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[JimmyT]

Hello, I have a curious problem. I am have been doing some DIY soundproofing, which is working to an extent, but I have come upon a problem with one of the windows. I have a double glazed unit, and have stuck a 6.4mm laminated pane over the top of that (so an air gap of only 10mm or so, the I have a 120mm gap and then a 10.8mm laminated piece of glass. This does a good job of blocking most noise.

But the 10.8mm glass seems to have the curious effect of making some very low frequency noise (specifically when nights buses drive past at speed) louder than they are without it glass on!

I use my phone and the SPLnFFT app to measure the sound using db(B) filter. so without the 10.8mm piece I gwet lots of traffic noise around 47-57db(B) but never any higher. But with the 10.8 piece on the general noise is low - it keeps it below 50db(B) which is great, but every 15mins or so I'll register a 65db(B) when a bus goes by at speed. without the 10.8mm on I never register sounds as loud as this, it seems the 10.8mm glass is somehow resonating and amplifing this very low frequency sound. It is only this one very specfic incidents of the buses accleratring at speed that causes it.

The question is how can I stop this happening? Should I replace the 10.8 with a thicker 12.8mm piece or a thinner 8.8mm piece? any ideas how to sort this one out? I should pint out that I don't need to see through this window, so other options are opened up.

Many many thanks

James

[Randy S]

Lets do test to make sure that it is not the final pane of glass creating the increase of noise.
Put something in between the panes that is absorbent like a pillow, beach towel, sleeping bag or a large piece of foam...
Let me know what the result is.

Randy S.
 

[JimmyT]

Thanks Randy, I'll give that a go. I'll throw in a couple of sleeping bags and some towels and foam.

[JimmyT]

Hi Randy, I put in a 4cm thick piece of latex from an old mattress topper. I stuck it against the 6.4mm glass. So from the outside there is the double glazinng, then stuck on the inside the 6.4mmthen the latex on top a larger gap then the 10.8mm.

The latex has made an improvement, which is great, thanks. I tested the sound over a 15 minute period and the loudest bus noise was 59 db(B), whereas without the latex I would usually get a couple of readings between 60 and 65db(B). The other general readings were lower too, more around 50 rather than 55.

What would you suggest to do now to try and get the sound down a little more? I have some more latex so could do a double layer.

Many thanks

James

[Randy S]

It sounds like we are getting a sympathetic vibration, lets go ahead and do another layer to see if we can get a bit more improvement or change to a thicker value for the next layer...
let me know the results..
looks like we are heading in the right direction.

Randy S.

[JimmyT]

Hi Randy, I put on another layer. I didn't quite work out. I didn't have enough latex left over for a full second layer, so about 2 thirds has a double layer of latex (4cm x 2) and the other third I put some closed cell foam on. The foam is only 5mm thick though. When I put the 10.8mm glass back on and tested the sound it registered the same figures as before. Do you think the 5mm foam is just too thin to have any impact at all? I should maybe get hold of something a lot thicker and try again?

Many thanks

James

[JimmyT]

With the sympathetic vibration would it be the case that if I put a thinner piece of glass on in place of the 10.8 mm piece I would still get a sympathetic vibration only at a higher frequency? And if I put on a thicker piece I'd get a sympathetic vibration at a lower frequency?

[JimmyT]

Also does the size of the glass increase the level of sympathetic vibration? The 10.8mm glass measures 174cm by 74cm (68.5 by 29 inches). If I got it cut into two pieces would that reduce the vibration?

[Randy S]

do not go with thinner glass, the size of the pane is where it is at...look at the window like a speaker..the bigger the window the bigger the speaker.
So If you are able to cut it down in size and maintain the air tight seal this would work better.

There will come a point of diminishing return.

Randy S.

[JimmyT]

I finished a second layer of foam , so there is now 8cm of latex and foam, but I am still getting the same readings of 58db(B).

I don't think I could make the glass sheet significantly smaller unfortunately.

Would a thicker 12.8mm piece sort the issue out? Or should I try another 4cm layer of foam?

Many Thanks Randy. I really appreciate the assistance

[Randy S]

Jimmy,

Here what we need to do....going to take some patience but what I need you to do is to put your hand on the center of the window pane and push when a truck or bus drives by when you are taking a reading and see if we get a change before I can proceed forward.
If that gives us more reduction then yes thicker pane might get us a few more points..

Randy S.