A C O U S T I C Treatment for the Ham Studio...

What some of us are using for audio gear on our Class E Rigs.
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A C O U S T I C Treatment for the Ham Studio...

Postby BEAR » Mon Jan 26, 2004 3:47 pm

A recent QSO on 75 here in the NE brought up the subject of how to acoustically treat the space in your AM Studio. With the use of high quality mics and modulators that have response that is reasonably wide range, all sorts of nasty stuff starts to be heard...

Here's a short core dump... :D

One of the suggestions is to use F O A M .
Both the commercially made foam (SONEX is one brand) and the use of packaging foam (the pink or grey stuff) or the stuff that is sold to put under you in bed was suggested. That along with regular old cardboard "egg crates" (actually egg packages) were suggested.

I am very much against the use of FOAM for this purpose.
Several reasons:

- it is VERY dangerous if ignited. (Note the club in Rhode Island where people died a year ago - all foam on the walls and ceiling) :shock:
- it outgasses nasty stuff :cry:
- it degenerates with UV and Oxygen exposure and crumbles :o
- mice love it :wink:
- it isn't particularly good *acoustically* compared to other materials of
lower or equal cost and ease of application. :roll:

Just on the basis of acoustical comparison, foam (thickness for thickness) loses its effectiveness at a higher frequency compared to some other materials. (doesn't *work* as low )

But even if it were equal or slightly better, I wouldn't want it around for the other reasons.

Ok, so what are the alternatives? Several, but let me come to that after taking a look at what problem is trying to be solved.

There are two separate and distinct problems that were talked about:
- low frequency reduction (furnace rumble, for example)
- mid/high frequency echo and slap

These two in practical terms require somewhat different approaches since LF sound tends to radiate *through* materials far better than higher frequency sound. This is in large part due to the wavelength of LF sound.

So, while a solution for Bass "rumble" *could also* address higher frequency problems, it is unlikely that a solution for the higher frequency problems will do much at bass freqs.

Generally speaking straight absorbers (things that "suck energy up" like a sponge) need to be *thicker* to be effective at lower frequencies. Most of the foam sold for "studios" and "rock -n- roll" is way too thin to be effective at bass frequencies. Those thicknesses are effective (though not
"flat" in their absorption) at mid/high and high frequencies.

Talking *only* about blocking or absorbing things like "furnace rumble" there are three goals.
- if possible decouple the *source* of the rumble from surfaces
(this reduces the transmission/size of the radiating surface)
- isolate the spaces ( make the studio and the furnace NOT mechanically coupled at low frequencies)
- absorb as much LF energy as possible

You see the first item if you look at those big airconditioner/heatpump units that are flown in commercial spaces - they generally are isolated from the hangers by an isolation mount. This reduces the transmitted rumble dramatically. It may or may not be reasonable to "float" your furnace - but if you could, it would help. Things like pipes or ducts that run past your operating position serve to transmit LF energy - so isolating them from a "hard" connection to the building structure also will help.

The next step is to provide some sort of isolation/absorption.

This involves a wall or other barrier that both absorbs bass energy AND does not re-radiate it!


This leaves you with only a few materials to chose from. Those being sheetrock on the furnace side and fiberglass on the reverse. Done properly, this can be very effective. The trick is that you'll need a fairly thick wall that *floats*. How to do this best can be researched in any one of a number of books that deal with standard construction techniques, acoustics and/or studio construction. There are several methods to chose from...

What you do on the shack side of the new barrier can be fairly flexible and depends in large measure upon what the acoustical needs are there.

The other possible solution *in* a given space is to build some helmholtz resonators into the ceiling or walls. You'd have to check the spectrum of the furnace rumble using a mic and a computer sound program (they're out there and free...) Then build the helmholtz boxes (they're just boxes usually with perf holes and a plastic diaphragm (like saran wrap or PVC sheet) and some other stuffing) and mount 'em... they can be extremely effective. If ur in the basement, you can make them into the floor joists
and it becomes a nice ceiling in the shack!

