The Theory of Sound 1 by John William Strutt, 3rd Baron Rayleigh
1. THE sensation of sound is a thing sui generis, not comparable with any of our other sensations. No one can express the relation between a sound and a colour or a smell. Directly or indirectly, all questions connected with this subject must come for decision to the ear, as the organ of hearing; and from it there can be no appeal. But we are not therefore to infer that all acoustical investigations are conducted with the unassisted ear. When once we have discovered the physical phenomena which constitute the foundation of sound, our explorations are in great measure transferred to another field lying within the dominion of the principles of Mechanics. Important laws are in this way arrived at, to which the sensations of the ear cannot but conform.
2. Very cursory observation often suffices to shew that sounding bodies are in a state of vibration, and that the phenomena of sound and vibration are closely connected. When a vibrating bell or string is touched by the finger, the sound ceases at the same moment that the vibration is damped. But, in order to affect the sense of hearing, it is not enough to have a vibrating instrument; there must also be an uninterrupted communication between the instrument and the ear. A bell rung in vacuo, with proper precautions to prevent the communication of motion, remains inaudible. In the air of the atmosphere, however, sounds have a universal vehicle, capable of conveying them without break from the most variously constituted sources to the recesses of the ear.
3. The passage of sound is not instantaneous. When a gun is fired at a distance, a very perceptible interval separates the report from the flash. This represents the time occupied by sound in travelling from the gun to the observer, the retardation of the flash due to the finite velocity of light being altogether negligible. The first accurate experiments were made by some members of the French Academy, in 1738. Cannons were fired, and the retardation of the reports at different distances observed. The principal precaution necessary is to reverse alternately the direction along which the sound travels, in order to eliminate the influence of the motion of the air in mass. Down the wind, for instance, sound travels relatively to the earth faster than its proper rate, for the velocity of the wind is added to that proper to the propagation of sound in still air. For still dry air at a temperature of 0°C, the French observers found a velocity of 337 metres per second. Observations of the same character were made by Arago and others in 1822; by the Dutch physicists Moll, van Beek and Kuytenbrouwer at Amsterdam; by Bravais and Martins between the top of the Faulhorn and a station below; and by others. The general result has been to give a somewhat lower value for the velocity of sound - about 332 meters per second. The effect of alteration of temperature and pressure on the propagation of sound will be best considered in connection with the mechanical theory.
4. It is a direct consequence of observation, that within wide limits, the velocity of sound is independent, or at least very nearly independent, of its intensity, and also of its pitch. Were this otherwise, a quick piece of music would be heard at a little distance hopelessly confused and discordant. But when the disturbances are very violent and abrupt, so that the alterations of density concerned are comparable with the whole density of the air, the simplicity of this law may be departed from.
5. An elaborate series of experiments on the propagation of sound in long tubes (water-pipes) has been made by Regnault 2 . He adopted an automatic arrangement similar in principle to that used for measuring the speed of projectiles. At the moment when a pistol is fired at one end of the tube a wire conveying an electric current is ruptured by the shock. This causes the withdrawal of a tracing point which was previously marking a line on a revolving drum. At the further end of the pipe is a stretched membrane so arranged that when on the arrival of the sound it yields to the impulse, the circuit, which was ruptured during the passage of the sound, is recompleted. At the same moment the tracing point falls back on the drum. The blank space left unmarked corresponds to the time occupied by the sound in making the journey, and, when the motion of the drum is known, gives the means of determining it. The length of the journey between the first wire and the membrane is found by direct measurement. In these experiments the velocity of sound appeared to be not quite independent of the diameter of the pipe, which varied from 0m·108 to 1m·100. The discrepancy is perhaps due to friction, whose influence would be greater in smaller pipes.
6. Although, in practice, air is usually the vehicle of sound, other gases, liquids and solids are equally capable of conveying it. In most cases, however, the means of making a direct measurement of the velocity of sound are wanting, and we are not yet in a position to consider the indirect methods. But in the case of water the same difficulty does not occur. In the year 1826, Colladon and Sturm investigated the propagation of sound in the Lake of Geneva. The striking of a bell at one station was simultaneous with a flash of gunpowder. The observer at a second station measured the interval between the flash and the arrival of the sound, applying his ear to a tube carried beneath the surface. At a temperature of 8°C, the velocity of sound in water was thus found to be 1435 metres per second.
