Department of Physics, Chelsea College, London. Summary. Portable equipment for the detection and recording of infrasonic noise is described. The results of ...
Field Measurement of Infrasonic Noise by R. A. HOOD and H. G. LEVENTHALL
Department of Physics, Chelsea College, London Summary Portable equipment for the detection and recording of infrasonic noise is described. The results of analyses show that infrasonic noise at high levels occurs in a wide variety of locations. Feldmessung von Infraschallgerauschen Z u s a m m e n f a s s u ng Eine tragbare Ausriistung fiir die Messung und Aufzeichnung von Infraschallgerauschen wird beschrieben. Untersuchungsergebnisse zeigen, daB Infraschall mit hohem Pegel an vielen verschiedenen Orten auftritt. Mesure du champ d'un bruit infrasonore S o mm a i r e On decrit nn equipement portatif pour la detection et l'enregistrement d'un bruit infrasonore. Les resultats des analyses montrent qu'un tel bruit se presente, a des niveaux eleves, dans une large variete de circonstances. 1. Introduction The first scientific studies in the field of infrasound were carried out during the 1914 —1918 war, but interest in the subject has recently been revived due to generation of high levels of infrasound by GAVREAU [1] and DUNN'S attempt to correlate road accidents and absenteeism with naturally occurring infrasound [2]. A review of work on infrasonic noise has been given by STEPHENS
[3].
Infrasonic waves as low as 0.001 Hz have been studied by COOK [4], FEHR [5] and others, but the present work is restricted to a lower limit of 2 Hz. 2. Description of the apparatus 2.1. Microphone A variety of infrasonic detectors have been described including the capacitance type, JOHNSON [6],
COOK
[4]
and
BEAVER
[7],
the
electro-
chemical type, COLLINS [8] and the thermistor type FEHR [9]. Other transducers used include pressure sensitive paints and hot wire sensors. The system chosen for the present work was based on a Briiel and Kjaer 4117 piezoelectric microphone. This has a nominal capacitance of 4000 pF and was used with a high input impedance amplifier employing a field effect transistor input stage. The amplifier impedance of 109 Q gives the electrical cut-off point as 0.04Hz {2nfRC=l). However, the microphone equalization hole controls
the acoustic low frequency cut-off and is 3 Hz in the standard capsule. The equalisation hole was partially blocked by inserting a fine wire to give the acoustic low frequency cut-off at 0.1 Hz. 2.2. Recording system A portable Uher stereo tape recorder forms the basis of the recording system. One channel of the recorder is used in the normal manner to record audio frequencies. The other channel is preceded by a frequency modulation unit with a carrier centre frequency of 7.5 kHz and a bandwidth of 500 Hz, giving a good region of overlap between the two channels. On reply the output of the audio channel is obtained in the normal manner whilst the frequency modulated output is passed through a demodulator to give the infrasonic region of the recording. The modulator and demodulator are based on a design of TEMPEST and BRYAN [10]. The self-noise level on the infrasonic channel has a spectrum with peaks at 2.5, 5, 11 and 16 Hz, with a maximum level equivalent to an input of about 74 dB (re 0.00002 N/m 2 ). This noise is largely due to tape recorder speed fluctuations which occur in the infrasonic region and measurements by the system are considered valid down to about 80 dB. The record and replay system is shown schematically in Fig. 1, and the response of the system in Fig. 2. 2.3. Calibration system Calibration is carried out with the simple system shown in Fig. 3, the closed chamber has a flexible
A.CUSTICA Vol. 25 (1971)
R. A. HOOD et al.: FIELD MEASUREMENT OF INFRASONIC NOISE
Frequency Modulation Unit Microphone
Q_O \
Amplifier
Tape Recorder
j
Infrasonic ' Output
F.M. Demodulator
Q_O
11
Audio Output
Tape Recorder
Fig. 1. The recording and replay system. Direct
F.M.
