Problem with sound traveling through air duct

[Cold Jazz]

Hi everyone,

I have a problem, which might have been covered in different threads, but I’ve done a fair amount of searching and haven't come up with a lot.

I have a basement studio which I’m quite happy with. The only main problem is that our neighbors (on the opposite side of the corridor) is an "old mans band” which has more skills playing loud than good. Most of the sound from them travel through the air duct (for the "out air") and into our control room - which sucks! When they play as loud as they can (I bet they are a bit deaf too), we can't even have a normal conversation in the control room.

I do have some hope though...
The only sound that travels through the air duct is of midrange and high frequencies. If I hold a record cover (for vinyl) over the air duct opening in the control room, it lowers the volume quite a lot! Therefore I believe I can sound proof the air duct and get rid of most of the sound pollution.

This is a “drawing" of how the air duct goes from the neighbor rehearsal room to our studio.



Number 1:
Here I’m thinking about installing a standard “noise killer” for ventilation. This image is an example of these “noise killers”:



Number 2:
Here I’m planning of making a “sound trap” in the air duct. Ill make it out of half circular MDF (half the diameter of the air duct) with high density rockwool glued to it. I might use 3-4 of these, with a distance of about 20 cm from each other. Also see the “detailed” illustration which explains what I mean a bit more.

Number 3:
This is the end of the air duct which goes straight out into the control room.
I’m thinking about removing this part, close to the point where it enters the control room. Instead Ill install a new air duct which goes in the other direction (to the left instead of to the right). Ill build this air duct into a sound proofed box which is installed on the roof.

But I have NO clue how to build this “box”. I've seen a few here on the forums, but never any blue prints or explanations how to build one.

So does anyone have any blue prints for this type of “box”?
And does anyone have comments or tips for my problem?

Many thanks in advance

Regards,
Anton

[gullfo]

you have some of the basic ideas for the sound trap (silencer/muffler). create a box from MDF with "duct liner" (instead of fiberglass or other insulation), make it 2x the area of the duct, not 1/2 the size - reduces velocity and increases the area of absorption, (square/rectangular works instead of round if its easier), and connect the existing duct to it with flexible duct. mount it outside (and sealed) to the control room, in case there is noise in the hallway.

[Cold Jazz]

My idea was to have three different points where the sound gets reduced.

“Point 1” is probably going to be one of those "mufflers" you can buy from a ventilation retailer. It will be installed in the hallway, around the area where I marked out the “1” in the illustration.

And "point 2" is in the area right before the spot where the air duct enters the control room ("point 2" will be right where the air duct enters the control room – but won't follow the air duct into the control room, just a few feet of the air duct which is outside the control room).

And point “3” is going to be inside the actual control room, and this part of the air duct is going to be installed in one of those MDF-boxes you mentioned.

The problem is that there isn’t not enough room outside, in the corridor, to install one of those MDF-boxes, therefore Ill have to install it in the control room. One option would be to install the muffler at the spot where the air duct enters the control room, and then install the MDF-box on the inside of the control room?

[gullfo]

you could build it inside the control room if that's where you have the space. the muffler can help, you might just start with the box and if you need to, you can still add the additional controls later.

[Cold Jazz]

Originally Posted by gullfo

you could build it inside the control room if that's where you have the space. the muffler can help, you might just start with the box and if you need to, you can still add the additional controls later.

Thanks, that’s very true. And a simple way of saving money if just an MDF-box is enough.
Do you know any web sites with blue prints for these MDF-boxes? The basic idea I have is to make something that looks like this:









OK, this drawing looks like it’s made by an 8 year old, but shows what I thought was the basic idea of one of those MDF-boxes.

Would make it out of MDF with high density rockwool glued to it on the inside. The air duct inside the box would also be one of the "sound proofed" ones you can order from a ventilation retailer. Around the air duct inside the box I would have rock wool of lower density.

If you know where I can get my hands on detailed information on how to make one of these boxes, I would be happy!

Thanks again!

[Cold Jazz]

...or maybe "duct liner" was what I should glue on the inside of the MDF-box rather than rockwool (glassfiber)? I didn't know duct liner was a material...

[gullfo]

something like that. John Manville makes a duct liner product you can use. (corrected)

[Cold Jazz]

But would it be possible (with a good result) to use the type of air duct that’s sound proofed in advance, and use that in the MDF-box (in the way I described it in my scribble) instead of creating a path for the air in the actual box? (in the way your picture described it).


In this way I could get a double sound proofing - with the MDF-box itself and with the sound proofed air duct. But still being able to create a similar path for the air duct as in the picture you posted.

Who/what is “John Mansfield”? I did some googling but didn’t come up with a lot (well, I came up with a lot, but not much about duct liners).

Thanks again!

[gullfo]

you could build isolation soffits to enclose the ducting. this would help. sorry i wrote mansfield instead of manville...

http://www.specjm.com/files/pdf/AHS-390.pdf