7. The conveyance of sound by solids may be illustrated by a pretty experiment due to Wheatstone. One end of a metallic wire is connected with the sound-board of a pianoforte, and the other taken through the partitions or floors into another part of the building, where naturally nothing would be audible. If a resonance-board (such as a violin) be now placed in contact with the wire, a tune played on the piano is easily heard, and the sound seems to emanate from the resonance-board. [Mechanical telephones upon this principle have been introduced into practical use for the conveyance of speech.]
8. In an open space the intensity of sound falls off with great rapidity as the distance from the source increases. The same amount of motion has to do duty over surfaces ever increasing as the squares of the distance. Anything that confines the sound will tend to diminish the falling off of intensity. Thus over the flat surface of still water, a sound carries further than over broken ground; the corner between a smooth pavement and a vertical wall is still better; but the most effective of all is a tube-like enclosure, which prevents spreading altogether. The use of speaking tubes to facilitate communication between the different parts of a building is well known. If it were not for certain effects (frictional and other) due to the sides of the tube, sound might be thus conveyed with little loss to very great distances.
9. Before proceeding further we must consider a distinction, which is of great importance, though not free from difficulty. Sounds may be classed as musical and unmusical; the former for convenience may be called notes and the latter noises. The extreme cases will raise no dispute; every one recognises the difference between the note of a pianoforte and the creaking of a shoe. But it is not so easy to draw the line of separation. In the first place few notes are free from all unmusical accompaniment. With organ pipes especially, the hissing of the wind as it escapes at the mouth may be heard beside the proper note of the pipe. And, secondly, many noises so far partake of a musical character as to have a definite pitch. This is more easily recognised in a sequence, giving, for example, the common chord, than by continued attention to an individual instance. The experiment may be made by drawing corks from bottles, previously tuned by pouring water into them, or by throwing down on a table sticks of wood of suitable dimensions. But, although noises are sometimes not entirely unmusical, and notes are usually not quite free from noise, there is no difficulty in recognising which of the two is the simpler phenomenon. There is a certain smoothness and continuity about the musical note. Moreover by sounding together a variety of notes - for example, by striking simultaneously a number of consecutive keys on a pianoforte - we obtain an approximation to a noise; while no combination of noises could ever blend into a musical note.
10. We are thus led to give our attention, in the first instance, mainly to musical sounds. These arrange themselves naturally in a certain order according to pitch - a quality which all can appreciate to some extent. Trained ears can recognise an enormous number of gradations - more than a thousand, probably, within the compass of the human voice. These gradations of pitch are not, like the degrees of a thermometric scale, without special mutual relations. Taking any given note as a starting point, musicians can single out certain others, which bear a definite relation to the first, and are known as its octave, fifth, &c. The corresponding differences of pitch are called intervals, and are spoken of as always the same for the same relationship. Thus, wherever they may occur in the scale, a note and its octave are separated by the interval of the octave. It will be our object later to explain, so far as it can be done, the origin and nature of the consonant intervals, but we must now turn to consider the physical aspect of the question.
Since sounds are produced by vibrations, it is natural to suppose that the simpler sounds, viz. musical notes, correspond to periodic vibrations, that is to say, vibrations which after a certain interval of time, called the period, repeat themselves with perfect regularity. And this, with a limitation presently to be noticed, is true.
11. Many contrivances may be proposed to illustrate the generation of a musical note. One of the simplest is a revolving wheel whose milled edge is pressed against a card. Each projection as it strikes the card gives a slight tap, whose regular recurrence, as the wheel turns, produces a note of definite pitch, rising in the scale, as the velocity of rotation increases. But the most appropriate instrument for the fundamental experiments on notes is undoubtedly the Siren, invented by Cagniard de la Tour. It consists essentially of a stiff disc, capable of revolving about its centre, and pierced with one or more sets of holes, arranged at equal intervals round the circumference of circles concentric with the disc. A windpipe in connection with bellows is presented perpendicularly to the disc, its open end being opposite to one of the circles, which contains a set of holes. When the bellows are worked, the stream of air escapes freely, if a hole is opposite to the end of the pipe; but otherwise it is obstructed. As the disc turns, a succession of puffs of air escape through it, until, when the velocity is sufficient, they blend into a note, whose pitch rises continually with the rapidity of the puffs. We shall have occasion later to describe more elaborate forms of the Siren, but for our immediate purpose the present simple arrangement will suffice.
12. One of the most important facts in the whole science is exemplified by the Siren - namely, that the pitch of a note depends upon the period of its vibration. The size and shape of the holes, the force of the wind, and other elements of the problem may be varied; but if the number of puffs in a given time, such as one second, remains unchanged, so also does the pitch. We may even dispense with wind altogether, and produce a note by allowing the corner of a card to tap against the edges of the holes, as they revolve; the pitch will still be the same. Observation of other sources of sound, such as vibrating solids, leads to the same conclusion, though the difficulties are often such as to render necessary rather refined experimental methods.
