Hydrophone

“ The hydrophone resembles a delicate telephone. It is so constructed that when the instrument is lowered over the side of a ship into the sea any noise, such as the movement of a submarine's propellers, can be heard on deck by an operator listening at an ordinary telephone receiver connected to the submerged microphone by an electrified wire. ”

“ One hydrophone was affixed to the instrument's titanium pressure case, and the other had a 3 m cable and was attached to a bracket allowing it to be positioned just a few centimeters from a black smoker vent orifice. We refer to these two hydrophones as the “case” and “remote” hydrophones, respectively. The first-generation system, used in 2004, sampled on three channels continuously at 1000 Hz with a 12-bit A/D converter and a fixed gain for each channel. ”

“ And fluid–structure interactions would generate dipole acoustic radiation with its locus situated within the chimney structure. The deployment of a hydrophone array in the near-field may therefore help discriminate between these possible sources by measuring the fall-off rate of the pressure fluctuations and by locating the sound sources.
Potential narrowband sources within black smokers are related to the excitation of the acoustic modes of resonators by unsteady flow. Such resonance typically involves flow past cavities, or flow impinging upon solid bodies. ”

“ The first hydrophone station which was established in the spring of 1915 was from Oxcars Lighthouse in the Firth of Forth; it was later in the year transferred to Inchcolm. Experimental work under Captain Ryan continued at Hawkcraig during 1915, and in 1916 a section of the Board of Invention and Research went to Hawkcraig to work in conjunction with him. This station produced the Mark II directional hydrophone of which large numbers were ordered in 1917 for use in patrol craft. It was a great improvement on any hydrophone instrument previously in use. ”

“ Binaural Listening. The principles underlying this method of direction-finding have been described already. In the present case the main difference is that the sound receivers have to be submerged hydrophones. It has been found to be equally necessary for success that these should be as completely as possible non-resonant. The simplest arrangement used in practice was two rubber cavities placed several feet apart horizontally, and joined by separate equal tubes to the two ears. The device could be rotated about a vertical axis, usually passing through the hull of the operating ship. ”

“ In twelve of the twenty-two areas into which the waters round the United Kingdom were divided, regular hunting flotillas were at work, comprising trawlers and motor launches fitted with hydrophones. Before the institution of the convoy system a few fast vessels, such as destroyers or "P" boats, had been formed into hunting flotillas, but the convoy work had necessitated the withdrawal of all these vessels, and the work of the flotillas had suffered in consequence, the speed of trawlers being too slow to offer the same prospect of success in such anti-submarine measures. ”

“ Thus for a 1-m tube closed at one end and filled with hot hydrothermal fluid, the fundamental frequency is ∼113 Hz.
Both Sully and Puffer produce acoustic tones at several different frequencies. The different tones might be produced by different types of resonators, or by several resonators of a single type but with different geometries and different relationships to the fluid flow. In either case, the tones that are generated will depend strongly on the vent's morphological structure, and each vent within the vent field is likely to have its own unique acoustic signature. ”

“ In an open pipe, the wave-length is theoretically equal to the length of the tube. The pitch of the note depends on the frequency per second with which each vibration or complete sound-wave reaches the drum of the ear. The longer the wave the lower the frequency. The velocity of the wave is independent of its length, being solely conditioned by the rate of vibration of the particles composing the conveying medium: while one individual particle performs one complete vibration, the wave advances one wave-length. ”

“ If the tube were stopped at its ends, the closed extremities would throw the air within the tube into a state of vibration; and if the velocity of sound in air were equal to its velocity in glass, the air of the tube would vibrate in synchronism with the tube itself. But the velocity of sound in air is far less than its velocity in glass, and hence, if the column of air is to synchronize with the vibrations of the tube, it can only do so by dividing itself into vibrating segments of a suitable length. ”

“ As a general result, it has been found that the most effective sound for coast-warnings is one having a frequency of 100, or a wave-length of about 10 feet. When dealing with the subject of waves in general, it was pointed out that the velocity of a wave depended upon the elasticity and the density of the medium in which it was being propagated. In the case of a sound wave in air or any other gas, the speed of wave-transmission is proportional to the square root of the elasticity of the gas, and inversely proportional to the square root of the density. ”

“ In selecting the position at which a fog-signal is to be mounted, the possible influence of a sound-shadow, and the possible extinction of the sound by the interference of the direct waves with waves reflected from the shore, must form the subject of the gravest consideration. Preliminary trials may, in most cases, be necessary before fixing on the precise point at which the instrument is to be placed.
The siren which has been long known to scientific men is worked with air ”

“ To sum up, we may then say that whenever rapid electrical oscillations are created in open circuit, such as the two rods above described, the arrangement constitutes a device for creating an impulse or effect in the surrounding space called an electric wave in the æther or electro-magnetic medium; just as an organ-pipe or piano-string or other musical instrument constitutes a device for creating waves in the air by means of mechanical oscillations. ”

“ Nevertheless, as matters stand at present, the moment that the wind is introduced, all the pipes would speak at once, to obviate which a moveable piece of wood, or sounding-pallet is inserted in the groove, the control over it being exercised by means of a wire connected with the key-note: the result is, that when the note is pressed, the wire acts on the pallet, allowing the air to escape into that particular groove, and thus produces a musical note, or, we may say, notes; for, as there are several pipe-holes to each groove, all those pipes would sound simultaneously. ”

