Investigation into how sound proofing actually works

[king oga]

Hey everyone

I'm doing some investigation into how sound proofing actually works, and I'm more or less stuck. I've found two terms(http://www.soundproofing101.com) which are used to decribe what happens when you sound proof. they are: noise reduction and noise absorption

"Noise reduction
noise reduction is achieved by stopping it, kilin it, not letting the noise get through the wall or window.

Noise absorption
Noise absorbation is achieved by chagnng the characteristics of the noise. Stop it from echoing.

What works for noise absorption does not ever work for noise reduction"

'wikipedia' also says something similar. http://en.wikipedia.org/wiki/Sound_proofing
wikipedia is written by ordinary people like you and me so maybe the person read the same resoucres i have.

does anyone else have any more indepth explanation? any links or places i should chek out?
perhaps anyone can try to explain what it means to 'change the acoustical propreties of sound'?

thanks for you attention so far, any response or help will be Greatly Appreciated!

 

[Chi Meson]

Noise reduction can be achieved by having an instrument create the same sound but "out of phase." This is also known as "sound cancelation." This is difficult to do over large range of frequencies, and the only way to cancel the sound everywhere is to create the "anti-sound" in the same place as the original sound. Otherwise you can only cancle sound in specific, small locations with each instrument. (Bose makes headphones that do just this,)

Sound absorption is done by having non-rigid materials take in the vibrations from the air, but instead of transmitting these vibrations, they turn it into heat. THis is kinda like shock absorbers in a car: they take away the "bounce" in the springs by introducing a lot of friction. THe energy becomes heat which makes the shocks warmer. As sound is absorbed, the material must get warmer.

"acoustical properties" include the frequency (pitch), the amplitude (loudness), and quality (combination of overtones). When reducing sound, you are primarily trying to reduce the amplitude.

More info is coming, I'm sure.

 

[king oga]

thanks for the infor so far. I've got a few question on what you've said. how does a rigid material absorb the energy of sound and change it to heat if rigid material are suppose to 'conduct' sound better than thigns like water and air. I was told this is so because the molecules are in close proximity and so the collionsions occur and spread more rapidly than in a liquid or gas. and so there is less distortion and the sound remains the same.

this is kinda weird becasue real-life evidence (that big cement walls reduce the sound [volume] of my music :D ) proves that what you are saying is true.

 

[Chi Meson]

I did say that non- rigid materials absorb sound better, right?

Big cement walls absorb sound usually because they are so massive. If you were to compare the same thinckness of material, say 6 inches of foam rubber to six inches of concrete, chances are that less sound will get through the foam rubber. There are of course, many other factors. For example, More sound will be reflected back by the concrete, and that means less sound going through the wall.

If you go to a library and pick up a book on house construction, there should be a section on what is done to soundproof rooms. The same stuff that is most often used for heat insulation (fiberglass batting) turns out to be great for sound insulation too.

 

[king oga]

so how can you tell if the sound is goign to reflect rather than go into the material?

unfourtuantly I am living in tanzania so we don't really have the best libraries, my school one is also pretty small. any good internet resources so can point me to? I've tried googling and i haven't been able to find any site which give real concrete explanations.

and I am moving house at the moment so it might be a bit be fore i next post. but thannks A LOT (and A LOT more!) for your help so far. thanks! :D

 

[Gokul43201]

I've refrained from getting into this thread because I'm not sure if you want this information for a school project or are you trying to achieve some practical task. If there is something specific you want to achieve, it would be best if you state clearly what you're trying to do.

To answer your last question, the fraction of sound reflected off medium 2, back into medium 1 is a function of a property known as the mismatch of acoustic impedance between the two media. The acoustic impedance depends the elastic properties of the medium. In air, the molecules have a large mean path length, but in steel, or glass, this is very small. In porous media, the mismatch is much smaller, making them good absorbers.

Also let me add that Chi Meson's method, known as active noise control, makes use of extremely expensive electronics (for signal detection, amplification and feedback circuitry) and is especially good at low frequencies. Most places around the world simply use passive noise control, which basically involves determining geometries and materials.

 

[rockolance]

i have a problem. i researching about sound absorption in office in terms of the material and design.can someone help me to tell how to conduct the experiment and collect the data.it is for my science project. i will appericiate your help.

 

[FredGarvin]

Rockolance, you should start your own thread. Don't hijack someone else's. It makes it difficult to follow the topic.

King oga, there are some other aspects to consider that have tricks used for quite a while. In terms of sound reduction, one of the best tricks in a room is to stop the transmission of the sound through the walls by limiting the contact area between the wall and the support structure that is shared between rooms. You are not absorbing the sound, you are limiting its path it can travel to other areas.

In a typical wall, you have 2x4 wood studs and two sheets of drywall on either side. That provides a rigid path for sound to go through one sheet-through the studs-into the other wall. The sound goes right through. In a staggered wall, the studs are staggered so that only half of the studs are against each sheet of drywall and there is a larger spacing between the walls. This adds a larger air gap between the walls and provides more room for insulation. Again, you are not absorbing the sound, just keeping it in the room it originated.

I highly recommend looking at Art Ludwig's website. He has some great work in the realm of acoustics and soundproofing:

http://www.silcom.com/~aludwig/index.htm

 

[Danger]

Just as a point of curiosity, how would an aerogel fit into the sound insulator/conductor spectrum?

 

[rockolance]

i have figure out the structure of the experiment. i just need a good research question .can u describe to me they do sound proofing in the car and for the plane as for the loud plane engine.is it has to do with sound cancellation.do they change frequencies based on the engine frequencies

 

[bartoneblaze]

One of the easiest ways to soundproof a room is by reducing the airflow into the room.The acoustical properties of sound means absorbing layers used in a gas turbine exhaust silencing component at high temperature with temperature gradient are investigated by analyzing the effect of temperature on the acoustical properties of porous materials.