22 Feb Critical Distance, Bass trapping, etc.

I would like to talk about the importance of understanding the Critical Distance, Bass trapping, and pressure zone of a room.


Critical Distance is defined as the distance that sound ceases to fall off at -6 dB per doubling of distance from the source. (Inverse square law) The source sound goes constant volume at the Dc (Critical Distance) of the room. – And the volume is the same everywhere in the room. This translates to ‘MUD’.

This is the reason that Control Rooms are heavily trapped – as well as Tracking Rooms. A control room being trapped is not an unsusal thing and most people assume that it must be done, but tracking rooms – often called ‘live rooms’ – need some serious trapping too.

You have to have a decent amount of trapping in a tracking room or you won’t be able to work multiple instruments or vocals in the room at the same time. You’ll need to use close-micing techniques – and then WHY the heck are you paying for such a large room that you can’t use?? You’re better off with a small, dead booth. Really.

Two of the most important things for recording musicians are Sight-Lines and Ensemble. They need to see each other. They need to HEAR each other, and NOT just in the headphones. You’ll see session musicians pulling ONE earphone off and working with the room. They need to be able to hear themselves in the room, as well as everyone else. They will work the room, the other musicians, and the mic. It is the job of the studio, producer, and engineer to create an atmosphere where the artist can excel and perform at his or her highest level of expression, creativity, and general ‘magic’. This is the point where the producer will stand and say, “That’s it. That’s the one. – That’s what we’re looking for!”

You’re not going to ‘fix it in the mix’. If your sessions are plagued with lackluster performances and mic positioning issues, the critical distance (Dc) of your recording room is likely a prime suspect.


Ensemble is what you get when the musicians can hear each other and themselves. If you experience sessions or practices where everyone keeps turning up, you will know you have neither ensemble nor a quality Dc. Ensemble is very difficult to achieve in either an ambient room or a completely absorbent room. This is an area of acoustics design where experience meets the math. We have to have reflective surfaces so that an individual musician can hear themselves and get feedback or feel on what he/she is doing. At the same time, there has to be enough absorption to cause a free-field fall-off to the next microphone! So it is critical to design a recording space with BOTH of these characteristics. It is not easy, nor is it arbitrary.

We use a variety of surfaces to provide ensemble. Rock/brick facade, quadratic residue and primitive root diffusors, angled panels, geometric shapes, optimized spline arrays, etc., etc., etc. The options chosen will depend on the room size, shape, and use.


Pressure Zone issues:

Smoothness of response in the Low Frequency range from 20 Hz to 250 Hz is the quality indicator of the room. This is the very reason why so many of the old control rooms sound very good. They are huge and the dimensions support a dense low frequency modal distribution.

It is extremely difficult to treat and/or trap sound that is in the pressure zone. Producing sound waves that exceed the dimensions of a critical listening room will only tend to mud things up. That is why I set the optimal dimensions for a 20 Hz room at about 38 feet. The full length of a 20 Hz sound wave is 56′ – 4″. Divide that by two and you get a half-wave. This is the lowest mode. A quarter-wave would fall into the constant volume of the pressure zone.
So, I take 20 Hz and multiply by the reciprocal of (SQRT)2 and get 14.1 Hz – The half-wave length of this is 39′ – 10″. That’s where I get the 38 Feet. – It ensures a smooth transition from 20 Hz to the pressure zone and allows an easy roll-off on the sub.

So, if you can’t get the size/volume then you have to be very careful about what to expect from frequencies below this dimensional threshold. (LOL – sounds like star trek) By dimensional, I mean Room Dimensions, room boundaries.

If your room is 14 feet at it’s longest dimension, you shouldn’t try to re-produce sound below 57 Hz. IF you must, only use a non-ported sub. It is possible to drive the pressure zone and obtain a very accurate response but caution is advised. The response can/will become erratic due to other factors within the structure in your building/room.

A note on ported speakers and subs: if the speaker or sub port frequency is close to or at the room boundary threshold, it will ‘hang’. By hang, I mean that air movement stops and speaker cone movement also stops, even though you might be pumping 200 watts of power into that speaker. Think about it. Without cone movement, the drive coil in the speaker becomes like an incandescent light bulb. It will heat up with no air movement to cool it. You know what happens next.


rant note:
Far too many people build look-alikes or copies. These usually have a diamond shape copied from Peter D’Antonio’s RFZ designs, others copy different pieces of other designs that they like a lot. A few get lucky, most do not. What is usually missing is the ‘understanding’ of what any particular design parameter does to the whole. Because a studio is not a collection of rooms, treatment, and equipment; it is a system. This system must function as a whole to work properly.


More in the next blog.

Enjoy your weekend!