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lec18reading - Chapter Sound Reception SOUND RECEPTION IS...

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Unformatted text preview: Chapter Sound Reception SOUND RECEPTION IS THE OPPOSITE PROCESS from sound production: sound vibrations propagating in the medium must first be coupled to the organism, modified as necessary, and then converted into useful nerve signals. Ideally, a perfect sound receiver would have a very wide range of detectable frequencies (frequency range), a very wide range of detectable amplitudes (dynamic range), very fine fre— quency resolution (Af), very fine temporal resolution (At), very ac— curate measurement of signal amplitudes, very accurate determina— tion of the angles of a sound source in the horizontal and vertical planes, very accurate determination of the distance to the source, and. the ability to monitor very rapid changes in the time domain waveform of signals (pattern recognition). 141 142 Chapter 6 Not surprisingly, it is impossible to achieve ideal conditions for all fea- tures in a single sound receiver. Evolutionary investment in one feature in~ variably curtails perfection in another. We have already discussed the physi- cal tradeoff that must exist between having a low Af and a low At. As we discuss later, there are other serious tradeoffs that exist between the various features. In addition, the environmental conditions under which organisms live may impose severe constraints on the perfection of specific features. Some of these constraints may be direct limits on the ways in which ears work; oth— ers may arise because the signals that are most easily produced or propagated in a given medium may not be the ones most easily detected and analyzed. Clearly, some tradeoffs between sound production, propagation, and recep- tion will exist. In this chapter, we review the more common mechanisms for coupling sounds to organisms and converting them into neural signals. We then review the major taxa that use acoustic communication and contrast their success in designing sound receivers. COUPLING OF PROPAGATING SOUNDS TO ORGANISMS Ideal couplers would transfer a large fraction of incident sound energy in the propagating medium to the detector organs of the receiver organism while preserving the amplitude, phase, frequency, and directional information con- tained in the original signal. There are basically three kinds of couplers found in animals. Each of these varies in its ability to meet these requirements. Particle Detectors Particle detectors are long thin organs that move when barraged by many molecules all moving in the same direction. As we have seen, such mass movements of particles due to sound propagation occur only in the near field. Sensory cells attached to the base of such organs are alternately stretched and compressed as the oscillating sound field forces the organ back and forth. Most arthropods have a variety of hairs on their bodies that provide chemosensory and tactile information. In addition, many also have very fine hairs (called trichobothria in spiders and trichoid sensilla in insects); these hairs are particle detectors sensitive to air or water currents and to near field sounds (Figure 6.1). Some caterpillars use these hairs to detect the wing-in~ duced air currents of approaching predatory wasps (Tautz and Markl 1978). Crickets and cockroaches have long cerci at the tips of their abdomens that are covered with many fine hairs. Variation in the lengths of these hairs is corre- lated with the particular near field sound frequencies to which each hair is sensitive (Petrovskaya 1969). In other insects, thin antennae or mouth parts are used as particle detec~ tors. The courtship dance of male fruit flies produces vibrations at about 200 Hz. Females within 0.5 cm of a dancing male can detect and monitor the near field signals with a fine segment of each antenna called the arista. The plumose antennae of male mosquitoes and midges are used to detect the species—specific wing beat frequencies of females. Water boatmen have a club Sound Reception 143 (A) I (B) Figure 6.1 Particle detectors (C) (D) in arthropods. (A) Antennal ‘ arista of a fruit fly. (B) Tri— choid sensilla on a cockroach cercus. (C) Antenna of a male H mosquito. (D) Trichoboth- rium of a spider, T, and a larger sensory hair, H. (A after Burnet et a1. 