soundproofing materials

[donquixote]

does any one know what good soundproofing materials there are for structurebourne noise? I'm having trouble with something that's too loud.

thanks,
joe

 

[Hatch]

The best soundproofing material (transmission loss) is a concrete block wall or enclosure, based on cost and performance. However, it may not be useful to your application if you don't have the space, care about looks etc.

It can also be important to control reverberation noise buildup inside your enclosure, so absorption materials may be required inside your enclosure.

In summary, more information is required to help you. What is the noise source, where is the listener and what exactly is the complaint?

 

[donquixote]

Hi, sorry I wanted to delve into the detail of the design, and in fact, I did, but somehow both of my original posts (that were very long) got lost--I don't know how. I wanted to make sure that at least _something_ made its way to this forum (so I posted a very obscure question).
Anyway, now that I know that this works (somewhat), I'll try to explain things a little better...

I'm trying to reduce the noise of a pill-crusher (a project in our design course). Right now it is very loud (120 dB!), and it was disturbing the patients (they thought it was a gun going off!). The crusher uses an impact force to crush the pills. A cordless screwdriver motor turns a little cam. The spring loaded follower has the ram on the end of it to crush the pills-- not a very efficient design, but now it has been bought and paid for so we gotta work with it. It is made of abs polymer.

Actually, I've placed photos of the pill-crusher on my webspace:

http://www.engr.uvic.ca/~jbiggar

Sorry, some of the images are blurry-- but it was my first time using a digital camera.
The blue casing on the crusher is not sound proofing (at least I don't think so-- because it doesn't work).

As I said before the crusher is very loud. It sometimes reaches noise levels up to 120 dB. The peak frequencies centered about (from loudest to quieter) 375, 2000, 3000, and 6000 Hz. I asked in my earlier post about damping structurebourne noise, but now, after some more testing, I think there is considerable airbourne noise too.

I'm still not _exactly_ sure how to distinguish structurebourne noise to airbourne noise--- any ideas??

I'm trying to find out about materials to reduce the noise. The goal is to be about a normal conversational sound level (~60 dB).
Is that plausible?

Does anyone know how to damp airbourne noise and structurebourne noise at the same time?

From what I've gathered, airborne damping is best done with a light airy foam-like material, and structurebourne damping material is much more dense rubber-like material.

If I need to damp structurebourne and airbourne noise at the same time, which damping material would be placed on the part first? (my guess would be the more dense stuff)

So, in response to the last post. Concrete would not be possible because this crusher device needs to be portable and hand held (as it is about 10X8X2 inches w/o sound proofing).


Also, on an unrelated note, I've been working with alot of sound editing software lately and also have worked with sound equipment (mixers, amps etc.). I've been confused my the way they indicate sound level. Is there some sort of gage or reference decibel level that people use?
According to the software that I was using, the loudest sound I could put out was 24 db! For mixers and eq's and stuff I'm mostly working with sound levels with negative dB levels! Does anyone know how these "levels" are referenced to the absolute dB scale?
ie 4 dB is a pin drop, 60dB conversation, and 150 dB breaks eardrums etc...


Sorry this post is really long-- I hope it makes it onto this forum.

Thanks a lot for your help...

Joe

 

[Hatch]

A short answer to the bottom part of your post. My electrical engineering co-workers say that there are many standards for reference voltage in the audio equipment industry, and so unless you have an owners manual, it is not easy to determine the reference voltage.
dB(voltage)= 10log (Volts absolute/ Volts reference)
Some instruments do use a 1 Volt rms reference voltage but then the your output voltage would be 251 volts which is not likely. If a reference voltage of .01 volts is used then the output voltage is 2.5 volts (looks good). What you can do is set the output to a known dB level, measure the output voltage and calculate the reference voltage.

The answer to the second part of the above question is that you cannot relate the VU meter dB voltage to dB sound pressure level. In order to do so you would need to know the entire signal chain. i.e. output voltage out of mixer and then amplification factor (if a power amplifier is used) and then the speaker output which depends on speaker efficiency. I can only remember that Klipshorn speakers which were very efficient could produce 98 dBA with a 1 Volt input. Most speakers are much more inefficient.

Now here are my comments to your project.

Damping of an object requires energy disipation. It can be obtained by adding mass (energy dissipated by movement of the mass) or by constrained layer damping (energy dissipated by bending of the constrained layer composite).

Therefore if you want to damp a flat surface material add the heavier material directly to the surface. The more contact the better. Some older cars have heavy tar paper glued to the inside of the door and they close with a solid thunk instead of a hollow twang.

For airborne sound, an absorbing material is required, either fibreglass (cheap but is a lung and skin irritant) or foam (expensive).

Think of airborne noise as noise which directly reaches your ear from the sound source. Structure borne noise is sound which is created by vibration of the surface started by energy from the sound source. Note that the vibration can be started by sound from the source or vibration from the source, that is why I used the term energy.

So if you want to damp structure borne noise and air borne noise at the same time you must eliminate the vibration path to the vibrating surface (your pill machine case) and
also surround the unit with sound absorbing material.

So put your hand on the pill crusher box, the white areas and/or the blue cover. Do they vibrate a lot. If they do then some damping material can be used. There are composites available which provide damping and absorption but when applied to the surface you also want a barrier composite. A barrier material stops noise from passing through (transmission loss).

I don't know how elaborate a solution you want for the pill crusher, but as a test, I would build a plywood box out of 1/2 inch plywood, line the box with 1 or 2 inches of foam and then install the pill crusher inside. Try not to let the pill crusher touch the plywood box directly. Suspend it on 2 or 3 inches of foam sitting on the bottom of the box. If the box is airtight, I would expect a very good noise reduction. Then you can fine tune the box for looks, size, suspension method for the pill crusher, required openings etc.