0-
-4-
-
/
-
\
/
\
-12 -
V ,1
\
-20 1
2
5
101
2
5 102 Frequency
.. 1 2
5
103
2 Hz 5
*-
Fig. 2. Response of the recording systems. Static pressure measuring point Eccentric wheel
Microphone
Fig. 4. Apparatus. Fig. 3. Low frequency calibrator. diaphram which is depressed by an eccentrically pivoted wheel. Pressure fluctuations are produced in the chamber up to 130 to 140 dB and are detected by the microphone. The pressure level is obtained by connecting a sensitive manometer to the pressure measuring point and turning the wheel to the maximum pressure position. The decay rate of the microphone output subsequent to this adjustment of position gives the low frequency cut-off of the system as 0.1 Hz. In normal use the handle is turned several times a second and the microphone output monitored on a calibrated oscilloscope. The complete recording and replay system, which is designed for portability, is shown in Fig. 4. 2.4. Analysis This is carried out using a Dawe Instruments Vibration Analyser, which has a lower limit of 2 Hz and an approximately constant percentage
bandwidth of 6 to 1%. The analysis range can be extended down to 0.5 Hz by recording at one fourth the normal speed with a modulation frequency of 2 kHz. The recording signal is then played back at the normal speed and analysed. This method unfortunately gives rise to a greater noise level and a smaller bandwidth. 3. Experimental results 3.1. Infrasonic noise of transport (Fig. 5) The upper trace is the fine frequency analysis of the infrasonic noise inside a car travelling at about 100 km/h, with the quarter-light front windows open. The low frequency content is wind noise, which depends on how much the window is open and in some cars is higher than shown here. The peak at about 25 Hz, shown dotted, was not always present with the car and is probably due to wheel tremor, caused by unbalanced wheels, energised by
12
ACUSTICA Vol. 25 (1971)
R. A. HOOD et al.: FIELD MEASUREMENT OF INFRASONIC NOISE 120
The other trace was taken in the pumping station of a sewage works. There is a peak of about 105 dB at 20 to 30 Hz near the pumps and the level remains well over 90 dB for most of the range up to 400 Hz.
dB 110
i I/
\
i Y
. I ~ •J \
100 SPL 90
A,
j
///\V.\1
3.3. Infrasonic noise on ships (Fig. 7)
1
\
^
'A
80
a) A small car ferry, 400 tons (full line). The noise is that found in the engine room whilst under full power. There is a narrow peak of 133 dB at about 13 Hz, which corresponds to the firing rate of the ship's diesel engine.
70
5
Fig. 5.
10
20 Frequency
50
100
200 Hz 500
— Noise in a car travelling at 100 km/h with quarter-lights open. — • — Noise in a car travelling at 100 km/h v^th back windows open. (dbre2XlO- 5 N/m 2 .)
road bumps. The noise level reduces rapidly above 40 Hz, but is still over 80 dB at 400 Hz. The lower trace is the noise inside a car with one of the back windows fully open, travelling at about 100 km/h. There are peaks at 8, 12, 18 Hz. It is probable that one of these peaks is due to the window acting as the orifice of a HELMHOLTZ resonator, the enclosed volume being the volume inside the car. 3.2. Infrasonic noise in industry (Fig. 6) The upper curve, which has a predominant peak of 115 dB at about 7 Hz, is the noise near a blast furnace in a steel works. The peak is thought to be a resonance effect. The audio-frequency content is high with peaks at about 100 Hz and 300 Hz of over 100 dB (due to the blast furnace fan).
10
Fig. 7.
