re:Re: Sound proof box

Graham,
One more comment on sound proofing:

Mass (for highs) and damping (for lows) are key in preventing noise from
exiting a box. As a former maker of woofer exclosures, the best thing I
found for a totally dead cabinet was an old Wharfdale trick:
Two sheets of ply with an inch or more of loose sand between them. The sand
has both mass and damping.
More sand is better. It is possible to make the box in fillable panels, so
that it can be assembled on site and used. If needed, the sand can be
drained out before moving. This way you could have filled panels that weigh
hundreds of pounds and still be able to easily fabricate them and tote them
around.

10,000 rpm = 166 rev/second. A two flute cutter (10krpm) will produce sound
with a fundemental frequency of 333 Hz. Wavelength in air about 1 m,
wavelength in wood between 10 - 30 cm depending on direction. So this
frequency is in the range where things get interesting: not a point source,
components several but not hundred of wavelengths in size, not clearly mass
or compliance limited.

It is probably best to experiment. Make a sand filled panel and anything
else you might want to use, see which one has the lowest sound
transmissivity.

regards-
Elliot B.


Graphma wrote:
Thanks every one for the ideas. I should have mentioned that the
Proxxon is a desktop mill, smaller than a sherline with a fairly high
speed spindle.

As Tim stated it is quiet and this is something that impressed me.
The problem is that my when the machine is milling I am working at my
PC and of course the there is the problem of the neighbours. I just
wanted to be playing with it at midnight.

I have added the eggbox foam and I think it has helped. Outside of
my room at full speed it sounds a bit like a hairdryer. It has
certainly cut out the high frequencies but the lower ones remain. I
will try to to some suitable matting for the base but it is doing ok
at the moment with some thin foam. I may make a wood foam sandwich
later. The other requirement is that the box be storable, light and
cheap. I am not sure I want to clad it with anything heavy but
perhaps I can add a second isolated skin later.

Graham
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