Underwater SoundAuthor(s): P. VigoureuxSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 152, No.946, A Discussion on the 'Ear' Under Water (Apr. 26, 1960), pp. 49-51Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/75361 .

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Single statocyst receptors 49

Lowenstein, O. & Sand, A. 1940 The mechanism of the semicircular canal. A study of the responses of single-fibre preparations to angular accelerations and to rotation at constant speed. Proc. Roy. Soc. B, 129, 256-275.

Prentiss, E. W. I90I The otocyst of decapod Crustacea. Bull. Mus. comp. Zool. 36, 167-251. Ripley, S. H. & Wiersma, C. A. G. I953 The effect of spaced stimulation of excitatory and

inhibitory axons of the crayfish. Physiol. Comp. et Oecol. 3, 1-17. Schone, H. 1951 Die statische Gleichgewichtsorientierung dekapoder Crustaceen. Verh. dtsch.

zool. Ges., Suppl. 16, 157-162. Schone, H. I954 Statozystenfunktion und statische Lageorientierung bei dekapoden

Krebsen. Z. vergl. Physiol. 36, 241-260. Schone, H. I957 Kurssteuerung mittels der Statocysten (Messungen an Krebsen). Z. vergl.

Physiol. 39, 235-240. WViersma, C. A. G. 1952 Neurons of arthropods. In: Cold Spr. Harb. Symp. quant. Biol. 17,

155-163. Wiersma, C. A. G., Furshpan, E. & Florey, E. 1953 Physiological and pharmacological

observations on muscle receptor organs of the crayfish, Cambarus clarkii Girard. J. Exp. Biol. 30, 136-150.

Underwater sound

BY P. VIGOUREUX

National Physical Laboratory, Teddington, Middx.

The following is an introduction to the differences between water and air as media for propagating sound. What follows and more information on underwater sound can be found in Horton (1957).

The main difference between air and water as media for propagating sound lies in their characteristic impedance. This quantity Z is the quotient of excess or acoustic pressure and particle velocity and is equal to pc where p is density and c is velocity of propagation. Then

Zw (pc), _ 1000 x 1500

Za (pc)a 1*22 x 330 -

Thus, in the language of transmission engineers, there is considerable ' mismatch' between air and water. We shall consider some of the consequences presently.

The second great difference lies in absorption, which is much less in water than in air. Absorption is due to conversion of mechanical energy into heat and it gives rise to a decrease in intensity over and above that due to geometrical spread of the

rays from the source. In water it is due to viscosity in the broad sense, and in air to viscosity and to thermal conduction, but in water it is only about a thousandth

part of what it is in air. In sea water there is also absorption due to the ionization of magnesium sulphate, and the complete expression is

40of2_- + 2-75 x 10-4f2 decibels kiloyard-1, 4000 +ff where f is the frequency in kc/s. The additional absorption represented by the first term is also much less than that of air, which is roughly 0-2f2 db kyd-l; that is

why sound carries such long distances in water.

Single statocyst receptors 49

Lowenstein, O. & Sand, A. 1940 The mechanism of the semicircular canal. A study of the responses of single-fibre preparations to angular accelerations and to rotation at constant speed. Proc. Roy. Soc. B, 129, 256-275.

Prentiss, E. W. I90I The otocyst of decapod Crustacea. Bull. Mus. comp. Zool. 36, 167-251. Ripley, S. H. & Wiersma, C. A. G. I953 The effect of spaced stimulation of excitatory and

inhibitory axons of the crayfish. Physiol. Comp. et Oecol. 3, 1-17. Schone, H. 1951 Die statische Gleichgewichtsorientierung dekapoder Crustaceen. Verh. dtsch.

zool. Ges., Suppl. 16, 157-162. Schone, H. I954 Statozystenfunktion und statische Lageorientierung bei dekapoden

Krebsen. Z. vergl. Physiol. 36, 241-260. Schone, H. I957 Kurssteuerung mittels der Statocysten (Messungen an Krebsen). Z. vergl.

Physiol. 39, 235-240. WViersma, C. A. G. 1952 Neurons of arthropods. In: Cold Spr. Harb. Symp. quant. Biol. 17,

155-163. Wiersma, C. A. G., Furshpan, E. & Florey, E. 1953 Physiological and pharmacological

observations on muscle receptor organs of the crayfish, Cambarus clarkii Girard. J. Exp. Biol. 30, 136-150.

