208 



NATURE 



[June 28, 1888 



DIFFRACTION OF SOUND. 1 



r pHE interest of the subject which I propose to bring before 

 •*■ you this evening turns principally upon the connection or 

 analogy between light and sound. It has been known for a 

 very long time that sound is a vibration ; and everyone here 

 knows that light is a vibration also. The last piece of know- 

 ledge, however, was not arrived at so easily as the first ; and 

 one of the difficulties which retarded the acceptance of the view 

 that light is a vibration was that in some respects the analogy 

 between light and sound seemed to be less perfect than it should 

 be. At the present time many of the students at our schools 

 and universities can tell glibly all about it ; yet this difficulty is 

 one not to be despised, for it exercised a determining influence 

 over the great mind of Newton. Newton, it would seem, 

 definitely rejected the wave-theory of light on the ground that 

 according to such a theory light would turn round the corners of 

 obstacles, and so abolish shadows, in the way that sound is 

 generally supposed to do. The fact that this difficulty seemed 

 to Newton to be insuperable is, from the point of view of the 

 advancement of science, very encouraging. The difficulty which 

 stopped Newton two centuries ago is no difficulty now. It is 

 well known that the question depends upon the relative wave- 

 lengths in the two cases. Light-shadows are sharp under 

 ordinary circumstances, because the wave-length of light is so 

 small ; sound-shadows are usually of a diffused character, because 

 the wave-length of sound is so great. The gap between the two 

 is enormous. I need hardly remind you that the wave-length of 

 C in the middle of the musical scale is about 4 feet. The 

 wave-length of the light with which we are usually concerned, 

 the light towards the middle of the spectrum, is about the forty- 

 thousandth of an inch. The result is that an obstacle which is 

 immensely large for light may be very small for sound, and will 

 therefore behave in a different manner. 



That light-shadows are sharp is a familiar fact, but as I can 

 prove it in a moment I will do so. We have here light from the 

 electric arc thrown on the screen ; and if I hold up my hand 

 thus we have a sharp shadow at any moderate distance, which 

 shadow can be made sharper still by diminishing the source of 

 light. Sound-shadows, as I have said, are not often sharp ; but 

 1 believe that they are sharper than is usually supposed, the 

 reason being that when we pass into a sound-shadow — when, for 

 example, we pass into the shade of a large obstacle, such as a 

 building — it requires some little time to effect the transition, 

 and the consequence is that we cannot make a very ready 

 comparison between the intensity of the sound before we enter 

 and its diminution afterwards. When the comparison is made 

 under more favourable conditions, the result is often better than 

 would have been expected. It is, of course, impossible to 

 perform experiments with such obstacles before an audience, and 

 the shadows which I propose to show you to-night are on a 

 much smaller scale. I shall take advantage of the sensitiveness 

 of a flame such as Professor Tyndall has often used here — a 

 flame sensitive to the waves produced by notes so exceedingly 

 high as to be inaudible to the human ear. In fact, all the 

 sounds with which I shall deal to-night will be inaudible to the 

 audience. I hope that no quibbler will object that they are 

 therefore not sounds : they are in every respect analogous 

 to the vibrations which produce the ordinary sensations of 

 hearing. 



I will now start the sensitive flame. We must adjust it to a 

 reasonable degree of sensitiveness. I need scarcely explain the 

 mechanism of these flames, which you know are fed from a 

 special gas-holder supplying gas at a high pressure. When the 

 pressure is too high, the flame flares on its own account (as this 

 one is doing now), independently of external sound. When 

 the pressure is somewhat diminished, but not too much so — 

 when the flame " stands on the brink of the precipice" were, I 

 think, Tyndall's words — the sound pushes it over, and causes it 

 to flare ; whereas, in the absence of such sound, it would remain 

 erect and unaffected. Now, I believe, the flame is flaring under 

 the action of a very high note that I am producing here. That 

 can be tested in a moment by stopping the sound, and seeing 

 whether the flame recovers or not. It recovers now. What I 

 want to show you, however, is that the sound-shadows may be 

 very sharp. I will put my hand between the flame and the 

 source of sound, and you will see the difference. The flame is 

 at present flaring ; if I put my hand here, the flame recovers. 



1 Lecture delivered by Lord Rayleigh, F.R.S., at the Royal Institution, 

 on January 20, 1888. 



