R. H. Nichols, Jr. Bell T½ltphone Laboratories, Inc., Whippany, New Jersey 07981

TECHNICAL NOTES AND

RESEARCH BRIEFS

7.10

REPEATABILITY OF GROUND-TEST NOISE MEASUREMENTS ON AN AIRCRAFT ENGINE

Howard A. Thorpe

Rohr Corporation, Chula Vista, California 92012

Measurement repeatability, accuracy, and effects of elevating both microphones and a JT8D engine are discussed briefly. Also data acquisition and processing techniques are indicated.

ROHR CORPORATION has been making mea- surements of noise from a static JT8D turbojet engine at a test site near Brown Field southeast of San Diego. These mea- surements started in April 1969 and are continuing as a part of Rohr's noise-sup- pression hardware-development program. Standard procedure has included measur- ing, more or less periodically, for an un- suppressed basic engine configuration. These checks on repeatability of the engine as a noise source are continuing. Results of some past repeatability measurements along with certain independent evidence regarding accuracy may be of interest to others. Also the principal acoustical effects of elevating the engine are indicated.

Our audiofrequency noise-measuring equipment features Briiel & Kja•r con- denser microphones and associated equip- ment except that a Honeywell 14-channel tape recorder is used. Microphones are located 200 ft from the center of the engine and at engine centerline elevation. Up to 14 microphones, normally 10 ø apart, have been used simultaneously. Frequently ad- ditional microphone stations are recorded but in a sequential fashion. Usually, engine rpm is held constant, while a 50-sec sample of noise is recorded. On any given day, each of several engine rpm values is estab- lished at least twice, and a noise sample recorded for each. Noise measurements

normally are made only when wind speed does not exceed 5 mph, and relative humidity is not less than 30%. Usually relative humidity values above 90% are avoided because of water condensation on

microphones. Presently, our data is pro- cessed over the 45-11 200-Hz band by a

local contractor who employs «-oct digital filtering and 30-sec integration time. Pro- cessing includes sound-absorption correc- tion to standard-day conditions.

From April through October 1969, eight sets of measurements were made for the

basic engine configuration. The maximum value of perceived noise levels (PNL) ob- served provides both a highly significant and a concise index for repeatability com- parisons. For these eight sets of PNL (max) values, the grand mean deviations are 0.3, 0.5, 1.0, and 1.1 PNdB, respec- tively, for engine speeds (% N1) of 91=t:1%, 86%, 70%, and 57%. For only one set of measurements were some of the deviations in excess of about 1.5 PNdB. For the two

lower rpm values of this set, the deviation reached about 3 PNdB. Mean values of

PNL (max) varied from 128.7 to 116.7, respectively, for the highest and lowest engine rpm values set. Both engine and microphones were at 8 ft elevation. Tem- perature and relative humidity ranges actually involved were 56ø-74øF and 38%-86%.

The principal field-calibration techniques used routinely were based on the B & K pistonphone (used each day) and the B & K electrostatic actuator (used about monthly). As a completely independent check on probable accuracy of our mea- surements, however, a limited (OASPL) comparison has been made with a nearly corresponding set of measurements made elsewhere on another JT8D engine by another organization. Agreement is con- sidered excellent. The other test instru-

mentation (also wide band) is not Brtiel & Kja•r, and the test distance was 150 ft

compared to our 200 ft. The Rohr engine rpm involved was 86% N1, while that of the other organization was 85% N1. The other engine was fitted with a bellmouth inlet, while Rohr used a more nearly flight- type of inlet. Only inverse-square-spreading correction from 150 ft to 200 ft (2.5 dB) has been applied to the other set of OASPL levels before comparing them with cor- responding Rohr levels. For nine corre- sponding angular locations, in or near an aft quadrant, the largest difference in OASPL was 1.5 dB, and level differences were not all the same sign. At more forward angles, level differences observed reached a maximum of 2.5 dB; the distinct difference in inlet hardware may have caused this larger difference. At forward angles also the sign of the level differences varied.

Recently, Rohr's JT8D engine and micro- phones have been elevated to 30 ft. Pre- liminary analysis indicates that this eleva- tion has not significantly changed the maxi- mum value of PNL observed but does

significantly change spectrum details. Most of these changes appear to be associated with the well-known image-interference effect. The major spectral minimum for the 8-ft elevation occurs at the 800- and'1000- Hz bands, while for the 30-ft eleval•ion, it occurs within the 63-Hz band. Two or more

minima at odd-harmonic frequencies are also observed for both elevations. All this

behavior is predictable (approximately) on the basis of image-interference theory, assuming a hard flat reflecting surface and a point source of sound near the center of the engine. Actually, the surface is old and rough asphalt but still is moderately flat.•,•

The Journal of the Acoustical Society of America 1485

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