Noise Reduction with Fiber glass 1

[MedicineEng]

Dear All:
I am not shure if this is the most apropriate forum to post my question, but I believe most of you that work with HVAC also had this noise problems. My question is the following:
This was my first acoustic insulation work so I don't have any past experience with this issue. Two months ago I asked one subcontractor to insulate a extraction blower that I have in my site. For that, he used a 0.6mm G.I perforated sheet in the inner face, 1mm GI flat sheet in the outer face and in between a 50mm thick fiberglass (density 48Kg/m3).
I measured before and after the work done and I realized that the noise reduction was only 10Db. In fact I was expecting much more reduction. After contact the supplier he told me that the reduction was inside the expected values.
What I would like to ear from you is if you have experience with this material and if the supplier is giving me the correct information regarding the noise reduction.
Also, is there any site that you recommend for me to get more knowledge about the issue?
Thanks a lot for your help.

PR

 

[TheHub]

I am not sure what type of noise reduction you are looking for, but after installing insulation a reduction of 10 DB is outstanding.

Sound level in decibels is a logarithmic rather than a liner measure of the change in pressure with respect to a reference pressure level. A small decrease or increase in decibels and represent a large change in sound energy. Technically, an increase of 3 dB represents a doubling of sound energy, and an increase of 10 dB represents a tenfold increase.

In addition, you provided no frequency weighting for your dB reduction. I assume most of the reduction was in the high range. Sound insulation will provide little if any reduction in low frequency range. You also did not provide over-all dB measurement for the area and the goal you are looking for.

You might want to check Mark's ME Handbook for Sound-Absorption Coefficients for your room before treatment and compare with sound reduction coefficients after treatment (sound reduction coefficients should have been provided by insulation manufacture). The general equation is,

Noise reduction in dB=10 log (total absorption after treatment/total absorption before treatment)

Mark's on the whole provides a easy to follow approach to making this determination.

Hope this helps
Jim

 

[ChrisConley]

The rule of thumb I use for sound calculation is that every 3dB of change is the result of doubling (or halving) the sound intensity.

 

[imok2]

This doesn't answer your question directly bue is a good source of information, hope it helps your understanding

http://www.domesticsoundproofing.co.uk/soundproofing.htm

amicus humani generis

 

[MedicineEng]

Dear All:
Thanks a lot for your feedback. Regarding some of the doubts raised let me try to explain better:
The measures that I did were with a handheld sound meter from Quest technologies and this instrument doesn't have the range of the sound, that is, only gives me the sound level in Db
The settings that I put to measure were:
Weighting A (simulates the response of the human ear);
Range: 50-120 Db;
Response :F (fast-125ms time constant) Decay rate-34.7Db/s.
Values obtained:
Pos1. 99Db before; 89Db after;
Pos.2 101Db before,91Db after.

PR

 

[pennpoint]

PauloRibeiro
I do not mean to prolong your answer. But you might want to post your query to "Mechanical Acoustics/Vibration engineering" on this forum; where specialist in acoustics may be able to give you a more direct answer. Not saying the group here are not helpful in any way. I am just attempting to broaden your audiance.

Good Luck
pennpoint