Analysis of on-ground and in-flight sound levels produced by Chetak and Pratap helicopters

Materials & Methods

Sound Level Meter. A precision handheld and battery operated SLM cum Octave Band Analyzer was used for real-time sound data acquisition, analysis and storage (System 814, Larson-Davis Laboratories, USA) [3]. A random incidence and pressure response condenser microphone of 1/2 inch diameter (Larson Davis Model 2559) having a flat frequency response between 4 Hz to 21 kHz was used for the measurement of sound pressure levels.

The SLM in the integrating mode samples the sound data at regular intervals of time to find out the rms and peak values of the time varying sound pressure level. It can also compute a single continuous sound pressure level (SPL) that would produce the same acoustic energy as the actual time varying sound signal, over the measurement period. This average value of SPL, the Equivalent Sound Pressure Level (L), is calculated in decibel as

Where t,

= the start time of measurement

= the end time of measurement

= the instantaneous sound pressure of the sound signal and

= 2 x 10"⁵ N/m², the reference sound pressure

The System 814 uses the A-weighting network to modify the SPL in order to obtain the same loudness as perceived by the human ear. The equipment then provides the L_eq value of the A-weighted sound level for the measurement period.

Microphone Calibrator. A Type 1 L precision Microphone Calibrator (Larson Davis Model CAL 200) capable of generating sound pressures of 1 Pa and 10 Pa (SPLs of 94.0 dB and 114.0 dB respectively) at a frequency of 1000 Hz was used for the calibration of System 814.

Helicopters used for the study. On-ground and in-flight sound measurements were carried out at AFA, Hyderabad and at ASTE, AF, Bangalore on Chetak (Allouette III) and Pratap (Mi-8) helicopters. The physical characteristics of these two helicopters are given in Table 1.

On-Ground Sound Measurements

The System 814 was configured in the integrating SLM mode. The time constant of the detector and the frequency weighting were set to 'Slow' and 'A' respectively and the instrument was set to store the L_eq value automatically every l/8^th of a second during the measurement period. On-ground sound was measured from eight locations around the two helicopters with the engine(s) at maximum rpm and the rotors engaged. The locations were at angular separations of 45° along the circumference of a circle of radius 10 m from the axis of the main rotor, as shown in Fig 1. 0 represents the location in front of the aircraft along the longitudinal axis while 180° represents the position behind the tail rotor. Sound data was collected for more than 10s from each location. In-Flight Sound MeasurementsChetak and Pratap helicopters were flown for one sortie each for acquiring in-flight sound data. The System 814 was set to the integrating mode with the time constant of the detector and the frequency weighting set to 'Slow' and 'A' respectively. The equipment was positioned inside the cockpit at the ear level of pilot. A-weighted sound data was acquired continuously at intervals of 2 min 30 s. The L value, the maximum rms (L ) value and the peak sound level were calculated for each of these intervals from start up to switch off of the engine(s). The sortie-profile showing the event versus time for sorties undertaken by Chetak and Pratap helicopters are given in Tables 2 and 3. Continuous sound data was recorded for 58 min 28 s for Chetak and 1 hr 10 min 20 s for Pratap helicopter. Acquired data was then downloaded to a computer for analysis.

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Fig 1.

On-ground sound measurement locations

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Results

On-Ground Sound Measurements

The maximum L values of A-weighted eq ° sound levels recorded during the ground run from the different locations around the two helicopters are given in Table 4 while a graphical representation of these sound levels are shown in Figure 2.

Table 4. Maximum L values of A-weighted sound levels for eq ° various locations around Chetak and Pratap helicopters

Location (degree)	Maximum values of A-vveighted sound level in dB(A)
	Chetak	Pratap
0	93.1	108.3
45	107.7	106.4
90	107.6	105.9
135	113.9	106.6
180	95.9	105.7
225	103.9	106.3
270	112.7	105.3
315	103.8	103.7

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Fig 2.

Maximum L values of A-weighted sound levels around Chetak and Pratap helicopters

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Mapping of on-ground sound levels along a circle of 10 m radius around Chetak helicopter with the rotors engaged and the engine running at max rpm shows that maximum Lc values of 113.9 and 112.7 dB(A) were recorded at the locations 135° and 270 while the minimum L values of 93.1 and 95.9 dB(A) were recorded at 0° and 180" locations respectively. For Pratap helicopter, a maximum L value of 108.3 dB(A) was recorded at 0° position while the other locations had more or less similar sound levels.