But in saying that pitch depends upon period, there lurks an ambiguity, which deserves attentive consideration, as it will lead us to a point of great importance. If a variable quantity be periodic in any time , it is also periodic in the times 2, 3, &c. Conversely, a recurrence within a given period , does not exclude a more rapid recurrence within periods which are the aliquot parts of . It would appear accordingly that a vibration really recurring in the time ? (for example) may be regarded as having the period , and therefore by the law just laid down as producing a note of the pitch defined by . The force of this consideration cannot be entirely evaded by defining as the period the least time required to bring about a repetition. In the first place, the necessity of such a restriction is in itself almost sufficient to shew that we have not got to the root of the matter; for although a right to the period may be denied to a vibration repeating itself rigorously within a time 1/2 , yet it must be allowed to a vibration that may differ indefinitely little therefrom. In the Siren experiment, suppose that in one of the circles of holes containing an even number, every alternate hole is displaced along the arc of the circle by the same amount. The displacement may be made so small that no change can be detected in the resulting note; but the periodic time on which the pitch depends has been doubled. And secondly it is evident from the nature of periodicity, that the superposition on a vibration of period , of others having periods 1/2 , 1/3 ...&c., does not disturb the period , while yet it cannot be supposed that the addition of the new elements has left the quality of the sound unchanged. Moreover, since the pitch is not affected by their presence, how do we know that elements of the shorter periods were not there from the beginning?
13. These considerations lead us to expect remarkable relations between the notes whose periods are as the reciprocals of the natural numbers. Nothing can be easier than to investigate the question by means of the Siren. Imagine two circles of holes, the inner containing any convenient number, and the outer twice as many. Then at whatever speed the disc may turn, the period of the vibration engendered by blowing the first set will necessarily be the double of that belonging to the second. On making the experiment the two notes are found to stand to each other in the relation of octaves; and we conclude that in passing from any note to its octave, the frequency of vibration is doubled. A similar method of experimenting shews, that to the ratio of periods 3:1 corresponds the interval known to musicians as the twelfth, made up of an octave and a fifth; to the ratio 4:1, the double octave; and to the ratio 5:1, the interval made up of two octaves and a major third. In order to obtain the intervals of the fifth and third themselves, the ratios must be made 3:2 and 5:4 respectively.
14. From these experiments it appears that if two notes stand to one another in a fixed relation, then, no matter at what part of the scale they may be situated, their periods are in a certain constant ratio characteristic of the relation. The same may be said of their frequencies 3 , or the number of vibrations which they execute in a given time. The ratio 2:1 is thus characteristic of the octave interval. If we wish to combine two intervals, - for instance, starting from a given note, to take a step of an octave and then another of a fifth in the same direction, the corresponding ratios must be compounded:
2 3 3
- x - = -
1 2 1
The twelfth part of an octave is represented by the ratio :1, for this is the step which repeated twelve times leads to an octave above the starting point. If we wish to have a measure of intervals in the proper sense, we must take not the characteristic ratio itself, but the logarithm of that ratio. Then, and then only, will the measure of a compound interval be the sum of the measures of the components.
15. From the intervals of the octave, fifth, and third considered above, others known to musicians may be derived. The difference of an octave and a fifth is called a fourth, and has the ratio 2 ? 3/2 = 4/3. This process of subtracting an interval from the octave is called inverting it. By inverting the major third we obtain the minor sixth. Again, by subtraction of a major third from a fifth we obtain the minor third; and from this by inversion the major sixth. The following table exhibits side by side the names of the intervals and the corresponding ratios of frequencies:
Octave ...................... 2:1
Fifth ....................... 3:2
Fourth ...................... 4:3
Major Third ................. 5:4
Minor Sixth ................. 8:5
Minor Third ................. 6:5
Major Sixth ................. 5:3
These are all the consonant intervals comprised within the limits of the octave. It will be remarked that the corresponding ratios are all expressed by means of small whole numbers, and that this is more particularly the case for the more consonant intervals.
The notes whose frequencies are multiples of that of a given one, are called its harmonics, and the whole series constitutes a harmonic scale. As is well known to violinists, they may all be obtained from the same string by touching it lightly with the finger at certain points, while the bow is drawn.
