"284: Lord Rayleigh : Acoustical Notes. 



The general results of the experiments do not appear to me 

 to exclude the view that the comparatively feeble beats heard 

 under these conditions may be due to the passage of sound 

 from one ear to the other through the bones of the head or 

 perhaps through the Eustachian tube. 



Loudness of Double Sounds. 



Observations upon the double syrens (with separate horns) 

 used by the Trinity House have given the impression that as 

 heard from a distance the two syrens are no better than one, 

 even though the horns are parallel, and the observer situated 

 in the direction of the axis. Dr. TyndalFs experience was 

 similar. In his Report of 1874 he remarks (June 2), " There 

 was no sensible difference of intensity between the single horn 

 and the two horns ;" and again (June 10), " Subsequent com- 

 parative experiments even proved the sound of the two horns 

 to be more effective than that of the three." 



These conclusions are rather startling, suggesting the 

 query as to what then can be the use of multiplying pipes in 

 an organ or voices in a chorus. In order to clear the ground 

 a little, I have recently tried some small-scale experiments 

 with organ-pipes. 



Two stopped pipes of pitch about 256 were mounted near 

 the window of a room on the ground-floor. When the window 

 was open the sounds could be heard (over grass) to about 

 200 metres ; but when the window was closed the range was 

 much less. Some difficulty was experienced in getting equal 

 effects from the two pipes. According to the instructions of 

 the observer, one or other supply-pipe was more or less 

 throttled with wax. 



With approximate equality of intensities and with such 

 tuning that the beats were at the rate of about two per second, 

 the results were very distinct. The beats were much more 

 easily audible than either of the component sounds. Doubtless 

 part of the advantage was due to the contrast provided by the 

 silences ; but it was thought that, apart from this, the swell of 

 the beat was distinctly louder than either sound alone. 



The result of the experiment is, of course, just what was to 

 be expected from a mechanical point of view. According to 

 theory the intensity (reckoned according to energy propa- 

 gated) at the loudest part of the beat should be four times 

 that of the (equal) component sounds heard separately. 



In another set of experiments the pipes were mistuned until 

 the interval was about a minor third, no distinct beats being 

 audible. In this case the intensity of the compound sound 



