Demonstrations. - Hold a toy music box in the hand and play it. Place it upon a table and play it. Hold it against the glass door of a bookcase and play it. Describe the differences in the effects.
When the music box is played upon the table, there is a greater volume of sound because the vibrations of the box are communicated to the table top and put it in motion although their rates of vibration are different.
Since all the tones played are increased in volume, it is evident that certain parts of the table were forced to vibrate in time with the tones given by the box. If the end of a vibrating tuning fork is pressed upon a table, the sound will increase in volume but will soon die out, showing that the energy of the fork is rapidly used up in making the table vibrate.
A thin tone is produced by any vibrating body that puts only a small quantity of air in motion. Fullness of tone can be secured only by putting a large mass of air in motion. For this reason, in all stringed instruments, as the violin,
FIG. 1. - Photographs of cylindrical sound waves made by Professor Arthur L. Foley and Mr. W. H. Souder, Indiana University. The source of sound was an electric spark, directly behind the black center and perpendicular to the page. Used by permission.
the guitar, and the piano, the strings are fastened to posts in a sounding board or wooden resonator box.