In-Flight Sound Measurements

Interval-wise break-up of L, L and peak ¹ eq max ¹ values of A-weighted sound levels inside the Chetak cockpit during the sortie are given in Table 5. The average value of A-weighted sound level for the various sub-intervals during the measurement period varied between 92.4 and 101.5 dB (A). Similarly L_max and peak values are in the range 98.1 to 108.6 dB (A) and 109.8 to 129.4 dB (A) respectively. The L and L values of A-weighted r* eq max ° sound levels in relation to time of the sortie for the same helicopter are graphically illustrated in Figure 3.

Table 5. Interval-wise break-up of L, L and peak values of A-weighted sound levels eq7 max

Time of sortie (hr : min : s)	A-weij	•hted sound level in dB(A)
	L eq	L max	Peak
13:18:00	92.4	98.1	109.8
	97.9	99.7	113.0
13:20:30
	98.5	100.1	113.5
13:23:00
	100.7	103.7	116.0
13:25:30
	98.6	101.7	120.6
13:28:00
	99.1	102.7	129.4
13:30:30
	101.5	108.6	124.4
13:33:00
	98.8	102.6	117.6
13:35:30
	100.1	102.9	115.2
13:38:00
	99.1	102.5	116.7
13:40:30
	99.8	103.8	123.6
13:43:00
	101.2	103.7	117.4
13:45:30
	100.6	103.2	114.4
13:48:00
	7.6	101.0	113.3
13:50:309
	98.5	102.3	118.3
13:53:00
	100.2	101.9	1165
13:55:30
	97.5	102.1	119.5
13:58:00
	982	103.2	124.4
14:00:30
14:03:00	97.2	101.4	116.9
14:05:30	97.8	101.8	113.6
14:08:00	98.3	100.1	116.4
14:10:30	98.6	1045	117.6
14:13:00	96.1	99.2	1125

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Fig. 3:

Leq and Lmax values of Chetak helicopter vs Time of flight

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In-flight L, L and peak values of A- ° eq max r weighted sound levels inside the Pratap cockpit during the sortie for different intervals are given in Table 6. It can be seen that the L, L and eq max peak values inside Pratap for the different intervals ranged between 91.8 to 98.6 dB (A), 94.5 to 108.6 dB (A) and 111.5 to 124.7 dB (A) respectively. L and L values recorded for the different eq max sub-intervals in relation to actual time of flight for Pratap helicopter are shown graphically in Figure 4.

Table 6. Interval-wise break-up of L, L and peak values of A-weighted sound levels for Pratap helicopter during the sortie

Time of sortie (hr : min : s)	A-weij	•hted sound level in dB(A)
	L eq	L max	Peak
11:10:30	91.8	97.8	115.4
	94.7	96.7	117.1
11:13:00
	96.1	99.4	114.3
11:15:30
	93.3	97.0	113.2
11:18:00
	93.4	95.3	113.0
11:20:30
	93.6	95.8	116.1
11:23:00
	95.5	98.0	114.7
11:25:30
	93.5	94.9	112.6
11:28:00
	93.6	95.3	111.9
11:30:30	92.9	945	111.5
11:33:00
	95.2	98.5	116.7
11:35:30
	94.1	96.2	113.9
11:38:00
	93.1	95.6	1135
11:40:30
	93.1	952	114.1
11:43:00
	92.9	96.9	118.2
11:45:30
	95.5	98.6	114.0
11:48:00
	93.4	95.1	112.8
11:50:30
	93.6	95.2	) 112.2
11:53:00
	93.6	95.2	112.2
11:55:30
	' 95:9	96.7	113.4
11:58:00
12:00:30	95.0	96.9	112.1
12:03:00	94.7	96.6	115.9
12:05.30	94.3	96.3	112.7
12:08:00	94.2	962	113.6
12:10:30	94.6	96.7	118.6
12:13:00	96.8	98.7	119,9
12:15:30	98.6	108.6	124.7

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Fig. 4:

