection points and ceiling, one on the ceiling, and three corner superchunks. I tried clapping again with some further panels on the side walls directly to the left and right of where I was sitting, and the noise disappeared. I understand enough to realise the sound is the clap bouncing back and forth between the two walls, and I’m guessing that this is what folk refer to as flutter echo. What I’m a little less sure about is whether it is a problem, and what — generally — a hand clap should sound like in a well-treated room.

Via SOS web site

SOS Technical Editor Hugh Robjohns replies: If we’re talking about the sound in a control room, the point is what the room sounds like when listening to sound from the monitor speakers. It is conceivable that, by design (or coincidence), the acoustics could well sound spot on for sounds from the speakers, but less accurate or flattering for sources elsewhere. And, unless you’re planning on recording sources in the control room at the position you were clapping your hands, those flutter echoes might not represent a problem or require ‘fixing’.

However, in general, strong flutter echoes are rarely a good thing to have in a control room and I’d certainly be thinking about putting up some absorption or diffusion on those bare walls to prevent such blatant flutter echoes.

Flutter echoes in a studio can be distracting and fatiguing, so it’s often worth putting up some absorbent foam on bare walls to reduce them. Don’t overdo it, though: you need to maintain a balanced acoustic.

You shouldn’t go overboard with the room treatment, though, because while working in a control room that has ‘ringy’ flutter echoes or an ultra-live acoustic can be very distracting and fatiguing, so too is trying to work in a room that sounds nearly as dead as an anechoic chamber!

Of course, traditional control rooms are pretty dead, acoustically speaking, and that is necessary so that you can hear what you are doing in a mix without the room effects dominating things. But the key is to maintain a balanced acoustic character across the entire frequency spectrum. The temptation in your situation might simply be to stick a load of acoustic absorbers on the walls, and that would almost certainly kill the flutter echoes, but in doing so there is also a risk that you’d end up with too much HF and mid-range absorption in the room (relative to the bass-end absorption).

That situation would tend to make the room sound boxy, coloured and unbalanced, and that’s why a better alternative, sometimes, is to use diffusion rather than absorption; to scatter the reflections rather than absorb them. The end result is the same, in that the flutter echoes are removed, but the diffusion approach keeps more mid-range and HF sound energy in the room.

The question of which approach to use — diffusion or absorption (or even a bit of both) — depends on how the rest of the room sounds, but from your description I’d say you still had quite a way to go with absorption before you’ve gone too far.

To sum up, I’d suggest that you’re not worrying unnecessarily, and that it would help to put up some treatment to reduce those flutter echoes.  

Published in SOS February 2012