When the adjustment is correct, my hand is a sufficient obstacle 

 to throw a most conspicuous shadow. The flame is now in the 

 shadow of my hand, and it recovers its steadiness : I move my 

 hand up, the sound comes to the flame again, and it flares. 

 When the conditions are at their best, a very small obstacle is 

 sufficient to make the entire difference, and a sound-shadow may 

 be thrown across several feet from an obstacle as small as the 

 hand. The reason of the divergence from ordinary experience 

 here met with is, that while the hand is a fairly large obstacle 

 in comparison with the wave-length of the sound I am here 

 using, it would not be a sufficiently large obstacle in comparison 

 with the wave-lengths with which we have to do in ordinary 

 life and in music. 



Everything then turns upon the question of the wave-length. 

 The wave-length of the sound that I am using now is about half 

 an inch. That is its complete length, and it corresponds to a 

 note that would be very high indeed on the musical scale. The 

 wave-length of middle C being four feet, the C one octave above 

 that is two feet ; two octaves above, one foot ; three octaves above, 

 six inches ; four octaves, three inches ; five octaves, one and a 

 half inch ; six octaves, three-quarters of an inch ; between that 

 and the next octave, that is to say, between six and seven octaves 

 above middle C, is the pitch of the note that I was just now 

 using. There is no difficulty in determining what the wave-length 

 is. The method depends upon the properties of what are known 

 as stationary sonorous waves as opposed to progressive waves. 

 If a train of progressive waves are caused to impinge upon a 

 reflecting wall, there will be sent back or reflected in the reverse 

 direction a second set of waves, and the co-operation of these 

 two sets of waves produces one set or system of stationary waves ; 

 the distinction being that, whereas in the one set the places o f 

 greatest condensation are continually changing and passing 

 through every point, in the stationary waves there are definite 

 points for the places of greatest condensation (nodes), and others 

 distinct and definite (loops) for the places of greatest motion. 

 The places of greatest variation of density are the places of no 

 motion : the places of greatest motion are places of no variation 

 of density. By the operation of a reflector, such as this board, 

 we obtain a system of stationary waves, in which the nodes and 

 loops occupy given positions relatively to the board. 



You will observe that as I hold the board at different distances 

 behind, the flame rises and falls — I can hardly hold it still enough. 

 In one position the flame rises, further off it falls again ; and as 

 I move the board back the flame passes continually from the 

 position of the node — the place of no motion — to the loop or 

 place of greatest motion and no variation of pressure. As I move 

 back, the aspect of the flame changes ; and all these changes are 

 due to the reflection of the sound-waves by the reflector which I 

 am holding. The flame alternately ducks and rises, its behaviour 

 depending npon the different action of the nodes and loops. The 

 nodes occur at distances from the reflecting wall, which are even 

 multiples of the quarter of a wave-length ; the loops are, on the 

 other hand, at distances from the reflector which are odd 

 multiples, bisecting therefore the positions between the loops. I 

 will now show you that a very slight body is capable of acting as 

 a reflector. This is a screen of tissue-paper, and the effect will 

 be apparent when it is held behind the flame and the distances 

 are caused to vary. The flame goes up and down, showing that 

 a considerable proportion of the sonorous intensity incident upon 

 the paper screen is reflected back upon the flame ; otherwise the 

 exact position of the reflector would be of no moment. I have 

 here, however, a different sort of reflector. This is a glass plate 

 — I use glass so that those behind may see through it — and it 

 will slide upon a stand here arranged for it. When put in this 

 position the flame is very little affected : the place is what I call 

 a node — a place where there is great pressure variation, but no 

 vibratory velocity. If I move the glass back, the flame becomes 

 vigorously excited : that position is a loop. Move it back still 

 more, and the flame becomes fairly quiet ; but you see that as the 

 plate travels gradually along, the flame goes through these evolu- 

 tions as it occupies in succession the position of a node or the 

 position of a loop. The interest of this experiment for our 

 present purpose depends upon this — that the distances through 

 which the glass plate, acting as a reflector, must be successively 

 moved in order to pass the flame from a loop to the next loop, or 

 fiom a node to the consecutive node, is in each case half the 

 wave-length ; so that by measuring the space through which the 

 plate is thus withdrawn one has at once a measurement of the 

 wave-length, and consequently of the pitch of the sound, though 

 one cannot hear it. 



