Building a dead room.

Albert · Post by **Albert** » Tue Jul 08, 2003 4:49 am

I need to build a room with as little reflections as possible for making acoustical measurements. Just browsing through the site here I came across the absorbtion coefficient chart which shows that 4 inch fiberglass board has very good absorbtion properties at all frequencies. What exactly is fiberglass board? what does it look like? Is it the same as rockwool? and can I fix it directly to the walls

Post by **John Sayers** » Tue Jul 08, 2003 10:40 am

I need to build a room with as little reflections as possible for making acoustical measurements.

you mean an anechoic chamber??

cheers
john

Albert · Post by **Albert** » Tue Jul 08, 2003 6:14 pm

Yes, an anechoic chamber would be ideal, although I'm on a very tight budget (hundreds rather than thousands of dollars). I realise that what I can build on a budget is not going to be perfectly reflection free. The room I am stuck with at the moment is pretty reverberant and if I can reduce the amount of reflected sound by absorbtion in all frequency ranges by using those 4 inch fiberglass boards it would improve matters tremendously. The acoustical measurements I am making are relative rather than absolute so perfection isn't necessary.

Any tips on how to apply those boards or even what they look like etc would be appreciated

knightfly · Post by **knightfly** » Wed Jul 09, 2003 6:09 am

Albert, what about OUTSIDE isolation? Do you also need serious sound proofing? The more info on what you're trying to accomplish, the closer we can get to your ideal solution... Steve

Albert · Post by **Albert** » Wed Jul 09, 2003 7:19 am

Thanks so far for the replys. I don't need to worry about the sound I'm making getting out of the room if that's what you mean, as the measurements I am making are not loud (I am a string instrument maker and am trying to measure the sound radiation of the instruments I make. This involves tapping the instrument with an instrumented impact hammer which causes broadband excitation of the instrument and measure the response with a microphone. What i've found is that taking several measurements in different positions of the room but without moving the microphone in relation to the instrument, I get massively different reading which must be caused by room modes, reflected sound etc) Also the place I work in is pretty quiet so outside noises also are not a problem (If i take a measurement and then half an hour later do the measurement again without moving anything I get the same result) any ideas to cut down the reflected sound in the frequency range 100Hz to 15000Hz would be greatly appreciated. I do realise that what I'm trying to do probably can't be done very well on a tight budget. Thanks.

Post by **John Sayers** » Wed Jul 09, 2003 9:49 am

AH Albert - what you need to do is go out and purchase heaps of 4" 703 insulation and line the walls and ceiling with it. Dead as hell

:)

cheers
john

knightfly · Post by **knightfly** » Wed Jul 09, 2003 10:23 am

Absolutely, Albert - Also, this is probably the ONLY time I would recommend carpeting an entire floor -

I didn't have any luck finding pix of 703 yet - maybe John has some somewhere?

You can mount it with anything from long nails to construction adhesive to used bubble gum, it's rigid enough not to sag. Usually comes in 2 ft x 4 ft panels, anywhere from 1" to 4" thick. They also make a 705 FRK, which has even better absorption in most frequency bands. For what you want, 4" is the way to go, everywhere but the floor... Steve

Albert · Post by **Albert** » Wed Jul 09, 2003 5:58 pm

Thanks for the advice. Just a couple of questions more, Are there particular firms who manufacture the 703 insulation boards? I did a google search under "703 insulation" but came up with nothing. Is it a specialist acoustic material or just a plain old insulation material that I could get at the builders yard? Also reading through your website I came across the principle that the absorber should idealy be placed at 1/4 wavelength for maximum efficiency, so to cover all frequencies down to 100Hz the absorber would need to be approx 34 inches thick, have I understood this correctly?

Thanks again for the help

giles117 · Post by **giles117** » Wed Jul 09, 2003 11:58 pm

owens corning does, call their 800 # and they will put you in touch with a rep who will put in touch with a local supplier.

That is how I found the 703 I am buying for my live room.

