directional hearing of a harbor seal in air and water

Directional hearing of a harbor seal in air and water J. M. Terhune

Zoological Institute, Aarhus University, 8000 Aarhus C, Denmark (Received 25 July 1973; revised 5 August 1974)

Minimum audible angles (MAA) of a harbor seal were determined using a click-train stimulus (left-right, forced choice) and found to be 9øñ4 ø underwater and 3øñ4 * in air. After accounting for the sound-speed differences, the MAAs and the similarities of the psychometric functions suggest that interaural time differences provide the most important clue. The findings indicate that the interaural distance is the same in both media. An implication of this is that nonbony tissues are not sound transparent in water. MAAs in air were ,determined using other signal types. Click trains and 1/3-octave noise sources above 3 kHz were localized with greater acuity than sinusoidal or low-frequency noise stimuli. The findings indicate that signals containing well-defined time clues are localized more readily than signals containing only intensity and/or phase information.

Subject Classification: 65.62; 80.50, 80.60.

INTRODUCTION'

The primary purpose of this study 'was to determine the abilities of a harbor seal (Phoca vitulina) to localize complex and sinusoidal sounds both in air and underwater. Sounds which would present only phase or intensity (low-and high-frequency sine waves), or time, phase, and intensity (click trains and noise bands) interaural clues were used. The abilities of the seal to localize

these various sound types would indicate which of these clues are important. The 1ocalizationa1 ability was measured in terms of the minimum audible angle (MAA) (Mills, 1958).

I. APPARATUS AND PROCEDURE

The subject was a healthy three-year-old male harbor seal.

Underwater testing was done in a pen acoustically open to a 2.5-m-deep harbor. Testing in air was done in a sound-dampened room. The seal was trained to push a switch (designated No. 1) with its nose (M•hl, 1964). While the switch was closed, a sound was produced from a source located along a horizontal plane in front of the seal. The seal responded correctly to a soun d from its subjective right by pushing a second switch and to a sound from its subjective left by pushing another switch. Each correct response was rewarded with a piece of fish. If the response was incorrect, a lamp (visible to the seal) 1it and reward was withheld.

Two transducers were used underwater. They were placed nominally 3.8 m from switch No. I and could be

o 4 ø 8 ø 12 ø or 16 ø (+0.5 ø ) on either positioned at 2 , , , , side of a point directly ahead of the switch. Both sound sources were visible to the seal. Source diameter was

0.4 ø viewed from switch No. 1.

In air, a single loudspeaker mounted on a track was used. The loudspeaker could be positioned directly

o 4 ø ahead of switch No. I (0 ø or center) and at I , 2 ø, , 6 ø , 8 ø , 10 ø , or 12 ø (+ 0. 5 ø ) to the left of center. The test distance was 195 cm. Source diameter was 0.8 ø

viewed from switch No. 1. A sound-transparent, visu- ally opaque, cloth screen prevented the seal from seeing the loudspeaker.

Signals were produced by a function generator or a random-noise generator, and were, in-some cases, shaped by filtering. The signals were gated, with rise and fall times of 0.26 and 0.27 sec, respectively. The gate was opened and closedby the seal's depressing and releasing switch No. 1.

Two sound types were presented underwater' a click train, generated by a rectangular pulse produced 30 times a second, and a 10-kHz sine wave. Sound levels were measured at the location of switch No. I by using a calibrated hydrophone (Br•el & Kjaer type 8100). Rise time of the click signal was 30 dB in 10 gsec. Total pulse duration (presumably caused by ringing)was 0. 2 msec. Peak signal pressure (unfiltered) was about 130 dB re 1 /•Pa.

The variation in signal levels measured at switch No. 1 for the six transducer positions was 2 dB. Several reflected signals were noted. The first, from the air- water interface, was ? + 2 dB below the direct signal level. A further six to eight reflected pulses, averaging 20 + 5dB below the signal level, were also present. The number and their intensities varied with the position of the transducer.

The intensity of the 10-kHz sine wave signal measured at switch No. I was nominally + 100 (lB re I gPa. The level of the most intense harmonic, 30 kHz, was 26 dB below that of the fundamental.

Three types of signals were presented in air: a click train (as before), band-limited noise, and sine waves. Sound pressure levels and signal spectra were measured by placing a calibrated microphone (Briiel & Kjaer type 4144 or 4135) at the location of switch No. 1.

Peak sound pressure level of the click signal was nominally 80 dB re O. 0002 dyne/cm •' (bandwidth 0. 07- 20 kHz). The rise time of the received click wavefront was 7 dB in 10 g•ec. Total pulse duration, presumably a result of ringing by the loudspeaker, was 0. 5 msec.

