Absolute pressure calibration system using a Michelson interferometer calibrates phase characteristics and pressure sensitivities of the transducers that monitor acoustic or aerodynamic pressure fields. The interferometer uses a helium-neon laser light source and interchangeable acoustic signal generators to produce acoustic waves

Pernet, D. F.

English

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djustment 
tical System 
oustic Tube 
April 1967	 Brief 67-10099 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
System Enables More Complete Calibrations of Dynamic-Pressure Transducers 
The problem: 
To devise a system for absolute pressure calibration 
of high-intensity, wide frequency-response transducers 
used to monitor acoustic or aerodynamic pressure 
fields. The system must enable calibration of the phase 
characteristics as well as the pressure sensitivities of 
the transducers. Conventional calibration systems used 
for such transducers seldom provide information about 
the phase characteristics of the transducers. In experi-
mental acoustic or aerodynamic pressure-field studies, 
where the signal outputs of one or more transducers 
must be correlated, it is essential that the phase 
characteristics of the transducers be known. 
The solution: 
A system incorporating a Michelson interferometer 
using a helium—neon laser light source and inter-
changeable acoustic signal generators which produce
acoustic waves in an acoustic cavity at pressure ampli-
tudes between 140 and 175 db and frequencies be- 
tween 0.5 to 40kc/s. 
How it's done: 
The Michelson interferometer incorporates the 
acoustic cavity in one of the two optical beam paths. 
Density fluctuations in the acoustic cavity, which are 
associated with the pressure fluctuations of the cali-
brating acoustic signal, produce a time-varying optical 
interference pattern. This pattern is monitored with a 
photodiode detector, which produces an electrical out-
put signal characterizing both the absolute amplitude 
and phase of the pressure fluctuations in the acoustic 
cavity. Comparison of this signal with the electrical 
output signal of a transducer positioned in the cavity 
enables the sensitivity and phase characteristics of the 
transducer to be determined.
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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The acoustic signals are generated by one of three 
interchangeable units: two horn drivers for frequencies 
up to 16 kc/s and a St. Clair generator for frequencies 
above 20 kc/s. The acoustic energy from these genera-
tors is propagated into a 0.25-inch-diameter telescopic 
tube at the end of which a 0.25-inch transducer to be 
calibrated is positioned. Adjustment of the length of 
the tube allows longitudinal tuning of the cavity, which 
produces an increased pressure amplitude at the 
transducer diaphragm. Because longitudinal pressure 
gradients will exist in the tube, the laser beam scans 
the cavity in a transverse direction immediately in 
front of the transducer diaphragm for minimal calibra-
tion error. The system is capable of being operated 
with air or other gases at atmospheric or reduced 
pressures.
Note: 
Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10099 
Patent status: 
No patent action is contemplated by NASA. 
Source: D. F. Pernet 
of Illinois Institute of Technology Research Institute 
under contract to 
Marshall Space Flight Center 
(M-FS-2 063) 
Brief 67-10099	 Category 01


System enables more complete calibrations of dynamic-pressure transducers

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