What Are The Best Sound Absorbing Materials – www.AcousticFields.com
Hi everyone, Dennis Foley from Acoustic Fields.
Today we’re going to talk about sound absorbing materials. There’s a lot of confusion out
there of what constitutes a sound absorbing material, so we need to look at the definition
and see if we can get a better understanding of things as it goes.
Sound absorption is a process where acoustical energy, airborne energy is converted to heat
through a process. It’s complicated but let’s just keep it simple for the sake of this discussion
and we’re changing airborne, acoustical energy, sound energy if you will into heat, so 2 things
going on, 2 variables and 1 process. We’re converting sound energy into nothing basically
through absorption and converting it to heat. We’re losing sound energy. We’re actually
managing it by losing it and that’s a debate we could have in a whole another video on
a whole another session. We don’t feel it’s necessary to absorb all that energy in order
to manage it correctly, that there are other ways to do it but we could talk about that
in another video. There’s 2 basic kinds of sound absorption
processes that go on. There’s air movement or molecular velocity, if you will, which
most of your foams, your open celled acoustic foams and your building insulations, your
limp mass absorbers use that principle. It requires air movement moving through the particular
sound absorbing material and through that movement, through the material, you get sound
absorption. You get the heat process and then you got loss of amplitude, so you slow the
reflection down, sometimes too much. Open celled foams, limp mass products are really
good at that. Anything that air move through, for this process to work, and thus it’s usually
only pertains to middle and high frequency, you need molecular velocity or air movement.
What’s the other one? It’s the pressure. When you’re dealing with low frequency waves, you’re
really not dealing with air movement. You’re dealing with air pressure, completely different.
Pressure and air movement are one and the same but basically different. You have air
movement with pressure but pressure is something that must be dealt with differently because
it’s just too much, too overwhelming and too great of a mass, so to speak, to work with
and that’s why acoustical foams and limp mass products and things like that won’t work in
the 30, 40, 50 cycle range. It simply won’t because it just overpowers the sound absorbing
technology. To deal with pressure, to deal with low frequency waves and long wavelengths,
you need pressure reactive devices — Helmholtz resonators, diaphragmatic absorbers, these
are just 2 that work really well. Membrane absorbers would be another one; all with various
different degrees of success, all with different rates and levels of performance. You first
have to assess your needs and apply the correct technology for that particular need.
Remember it’s a combination. Sound absorption is a process where airborne acoustical energy
or if you will vibrational energy through solids is changed to heat and thus a reduction
in the amplitude or the signal occurs but there’s a trade off. We actually lose energy
when we change it to heat. We don’t destroy it but we lose it and losing energy in order
to manage it is very problematic. You have to be very careful about that. You have to
choose the sound absorbing material that fits either the air movement or the air pressure
and make sure you have the correct rates and levels to deal with.