260 



NA TURE 



\July 12, 1883 



influenced by the presiure of the ends of the post against the 

 upper and lower plates ; (3) that Ilelmholtz is right, at least so 

 far that the leg of the bridge under the fourth or G string 

 has much more power than the other in setting the belly into 

 vibration. 



The usual way of investigating vibrations by the scattering of 

 sand over the surface of the agitated body is difficult of appli- 

 catim to the violin, on account of the curved form of the upper 

 and lower pUtes. I found a convenient method to be by the 

 use of what I may call a touch-rod. It consists of a small round 

 stick of straight-grained deal a few inches long ; the forefinger 

 is placed on one end, and the other end is put lightly in contact 

 with the vibrating surface. The finger soons becomes very sen- 

 sitive to small differences of agitation transmitted by the rod. 



The experiments were made on a strongly made modern violin, 

 and in some cases repeated on a fine violin by Stradiuarius in 

 the possession of the writer. 



The sand method, and also the touch-rod, showed that the 

 position of maximum vibration of the belly is close to the foot 

 of the bridge under the fourth string. The place of least vibra- 

 tion is exactly over the top of the sound-post behind the other 

 foot of the bridge. The back is strongly agitated, the vibra- 

 tions being least powerfully felt where the sound-post rests, 

 which is at nearly the thickest part of the back. These effects 

 were very satisfactorily observed on a violoncello, where the 

 phenomena are on a larger scale. 



When the sound post was removed from the violin the large 

 difference of the amount of vibration on the two sides of the 

 belly was no longer present, the belly was about equally strongly 

 agitated on both sides, making allowance for the string which 

 was bowed. The tone became very poor and thin, as is well 

 known to be the case when the sound-post is removed. The 

 vibration of the back was now very feeble as compared with its 

 vibration when the sound-post was present, a circum tance in 

 favour of the view that the sound-post conveys vibrations to the 

 back. 



A clamp of wood was prepared which could be so placed on the 

 violin as to connect by an arch of wood outside the violin the 

 place of the belly behind the bridge where the top of the sound- 

 post presses with the plice of the back where it rests. It was 

 expected that the woolen arch would restore to some extent the 

 connection of belly and back which was broken by the removal 

 of the post, and carry, though imperfectly, vibrations from the 

 upper plate to the back. 



When this clamp was put on, the poor and thin sound was 

 altered to the fuller character of tone which belongs to the 

 violin when the sound-post is in its place. On testing the con- 

 dition of the back its normal state of vibration was found to be 

 in a large degree restored. If, while the strings were being 

 bowed, the clamp was suddenly removed, the tone at the same 

 moment fell to its poor character, and the vibration of the back 

 as instantly diminished. 



It was further observed that, if the upper part of the clamp 

 pressed upon the belly without the lower part coming into 

 contact with the back, the tone is altered in the direction as 

 when the sound pjst was present, but it was not until the lower 

 part of the clamp was in contact with the back that the normal 

 character of the tone was fully restored. A similar effect to 

 that resulting from the pressing of one end of the clamp only 

 was produced by firmly placing one end of a woiden rod at this 

 part of the belly. This effect may be due to the setting-up in 

 the belly, by pressure at this part, of the peculiar nodal arrange- 

 ment which the post produces when in its place. 1 



There could be no doubt that vibrations were carried by the 

 clamp, for the lower end was powerfully agitate! when the 

 upper end rested upon the belly. If the sole function of the 

 sound-post is to serve as a firm prop for the foot of the bridge, 

 it should fulfil this condition most fully when placed under the 

 foot of the bridge. In this position of the sound-post, however, 

 as is well known, the tone is much injured. 