“ Not only so, but the amplitude of vibration in the one will determine the amplitude of vibration in the other, for the intensity of the induced current is determined by the amplitude of the inducing vibration, and the amplitude of the vibration at the receiving end depends upon the intensity of the attractive impulses. When we sing into a piano, certain of the strings of the instrument are set in vibration sympathetically by the action of the voice with different degrees of amplitude, and a sound, which is an approximation to the vowel uttered, is produced from the piano. ”

“ The theory of acoustics shows that the length of each complete vibration is four times that of the depth of the closed pipe, and since experience proves that all sound, whatever may be its pitch, is propagated at the same rate, which under ordinary conditions of temperature and barometric pressure may be taken at 1120 feet, or 13,440 inches per second,--it follows that the number of vibrations in the note of a whistle may be found by dividing 13,440 by four times the depth, measured in inches, of the inner tube of the whistle. ”

“ These succeed one another so rapidly that the intervals of darkness which intervene are not perceived, and the quantity of gas which explodes is too small to produce any audible noise; but on bringing a tube, the air in which is capable of vibrating with the same quickness as the little explosions are produced, the air is thrown into vibrations which reach the ear, and produce the peculiar musical tone. With a selection of gas jets and tubes a variety of notes may be produced, so great that a musical instrument has been constructed by their means. ”

“ But there is another reason why falling water makes a sound, and often even a loud roaring noise in the cataract and in the breaking waves of the sea. You do not only hear the water dashing against the rocky ledges or on the beach, you also hear the bursting of innumerable little bladders of air which are contained in the water. As each of these bladders is dashed on the ground, it explodes and sends sound-waves to your ear. Listen to the sea some day when the waves are high and stormy, and you cannot fail to be struck by the irregular bursts of sound. ”

“ Although light is badly broken up in its passage through water, sound-waves will travel through water much better than in air. The first listening-devices used were crude affairs and did not amount to much, particularly when the U-boats muffled their motors and engines so that they were virtually noiseless. But the French invented a very sensitive sound-detector. It consisted of a lot of tiny diaphragms set in a big hemisphere. There were two of these hemispheres, one at each side of the boat. When sound-waves struck these hemispheres, the diaphragms would respond. ”

“ In a sufficiently narrow tube the waves are rapidly stifled, the mechanical energy lost being of course converted into heat. * * * * When a sound wave impinges on a slab which is permeated by a large number of very minute channels, part of the energy is lost, so far as the sound is concerned, by dissipation within these channels in the way just explained. The interstices in hangings and carpets act in a similar manner, and it is to this cause that the effect of such appliances in deadening echoes in a room is to be ascribed, a certain proportion of the energy being lost at each reflection. ”

“ At each oscillation some energy is frittered away into heat in the conductor, and if the conductor and condenser have a special form, energy may be rapidly removed from the system by the electric waves which are formed in the surrounding æther or dielectric. These waves consist in the propagation through the medium of lines of electric strain, just as an air wave consists in the propagation through the air of regions of air-compression, or a water wave consists in the propagation of an elevation on the surface. ”

“ The sea-surface acts somewhat like an optical reflector or mirror, and the electric waves glide along its surface. The rotundity of the earth within certain limits hardly makes any perceptible effect upon the ease of communication. The waves sent out by the transmitter of a long-distance wireless station are from 3000 to 20,000 feet in length, and there is, therefore, a considerable amount of bending or diffraction. It is a familiar fact, as already explained, that a wave-motion, whether on water or in air, spreads round an obstacle to a certain extent. ”

“ The wind always howled around the lamasery, up so high. From what we knew about acoustics and how they shaped the old phonograph horns to increase sound amplification, we worked it out that we were in a sort of wind-tunnel or horn, and it didn’t seem that they opened any rock at the image or we would have heard it. If the far end of the tunnel opened, and wind howled in and through the hollow image, it could make those weird howls, high and low, moans and screeches. So we followed the tunnel down, and by using Tip’s pencil flashlight we located a lever, and risked making the sound. ”

“ Each night and morning the nets are put over, for a surface haul and for a deep one a sounding is taken—a curious lead on a wire, weighted down with an iron weight which forces the sound lead down to the sea bottom and brings up a sample of the bottom, so we can see if it is muddy or rocky or sandy. Each time the weight automatically drops off when the bottom is reached, because the sounding wire isn’t strong enough to pull it up. ”

“ Our eyes are wonderful instruments for detecting wave-motion in the æther, and our ears are appliances for detecting wave-motion in the air. The ear, however, is, as explained in a previous chapter, sensitive to air-vibration forming musical tones which lie between 30 and 30,000 per second, and these numbers are in the ratio of 1000 to 1, and cover a range of about ten octaves. The eye, however, is only sensitive to æther-vibrations which lie in frequency between 400 and 700 billions per second, and these numbers are in the ratio of nearly 2 to 1, or comprise only one octave. ”

“ On the whole, from a literary point of view, the hydros come out better than the mere hotels. But of course they have unequalled advantages. With such material as Dowsing Radiant Heat, D'Arsonval High Frequency and Fango Mud Treatment almost any writer could be sensational. What is High Frequency, I wonder? It is clear, at any rate, that it would be madness to have a hydro without it.
Well, I have selected my hotel—on purely literary grounds. ”

“ In times of fog or at night a code of signals would be produced by it which would be transmitted in all directions through the water, with a velocity four to five times that in the air. Each vessel, in addition to the sound-producing apparatus, would be provided with a sound-receiving apparatus, which would take up out of the water the signals arriving from neighboring vessels. As boys in swimming communicate the sound of the striking of stones together under water, so is it possible to send musical tones from one ship to another. ”