The establishment of the connection between musical intervals and definite ratios of frequency - a fundamental point in Acoustics - is due to Mersenne (1636). It was indeed known to the Greeks in what ratios the lengths of strings must be changed in order to obtain the octave and fifth; but Mersenne demonstrated the law connecting the length of a string with the period of its vibration, and made the first determination of the actual rate of vibration of a known musical note.
Lord Rayleigh was born John William Strutt into a barony begun in 1821 on the occasion of King George IV's coronation. He was the eldest of seven children, born on the 12th November 1842. His father, John James Strutt, had been Second Baron for only six years, during which time he had married Clara Latouche Vicars, a lady over twenty five years his junior. His inquisitive scientific mind showed itself when he was four (despite the fact that he had seemed rather unintelligent when he was unable to speak at the age of almost three). His aunt Emily complained at his constant questioning, such as: "What becomes of the water spilt on the tablecloth after it has dried up?"
He attended Eton College at ten years of age, only to catch smallpox, and then whooping cough. His parents decided a home education would be best, and so a private tutor educated him in mathematics, trigonometry and statics. His short stay at Harrow (West Acre, 18571 - 18572) was his last at school, as he caught a chest infection which left him in ill health for the rest of his life. He was taught from the ages of fourteen to eighteen by Rev. G.T. Warner at Torquay. He entered Trinity College, Cambridge in October 1861 having passed his entrance exams with great success. His mathematics course was vital to his future career in understanding physics. He graduated in 1865 with awards which displayed his promise, a promise which he amply fulfilled. He gained a fellowship at the college the following year, which he held for five years before he married. In 1873 his father died, so he became Third Baron Rayleigh and inherited Terling Place, Essex, as well. For the next three years he felt compelled to look after the estate so his scientific research was little. In 1876, he left the job to his younger brother.
He spent five years as the second Cavendish Professor of Physics at Cambridge. He first researched optics and vibrations, both rather mathematical topics. Later he considered physics as a field of work in itself and investigated wave theory, light scattering, electrodynamics, hydrodynamics, viscosity and photography. His careful, precise work led to the establishment of standards for resistance, current and electromotive force. Lord Rayleigh was the cause of a radical new way of instruction of physics experiments at Cambridge, increasing his students from six to seventy. After Cambridge, Rayleigh returned to his country seat. For much of his career he divided his time between his laboratory at Terling Place and the Royal Institution in London where he was Professor of Natural Philosophy from 1887 to 1905. The experiments for the isolation of argon, for instance, were first carried out at the Royal Institution, but the final production was made at Terling Place. Lord Rayleigh's first researches were mainly mathematical, concerning optics and vibrating systems, but his later work ranged over almost the whole field of physics, covering sound, wave theory, colour vision, electrodynamics, electromagnetism, light scattering, flow of liquids, hydrodynamics, density of gases, viscosity, capillarity, elasticity, and photography. His patient and delicate experiments led to the establishment of the standards of resistance, current, and electromotive force; and his later work was concentrated on electric and magnetic problems. Lord Rayleigh was an excellent instructor and, under his active supervision, a system of practical instruction in experimental physics was devised at Cambridge, developing from a class of five or six students to an advanced school of some seventy experimental physicists. His Theory of Sound was published in two volumes during 1877-1878. Volume I covers harmonic vibrations, systems with one degree of freedom, vibrating systems in general, transverse vibrations of strings, longitudinal and torsional vibrations of bars, vibrations of membranes and plates, curved shells and plates, and electrical vibrations. Volume II covers aerial vibrations, vibrations in tubes, reflection and refraction - of plane waves, general equations, theory of resonators, Laplace’s functions and acoustics, spherical sheets of air, vibration of solid bodies, and facts and theories of audition. His other extensive studies are reported in his Scientific Papers - six volumes issued during 1889-1920. He has also contributed to the Encyclopaedia Britannica.
The honours he received for his work were numerous. He was made a Fellow of the Royal Society in 1873, was Secretary between 1885 and 1896, and President between 1905 and 1908. He was made a Privy Counsellor in 1905. The Royal Society awarded him the Copley, Royal and Rumford medals for his work in Physics. All of this was overshadowed by his being awarded the Nobel Prize for Physics in 1904 for the isolation of the inert gas argon. He is the only Old Harrovian to achieve this award. Rayleigh had three sons, the eldest of whom, Robert John Strutt, succeeded him at his death on 30th June, 1919. The name of Rayleigh in science continued through the Fourth Baron who became Professor of Physics at Imperial College of Science and Technology, London. However, the work of his father is far more famous and few physicists, and no Old Harrovians, have had quite as dramatic and lasting effect on the world of science.
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