Underwater sound

BY P. VIGOUREUX

National Physical Laboratory, Teddington, Middx.

The following is an introduction to the differences between water and air as media for propagating sound. What follows and more information on underwater sound can be found in Horton (1957).

The main difference between air and water as media for propagating sound lies in their characteristic impedance. This quantity Z is the quotient of excess or acoustic pressure and particle velocity and is equal to pc where p is density and c is velocity of propagation. Then

Zw (pc), _ 1000 x 1500

Za (pc)a 1*22 x 330 -

Thus, in the language of transmission engineers, there is considerable ' mismatch' between air and water. We shall consider some of the consequences presently.

The second great difference lies in absorption, which is much less in water than in air. Absorption is due to conversion of mechanical energy into heat and it gives rise to a decrease in intensity over and above that due to geometrical spread of the

rays from the source. In water it is due to viscosity in the broad sense, and in air to viscosity and to thermal conduction, but in water it is only about a thousandth

part of what it is in air. In sea water there is also absorption due to the ionization of magnesium sulphate, and the complete expression is

40of2_- + 2-75 x 10-4f2 decibels kiloyard-1, 4000 +ff where f is the frequency in kc/s. The additional absorption represented by the first term is also much less than that of air, which is roughly 0-2f2 db kyd-l; that is

why sound carries such long distances in water.

Vol. 152. B. Vol. 152. B. 4 4

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P. Vigoureux (Discussion Meeting) As a result of the big mismatch between air and water very little sound crosses

the free surface; most of it is reflected. At normal incidence, which is the most favourable direction for transmission, the ratio of transmitted to incident energy is

4/ A ? Za )

which with the value given above for Zw/Z is about 1/1000. In other directions the rays issuing from water into air are bent towards the vertical according to the law sin O/sin 0, = /,

which gives 13? for 0a when w is 90?. Thus, after crossing the free surface from water to air, sound is propagated in an almost vertical direction independently of the direction of propagation below the surface.

Here there is no critical angle and no total reflexion, unlike what happens with

light, for which water is the 'soft' medium and air the 'hard' one. Conversely there is a critical angle of 13? for propagation from air into water, with total reflexion at all greater angles of incidence, so that if sound is to be transmitted from air into water it must reach the surface at almost normal incidence. Although even then only about 1/1000th part of the energy is transmitted, and although this amount is now distributed throughout a much larger solid angle, the ear is sensitive

enough for a person in water to hear sound from a powerful source in the air above, e.g. a helicopter hovering overhead.

Reflexion also occurs at the sea bed, and in a shallow sea propagation can take

place in successive hops somewhat in the way that radio waves are propagated by hops between the ground and the ionosphere. But because mismatch between the sea bed and the sea is much less than that between the sea and the air, at each reflexion an appreciable part of the incident energy is transmitted into the ground and lost unless there is a critical angle. In general the velocity of sound in the sea bed is rather higher than in the sea, and there is a critical angle beyond which total reflexion occurs. Thus rays which are near to the horizontal are reflected with little loss at the surface and no loss at the sea bed, and sound can carry a long distance.

In water as in air there can be velocity gradients which bend the rays. An interesting case occurs when the sea having been well mixed, e.g. by a moist, warm south-westerly wind, the weather turns calm, fine and cold: the temperature near the surface is then less than the temperature lower down, and sound rays are bent upwards. After reflexion they travel in a path concave upwards and are reflected again and again, thus hugging the surface and never reaching the bottom. There is little loss and the sound can carry many miles. The more general case however is that of temperature decreasing with depth, when the rays are then convex upwards and transmission, at least in a not very deep sea, occurs by alternate reflexions at the surface and the sea bed.

Lastly there is an effect analogous to one sometimes observed in air, although the cause is not exactly the same. The velocity gradients in still air are due solely to temperature gradients, for since air acts nearly like a perfect gas, p V is constant

50

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Underwater sound Underwater sound

for a given temperature, thus p is proportional to the density p and velocity, which is the square root of yapl/p, is constant. But there are days when the temperature decreases with height up to a certain height, after which it increases. The ray is then concave upwards at the start, it goes through a point of inflexion at the height of minimum temperature, after which it is convex upwards until it is turned back to the same height, when it resumes its original concave upwards shape for the last lap of its journey back to the ground, where the intensity may be greater than at intermediate points on the ground.

