344 BUIiLETIN OF THE 



INVESTIGATIONS IN ACOUSTICS. 



During the last quarter of a century, among the many interests 

 which demanded and engaged his attention, Henry studied with 

 much care various y^henomena of acoustics, and added much to 

 our practical as well as theoretical knowledge of this important 

 instrumentality. In 1851, he read a communication before the 

 American Association, " On the Limit of Perceptibility of a direct 

 and reflected Sound," in which he gave as the result of experi- 

 mental observations, the subjective fact that a wall or other reflect- 

 ing surface if beyond the distance of about 35 feet from the ear, 

 or from the origin of the sound, gives a distinguishable echo from 

 the sound ; but that if the ear or the sounding agent be placed 

 within this distance, the reflected sound appears to blend com- 

 pletely with the original one. From a number of experiments, he 

 found that under the same circumstances, this limit of percepti- 

 bility did not vary more than a single foot ; but that under differ- 

 ing conditions the limit of distance ranged from 30 to 40 feet, 

 (equivalent to a difference of from 60 to 80 feet of sound travel,) 

 depending partly on the sharpness or clearness of the sound, and 

 partly on the pitch or the length of the soniferous v/ave, which 

 affected the amount of overlapping of the two series. These re- 

 sults imply a duration of acoustic impression on the ear of about 

 one-sixteenth of a second; serving to show that 16 vibrations to 

 the second must be about the lower limit of a recognizable musi- 

 cal tone.* As applied to lecture-rooms, he pointed out that the 

 ceiling should not be more than about thirty feet high, within which 

 elevation, a smooth ceiling would tend to re-inforce the sound of 

 a speaker's voice, f 



Many experiments were afterward made on the resonance of dif- 

 ferent materials, by means of tuning forks. While a tuning fork 

 suspended by a fine thread continued to vibrate for upward of four 

 minutes with scarcely any appreciable sound, if placed in contact 

 with the top of a pine table, the same vibration continued but 

 ten seconds, but gave a loud full tone. On a marble topped 

 table the sound was much more feeble, and the vibration continued 

 nearly two minutes. While the tuning fork against a brick wall 

 gave a feeble tone continuing for 88 seconds, against a lath and 

 plaster partition it gave a sound considerably louder but continu- 



* This does not seem to agree with results obtained by Savart some 

 twenty years previously; who concluded from observations with the siren, 

 " that sounds are distinctly perceptible, and even strong when composed 

 of no more than eight vibrations in a second." (Rev. Encycl. July, 1832. 

 Quoted in Sill. Am. Jour. Sci. for 1832, vol. xxii. p. 374.) This latter de- 

 termination is somewhat difficult to reconcile with ordinary observations, 

 as it is certain that intervals of one-eighth of a second would give a very 

 appreciable rattle to almost every ear. 



t Proceed. Am. Assoc. Cincinnati, May, 1851, pp. 42, 43. 



118 



