Why is sealing so important in acoustics?

Acoustics is based on vibrating air pressure. The higher the amplitude of the vibration the higher the SPL. To create these pressure changes a moving membrane is used in loudspeakers.

Paying a closer look at the situation, one can see, that when the membrane moves forward, an over-pressure created in front of the membrane. On the other hand, on the backside, an under-pressure emerges. The natural tendency of the air is now to balance out this difference by flowing around the speaker from the front to the backside. If the target is to get a high SPL, it is crucial to avoid this effect.

If there is a direct way for the air to balance out the pressure between the back and front side of a loudspeaker we call this “acoustic short circuit”. This poses the worst-case scenario and will result in a strong reduction of SPL, compared to the separated case.

Two ways are possible to avoid the acoustic short circuit:

There are exceptions when some kind of acoustic short circuits is allowed:

In case of In-ear / artificial ear, sealing of the ear channel is even more critical, because it determines how much air pressure is produced. Probably everyone has experienced this him/herself: if In-ear phones are not inserted correctly the bass is gone, and the overall level is much lower. This is because the pressure is much higher if the volume is completely closed, and no balancing with the outside air pressure is possible.