1971 ; B after Autumn 1942; C after Hutchings and Lewis 1983; D after Foelix 1982.) that extends from their ear and responds to near field Vibrations generated in the water by other individuals. Note that aquatic arthropods are more often within near fields than are terrestrial forms because of the longer wavelengths for any given frequency in water. The frequency response of a particle detector depends upon its ability to oscillate in phase with the particles of the surrounding medium. Massive hairs will be unable to keep up with high frequencies or large amplitudes, and even the smallest particle detectors appear to be limited to frequencies well below 1 kHz. Insects such as male mosquitoes and female fruit flies use this constraint by adjusting the mass, hing-mg, and shape of their antennae to limit their responsiveness to the species—specific frequency band. Where they have been studied, the nerves arising from antennal particle receptors appear able to track the waveforms of conspecific sounds well as long as very high frequencies are not present. Thus they can provide considerable information to higher brain centers with only minimal distortion. Long hairs and antennae that are attached at the cuticle of the animal pro— vide a significant mechanical advantage to the sensory cells: a very small force distributed over the long axis of the hair or antenna is transmitted to the sensory cells as a very small displacement but one of great force. Thus very weak sound fields can be detected by such receivers. The enormous mechani— cal advantage of the mosquito antenna makes it one of the most sensitive au« ditory receptors known among the arthropods and similar in sensitivity to the far field response of the human ear (0 dB SPL). in water, the much smaller displacement of molecules for a given sound energy makes it challenging to produce a particle receptor response. In addi— tion, because most animal tissues have similar acoustic impedances to that of 144 Chapter 6 water, near field displacements cause most of the molecules in an organism to move in concert with the sound. When the tips and bases of particle detectors move together, there can be no flexing of the detector and thus no stimulation of the sensory cells. To get around this limitation, some organisms Whose de~ tectors receive Vibrations through water attach the tips of sensory hairs to ob- jects that have quite different masses and acoustic impedances from the sur— rounding tissues. The objects then vibrate either with a lag or with a decreased amplitude when compared to the base tissues of the hairs. The greater the dif- ferential in movement between the heavier objects and the surrounding tis~ sues, the greater the stimulation of the sensory cells. Since the objects will have their own acoustic properties, such objects may limit the response of such a system to specific frequencies. Where heavy objects such as calcium carbonate masses are used, the response will usually be limited to lower frequencies. Particle detectors are inherently directional (Figure 6.2). When the axis of particle movement is parallel to a sensory hair, the hair will not move; when the axis of particle movement is perpendicular to a hair, it will experience the greatest force upon it. Angles of incidence between 0° and 90° will create inter- mediate forces on the hair depending upon the value of the perpendicular component. Thus the amplitude of stimulation from a single hair could be used as an index of the direction of the signal source. However, there are sev- eral problems with using particle detectors to determine source direction. One is that the animal cannot know from a single hair whether variations in hair displacement are due to differing angles of incidence or to different ampli- tudes of the signal at the source: signal amplitude and signal direction are con~ founded. A second is that near fields that are dipolar or of higher—order geome- lncident wave Hair —— Vertical force on hair Figure 6.2 Components of force on a hair shaped particle detector. Incident sound will have two components of force, one perpendicular to the hair and one parallel to the hair. Only the per— V 3 pendicular component will move the Guide V hair. As the angle of incidence changes from parallel to perpendicular to the hair, the movement of the hair for a sound of a given amplitude Will increase. \ Horizontal force on hair Sensory cell nucleus Sensory nerve Sound Reception 145 try, certainly the most common source of sounds, do not necessarily produce particle movements whose axis points at the source. Even if an organism can uncouple source amplitude and directional information, it must somehow map the sound field’s geometry before extrapolating the location of the source. A common method of decoupling source angle and signal amplitude is to have multiple particle detectors. Many hairs can be spread over the body, and two or more antennae can be placed on the head. Particle detectors can be hinged or curved so that they move only along a single axis with different de— tectors aligned along different axes. Since the alignment and body curvature is given, a reasonable brain ought to be able to combine the information from such an array of detectors to provide separate estimates of source signal am- plitude and source location. Particle detectors thus meet some of our ideal conditions and not others. They are primarily limited to lower-frequency signals and have dynamic ranges and temporal tracking abilities limited by their mass and inertia. Within these limited frequency and amplitude ranges, they track slowly vary- ing waveforms well and can thus provide relatively undistorted signals to