I found these interesting web sites with information relating to
quieting the noise produced by a rotary positive displacement blower,
such as is often used to supply air to a calliope.  It is possible
to achieve more than 35 decibels attenuation using silencers.

Robbie Rhodes

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http://www.rootsblower.com/rotpos.html

A rotary positive blower, sometimes called a Positive Displacement,
or PD blower, is a machine which "pushes" a relatively constant volume
of the gas handled at the inlet through to the discharge.

It doesn't internally compress the gas, so it isn't considered a
compressor.  The amount of gas the machine handles is fixed by the
configuration of the casing and rotating parts.

Flow through a PD machine is relatively constant, regardless of
pressure changes.  This is different from a centrifugal compressor,
which maintains a relatively constant pressure while the flow varies.

 - - -
http://www.universal-silencer.com/01blower.htm

The RPD blower is a two- or three-lobe rotating device that delivers a
large quantity of gas or air relative to the individual pulses.  Blower
capacities are expressed in cubic feet per minute (CFM) at the inlet
conditions, indicated as ICFM.  Blower size is usually expressed as the
lobe (or timing gear) diameter by the rotor length.

The blower presents two problems:

   Pulsation within the piping system
   Noise radiation in the vicinity of the blower and piping

The importance of these relative to each other is a function of blower
size and speed; both increase approximately proportionately to the size
and as the square of the speed.

 - - -
http://www.stoddardsilencers.com/std_002.htm

Rotary Blower Discharge Silencers

Discharge silencers are essential to a good system performance on all
rotary blower systems.  The belief that the discharge creates less
noise than the inlet is erroneous because the discharge pulsations and
noise are normally contained in a closed system.

The rotary positive blower does not compress the air as it moves from
the blower inlet to the blower discharge.  Compression takes place when
line pressure backs up into the open port - thus compressed air is then
pushed out into the line.  This action takes place in a very short
period of time and produces steep wave forms that can be destructive to
piping and other equipment unless properly treated.

Blower displacement and speed are the major parameters in designing
discharge silencers that perform their assigned function and will live
throughout the useful life of the blower.  Normally for good silencing
the discharge silencer should have an internal volume to blower
displacement ratio of 18 to 1.

 - - -
http://www.aiha.org/abstract/6hearcon.html

Hearing Conservation Program Administration: Assess,
Evaluate, and Protect Papers (236-243)

243.  The Successful Use of Active Noise Control on the Rotary Positive
Displacement Blower.

R. Posey, Walker Manufacturing Company, Linthicum, MD

This paper describes the successful use of active noise cancellation
(ANC) systems on several different manufacturer's rotary positive
displacement (PD) blowers.  The PD-blowers in this study were
Roots-Whispair 1024, URAI-47, MD-Pneumatics 9020-17T2, Duraflow 7012
V-B, 4512, and Aerzen 287-314 units.  Work was conducted with several
Fortune 500 companies within the United States.  All had a desire to
reduce harmful noise in operating areas or to reduce unwanted boundary
noise.

The ANC System consists of a passive silencer, active silencer, and
associated electronics.  The passive silencer is a low flow restriction
design which effectively eliminates high frequency noise but lacks low
frequency attenuation.

The active silencer consists of tuned speaker enclosure, controller,
amplifier, sensor, and microphone.  The microphone senses low frequency
PD-blower tones emitted from either the exhaust (vacuum), or intake
(pressure).  In response to these tones, the control generates a
signal, when amplified and applied to the active silencer, produces
an acoustic signal equal in amplitude but opposite in phase to the
original noise source which is commonly called "anti-noise."  The
generation of anti-noise in close proximity to the original source
causes reduction in that noise.

The combination low frequency attenuation of the active silencer and
high frequency attenuation of the passive silencer results in better
overall reduction in noise than offered by conventional reactive
passive silencers.  The implementation of active noise on the study
PD-blowers resulted in noise level reductions up to 24 dBA or 92%
reduction in sound pressure levels (SPL) than obtained by conventional
passive silencers with specific tonal reductions to 31 dBA.

It is concluded that active noise control provides an alternative
pathway to conventional noise reduction methods.