Noise Reduction in a blower with fiberglass

[MedicineEng]

Dear All:
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.
The measures that I did were taken 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.

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.

 

[okdb]

Hello Paulo,

you should expect between 10 and 20dB attenuation with an enclosure. that is 10dB attenuation isn't that bad but there's place for improvement.

what I see at first sight is that your enclosure might not be good enough to block the low frequencies related to a big fan. You should use 100mm fiberglass and a steel plate at least 2-3mm thick.

Look for leackage, maybe the enclosure isn't sealed enough around the ducts.

Another point, it can be a lot of fan noise going through the ducts.

Hope this will help.

OkdB

 

[MedicineEng]

okdb:
Thanks a lot for your reply. I am already more relieved to know that at least the enclosure is inside the expected noise reduction values. Regarding the duct, we also have insulated it with a internal silencer but due to the configuration of the duct, the fiberglass thickness used was 25mm.
I will also check the leakage.

PR

 

[ccw]

It has been a while, but a few years back I would have been happy to get 10dB reduction out of a fairly simple two inch barrier application. Your noise reduction guy has effectively done a little better than cutting the sound power in half, three times. To do a better evaluation of the barrier performance, you would need to know the before and after power spectral density (frequency content) of the noise. Qualitatively, have you noticed a change in the nature of the sound, more low frequency rumble, less harsh, etc.?