<snip>
In order to minimize telegraphed vibration, the plan is to take the motor and squirrel cage assembly, and create an enclosure and air flow plenum by cutting and laminating several sheets of one-inch thick styrofoam. This assembly will then be enclosed in two-inch thick cellular foam rubber, before mounting in the rack cabinet.
To reduce the intake and exhaust airflow noise, a labyrinth airflow path is planned. The entire transmitter will be enclosed in a six-foot high rack cabinet, 22 inches wide, and 33 inches deep. The floor of the cabinet is 80% open, and the cabinet frame is made up of cast and extruded aluminum sections, with removable sheet aluminum skins for the sides. A sheet of 3/4 inch thick plywood will be used as a base for the plate transformer and other HV power supply components. Around these components, 3/4 inch holes will be drilled to allow air to enter from beneath the plywood, through a pleated air conditioner filter. By providing many moderate sized holes around the components, a large volume of air may enter, flowing at a lower speed than the intake or exhaust ports of the blower assembly.
The sides of the cabinet, and the front and rear, where possible, will also be lined with two-inch thick cellular foam rubber sheets.
Air will flow into the enclosed blower assembly, and then on to a plenum enclosing the 3CX2500F3 modulator, and the 4-400A driver tube. The exhaust side of the plenum will also be perforated with 1/2 inch to 3/4 inch holes over a large area.
Two or three layers of fiberglass sheet insulation will be placed above the tube plenum, in a manner creating a labyrinth path whereby the air flow much change direction several times, thus muffling the noise from the motion of the air, as well as the higher frequency noise caused by the tube heat radiator fin assembly. The airflow will be sufficient that the cellular foam rubber and styrofoam are not exposed to extreme temperatures. </snip>
A few thoughts.
I would advise strongly
against using any plastic foam inside a transmitter.
It's a severe toxic hazard in the event of a thermal event (aka fire).
In addition it is a
poor material for sound absorption, and poor for blocking
sound transmission.
Also, it degrades with temperature and exposure to oxygen and UV.
To drop noise in a duct there are a variety of methods - they tend to be documented
in acoustics books and the more common ones in HVAC texts and papers.
The simplest idea is to create what works out to be a low pass filter. The simplest
one is in the form of a duct that opens briefly to a larger volume, and then continues
out to a smaller volume. Turns in a duct that are NOT
lined with absorptive material that is
effective at the frequencies of interest, and
effectively so, will not attenuate
much of anything.
What is an effective sound absorbing material?
Most are marginal.
Foam is one that is reasonably poor.
Every material that is commonly available works best in fairly thick applications.
Thinner applications work mostly at HF, and even then not so well.
One of the best turns out to be
wool felt. If that can not be found then
cotton felt is not bad. There is also nowadays sold synthetic and combination
synthetic & cotton felts. Some of it has been fire retardant treated for domestic
and commercial installations.
Just like car mufflers, a tuned
Helmholtz type filter system can be effective, and made
mostly from metal or similar. This is done all the time in HVAC installs to get rid of
fan noise, especially where high velocities are needed. Should be possible to apply
for us.
Rockwool and fiberglass are actually
poor acoustically, and not effective
at lower frequencies until they are used in fairly thick applications.
There is "solid" material sold for use in domestic and commercial buildings similar
to the ceiling tiles but thicker, in 1 to 4" thick sheets. It's either fiberglass or
rockwool with some sort of binder. In some situations these might be useful.
So, stay away from styrofoam or Urea based "foams", latex "foam rubber" has
a short half-life so not a good one either.
Preventing
sound transmission through surfaces is similar but a bit different.
The people who do sound studios and speaker enclosures have studied this in
depth.
Two main methods:
- decoupling of two surfaces
- constrained layer
The constrained layer idea is a way to turn vibrational energy into heat within
a composite surface. Essentially it's a high pass - low pass - high pass - low pass,
etc. So, a soft layer, stiff layer, soft layer, stiff layer... etc.
Sometimes space permitting one can incorporate one layer of granular material, like
fine sand as a layer. The sand which vibrates turns the vibration into heat. So that
would be between layers of something like metal - silicone rubber - metal - silicone rubber -
metal - sand - metal, silicone rubber - metal. Or wood/rubber/wood etc...
The decoupling of two surfaces is the classic method - aka "shock mounts".
Even if it holds up a wall or the entire floor or ceiling. For example.
We'd engineer the operation of a tube, or the design of a tank circuit, it's a good
idea to apply the same to sound control and reduction...
_-_-bear
The absorption of and resistance to transmission of sound is often shown in graphs
of attenuation vs. frequency WRT thickness...fyi.
EDIT: forgot to mention many foams are mildly electrostatically charged, and even those
that are not tend to pick up airborne dirt and grease quickly, and are not easily cleaned.
This creates an additional concern.