# A simple question about frequencies, vibrations and engineering

I am a composer that likes science a lot but I am less than amateur at it.

I am trying to start understanding how low frequencies (below 100hz let's say) behave. The intent is to find solutions to block and absorb it. Yes, I know there are many solution for this already, but if I can understand how those frequencies work affecting the environment where they are being generated, I can understand the solutions as well, as opposed to simply hear people say "do this, do that" without exactly knowing why.

A practical example: I live in a building and have a studio in a 12' x 12' room. I know, for professional standards, it is far from ideal. Reality most of the time is. And of course after I moved in here, the low frequencies of my subwoofer did not take long to bother my neighbor downstairs. Since there is a minimal listening volume for a even hearing along the frequency spectrum, I would have to find a solution to 'treat' the low frequencies.

The problem is: the word of mouth, from amateurs to specialists is 'there's no solution to your room or it would be unworthy cost/benefit-wise". Ok, I could go with that and give up, wait until my lease is finished in a year from now and shut up or... I could try finding other solutions.

Who knows, maybe going into the core of the problem, learning the physics of it, I could A - come to the conclusion that there is not a solution for this problem (bah...) B - Find a solution, be happy and possibly make a lot of other people happy too.

Would anyone want to help me on this journey?

I have at least some math foundation to understand what is going on if someone explain to me.

Thanks!

• There are many bass traps already on the market, with a great deal of expertise, money, and trial-and-error that's gone into them. I feel that your time could be better spent. To answer the physics though: you want to introduce something into your room that the sound waves will cause to oscillate and thus dissipate the energy. The problem with low frequency (high wavelength) sounds is that they resonate with big things like walls... Commented Feb 21, 2015 at 12:08
• I would talk to the neighbours and explain your problem. Maybe you can agree with them not to use your subwoofer within some time of the day/night when they want peace and quiet. When I was learning the trumpet, the deal was "Ok, but not after 21h". Commented Apr 12, 2015 at 19:54

I will second what everyone has said... you will not like what you hear.

At the most general level, sound is governed by wave equations, which are differential equations. Feel free to look the details up, I'm just looking to provide an overview.

Energy is never created nor destroyed (in known physics). It just changes form. Your subwoofer has changed electrical energy into physical movement energy (sound waves). That's what you paid it to do. However, once it gets past your head, you don't care for that energy anymore. You want to get rid of it.

The answer is always that you convert the sound energy to heat. There's some really cool technologies to redirect that energy, but that's the realm of stealth fighters and nuclear submarines (not joking... they spend a lot of money on it). The best that is available to you is to try to convert all that energy to heat.

The tried and true process for this is called a "damped mass." The clearest example I have found of this is a soundproofing technology where you glue an extra layer of drywall onto your room (Green Glue is one company that produces such glue). The idea is that the drywall acts like a "mass" which vibrates with the sound waves. The glue is a damping material that is very good at turning vibrations into heat. It then connects to the existing layer of drywall for structural support.

For bass frequencies, the second layer of drywall is important. The glue could not effectively stop low bass notes just with its own mass and rigidity. The second layer provides the mass and the rigidity needed to help convert those bass notes to heat.

Nearly every technology you will find works like this. Even if you have some fancy electromagnetic gizmos, they'll eventually boil down to trying to act like a damped-mass (just electronically rather than physically).

Bass frequencies are not special. They obey the same laws as trebble frequencies. However, lower frequencies require larger damped masses, and our hearing is of trouble as well. We tend to need the VAST majority of our sound energy in the bass frequencies to hear them. It is not unheard of to have a subwoofer driven by a 5kW amplifier in the same stack as a 25W tweeter, and have to turn the trebble down!

• Nice explanation. Although, one has to keep in mind that in many cases, smaller attenuations in the bass frequencies is quite adequate since the equal loudness contours tend to become steeper in the low frequency regime when the amplitude is lowered. This of course is more than compensated in the higher frequencies with the increased damping provided by materials and structural elements. Commented Feb 12, 2020 at 12:05

Would anyone want to help me on this journey?

Probably but they all will probably give you the answer you don't want to hear. Which is that technology to prevent sound from affecting walls without any construction does not exist.

• Can you expand on this a bit, i.e. explain why the technology can't exist? As it currently is, this comes off as a bit rude. Commented Apr 12, 2015 at 21:36