20 Frequency
50 >
100
200 Hz 500
Noise in engine room of small car ferry of 400 tons. — • — Noise in engine room of British Rail ferry of 8000 tons.
b) A British Rail car ferry of 8C00 tons. The noise was that found in the engine room and shows peaks at approximately 7 Hz, 14 Hz and 20 Hz. These frequencies again correspond to the firing rate of the large diesel engines. 3.4. Infrasonic noise in helicopters (Fig. 8) a) A medium-size five-seat helicopter, cruising at 100 knots, shows a peak of 120 dB at about 28 Hz. This is the blade passing repetition frequency. There is a lower level harmonic at 56 Hz. 5
Fig. 6.
10
20 Frequency
50
100
200 Hz 500
*•
Noise near blast furnace. Noise near pumps in sewage works pumping station.
b) A two-seat helicopter, cruising at 70 knots, shows a peak of 118 dB at 11.5 Hz, with second harmonic, again associated with the blade passage frequency. Peaks at higher frequency are probably engine noise.
ACUSTICA Vol. 25 (1971)
R. A. HOOD et al.: FIELD MEASUREMENT OF INFRASONIC NOISE
13
There seems little doubt that noise assessment which neglects the 7 Hz and 13 Hz peaks is inadequate. d) The noise in helicopters (Fig. 8) also Contains large low frequency peaks which would be neglected in a conventional noise assessment. The subjective effects of infrasonic noise of the types measured in this field study are at present being investigated.
SPL 90
Acknowledgement The work described in this paper was supported by a grant from the Medical Research Council. -10
Fig. 8.
20 Frequency
(Received August 10 th , 1970.)
Noise in 5-seater helicopter. — • —• Noise in 2-seater helicopter. [1] 4. Discussion
Infrasonic noise occurs at high levels in a variety of situations. The subjective effects of the noise are not fully understood, but some general comments can be made. a) The noise of turbulence within a car (Fig. 5) with the quarter-light windows open rises as the frequency reduces. It is quite possible that there is a D. C. pressure change within a moving vehicle. The high levels at a few Hertz are of special interest since infrasonically-induced dizziness may be a hazard in fast driving. b) The noise from a blast furnace (Fig. 6) has interesting characteristics. The blast furnace is very noisy and a standard octave band analysis to determine its noisiness would show little of the energy below the dip in the spectrum at 30 to 40 Hz. One must ask whether the low frequency peak is a factor which should be considered in assessing the subjective effect of the noise. c) The noise in the ship's engine room (Fig. 7) has predominant peaks, which are more than 30 dB higher than much of the low frequency spectrum.
[2]
[3] [4] [5] [6]
References GAVREAU, V., CONDAT, R., and SAUL, H., Infrasons: generateurs, detecteurs. Acustica 17 [1966], 1. DUNN, F., and GREEN, J. E., Correlation of naturally-occurring infrasonics and selected human behaviour. J. Acoust. Soc. Amer. 44 [1965], 1456. STEPHENS, R. B. W., Ultrasonics. 7 [1969], 30. COOK, R. K., Strange sounds in the atmosphere. Sound 1 [1962], Part I p. 12; Part II p. 25. FEHR, U., Measurement of infrasound from artificial and natural sources. J. Geophys. Res. 72 [1967], 2403. JOHNSON, C. R., The N.E.L.T21 Microbarographic Recording System. NEL Rep., San Diego, Calif., 1967.
[7] BEAVER, B. R., and MAYER, N. J., VLF Adjust-
able Acoustic Bandpass Filter Microphone System. 73rd Meeting, Acoustical Society of America, New York, 19 April, 1967. [8]
COLLINS, J. L., RITCHIE, W. C,
and ENGLISH,
G. E., Solion Infrasonic Microphone. J. Acoust. Soc. Amer. 136 [1967], 1283. [9]
FEHR, U., BEN-ARY, B., and RYAN, J. D.,
New
Instrumentation Techniques Jor the Measurement of Infrasonic and Gravity Waves. Rev. Sci. Instr. 38 [1967], 778. [10] TEMPEST, W., and BRYAN, M. E., A Simple Frequency-Modulation Tape Recording System. Electron. Eng. 39 [1967], 87.