Bryan Giles

knightfly · Post by **knightfly** » Thu Jul 10, 2003 12:22 am

Right, Albert - for best absorption to that low a frequency the 703 would need to be stood off the wall by 34". However, that formula pre-supposes that all sounds will be incident at 90 degrees to the absorbent material, when the reality is that very FEW sound waves will actually be traveling parallel with room dimensions. Also, depending on the size of the room you're using it might not even support 100 hZ - can you give us the dimensions (all three) of your room, and how much of it you can give up for acoustic treatment? Steve

Ethan Winer · Post by **Ethan Winer** » Thu Jul 10, 2003 12:58 am

Steve,

> depending on the size of the room you're using it might not even support 100 hZ <

I'm not sure what that means. Even small rooms can reproduce very low frequencies if the reflections are adequately trapped.

--Ethan

Albert · Post by **Albert** » Thu Jul 10, 2003 5:27 am

The room is 4 foot 9 inches by 9 foot 10 inches by 8 foot 2 inches high, so its pretty small, but would be physically big enough with 34 inches of insulation installed all round to still do my measuring.

I'm confused by your statement about the room possibly not supporting frequencies down to 100Hz. Do you mean that even if a very small room were totally reflection free, these low frequencies won't be heard or not be measurable? can you explain further what you mean as I already can measure some frequencies around 95Hz on my bass string instruments that are definately emitted by the instrument, but maybe my room is big enough?

Thankyou to everyone who replied, it's much appreciated.

knightfly · Post by **knightfly** » Thu Jul 10, 2003 9:34 am

Guys, sorry for the quick blip with no explanation, I was headed to bed after 12 hours of graveyard shift - all I meant was, that a room would have to have a certain minimum dimension before it would RESONATE at 100 hZ - that dimension is only about 5'8" (for an AXIAL mode) but you'd be surprised at the small rooms people want to put a full control room in.

"The room is 4 foot 9 inches by 9 foot 10 inches by 8 foot 2 inches high, so its pretty small, but would be physically big enough with 34 inches of insulation installed all round to still do my measuring. " -

Now I'm starting to question my own math. How can you put almost 6 feet (34" times 2) into a 4 foot 9 inch dimension and still have room to work?

I've not built any anechoic chambers, and I've only been in one. It was at HH Scott's test lab in Maynard, Mass, when I worked (briefly, part time) for Daniel Von Reklinghausen (never could spell his name, sorry Danny) - Anyway, that one definitely wasn't a $100 room, but it certainly was strange to be able to hear your own heart beating, and the air moving past each cilia in your nose, etc...

I'm thinking that if you used the 4" Owens Corning 705, spaced out from the wall (with no paper backing) on ONE of each pair of walls, the same stuff spaced as far down from the ceiling as you can (but a DIFFERENT amount than the spacing from walls) - then used 4" 703 AGAINST all walls/ceiling, even those with spaced out 705, that you should end up with a room that's as close to anechoic as you'll get for less than paying a professional acoustician to design it and his favorite builder to build it.

If there's room between the two absorbers, say on the long wall, you could use John's bass hangers in between the spaced out layer of 705 and the wall-mounted layer of 703 - this would further kill the lows. I'm not sure where, but maybe John can come up with a picture of the bass hanger construction - I'm nearing the end of my free time for now.

Your room dimensions do have some modal problems, but because you need TOTAL absorption of EVERYTHING there's no point in targeting specific frequencies more than others. Just for reference though, here's a couple of screen shots showing your room dimensions and the resultant modal distribution - the first is from studiotips' very useful sheet, modesv2, and the second is from an old excel sheet (roomtune) I wrote nearly 20 years ago.

On the Bonello chart, any blue bar should be taller than its corresponding red bar or there is an uneven mode distribution that will cause coloration.

On the Axial mode chart, ideal is no two bars the same height, at least 4-5 hZ difference between them, and no more than about 20-25 hZ spread between successively taller bars.

knightfly · Post by **knightfly** » Thu Jul 10, 2003 9:36 am

And, just the Axial modes -

Albert · Post by **Albert** » Thu Jul 10, 2003 5:52 pm

Steve thankyou very much for all the information. I'm afraid I typed 4 foot 9 inches when I meant 14 foot 9 inches

, sorry to make you doubt your math!

Thanks again.