Reception of Sound

RECEPTION OF SOUND The manner in which sound-energy is abstracted from the medium carrying the sound-waves is dependent on a wide va riety of circumstances and each case must be treated accordingly. In the first place the choice of a receiver must depend on the medium of transmission—a receiver suitable for air being gen erally quite unsuitable under water or in a solid medium. Again the selection of a receiver will depend on the frequency and wave form of the vibration—sounds of frequencies 500 or 50,000, or a single explosion impulse, requiring entirely different treatment. If the sound-wave is employed in long-range sig nalling it is important that the receiver should be efficiently designed and tuned to the incoming sound, in order to obtain maximum response to weak signals. Distortion of wave-form in this case may be of secondary importance. On the other hand, if it is required to obtain a faithful record or reproduction of the sound-wave, energy considerations are relatively unim portant whereas an accurate reproduction of the wave-form is vital. In this connection we have to distinguish between resonant and non-resonant receivers. As a general rule a resonant re ceiver is employed where maximum sensitivity and efficiency are required, whilst a non-resonant receiver is used for faithful recording or reproduction of the sound.

All forms of sound-receivers involve the introduction of some obstacle into the path of the sound-waves, this obstacle either partaking of, or otherwise influencing, the motion of the particles of the medium, or responding in some way to the pressure varia tions on its surface. Energy may be lost due to reflection and re radiation from this obstacle, but in a good receiver a moderate amount of sound-energy is transferred from the medium and converted into a form convenient for observation. It is often desirable to transform the mechanical vibrations of the medium into another type of vibration or into some other Corm 05 energy. For example, longitudinal sound-waves passing through the air from the mouth of a speaker fall on the diaphragm of a telephone transmitter which is set into transverse vibration. This trans verse vibration controls electrical energy which is transmitted to a distant diaphragm receiver and reconverted into mechanical energy in a form suitable for transmission to the ear. Many well known forms of sound-recording apparatus (e.g., Webster's Phonometer, Millar's Phonedeik, and the Hilger Audiometer) employ a diaphragm receiver with some optical means of indi cating and recording the transverse vibrations. Other forms, exemplified by magnetophones, microphones, piezoelectric re ceivers, condenser microphones, etc., involve the conversion of mechanical into electrical energy. Sound-vibrations may also be observed by means of such devices as optical interferometry and sound shadow photography or by means of manometric and sensitive flames. Metrical forms of receiver such as the Rayleigh disc and the sound-radiometer (measuring radiation pressure of sound-waves) form another class, which might also include the various "phonometers," the hot-wire microphone, and piezo electric receivers. The amplification of received sounds by

mechanical devices, "mechanical transformers," mirrors, trumpets and resonators, or by electrical means, microphone and valve amplifiers, must also be considered. In certain cases special devices are required, as in sound-reception under water (hydro phones) or in the detection of sounds travelling through the ground (geophones). The above remarks will be sufficient to show the difficulty of attempting a comprehensive classification of sound receivers. In what follows we shall deal with the more impor tant types, some of which have already been mentioned.

The

Ear.—Sensitivity and Audible Limits.—By far the most important and most universal receiver of sound is the ear. This organ of hearing" has a marvellous range of frequency and sen sitivity; it can perceive vibrations of frequency varying from 20 to 30,00o cycles/sec., and can detect a sound of amplitude less than cm. whilst it is not destroyed by a vibration having an amplitude a million times as great. The ear with its asso ciated nerve endings has also remarkable powers of analysis. It can distinguish one complex sound from another and can "listen intelligently " to a number of different sounds at the same time. For a complete description of the complicated structure of the ear, reference should be made to text books of physiology or anatomy. Briefly, it consists of a device which brings the waves of sound to act on a terminal expansion of the auditory nerve. This device is divided anatomically into three parts, the external ear with the auditory meatus, the tympanum and the internal ear. The external ear in the case of the lower animals is very movable, presumably to assist in sound-location. This function in man is rudimentary so that he can hear almost normally with his ears cut off. The external ear is separated from the tympanum by the "drum " of the ear or membrane tympani, which is set in vibration when sound-waves fall upon it. These vibrations are transmitted with diminishing amplitude but increased pressure by a chain of bones (malleus, incus and stapes) acting as a system of levers, to the fluid (perilymph) in the internal ear. The vibra tions travel through the perilymph from the vestibule and the "oval window" up the turns of the cochlea and ultimately to the basilar membrane thus affecting the haircells and the ends of the auditory nerve. Many theories, notably the resonance Theory of Helmholtz, have been proposed to explain the func tion of the various parts of the ear, in particular the cochlea and the basilar membrane (with the "rods of Corti " and the haircells), but none appears to be really satisfactory. It is certain, however, that the more highly developed the cochlea the more perfect is the hearing of an animal. This is shown in birds and mammals which have a far more perfect cochlea than is found in reptiles and fishes.