Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 252

9.3.3 Siren ' p procedure using single and differential
transducers.

Consider the top drawing in Figure 9.9b, with the speaker in the middle of
the intermediate length tunnel, again without a blower. The waveform generator
should create a sinusoidal signal of frequency Z. Test frequencies should be less
than 100 Hz. Then, the wavelength O, equal to sound speed (approximately
1,000 ft/sec in air) divided by Hertzian frequency, will exceed 10 ft and remain
large compared to the diameter, so that wind tunnel testing remains valid.
Two transducer types will be considered. For the differential transducer,
each sensor should be placed at equal distances ahead and behind the speaker.
So that local three-dimensional effects can dissipate before affecting
measurements, a recommended separation is at least five tube diameters. For
this test, the electronic displays should show both the signal source and the
differential pressure measurement versus time on the same screen. The
differential pressure measurement is straightforward. For a given siren
configuration, our experimental objective is the relation p s = p s (Z).

x' = 0

Differential
transducer

-L' +L'

Open Open

Single
transducer

Open

Differential
transducer

-L +L

Blower Open

Open

x = 0

Differential
transducer

-L + L

Op en

Air out

Motor-driven siren
with flow straighteners

Open

Single
transducer

Air in

Figure 9.9b. Siren signal test set-up explained in detail.