Amazing!! The demonstration is often repeated to assure that the same surprising results are observed. Only now the sound is being produced by the second tuning fork - the one which wasn't hit with the mallet. Then the tines of the tuning fork are grabbed to prevent their vibration and remarkably the sound of 256 Hz is still being heard. Surrounding air particles are set into vibrational motion at the same natural frequency of 256 Hz and every student in the classroom hears the sound. These vibrations set its sound box and the air inside the sound box vibrating at the same natural frequency of 256 Hz. Suppose the first tuning fork is struck with a rubber mallet and the tines begin vibrating at its natural frequency - 256 Hz. Neither of the tuning forks is vibrating. Suppose that a tuning fork is mounted on a sound box and set upon the table and suppose a second tuning fork/sound box system having the same natural frequency (say 256 Hz) is placed on the table near the first system. Now consider a related situation that resembles another common Physics demonstration. A louder sound is always produced when an accompanying object of greater surface area is forced into vibration at the same natural frequency. This principle of forced vibration explains why demonstration tuning forks are mounted on a sound box, why a commercial music box mechanism is mounted on a sounding board, why a guitar utilizes a sound box, and why a piano string is attached to a sounding board.

The vibrating whiteboard or overhead projector panel in turn forces surrounding air particles into vibrational motion and the result is an increase in the amplitude and thus loudness of the sound. The tuning fork forces surrounding glass (or vinyl) particles into vibrational motion. However, if the tuning fork is set upon the whiteboard panel or the glass panel of the overhead projector, the panel begins vibrating at the same natural frequency of the tuning fork. The sound produced by the tuning fork is barely audible to students in the back rows of the room. If the tuning fork is held in your hand and hit with a rubber mallet, a sound is produced as the tines of the tuning fork set surrounding air particles into vibrational motion. This same principle of a forced vibration is often demonstrated in a Physics classroom using a tuning fork. This causes an increase in the amplitude and thus loudness of the sound. In the case of the guitar string mounted to the sound box, the fact that the surface area of the sound box is greater than the surface area of the string means that more surrounding air particles will be forced into vibration. The tendency of one object to force another adjoining or interconnected object into vibrational motion is referred to as a forced vibration. The entire system (string, guitar, and enclosed air) begins vibrating and forces surrounding air particles into vibrational motion. The sound box in turn forces air particles inside the box into vibrational motion at the same natural frequency as the string. On the other hand, if the string is attached to the sound box of the guitar, the vibrating string is capable of forcing the sound box into vibrating at that same natural frequency. If you were to take a guitar string and stretch it to a given length and a given tightness and have a friend pluck it, you would hear a noise but the noise would not even be close in comparison to the loudness produced by an acoustic guitar.

This input of energy disturbs the particles and forces the object into vibrational motion - at its natural frequency. Whatever the case, a person or thing puts energy into the instrument by direct contact with it.

For instance, a guitar string is strummed or plucked a piano string is hit with a hammer when a pedal is played and the tines of a tuning fork are hit with a rubber mallet. Musical instruments and other objects are set into vibration at their natural frequency when a person hits, strikes, strums, plucks or somehow disturbs the object.

440 tuning fork