Leq and Lmax values of Pratap helicopter vs Time of flight

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Table 7 gives the overall summary of the two sorties viz., duration, L, peak and percentile distribution (L_n) of A-weighted sound levels recorded for the two helicopters. L_n is that level of sound which has exceeded for 'n' percent of the total measurement duration. The maximum values of A-weighted peak for Chetak and Pratap helicopters were 129.4 and 124.7 dB(A) respectively while the maximum L value for both the max helicopters were 108.6 dB (A). The maximum value of L_max for Pratap was recorded during the landing phase while for Chetak it was registered during flight. The percentile distribution of A-weighted sound levels indicate that for one-third of the sortie duration (L₃₃₃₃), the sound levels were in excess of 99.3 and 94.4 dB(A) inside Chetak and Pratap cockpits respectively. The analysis also shows that for two-third of the sortie duration (L^ „), the sound levels were in excess of 97.6 and ^V DO. Of 93.2 dB (A) for Chetak and Pratap, respectively.

Table 7. Summary of in-flight sound data and percentile distribution of sound levels inside the cockpits of Chetak and Pratap helicopters

Helicopter	Duration	Max L max	Peak	Percentile values in	dB(A)
	(hnrnin)	[dB(A)]	[dB(A)]	^ 1.67 ^8.33 ^33.33 30.00
Chetak	00:58	10S.6	129.4	102.9 101.3 99.3 98.3	97.6 94.8
Pratap	01:10	103.6	124.7	97.6 96.2 94.4 93.7	93.2 91.7

The percentile distributions of L values ^r max of A-weighted sound levels for six levels from L, 67 to L_l)(|, during the two sorties are shown graphically in Figure 5. The figure clearly shows that the rms value of sound levels inside the cockpits of Chetak and Pratap helicopters were consistently higher than 90 dB (A).

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Fig 5.

Percentile distribution of A-weighted sound level inside Chetak and Pratap cockpits

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Discussion

Noise is believed to act as a general stressor and possibly to have some effect on task concentration and performance efficiency. Extremely intense noise, be it continuous or impulse, can cause immediate damage to hearing. High frequency hearing is lost first, although the loss progresses to lower frequencies later and generally becomes greater in magnitude with increase in age. Although its causes are incompletely understood, it is believed to be brought about by deterioration of the inner ear, nerves to the brain and possibly cortex.

The effects of noise on human beings can be broadly divided into two: physiological and psychological effects. [4] Physiological responses, both auditory and non-auditory, involve changes in physiologic mechanisms or functions attributed to noise. One of the most important physiological effects of noise of sufficient intensity is to rise the threshold of hearing by a direct action upon the hair cells of the organ of Corti. This elevation of threshold may be temporary (Noise Induced Temporary Threshold Shift, NITTS) or permanent (Noise Induced Permanent Threshold Shift, NIPTS). The general consensus is that noise, at least at levels in the audible frequency spectrum which are tolerable in practice according to accepted standard criteria, has no untoward or permanent physiological effects upon man provided that the hearing is fully protected [5]. However, noise induced auditory pain occurs almost independent of frequency at sound levels of 130 to 140 dB and above.

In most countries, the trading relation between exposure time and sound level is based on 'equal energy rule' so that if 90 dB(A) for 8 hr is assumed as the acceptable limit, then for every halving of exposure time, overall level can be increased by 3 dB. Some countries have taken a less cautious approach by advocating 5 or 6 dB as appropriate halving factor. The United States' Occupational Safety and Health Administration (OSHA) Standards uses a 5 dB trading relation with 90 dB (A) for 8 hr per day in a 5 days/week working environment as the permissible noise exposure limit [6]. However, most countries place an upper limit on the peak SPL of 140 dB for any single exposure to loud noise as high intensity sound extend the physical response of the ear beyond its 'elastic limit'. If we go by the permissible noise exposure limits advocated by the United States' OSHA Standards, then without any ear protective devices, the daily noise exposure level will have to be limited to 100 dB(A) for 2 hr, 105 dB(A) for 1 hr and so on for conservation of hearing.

