Instantaneous signals from coherent random sound field are summed and time delayed to avoid introducing vectorial addition errors. Resultant statistically independent signals are applied to spectrometer. Displayed sound pressure level is proportional to square root of sum of squares of sound pressure levels taken over frequency range of interest

Nakich, R. B.

Woodbury, R. C.

English

NASA Technical Reports Server

June 1971
	 Brief 7l-10157 
.,
NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the. Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Digital Decorrelator Saves Time and Expense in Acoustic Testing of Structures 
The problem:	 Spatial averaging with respect to time and fre-
Devise a technique which will save time and
	 quency in one-third octave bands is required to 
money on acoustic. energy absorption tests on struc-
	 define the sound field accurately. This can be 
tures such as aircraft or spacecraft assemblies,
	 achieved by using a traversing microphone, or by 
No Delay (Reference Channel)
2 Akasing 38.4ms 0-A 
Filter Delay Channel 2 Converter ri 
Ahavng I	 6.8ms 7 D-A	 l Ether Delay Channel 3 F-i........+-1 Converter 
Ahavng Mu ltiplex AD
115.2Th I	
D-A 4d Filter Switch Converter Delay 4 Converter I Channel 
Ahaswg D- F4te,
I 153.6ms} 
Delay 
ChannelS Converter 
Alsng
21 kHz 192.0 ms D-A	 I 
Filter Delay ConverterF--1 Channel 6 
01	 05.21 kHz T 521 kHz
9.21 kHz1	 5.21 kHz 
62.5 kHz Multiplex Multiplex Multplen 1	 l	 Delavl] Two Phase L__..J Oscillator D ecoder ntrol Co Trigger I	 I	 Clock Drivers I Generator I	 (Maltiplened) I
Decorrelated 
Random 
Signal 
Output
(6 Channel) 
L3O 
Filter
1/3 Octave 
Spectrometer 
Because of the transmission characteristics of a 'using several microphones to measure the sound 
reverberation room, the sound pressure level in
	 pressure level in one-third octave bands and then 
the room is a function of measurement position and
	 averaging the readings with a computer or desk 
frequency. Also, a large test object absorbs sound,
	 calculator. Both techniques are expensive in terms of 
distorting the field. These problems make it im-
	 machine costs or operator time; neither technique 
possible to describe the sound field in terms of
	 allows real time presentation of data; and neither 
a single point of measurement, 	 is adapted to use in servo control systems. Aver-
(coniivued o' erleal) - 
Naw
This document was prepared under the sponsorship of the National 	 Governmtht assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such us 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19710000156 2020-03-17T02:23:49+00:00Z
aging the instantaneous signals by means of direct 
summation or scanning devices fails because of un-
resolvable vectorial addition errors associated with 
the coherent characteristics of the sound field. 
The solution: 
Instantaneous signals selected from a coherent 
random sound field are summed and are time de-
layed to avoid introducing vectorial addition errors. 
(The amount of delay is equal to or greater than 
T = n/213, where n is the nth spatial measure-
ment point and B is the lowest one-third octave 
band of interest.) The resultant statistically indepen-
dent signals are applied to a spectrometer. The 
displayed sound pressure level is proportional to the 
square root of the sum of the squares of the sound 
pressure levels taken over the frequency range of 
interest. This is possible because averages and var-
iances are both additive for statistically independent 
signals. 
How it's done: 
Microphones (typically six) are strategically placed 
about the test object in the reverberation room. 
Channel 1 carries the undelayed reference signal. 
Channels 2 through 6 are first filtered to prevent 
aliasing, then digitized after selection by the multi-
plex switch, and finally passed to appropriate delay 
registers. These dynamic shift registers introduce 
delays of from 38.4 to 192.0 milliseconds, in 38.4 
ms increments, to ensure decorrelation in the 50Hz 
one-third octave band. After digital-to-analog con-
version, all channels are summed and are divided 
by 6 to give an average sound field level with 
unity weighting. The resultant ensemble of statis-
tically independent signals is analyzed and then 
displayed on a one-third octave spectrometer. Since 
the delays introduce a lag of less than 0.2 second, 
changes in the sound field can be observed in vir-
tually real time, permitting servo control of the 
sound field. 
Notes: 
I. Although the method is described here for acous-
tical testing, the concept is also adaptable to 
multipoint vibration testing using random noise 
excitation. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP71-10157 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. C. Woodbury of

Caltech/JPL and

R. B. Nakichof
Time Zero Corporation
under contract to

NASA Pasadena Office
(N PO- 1 1542)
S 
Brief 71-10157	 Category 03


Digital decorrelator saves time and expense in acoustic testing of structures

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