As one of only a couple people manufacturing vacuum boxes these days, I
thought I would chime in on this topic.  In an effort to understand the
noise created by turbine impeller type pumps, I have spent considerable
time evaluating the existing design which was sold virtually unchanged
for years.

Both Spencer Chase and John Tuttle hit the nail right on the head.
There are three sources of noise: the turbine blades, the motor
impeller at the inlet, and the turbulence at the exhaust.  Proper
baffling and muffling work great on the exhaust as pointed out.

The intake port of the vacuum motors used in these pumps is a strong
source of noise, as John Tuttle demonstrates in his YouTube video.  The
impeller is chopping up the air right as it enters the motor, creating
quite a bit of turbulence.  In the PPCo vacuum boxes, this motor port
is right under the inlet flange on the top of the box, providing no
baffling and a direct path up the supply hose and into the player
system.  Moreover, in this design the flap valve is right in this
critical area, adding quite a bit of turbulence.

In my design, I have a small chamber above the motor which is lined
with acoustic dampener, and the inlet port is offset to one corner, not
in line with the noisy motor port.  Add to this a chamfered inlet port
to reduce edge-effect turbulence, and the inlet noise is addressed.
This also allows multiple ports to be added to operate devices such as
auto-rewind pneumatics.

The one source of noise that has not been discussed is the motor
itself.  The universal, brushed type of motor used is an inherently
noisy motor.  The brushes, no matter how well lapped they are to the
commutator, still create quite a bit of noise with every commutation.
Since these motors run at high RPMs (up to 8000 rpm), this translates
to a very high pitched whine, which in my opinion is the most obvious
and offensive noise of the whole system.  This noise is hard to damp
due to its high frequency spectrum, and this is where the lead sheeting
that John Taber mentioned would help substantially.

The cooling of a vacuum-box motor is accomplished by passing the
inletted air right through the motor armature.  Due to the high speed
of this armature, the air is again chopped up pretty badly and creates
a whole lot of noise in the form of turbulence.

So the ideal motor to use in a vacuum box would be a brushless,
externally cooled device that is capable of efficiently developing the
power needed to spin the turbine at these high speeds.  It must be
efficient, because overheating is a real concern.  I'm not sure whether
this combination even exists, and if so, would it be affordable and fit
into a box that is .75 cubic feet?

I use one of the old Welte pumps on my bench to test player components,
as it is a very quiet motor.  But it is also about 2 cu. ft. in size,
and would be impractical to use as an upright electrification box.  It
is sort of a box within a box, and the commutator is closed off and
ported, which helps subdue the brush noise.

I like the comment that the best way to quiet the system is to put the
pump in the garage and run forty feet of hose to the piano!

Eric Bergstrom
Bergytone Products
www.pianoproservice.com