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Is that it for passive vibration control? What next?
1/4" felt
vinyl barrier
1/8" felt and vinyl barrier
Yes, that is the basics of VIBRATION CONTROL. From here on, we will be treating NOISE directly.
At this point, I liken noise control as being similar to a boat on the water. The hull acts as a barrier to keep water out but once in, it is treated with absorbers (a sponge). Barriers are the next step in controlling noise and to draw another analogy, a barrier is to sound what an isolation mount is to vibration. They both get between the source and the receiver.
A noise barrier is exactly what it sounds like but to be effective, it needs to be massive (heavy) and limp (does not allow waves to move through it easily). And it ABSOLUTELY MUST BE ISOLATED FROM THE VIBRATING STRUCTURE! The reason is simple. If the barrier were placed directly against a vibrating surface, the barrier would also vibrate and generate the same noise that the structure does. Therefore, it is imperative that the barrier be isolated from the structure. Felt or some kind of fiberglass batting works well. The main thing is there should be something that is limp and transmits vibration poorly between the barrier and the vibrating surface.
As an aside, felt is one of our most popular materials for treatment since it is so versatile. It is good at absorbing noise and vibration and while porous, it still acts as a reasonable noise barrier. When used as a floor underlayment, it has a soft, comfortable feel and is generally durable enough to take the wear and tear of feet constantly pressing on it.
Obviously, weight is our enemy so we have to balance the benefits of weight gain vs. sound reduction.
We tend to use barriers as sparingly as possible and only where needed. Most of us have seen sound walls along freeways. These are made of heavy materials (brick and concrete) to stop sound from penetrating to the other side. Now try to imagine how ineffective these walls would be if they were made of fabric.
To operate effectively, barriers must also be very tightly fitted to eliminate noise from leaking in. Where air can go, so can noise so it is VERY IMPORTANT that in fitting a barrier, all gaps are as small as possible consistent with allowing free movement of all controls and control surfaces! In many cases, I will use soft closed cell foam as a gasket between openings and a barrier if I can achieve a better fit.
While it would be great to use limp barriers all over an aircraft, practically speaking it is best limited to the noisiest parts of the plane, i.e. floors, firewalls and over nose wheel wells (a VERY noisy structure!)