The shape of the spectrum of the widest noise band was like that of the click train. The sound pressure level was 75 dB re O. 0002 dyne/cm •' (bandwidth 0. 07- 20 kHz). A band-limited noise (0.2 to 20 kHz), with the highest intensity at 16 kHz, had a sound pressure

1862 J. Acoust. Soc. Am., Vol. 56, No. 6, December 1974 Copyright ¸ 1975 by the Acoustical Society of America 1862

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1863 J.M. Terhune: Directional hearing of a harbor seal 1863

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FREQUENCY (kHz)

FIG. 1. Spectra (analyzing bandwidth 1 Hz)of the in-air ambient noise ( ), and the 1/3-octave wide noise bands centered at 0.8 (----), 2 (...), 4 (. -.), and 8kHz (---).

level of 85 dB re O. 0002 dyne/cm •'.

The spectra (analyzing bandwidth 1 Hz) of the 0.8-, 2-, 4-, and 8-kHz center frequency band-limited noises and the ambient noise in the testing room are in Fig. 1. The sound pressure levels of the four noise bands were 55, 60, 60, and 55 dB re O. 00•02 dyne/cm •' (bandwidth 0. 07-20 kHz), respectively.

The sound pressure levels of the 1-, 2-, 4-, and 8-kHz sine waves were 60, 60, 65, and 60 dB re O. 0002 dyne/ cm •', respectively. All harmonics of each of the sounds were more than 20 dB down.

A. Experiment I

Sounds were presented from the left or right of center. Within any one session, sounds were produced at only two positions equidistant from center. Following a "warm-up" sound from each side, 25 presentations from the right side and 25 from the left were made. The order of presentation was predetermined and random within the limit of four consecutive sounds from the same side.

The click-train stimulus was presented at the angles indicated in Fig. 2. When testing with the 10-kHz sine wave underwater, ten sessions were run with the sound sources 16 ø from center.

The shape of the psychometric function was assumed to be a straight line (Mills, 1958). For the click-train stimuli, MAAs and standard errors were determined by taking the total number of correct responses to both right and left--at each of the angles--and, by the method of least squares, calculating the equation for the straight line best fitting these points. The MAAs were defined as the values (in degrees) responded to correctly 75% of the time.

Water and air temperatures were 10 ø and 18 øC, respectively. Sound speeds were 1500 m/sec in water and 340 m/sec in air.

B. Experiment II

The sounds (in air only) were presented from directly ahead of the seal (0 ø or the catch trial position) or from one of six specific angles to the seal's left. Each session began with warm-up presentations from the catch- trial position and from the farthest left position to be tested. The catch-trial left presentation order was ran- domized as in Experiment I. Ten sessions per stimulus type were run. Tests were conducted using the angles and stimuli indicated in Fig. 3.

The shape of the psychometric function was assumed to be a cumulative normal curve. This shape was cho-

sen because, by presenting catch trials, the task closely resembled that of a signal in noise threshold determin- ation. The MAA and standard error values were calcu-

lated using a relation based on uncoded values (Guilford, 1954). The method first transforms the percent correct value for each stimulus angle into its integral derivative, thus transforming the shape of the cumulative normal curve into a straight line. By the method of least squares, the equation for this straight line is calculated and the MAA is defined as that angle responded to correctly 50% of the time. The threshold determined in this way is basically the same as that of Mills (1958).

Responses to the catch trials were not considered when calculating the MAA thresholds. They were used to estimate the validity of the data by testing for nonran- domness.

II. RESULTS AND DISCUSSION

A. Experiment I

The results obtained with the click stimulus are shown

in Fig. 2. The MAAs are 9øñ 4 ø underwater and 3ø+ 4 ø in air. The distance between the meatal openings of the seal was 12 cm.

In this experiment, the seal had to establish a subjective center and classify sounds as coming from the right or left of this. Sometimes it appeared to change its reference, tending to favor responses to a particular side. Similar results with a harbor porpoise (Phocoe•a phocoe•a) were reported by Dudok van Heel (1962). An- derson (1970), with a harbor porpoise, and Mdhl (1964), with a harbor seal, found that responses to the right and left sides were not symmetrical. In this experiment,

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• •. • ,•' DEGREES FROM CENTER- WATER

FIG. 2. Ability of a harbor seal to detect sound direction in air (--) and underwater (o).

J. Acoust. Soc. Am., Vol. 56, No. 6, December 1974


1864 J.M. Terhur•: Directional hearing of a harbor seal 1864

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FIG. 3. Ability of a harbor seal to detect sound direction in air. [Note: C = click stimulus; WN=wideband noise; Numeral N = 1/3-octave noise band centered at n kHz; S = sine wave; numeral = frequency; + and 0 = percent correct responses at each angle; •-= percent correct responses to catch trails].

responses to the left and right for each angle were aver- aged.