In order to separate that part of the function of the sound- 

 post which serves as a support from the further function it may 

 posses; as a transmitter of vibrations, it was desirable to intro- 

 duce such alterations in the structure of the sound-post as would 



According to Daguin some similar experiments were made by Savart, 

 but I have failed to find them in those of his papers to which I have had 

 access. " On peut la (fame) mettre en dehors, en l'appuyant a une espece 

 d'arcade dont on colle les pieds de chaque cote - dn violin. ... On peut la 

 remplacer^par la pression d'un psids convenable appuye' sur la table supc- 

 rienre.' " Savart a conclu de la que Tame a pour effet de rendre normales 

 les vibrations de la table. . . . "— "Traite de Physique," tome i. p. 575. 



emble it to retain its supporting power, and yet greatly modify 

 and, if possible, stop its po-ver of transmitting vibrations. A 

 sound-post was made in which about half an inch of the middle 

 was cut out, and a piece of lead inserted, also a sound-post iu 

 which in-tead of lead sealing-wax was put in. The effect of these 

 compound posts which retained uninjured their prop power was 

 to modify greatly the quality of the tone, but not to diminish its 

 quantity in any marked degree, a result in favour of the view 

 that the character or the wood of which the post is made does 

 influence the tune, and that vibration is transmitted by the post. 

 As these compound posts could transmit vibrations freely, it was 

 desirable to contrive a post which would not carry vibrations and 

 yet form a firm prop. A po»t was made with a piece of hard 

 indiarubber inserted in the middle, but this post was found by 

 experiment with a tuning-fork to transmit vibrations to some 

 extent. Other materials were tried without success. A post 

 capped at each end with pieces of sheet vulcanised rubber 

 stopped almost completely the sound of a tuning-fork when the 

 foot of the fork rested on the rubber over one end of the post, 

 while the other end equally protected with rubber rested on a 

 body capable of reinforcing the sound of the fork. This rubber- 

 capped post was firmly fixed in position in the violin, so that it 

 would be able to support fairly well the belly and foot of the 

 bridge, and yet not be able to carry vibration ; unfortunately it 

 does not seem pos.-ible, from the nature of things, to have a rigid 

 prop which does not transmit vibrations, but this post, with thin 

 sheet rubber at the ends firmly forced into po-ition, must have 

 been fairly efficient in its supporting power. The effect nn the 

 tone was about the same as when the sound-post was removed. 

 When the wooden clamp was put on, then the normal tone 

 returned, and the back vibrated strongly. 



These experiments appear to show that the sound-post is more 

 than a prop, and that, besides its other functions, it does transmit 

 vibrations to the back in addition to those which are conveyed 

 through the sides. 



Experim?nts with sand and the touch-rod appear to me to 

 show that Helmholtz's statement is too absolute when he says 

 "it is unly the other leg of the bridge which agitates the elastic 

 wooden plates." Undoubtedly it is the fourth string foot of the 

 bridge which is the more powerful in agitating the upper plate, 

 but the other foot appears to me also to have an influence. 

 When the post is placed exactly under the foot of the bridge, 

 then the belly on this side is almost without vibration ; if the 

 post is absent, then this f 'Ot appears to agitate its own side of 

 the belly as strongly as the other foot. As there is no post on the 

 fourth string side of the fiddle, that foot stands in a position most 

 favourable for setting up vibrations in the belly, being nearly 

 half wav between the supports of the belly at the tail and the 



neck end of the violin. The other side of the belly, on the first 

 or E string side, where the other foot of the bridge rests, is 

 divided into two parts by the damping effect of the end of the 

 sound-post, namely, the part a and the part b. It is obvious that 



this foot of the bridge is unfavourably placed for setting the part 

 of the belly, b, into vibration, since it is so far from its central 

 mobile part. On the other hand, its position is favourable for a 

 portion of its energy of vibration t:> be transmitted through the 

 post to the back. 



Practically very small differences of position of the top of the 

 post behind the foot of the bridge are found to alter largely the 

 character of the tone of the fiddle, and in the case of fine instru- 

 ments the setting of the post is an operation demanding much 

 care and judgment. The explanation lies probably in the cir 

 c umstance that a small difference in the position of the post will 