sensory cells. Particle detectors are inherently directional and thus can pro— vide information about a signal source given independent measures of signal amplitude. Their use is limited‘to near fields. Pressure Detectors Nearly all terrestrial organisms that do not utilize particle receptors for sound coupling rely on some sort of membrane (often called a tympanum). When the pressures on the two sides of a tympanum are unequal, the tympanum is bent away from the side of higher pressure. This bending of the tympanum can then be coupled to sensory cells to produce neural impulses. A pressure detector consists of a tympanum stretched over a closed cavity (Figure 6.3). Sensory cell Drum membrane Figure 6.3 Pressure detector. A thin membrane is stretched over a closed cavity. When the incident sound pressure is greater than that inside the cavity, the mem— brane (called a tympanum) is forced into the cavity When the external pressure is lower than that inside the cavity, the tympanum is forced out. Movement of the tympanum is monitored by sensory cells. 146 Chapter 6 There must be no effective opening between the cavity and the outside world and the walls of the cavity must be reflective at the sound frequencies of inter‘ est. When the typanum is at rest, the pressures of the medium inside and out- side the cavity are similar. When a propagating sound arrives at the tympa- num, the rise in pressure outside the tympanum bends it into the cavity until the compression of the enclosed medium results in an equal but opposite pressure. Similarly, when the pressure outside of the tympanum is lower than that inside, the tympanum is bent outward until the increased cavity volume results in a compensatory lowering of cavity pressure equal to that outside. Sensory cells attached to the tympanum can then record the movements of the tympanum into and out of the cavity. Pressure detectors are used in far fields. Because terrestrial organisms consist of water and solids, they must cope with the fact that much of the sound energy carried in air will be reflected at their ears. Tympana have sev- eral advantages as terrestrial sound couplers. They can be made very thin, so that their acoustic impedance is closer to that of air. Because the force, F, on a pressure detector tympanum equals the product of the sound pressure, P, and the membrane area, A, large thin membranes are very efficient ways to trap incident sound energy. The thinness of the membrane and the strong restora— tive force created by compression of the closed cavity volume also help pres- sure detectors to track changes in incident waveforms rapidly. Like a string, a stretched tympanum has a number of resonant modes at which it responds strongly to incident sounds. The frequency of the lowest mode increases with the thickness of and tension on the tympanum, and de- creases with the diameter of the tympanum. Unlike a string, the frequencies of successive modes are not harmonics of the lowest mode, but tend to be spaced at intervals much less than the lowest mode’s frequency. Thus a very thin but large—diameter tympanum may respond to a much greater variety of frequen- cies within a given range than an equivalently sized string. This range makes a tympanum well—suited to respond to a wide variety of incident sound frequen- cies. In some organisms, however, this breadth of frequency response is undev- sirable. Such species have modified membrane thickness and use complex membrane shapes to constrain the resonant modes to preferred values. Sound pressure has no inherent direction. This means that a single pres~ sure detector lacks any intrinsic mechanism for determining the location of the sound source. One solution is to add a directional structure between the pressure detector and the ambient medium. Nearly all terrestrial mammals have a directional pinna that connects to the tympanum via a funnel—shaped tube (called an auditory meatus). Such a pinna can provide directionality only for wavelengths that are similar to or smaller than the size of its external opening. Thus the utility of a pinna is limited to higher frequencies. The aim“ plest use of a directional pinna is to aim it in various directions and search for a maximum in the received signal amplitude. Many mammals have elabo- rated the structure of pinnae with numerous fine folds, ridges, and protru~ sions (see Figure 6.13). Each of these ridges or folds reflects sounds of a partic- ular wavelength, or shorter. The relative phases of the separate reflections at Sound Reception 147 the tympanum, for any given wavelength, will vary depending upon the ver— tical and horizontal angle of the source. Thus the amplitude of any given fre- quency component at the tympanum will vary as a function of source loca- tion. By comparing the frequency spectrum received at one tympanum with that expected for a given type of source, the animal can often reconstruct the vertical and horizontal angle of the sender. The problem with this method is that frequency spectra of sounds are easily