fabric over perforated panels

[JSilver]

Hello, I'm trying to build some perforated panel. after I finish if I cover them with some fabric I risk to invalidate or limit the effect of the panel?

What's better many little holes or less bigger holes assuming that the perforation percentage is unchanged.

thanks

[Jens Eklund]

Originally Posted by JSilver

Hello, I'm trying to build some perforated panel. after I finish if I cover them with some fabric I risk to invalidate or limit the effect of the panel?

The best place for the fabric is behind the perforated panel but it’s important that the fabric does not introduce a high flow resistivity since the function of the system will change. You could of course place the fabric on the outside of the panel but then it might absorb the highest frequencies if not very thin.


Sincerely Jens Eklund

[Jens Eklund]

Originally Posted by JSilver

What's better many little holes or less bigger holes assuming that the perforation percentage is unchanged.

A very good question. I would assume that more (holes) is better (assuming that the other parameters are adjusted so the fc, Q and abs. coefficient, is predicted the same way in the calculator) but I really don’t know and I’m hoping to do some experiments regarding this.

Naturally it comes to a point where the flux lines for the flow from the holes cannot spread out over an area to big and I guess the question is how big this area is.


Sincerely Jens Eklund

[avare]

Originally Posted by JSilver

Hello, I'm trying to build some perforated panel. after I finish if I cover them with some fabric I risk to invalidate or limit the effect of the panel?

What's better many little holes or less bigger holes assuming that the perforation percentage is unchanged.

What are you trying to achieve acoustically? If it is a broadband absorber, then the finer the holes the better. The length of the solid material determines at what frequency the panel will start to reflect sound. The longer the web material, the lower frequency at which it starts to reflect. With a 20% open area, below the frequency where the lenght of the seb is significant, the panel is acoustically transparent.

Yes, panels with 20% open area are 100% transparent.

It is a bit daunting on first read, but it is detailed quite well in by Schultz here.

Andre

[Xander]

Originally Posted by avare

It is a bit daunting on first read, but it is detailed quite well in by Schultz here.

Thank you Andre for the link. I'm excited to start reading it.

[RLD]

Originally Posted by avare

What are you trying to achieve acoustically? If it is a broadband absorber, then the finer the holes the better. The length of the solid material determines at what frequency the panel will start to reflect sound. The longer the web material, the lower frequency at which it starts to reflect. With a 20% open area, below the frequency where the lenght of the seb is significant, the panel is acoustically transparent.

Yes, panels with 20% open area are 100% transparent.

It is a bit daunting on first read, but it is detailed quite well in by Schultz here.

Andre

Interesting...I've been doing some research as I ended up wrapping my panels in plastic so I wanted to be sure it was OK...
This book...Noise control manual for residential ... - Google Books
has a similar conclusion , but says 25% open is needed to be acoustically transparent...close enough. thumbsup
He also says plastic film less than 1 mil thick is acoustically transparent as well.

[avare]

Originally Posted by RLD

This book...Noise control manual for residential ... - Google Books
has a similar conclusion , but says 25% open is needed to be acoustically transparent.

Thanks for the link. thumbsup

I have seen test reults from BBC which indicate that 18% is still tranpsarent at low frequencies. So I gues the YMMV is very true in this case.

Andre