Mapping of sound levels during ground run at max rpm of the engine(s) with the rotors engaged, revealed that the L_eq value of A-weighted sound level for Chetak had maximum values at 135° location while for Pratap the max L^ was registered in front of the cockpit at 0° location. These maximum L_e values were around 110 dB (A). If one goes by OSHA Standards, the permissible duration, of noise exposure at this level for 5 days a week gets limited to 30 min per day.

Sound at levels high enough to cause damage to hearing is also undesirable as it creates other health problems, interferes with performance of task or produces annoyance. Sound below hazardous levels may still affect performance, cause annoyance or produce mental fatigue [7]. Man's perceptions, judgments, attitudes and opinions influence psychological response to noise. However, the effects of noise on cognitive and sensorimotor performance remain unclear and very complex. There have been many experiments and field surveys concerned with task efficiency at work, but no unequivocal simple generalization about the effects of noise on behavior have been drawn. Some of the studies have reported detrimental effects; a somewhat smaller number, beneficial effects; and a great many, no effects at all [8]. Intelligible communication between aircrew and air traffic controllers is essential during flight. The successful transmission of spoken messages or other auditory signals depends primarily upon the microphone picking up signals satisfactorily from the sender and an earphone or loudspeaker delivering the message without undue masking or distortion to the listener. The difficulty of understanding speech heard over communication a system in aerospace operations is quite common, even for a practiced listener. Extraneous acoustical noise picked up by the communication system can add to aircrew's difficulty in deciphering the message. Distortion can also result when high intense noise levels force the speaker to raise his voice. On the receiving end, the ground crew may have to increase the volume in their headsets, which could be hazardous over long duration exposures. The maximum A-weighted rms value (L ) ° of sound level recorded during the two representative sorties of Chetak and Pratap helicopters was 108.6 dB (A). The percentile distribution of A-weighted sound levels inside the helicopters indicate that for one-third of the sortie duration, the rms value of sound levels were in excess of 99.3 and 94.4 dB(A) for Chetak and Pratap respectively. For two-thirds of the sortie duration, the sound levels were in excess of 97.6 and 93.2 dB (A) while 90% of the time it was more than 94.8 and 91.7 dB (A) respectively for the two helicopters. At 95 and 100 dB (A) noise levels, OSHA Standards permit only 4 hr and 2 hr exposure per day over a 5 days/week. When many sorties are undertaken on the same day in these types of helicopters, the unprotected aircrew within the cockpit is liable to be subjected to hazardous noise levels.

Summary

1. Mapping of A-weighted sound levels around Chetak and Pratap helicopters were carried out with the engine (s) at maximum rpm, at eight positions at angular separation of 45° at a radius of 10 m from the axis of the main rotor. The maximum L_eqvalues averaged over l/8^th s were 113.9 & 108.3 dB (A) respectively for Chetak and Pratap helicopters.

2. In-flight sound data was recorded continuously inside the cockpits of Chetak and Pratap helicopters during two representative sorties. The A-weighted L_eq, L and peak values inside Chetak helicopter max r 1 for intervals of 2 min 30 s varied between 92.4 and 101.5 dB(A), 98.1 and 108.6 dB(A) and 109.8 and 129.4 dB(A) respectively. Similarly, the A-weighted L, L and peak values 1=1 eq max r inside Pratap for the different intervals ranged between 91.8 and 98.6 dB (A), 94.5 and 108.6 dB (A) and 111.5 and 124.7 dB (A) respectively.

3. The maximum L value recorded for the max a helicopter during the two representative sorties was 108.6 dB (A) while the maximum values of A-weighted peak were 129.4 and 124.7 dB (A) respectively for Chetak and Pratap.

4. The A-weighted sound levels at the pilot's ear were in excess of 99.3 and 94.4 dB (A) inside Chetak and Pratap cockpits respectively for one-third of the sortie duration. For two-thirds of the sortie duration, the sound levels were in excess of 97.6 and 93.2 dB (A) for Chetak and Pratap respectively.

5. The A-weighted sound level was always more than 90 dB (A) for both the helicopters on-ground as well as in-flight.

Conclusion

The ground crew and aircrew of Chetak and Pratap helicopters are likely to get exposed to A-weighted sound levels of hazardous magnitude (above 90 dB(A)) and as such they must adhere to mandatory use of efficient and properly fitting ear protective devices for hearing conservation.