altered during propagation and thus may never be similar to that expected. This makes it difficult to separate the effects of location and distortion when only a single ear is used. If, how— ever, the animal uses two ears, each with a pinna, similarities in the spectra at the two ears can be used to determine the likely spectrum of the incident sound, and differences between the ears can be used to extract information about the sound source’s location. Note that this type of localization mecha- nism requires an inner ear that can perform at least a crude Fourier analysis of the received sounds. The use of a pair of pressure detectors also provides a number of other di- rectional cues. If the incident sound has a sharp onset or ending, or has some other portion clearly defined by an amplitude or frequency modulation, the difference in time of arrival of that marker at the two ears can be used to esti- mate the location of the source in at least one plane. In the simplest case, an animal with two ears can rotate its head. until the time delay is minimal. The sound source will then either be directly in front of it or directly behind it. Ro- tating a quarter—turn more will then resolve this final ambiguity. The problem with this method is thatthe sound must persist long enough to perform the head rotations. An animal could react faster by computing the angle of a sound source using the delay in arrival of a single sound at the two ears (Fig- ure 6.4). To be useful, the time delay must be long enough to be resolvable by normal nervous systems. In humans, the maximal delay of arrival at the two ears (given a typical head diameter) is about 0.5 msec. For a small terrestrial animal with only 1 cm separating its two ears, the delay is at most 0.03 msec and for a similarly sized animal in water the delay is 22% of the terrestrial value. Although some animals can resolve very fine time delays (owls are ac- curate down to 0.006 msec), this detection usually takes a very large number of brain cells. Because only larger animals can afford to dedicate so many brain cells to time-delay resolution, and because larger animals will be able to separate their ears more and create longer maximum time delays, this mecha— nism of sound source localization is most common in larger species. A second method for determining source location using two pressure dec— tectors is to compare the relative phases at the two ears. Unless an animal is facing directly toward or directly away from a sound source, the phase of an incident sound differs at the animal’s two ears. Again, the animal can rotate the head until such differences are nulled out. Maximal phase differences will occur when the wavelength of the incident sound is twice as long as the dis— tance between the two ears. At much lower frequencies, the waves are so long relative to the distance between the ears that there will be little difference in perceived pressures at the two ears. At much higher frequencies, there will be 148 Chapter 6 Figure 6.4 Determination of sound source using two ears and time delay. The time domain waveforms of signals from two ears are shown as perceived by a center in the brain that receives signals from both. The white waveform shows a signal from the left ear and the black waveform shows a signal from the right ear. Arrows show the angle of source in the horizontal plane of the head. When the sound source is along a line perpendicular to the line joining the two ears and midway between them, the sound arrives at the two ears simultaneously. When the sound is opposite one ear and on a line connecting the two ears, maximum delay is achieved. If the dis~ tance between the ears is known, the horizontal angle to the source can be computed from the measured delay. Ambiguity concerning whether the source is in front of or behind the animal can be resolved by rotating the head. many different positions at which phase differences will null. out. Thus a ter— restrial animal with ears separated by 1 cm is likely to use phase information only if it typically responds to sounds of 12—22 kHz. An animal with ears sep- arated by 10 cm will be able to use phase information for sound frequencies down to a few kHz. Finally, when a far field sound encounters an object at least as large as one-tenth of a typical wavelength, diffraction causes a distortion of the sound field around the object. Sound pressure on the side of the object nearest the source will. build up above free field levels, and interference between directly reflected and creeping sound waves will generate a complicated pattern of pressure levels at other angles around the object. As the ratio between the ob~ ject and wavelength is increased to 5, the pressure build up on the side facing the source will approach an asymptote at about 6 dB relative to the free field values. Pressures on the far side of the object vary with location, shape of the object, and the object to wavelength ratio but are at most 8—10 dB lowe...
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