A similar effect can be observed in deep oceans even when the temperature does not go through a minimum. For the compressibility of liquids is not inde-

pendent of pressure, and the velocity of propagation increases with pressure, and therefore with depth. Thus even when the temperature, after decreasing with depth, reaches a fairly constant value, the rays, convex upwards to start with, become

concave, and if the sea is deep enough for the turning point to be in water, the

pattern resembles that in air upside down; there is very little loss and considerable

channelling of the sound, so much so that an explosion can be heard hundreds of miles away; but the observer must be at the right depth and range, for as in the

analogous case of air there are intermediate regions of lesser intensity.

REFERENCE (Vigoureux)

Horton, J. W. 1957 Fundamentals of sonar. U.S. Naval Inst., Annapolis, Md., U.S.A.

Hearing in bony fishes

BY S. DIJKGRAAF

Laboratory of Comparative Physiology, University of Utrecht

Definition In a recent definition of sound and hearing, primary stress is laid on the aspect of localization (Pumphrey 1950). However, there are fishes which react vividly to sounds and yet are unable to locate the sound source (v. Frisch & Dijkgraaf I935; Reinhardt I935). Therefore we would prefer a more conventional definition and ascribe the ability of hearing only to those animals whichfirst are shown to be sensi- tive to air- or water-borne sound, i.e. to a succession of pressure waves propagating with a characteristic velocity through the medium involved, and which secondly detect these stimuli with special receptors primarily used for this purpose. If the latter condition is not fulfilled we merely speak of sound reception. Responsiveness to sound or vibrations reaching the animal through the solid substrate will be termed vibration reception.

Performance

Sensitivity to sound has been shown in a great number of fishes, predominantly by the use of conditioning or training methods. The degree of sensitivity may differ considerably from one family to another. Roughly, we can distinguish two

4-2

for a given temperature, thus p is proportional to the density p and velocity, which is the square root of yapl/p, is constant. But there are days when the temperature decreases with height up to a certain height, after which it increases. The ray is then concave upwards at the start, it goes through a point of inflexion at the height of minimum temperature, after which it is convex upwards until it is turned back to the same height, when it resumes its original concave upwards shape for the last lap of its journey back to the ground, where the intensity may be greater than at intermediate points on the ground.

A similar effect can be observed in deep oceans even when the temperature does not go through a minimum. For the compressibility of liquids is not inde-

pendent of pressure, and the velocity of propagation increases with pressure, and therefore with depth. Thus even when the temperature, after decreasing with depth, reaches a fairly constant value, the rays, convex upwards to start with, become

concave, and if the sea is deep enough for the turning point to be in water, the

pattern resembles that in air upside down; there is very little loss and considerable

channelling of the sound, so much so that an explosion can be heard hundreds of miles away; but the observer must be at the right depth and range, for as in the

analogous case of air there are intermediate regions of lesser intensity.

REFERENCE (Vigoureux)

Horton, J. W. 1957 Fundamentals of sonar. U.S. Naval Inst., Annapolis, Md., U.S.A.

Hearing in bony fishes

BY S. DIJKGRAAF

Laboratory of Comparative Physiology, University of Utrecht

Definition In a recent definition of sound and hearing, primary stress is laid on the aspect of localization (Pumphrey 1950). However, there are fishes which react vividly to sounds and yet are unable to locate the sound source (v. Frisch & Dijkgraaf I935; Reinhardt I935). Therefore we would prefer a more conventional definition and ascribe the ability of hearing only to those animals whichfirst are shown to be sensi- tive to air- or water-borne sound, i.e. to a succession of pressure waves propagating with a characteristic velocity through the medium involved, and which secondly detect these stimuli with special receptors primarily used for this purpose. If the latter condition is not fulfilled we merely speak of sound reception. Responsiveness to sound or vibrations reaching the animal through the solid substrate will be termed vibration reception.

Performance

Sensitivity to sound has been shown in a great number of fishes, predominantly by the use of conditioning or training methods. The degree of sensitivity may differ considerably from one family to another. Roughly, we can distinguish two

4-2

51 51

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