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:
- Build the speaker in a sealed box. This way the created pressure on the back side is contained in a separate volume and will never reach the listener/microphone.
- Built a very large separation between both sides. This is typically called “baffle” and basically extends the way from the front to the backside to such lengths that the pressure wave from the other side is strongly diminished.
There are exceptions when some kind of acoustic short circuits is allowed:
- Acoustic Dipole: There this effect is used on purpose to create a strong canceling of the sound in some directions. Anyway, the mechanical design must be appropriately done. Otherwise, there are only disadvantages.
- Venting holes: This is needed in closed acoustic volumes. Imagine a change in static pressure (by weather or absolute height). If the volume were perfectly sealed, a constant pressure difference would occur. This is problematic if a speaker is placed as a barrier between this closed volume and the outside. The constant air pressure difference will deflect the membrane away from its ideal working point, thus increasing unwanted THD, robustness, etc.
- Bass reflex systems: In this special speaker housing the back volume is opened on purpose. The opening is designed in a way that a Helmholtz resonator emerges; this is used to extend the low-frequency region by approximately half an Octave. Anyway, below this resonance, an ideal acoustic short circuit will emerge.
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.