the control and measurement of a single-tone noise nuisance
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THE CONTROL AND MEASUREMENT OF A SINGLE-TONE NOISE NUISANCE
K. A. MULnOLLAND and W. A. UTLEV
University of Liverpool, Liverpool (Great Britain)
(Received: 1 November, 1967)
A method is described which can be used to estimate the loudness of noise nuisance when the noise in question apparently contains a discrete tone detectable by the human ear but not by instruments. A specific problem is considered and it is shown how the method was used to assess the improvement obtained when steps were taken to reduce the level of the discrete tone. The results indicate that the method is a valid one.
In this article a method is described that can be used to measure the effectiveness of noise-reducing treatments when a discrete tone is apparently present.
This problem concerned the noise being produced by a 700-h.p., single-stage centrifugal compressor at the chemica! works of Laporte Industries Ltd., Widnes. The compressor was producing a noise that sounded like a discrete tone. This noise was coming from the compressor inlet which was mounted on the roof of the com- pressor house (Fig. 1). Measurements using narrow band filters failed to detect the tone and it was concluded that the tone was, in fact, a set of harmonics each below the general background noise level. The human ear was capable of inte- grating these harmonics into a single tone, whereas measuring equipment was not.
After receiving complaints from people living near the plant a series of measure- ments was carried out in order to determine the sound levels at various distances from the plant. The levels were measured in octave bands using a portable sound level meter and the levels were compared with the standard acceptable levels. 1 Despite the fact that the measured levels were not unduly high using the above criteria, complaints were still received because of the presence of the tone. The company decided to take steps to try to reduce the sound coming from the com- pressor inlet and required measurements to determine the effectiveness of the treatment.
Applied Acoustics--Elsevier Publishing Company Ltd., England--Printed in Great Britain
138 K.A . MULHOLLAND, W. A. UTLEY
The initial measurements were carried out using a portable sound level meter in a position about 0.2 km from the plant and close to the houses from which complaints had been received. Whilst carrying out these measurements it was noticed that the general noise from the plant contained a discrete tone. The frequency of this tone was estimated to be about 500 Hz. This was confirmed by an examination of the fan which revealed that it had 10 blades rotating at 3000 rev/min giving a blade-passing
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frequency of 500 blade passings per second. Measurements were carried out using a narrow-band filter in order to try to determine the level of the tone. The level could not, however, be measured above the background. This could be due to one of two reasons:
(a) the filter was not narrow enough;
(b) the tone actually consisted of a large number of harmonics which were integrated by the ear to give the impression of a tone at 500 Hz.
CONTROL AND MEASUREMENT OF NOISE NUISANCES 139
From careful discussions with workers at the plant it was clear that this discrete tone was causing the annoyance to local residents and steps were taken to reduce the level of the tone in order to satisfy local residents. A quantitative estimate of any reduction in the loudness of the tone was also necessary to assess the probable reduction in complaints that would follow from the attenuation of the sound.
SOUND REDUCTION METHOD
Two methods of sound reduction are possible for a noise source of this type. The first is by means of internal sound absorption treatment within the duct itself. It is always possible that the fan is exciting a resonance in the ducting of the fan. In this case it was found from measurements of the dimensions of the duct that there was no probable location for such a 500-Hz standing wave. It was also undesirable to consider a sound-reducing treatment that would entail stopping the compressor because this would have meant stopping production on the main process in the factory; the economic repercussions would have been severe. The alternative treatment (which was chosen) consisted of a hood (see diagram) placed over the inlet. The hood has sound-absorbing properties and also removed the direct up- wards and outwards radiation of sound from the inlet.
Since the narrowest filter available failed to distinguish the tone from the back- ground noise, it was decided to measure the difference in the level of the tone before and after the noise reduction treatment using a subjective method.
The two types of subjective measurements of the level of the 500 Hz tone were carried out. Both methods required the use of an observer who judged the loudness of the tone and set the level of a locally produced tone according to certain criteria. In the first method the observer was required to alter the level of a 500 Hz tone from a nearby loudspeaker until it just masked the tone from the plant. In the second method the observer altered the level of a 1000 Hz tone until he judged it to be of equal loudness as the 500 Hz tone from the plant.
The first method yielded the difference in sound level before and after treatment directly from the difference in voltage fed to the loudspeaker before and after measurement (the linearity of the loudspeaker having been checked in the labora- tory). Using the second method the level of the 1 kHz tone at the ear of the observer was measured using a microphone. In order to raise the level of the tone out of the background noise the voltage fed to the loudspeaker was increased by a known amount and this was subtracted from the measured result. This method gives the level of the tone in phons directly.
Applied Acoustics. 1 (1968) 137-141
140 K. A. MULHOLLAND, W. A. UTLEY
The results were reasonably consistent within the groups of people and are shown in Tables 1 and 2.
LEVEL TO MASK THE SOUND (VOLTS SUPPLIED TO SPEAKER)
Mean dB re O. 1 volts
Before 0"45 0.32 0-36 0.45 0.55 0-426 12"6 After 0"16 0.24 0.125 0.081 0-091 0-139 2.9
Improvement 9.7 dB
TABLE 2 PHON TESTS
Level judged equal to the fan noise (dB) Mean Before 55 53 53 53.6 After 45 44 46 48 49 46"4 Correction for relative sensitivity microphone + 0-8 dB
Improvement 8"0 dB
Air turbulence due to wind is known 2 to cause quite severe attenuation of sound over distances as small as the 0-2 km used in this test. We therefore recorded the wind speed as recorded by Liverpool Airport Meteorological station at the times of the two sets of measurements and noted the figure for attenuation to be expected at these speeds.
Wind Attenuation Attenuation speed (dB/km) over 0"2 kra
Before 17 120 24 After 5 40 8
Prima facie correction for wind speed + 16 dB
However, this alteration would only apply under steady conditions. The wind was gusty and the level of the noise was observed to rise and fall, the maximum level being generally used in the two experiments. It is certain, therefore, that the wind speed correction must be less than the calculated value. A figur e of 5 dB seems reasonable.
CONTROL AND MEASUREMENT OF NO1SE NUISANCES 141
Despite the uncertainty about the wind speed it is clear that the sound-muffling device used by the firm to deal with the noise nuisance has resulted in an improve- ment of at least 9 dB and it is reasonable to suppose that the actual figure is nearer 15 dB improvement. Our measurements also showed that the level in phons to be expected at the windows of the houses affected by the noise under the most adverse conditions (i.e. at night with no wind) will be about 45 dB. Under such conditions the Wilson committee report indicates that 50 dBA should be an acceptable figure for pure tone noise. While phons and dBA are not strictly comparable it is probably reasonable to use these two figures as a rough guide to acceptability.
The noise level is thus 5 dB below the level at which complaints may be expected and up to the time of writing no further complaints have been reported to the company.
1. Final Report on Noise (Wilson Committee), Her Majesty's Stationery Office, London, 1966. 2. K. U. INGARD, Review of the influence of meteorological conditions on sound propagation, J.
Acoust. Soc. Am., 25 (1953) 405.
Applied Acoustics, I (1968) 137-141