Which brings me around to my favorite method for absorption that doesn't involve *foam*. This method can be tailored to meet almost any frequency requirement for lowest absorption (again this is mostly a question of thickness of the finished piece).

Commercial office dividers, you seem them commonly in banks and large companies... they are used to make those cubicles where all the stuff hangs off them? You also see stand alone dividers that are covered in a nice fabric?

These all work on the same principle and design - a good one that can be copied freely, and can be made to *fit* spaces as required.

They are simply this:

- layer of fabric (to taste)
- thickness of absorbing material
- backing
and sometimes (double sided)
- thickness of absorbing material
- layer of fabric

The nice thing is that about 2" of absorbing material will work fairly well
down to 300-400 Hz. 4" gets you pretty much into the mid bass, iirc.

Corning and some others sell "hard" batts of fiberglass with glued on designer fabric covers that are used to treat the interiors of high end offices and board rooms. But that is pricey stuff.

You can approximate the same stuff, if you find lowly ceiling tiles made of the hard fiberglass, and stack them to thickness required. They cut nicely too. OR, you can use regular good old fiberglass insulation (handle with gloves and respirator when installing) need be.

(...not talking about the composite ceilng tile stuff that crumbles and is sort of like the material used for bulletin boards... fiberglass or a material like it is required)

The basic design is to make a simple wood frame, backed with a thin but reasonably sturdy backer material (like the stuff you find on the rear of inexpensive furniture is fine). The frame is made to size to fit the requirements of your space. Fill the frame with the absorptive material, tack or glue or bind the material so that it won't FALL over time when hung, cover the face with fiberglass window screen. Then cover like a canvas on a painter's frame with a fabric to taste. (hardware wire mesh can also be used front or back)

Nothing wrong with making 10 small units or one big one... whatever fits and looks right.

These absorbers don't have to cover 100% of a wall. Some reflections won't be objectionable.

These framed units can be made in sizes that are fairly large (like 4' x 8') and affixed to ceilings and floor joists above. Or small and hung like

It is *best* and safest to pick a fabric that has been treated to be fire retardant if at all possible. Synthetics are usually not fire retardant...

You can make cross members inside the frame(s) for larger units for additional support and siffness.

Adding a % of hard surfaces to this construction makes for an absorber/diffuser - especially if the hard surfaces are not uniformly spaced and are not uniformly shaped or sized.

Doing this last bit is the "ne plus ultra" in acoustical treatment, btw.

Last bit of info - the best material for these units is thick felt, which can be literally "looped" into a shape that looks like the cross section of cardboard - the wavy part. This is extremely effective.

Carpet, open pile is effective too, but again mostly thickness determines the lowest effective frequency. Carpet has the down side of being very heavy.

Depending upon what you pick for the outer fabric, you end up with a very attractive and personalized, effective treatment for reflections off walls and ceilings. And, one that is non permanent, can be removed or moved, cleaned and changed in appearance as needed!

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Postby blaine » Mon Jan 26, 2004 4:26 pm

Excellent info Bear!
i made the post into a sticky for reference.
one note I would like to make is that from what i remember about the blaze in RI was that the sound treatment they used was packing foam, purchased from a shipping company, not fire resistant Sonex or Cutting Wedge.
I covered the walls in a studio with Cutting Wedge 2000, and you can smell the chemicals from the fire treatment they soak them in when you break open the boxes.
after i was finished the room had really quiet warm acoustics, it was quite nice but very expensive.
Blaine N1GTU
"The box said that I needed to have Windows 98 or better...
so I installed Linux." --- LinuxNewbie.org
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Postby frank carcia » Tue Jan 27, 2004 11:18 am

My building inspector told me foam insulation is a fire hazard and should
be covered with sheet rock. Go to Home Depot and look at their products
which are marked as needing to be covered. I was going to do my basement walls with foam and he talked me out of it. I've seen rug material hung on walls to do the same thing. Be Careful with the material selection.
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