A SELECTIVE HOT-WIRE MICROPHONE. 
423 
§ 9. Some Observations of Distribution and Intensity of Sound made with the 
H ot- Wire Microphone. 
The applications of the microphone to sound measurements are sufficiently numerous 
to justify a short description. It is quite obvious that the apparatus is not adapted to 
the measurement of the total quantity of a medley of sounds, since the microphones 
are selective, and the pitch or wave-length, of the sound measured must therefore always 
be given. 
Full advantage can be taken of the two alternative methods of using the microphone 
for sound measurement. In general, it should be laid down that the Wheatstone’s 
Bridge method should be adopted for cases in which the sound distribution can be altered 
by keeping the microphone fixed in space and changing the position of the source of 
sound or by movement of any screen, reflector, trumpet, etc., while source and microphone 
* 
occupy given positions. 
If, however, the microphone has to be attached to some moving object the amplifier 
method has to be employed, records of amplitude being given by a vibration galvano¬ 
meter, or in certain cases by rectifying the current and using a reflecting galvanometer. 
The simplest experiment to perform is that of observing the distribution of intensity 
of sound in a closed room. By the Wheatstone’s Bridge method the microphone can 
occupy some fixed position while a steady source of sound such as a tuning-fork is 
carried about the room. 
The effects are sufficiently strong for a pivoted galvanometer to be employed for 
observation. 
A variant of this experiment is to keep the source of sound in one place, and observe 
the effects of moving either one’s self, of altering the position of furniture in the room, 
or of opening of doors or windows. It is quite easy to vary these arrangements in such 
a manner as to reduce the intensity of sound as recorded by the microphone from a 
maximum value to zero. The position of nodes and antinodes in the room can be 
investigated by moving the microphone and employing the amplifier method with tele¬ 
phones or vibration galvanometer. The results obtained are sufficiently Striking to 
condemn any method of sound, measurement in a closed room—the mere movement of 
the observer being sufficient to vitiate any experimental results. Methods of ear 
testing, which so commonly employ tuning-forks, are equally unsatisfactory. All 
measurements must therefore be made in the open-air and full precautions must be 
taken to avoid obstacles presenting reflecting surfaces. 
Moreover, open-air work can only be performed under exceptionally calm conditions 
such as exist on certain nights or during a fog. 
These phenomena are of far reaching importance in architectural acoustics. It 
seems evident that in any room or concert hall there is a considerable difference between 
the musical piece as rendered by instruments and the sounds which the audience observes ; 
and it also follows that various members of the audience hear the same rendering 
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