Experiment I tested the ability of the seal to lateralize a complex sound in air and underwater. The click stimulus contained energy covering a large portion of the seal's hearing range and provided a discrete time clue. In Fig. 2, the abscissas have been normalized so that the interaural time differences are equivalent for the various angles in air and water. The similarity of the responses by the seal in these two media indicates that, for the click stimulus at least, the localizational ability of the seal is related directly to sound speed. This sug- gests that the most important factor involved is the ability to discriminate interaural time differences. Be- cause of the broad frequency range of the sound, both interaural intensity and spectral differences would be expected to occur in both media. If either of these were of major importance, then the seal's MAAs would be similar.

By observing the cochlear microphonic, Mdhl and Ronald (1974) found that sound entered the head of the harp seal at the meatal orifice in air and at an area just ventral to the meatal orifice when submerged. They also found (contrary to a proposal by Reysenbach de Haan, 1957) that the nonbony tissues of the head were not sound transparent underwater. These findings indicate that the auditory pathways originate on the surface of the head and are a similar distance apart in both air and water. If the nonbony tissues of the seal's head were sound transparent in water, the effective interaural distance would be about 6 cm, the distance between the bullae. A result of this would be that, for equivalent interaural time differences, the lateraliza- tion angle underwater would be nominally nine times that in air. If this were the case, then the underwater MAA would be 18ø-27 ø. Because the MAA was much

less than this, the assumption of a 12-cm interaural distance appears to be valid. The results of Experiment I, together with those of Mgfhl and Ronald (1974), indicate

TABLE I. MAAs of a harbor seal in air. SN=wideband noise; Numeral N=i/3- octave noise band centered at n kHz; C =click stimulus; S = sine wave; MAAs and standard errors given as -- indicate nonsignificant (random response to catch trials) determinations.

_

Frequency MAA Standard error Catch trial (kHz) Signal type (deg) (deg) % positive Testing order -- C 1.8 0.6 ,, 72 1 -- C 2.1 0.4 74 2

W N - 0.7 1.1 79 12 W N 3.3 0.5 78 10

0.8 N -- -- 49 9 2 N 6.6 2.2 68 11 4 N 0.7 1.0 60 6 8 N 1.7 1.5 64 5

16 N 1.2 0.8 80 7 1 S 4.6 1.8 61 3

2 S -- -- 44 13

4 S -- -- 54 4

8 S 5.5 4.1 57 8



1865 J.M. Terhune: Directional hearing of a harbor seal 1865

that the seal's head is not sound transparent underwater and that sound enters the head at places the same dis- rance apart in both air and water.

B. Experiment II

The results of Experiment II are presented in Table I; the data are plotted in Fig. 3.

In an attempt to reduce the amount of left-right bias apparent in Experiment I, the methodology was altered for this experiment. It differed in that an acoustical center position was presented. It was assumed that the seal could make use of this as a stable reference against which to compare the sounds from the left. The shape of the psychometric functions of the click-train determinations (Figs. 2 and 3) and the relative sizes of the standard errors suggest that the seal's responses were more stable when this method was used.

The results indicate that the seal was able to localize

click and wide-band noise stimuli readily. The data (Fig. 3) show a less pronounced ability to localize the 8- and 4-kHz centered noise bands and a much reduced

ability to localize the 2- and 0.8-kHz centered noise bands. The seal's ability to localize sine waves was very poor.

ACKNOWLEDGMENTS

The assistance offered by the Danish Research Coun- cil (for equipment provided to the Zoological Institute, Aarhus University) is gratefully acknowledged. The seal was kindly supplied by the Copenhagen Zoo. Lektor B. Mdhl is thanked for his discussions throughout the study. For assistance and advice regarding the preparation of the manuscript I would particularly like to thank Dr. K. Jerome Diercks of the Applied Research Laboratories, The University of Texas at Austin.

*Present address: College of Biological Science, University of Guelph, Guelph, Ontario, NIG 2W1, Canada.

Andersen, S. (1970). "Directional Hearing in the Harbour Por- poise, Phocoena phocoena," in Investigations on Cetacea, G. Pilleri, Ed. (Benteli AG, Berne, Switzerland), Vol. II.

Dudok van Heel, W. H. (1962). "Sound and Cetacea," Nether. J. Sea Res. 1, 407-507.

Guilford, J.P. (1954). Psychometric Methods (McGraw-Hill, New York),

Mills, A. W. (1958), "On the Minimum Audible Angle," J. Acoust. Soc. Am. 30, 237-246.

M•hl, B. (1964). "Preliminary Studies on Hearing in Seals," Vidensk. Medd. Dansk Naturh. Foren. 127, 283-294.

M•hl, B., and Ronald, K. (1974). "The harp seal, Pagophilus groenlandicus, (Erxleben, 1777). XVII. Peripheral Auditory System," in Symposium on the Biology of the Seal, 13-17 Aug. 1972, Univ. Guelph, Guelph, Canada (in press).

Reysenbach de Haan, F. W. (1957). "Hearing in Whales," Acta Oto-Laryngol. Suppl. 134.



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