152 HAND-BOOK OF ACOUSTICS.
The method of using the Tonometer is similar to that above described, in the case of the standard forks. In the experienced hands of Mr. Ellis, the tuning-fork Tonometer has given results at least equal in accuracy to those obtained by means of any other counting instrument (see table on page 39).
When two series of sound waves of the same lengths and amplitudes, traverse simultaneously the same mass of air:
(1) If the waves of the one series are in exactly the same phase as those of the other, resultant waves are produced of the same length, but of double the amplitude ;
(2) If the waves of the one series are in exactly the opposite phase to those of the other, the result is,—no wave;
(3) If the waves of the one series are neither in the same phase as, nor in opposite phase to, those of the other, the amplitude of the resultant waves will be intermediate between the two limits given above, viz., no amplitude at all, i.e., silence, and twice the amplitude of the constituent waves.
When two simple sounds of the same pitch and intensity are simultaneously produced the result is
(1) Silence; or
(2) A sound of the same pitch as, but of four times the intensity of, either; or
(3) A sound, intermediate in intensity between these two limits,
according as the corresponding sound waves are in (1) opposite phase, (2) the same phase, or (3) any relative position intermediate between these two.
When two sounds differing slightly in pitch are simultaneously produced, the flow of sound is disturbed by regular recurring throbs or alternations in intensity, termed beats. These beats are due to the alternate coincidence and interference of the two systems of waves.
If the two tones be of equal intensity, the maximum intensity of the beat, will be four times that of either sound heard separately, the minimum intensity being zero.