Suszanne Thwaites

Acoustical analysis of the auditory system of the cricket Teleogryllus commodus (Walker)

The basic auditory physiology of crickets, and particularly of Teleogryllus commodus (Walker) is examined and its behavior simulated by electrical analog networks, beginning from the simplest possible model and progressing by stages to the full system found in the real insect. It is found that the attenuation of sound in the auditory trachea plays a crucial role in the mechanism for directional hearing in even the simplest model and that the tracheal diameter is in fact appropriate to produce the desired attenuation. In a more complex model in which it is recognized that the auditory system probably responds to pressure changes in the tracheal sacs underlying the tympana rather than simply to tympanic motion, it is found that the phase shift produced by the combined effects of the central septum and the adjoining cavities leading to the spiracles is also important to hearing directionality. The final model which includes both tympana and spiracles is able to simulate both the hearing directionality and, in part, the frequency selectivity of the system. It appears, however, that a large measure of the observed frequency selectivity is due to some form of selectivity in the neural transducers themselves rather than in the simple acoustic components of the system.

Some notes on the clavichord

Various acoustical features of the fretted clavichord, in some ways the simplest of keyboard instruments, are investigated experimentally and theoretically. The unusual excitation mechanism, in which a metal blade strikes the string and holds it deflected, yields an excitation force spectrum level with a smooth slope of 6 dB/oct, though the radiated spectrum depends greatly on the properties of the soundboard. Energy loss from the paired strings occurs primarily through the bridge to the soundboard, interaction between the strings giving a two‐stage decay as described by Weinreich [J. Acoust. Soc. Am. 62, 1474–1484(1977)]. The uncomplicated soundboard configuration allows its measured response from 50 to 1000 Hz to be well accounted for theoretically. In this important range, a series of couplings between normal modes of the soundboard and of the enclosed air cavity considerably modifies the system response. The sound pressure level and sound decay time to inaudibility across the compass of the instrument...

Acoustic admittance of organ pipe jets

The admittance of a turbulent jet as the generator in organ flue pipes was investigated experimentally with respect to the relevant parameters, that is, the frequency, the jet efflux velocity, the mouth end‐correction, and the jet tip deflection at the edge. First, an experiment was performed using a jet with a pulsating velocity to drive a pipe to isolate and identify the various drive mechanisms occurring in the jet‐pipe interaction. The two established drive mechanisms, momentum drive and volume drive, were found to be operating, but the relative proportion of momentum drive was greater than expected. Turning then to a typical flue pipe arrangement, as a preliminary experiment, the jet admittance in the pipe was shown to be independent of the jet tip deflection so long as the deflection was less than about 0.7 jet widths. This constant admittance regime was then used to facilitate measurement of the pipe excitation as a function of the three remaining parameters. The admittance magnitude and phase were...

Propagant phase in reverberant environments

The phase of the transfer function between two points in an extended system can be easily measured if it is taken to be the accumulated phase obtained by smoothly raising the measurement frequency from zero to the reference frequency. Lyon et al., in an extended series of papers [most recently J. Acoust. Soc. Am. 95, 286–296 (1994)], have examined the behavior of this accumulated phase in systems of two and three dimensions and have elucidated the concept of a reverberant phase which is independent of the separation between the two measurement points, provided they are far enough apart, but which rises sharply with increasing frequency. In some applications, for example, in nondestructive testing of extended structures, it is important to be able to observe simple wave‐propagation behavior and in particular to measure the propagant phase as a function of frequency and position. The conditions under which this is possible are investigated, and are shown to impose constraints on the ratio between the propagation distance and the size of the structure under test, and either the material damping coefficient or the reflection coefficient at the domain boundaries. These results, which represent an extension of those of Lyon et al., are discussed in terms of the distribution of zeros of the transfer function in the complex frequency plane. Many platelike structures of practical interest are found to satisfy these conditions, so that measurement of propagant phase behavior can provide the basis for a useful technique of nondestructive examination.

Electrode surface profile and the performance of condenser microphones

Condenser microphones of all types are traditionally made with a planar electrode parallel to an electrically conducting diaphragm, additional diaphragm stiffness at acoustic frequencies being provided by the air enclosed in a cavity behind the diaphragm. In all designs, the motion of the diaphragm in response to an acoustic signal is greatest near its center and reduces to zero at its edges. Analysis shows that this construction leads to less than optimal sensitivity and to harmonic distortion at high sound levels when the diaphragm motion is appreciable compared with its spacing from the electrode. Microphones of this design are also subject to acoustic collapse of the diaphragm under the influence of pressure pulses such as might be produced by wind. A new design is proposed in which the electrode is shaped as a shallow dish, and it is shown that this construction increases the sensitivity by about 4.5 dB, and also completely eliminates harmonic distortion originating in the cartridge.

Multi-horn matching plate for ultrasonic transducers

Abstract The problem of matching a thick piezoelectric transducer to an air medium is considered and it is concluded that a suitable matching device can be designed and built in the form of a rigid plate, a few millimetres thick and pierced by a closely packed array of horn-shaped channels, the wide mouths of the horns facing the external world and the narrow throats facing the transducer. Design parameters, particularly the spacing between the multi-horn plate and the transducer surface, are found to be quite critical, but well within ordinary manufacturing capabilities. Methods of fabricating the multi-horn plates are described and measured results presented. The measured performance agrees quite well with design calculations. The experimental plates show a gain of ≈10 dB over a frequency range from ≈30 to 100 kHz. Similar matching plates for other frequency ranges can be made.