# HANDBOOK OF ACOUSTICS - online book

### A complete view of Acoustical Science & its bearings on music, for musicians & music students.

 272 HAND-BOOK OF ACOUSTICS. 4. Describe the changes in form observed in the Lissajous' figure obtained by using two tuning-forks, the pitch of one being nearly the octave of the other. 5.   (a) How would you produce a flame sensitive to sound. (5) Give a method of measuring the wave-length of a note of very high pitch by means of a sensitive flame. Am. (a) Burn coal gas from a pin-hole burner at a pressure of ten inches of water. Adjust the supply of gas by the tap, so that the flame is just on the point of flaring. The flame, which is now from 15 to 20 inches long, is very sensitive to any movement of the air just above the burner. (b) Place the source of sound at some distance from a wall. The direct and reflected waves will then interfere, producing stationary nodes along a line drawn from the source of sound at right angles to the wall; and if the jet of the sensitive flame is moved along this line it will flare everywhere except at the nodes, the positions of which are thus discovered. The wave-length is obviously twice the distance between two consecutive nodes. 6.    Describe the construction and uses of Helmholtz' Double Syren. Am.—For construction, see pp. 33 and 34. Uses : 1st, To ascertain the vibration number of any note, by means of a counting apparatus not shown in fig. 22. 2nd, To demonstrate the exact vibration ratios of the most important musical intervals. 3rd, To illustrate and prove the fundamental facts concerning the theory of beats. Beats are produced in this instrument by rotating the handle of the wheel shown in fig. 22 at the top right hand side. When this wheel is rotated in one way the upper sound chest revolves in same direction as the discs, thus diminishing the vibration number of the upper syren; when rotated the other way, the upper sound chamber turns in the contrary direction to the discs, so increasing the vibration number of this same syren. Beats are produced in either case. Honour Examination in Music. 1 and 2. Not acoustical questions. 3. (a) Distinguish between Difference and Summation Tones.