Such an ideal sound source may be made to vibrate as slowly or as rapidly as one chooses.

One of the primary questions in audition is that of how slowly or how rapidly the source may vibrate and still be heard by the human ear.

The answer to this question has been determined experimentally.

When the sound source is vibrating very slowly, at fewer than 20 complete vibratory cycles per second, one hears each individual sound wave separately; the experience is not one of hearing a continuous tone.

As the frequency of vibration increases one begins to hear a continuous low tone much like that of the deepest bass pipe in an organ.

As frequency is further raised the tone becomes increasingly higher until, at a frequency of 1,000 cycles per second, one hears a note much like that of a soprano's high C.

This note, however, is still far from the upper limit of human hearing.

It is not until the frequency reaches the region of 20,000 cycles per second that the normal human ear gradually fails and then can no longer perceive the sound.

The ear is not equally sensitive to each of these frequencies of vibration.

It is most sensitive to an intermediate range of frequencies, 1,000 to 4,000 cycles per second.

Here, indeed, sensitivity is so extreme that any substantial increase would actually be disadvantageous; the ear would then detect the constant noise of molecules moving about at random in the air.

As frequency decreases or increases from this central range of sensitivity, however, the acuity of the ear gradually diminishes.

At the extremes of audible frequency a sound must be very powerful to be heard, so powerful that it is sometimes physically felt before it is heard.

Sound and Sensation.

As has been implied, pure tones have two independent dimensions:

1) frequency and
2) strength or intensity.

Frequency is measured by the number of complete vibratory cycles per second.

Intensity is measured by the magnitude of the pulsating pressure that sound waves exert against any surface they strike; intensity is usually expressed in terms of a logarithmic, relative unit, the decibel.

It must be remembered that the terms frequency and intensity are used only in reference to sound, the external, physical stimulus to audition.

The terminology applied in speaking of the sensation, the internal, subjective auditory experience, is somewhat different.

Here, the highness or lowness of a heard sound is referred to as pitch, and the strength of the sensation as loudness.

Generally speaking, pitch is the attribute of the sensation that varies closely with the frequency of the sound: more intense sounds seem louder.

These relationships are not, however, invariable and absolute, as is often assumed.

To some degree, pitch is affected by intensity and loudness is affected by frequency.