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All About The Boundary Effect

Exploration By Paul White
Published December 1995

Accurate monitoring is a must in the recording studio, but the perceived level of bass depends on where you are standing and where the speakers are positioned. Paul White explains why in audio, a boundary literally means a six.

One of the most common sound‑related questions we are asked at SOS concerns how the level of bass seems to increase when you move towards the back wall of your studio. Is it a 'hot spot', is it a room‑specific problem, or what? The truth of the matter is that this is a perfectly normal phenomenon that will occur close to any solid wall, and is all to do with the so‑called boundary effect. If the term sounds familiar, this could be because the boundary principle is also used by PZM, or boundary, mics which utilise reflections from the surface they are mounted on to reinforce their low‑frequency response.

Hard surfaces reflect sound at all frequencies, but the boundary effect (as it affects the listener's position in a room) becomes more pronounced at low frequencies, where the wavelengths of sound are correspondingly longer. When you move very close to a reflective surface, the low frequencies which bounce back are almost in phase with the direct sound you're hearing which, in effect, doubles the amount of bass you perceive. A boundary creates a 6dB lift at low frequencies, and this isn't limited to the back wall of a studio — it's just where most people seem to notice it first.

The first lesson to be learned is that your monitoring position must be at least three feet from the nearest wall, otherwise you may be misleading your ears as to the level of bass that's really there. If you hear too much bass, you're likely to EQ some of it out, resulting in a mix which sounds bass‑light when played back in an average listening room.

If one wall can cause a 6dB bass rise, what sort of damage can two walls do? Position yourself in a corner, and you get in‑phase reflections from two nearby walls, producing a 12dB increase in bass. If you now move your head down to floor level, you'll find that the floor joins in and adds another 6dB just for good measure. This is an interesting experiment to try, and also makes you see just how dusty the corners of your studio really are — you may even find a few lost plectrums amongst the dead spiders!

As far as you, the listener, are concerned, the boundary effect isn't a problem as long as you are able to stay at least a couple of feet from a wall. If you must sit closer due to space constraints, then you really should use relatively small monitors that aren't too heavy in the bass, in order to avoid the frequencies that cause the most trouble. Furthermore, regardless of your system, you should play a few commercial recordings through it just so that you can get used to any little quirks your speaker/room combination may have.


As a typical loudspeaker projects low frequencies in all directions and not just forwards, the closer you stand your monitor to the wall, the more bass lift will be produced. Yes, the boundary effect comes into play for speakers too. Some small monitors are actually designed to be used close to a wall, and in their case the boundary effect is taken into account, so always read the manual that came with your monitors to determine their best position.

You should be able to put compact nearfield monitors, such as NS10s, about a foot from a wall without causing any serious problems, but getting them too near to a corner can cause significant low‑end inaccuracy, especially if the speakers have an extended bass response. If you must put them near to a corner, make sure that the distances from the monitor to the back wall, and the monitor to the side wall are as different as possible. If they are identical, then any peaks or troughs in the bass response, caused by wall reflections adding to or subtracting from the direct sound of the monitor, will all occur in the same part of the audio spectrum. Keeping the distances different will at least tend to randomise any undesirable effects.

Finally, bear in mind that it isn't only walls that reflect sound; any hard, flat surface can do it, including your mixer and effects rack. That's why mounting your monitors on your meter bridge isn't really a good idea, because you get a lot of sound bouncing back from the desk, which interacts with the direct sound to produce inaccuracies in the frequency response. It is far better to put the speakers on stands a little way behind the desk, so that any reflections that do occur are at a shallower angle, and therefore less likely to end up at your ears. Recording is supposed to be without limits, but it will always have its boundaries!