•  As a sound wave moves through a medium, each par5cle of the medium vibrates at the same frequency. This is sensible since each par5cle vibrates due to the mo5on of its nearest neighbor. The first par5cle of the medium begins vibra5ng, at say 500 Hz, and begins to set the second par5cle into vibra5onal mo5on at the same frequency of 500 Hz. The second par5cle begins vibra5ng at 500 Hz and thus sets the third par5cle of the medium into vibra5onal mo5on at 500 Hz. •  The process con5nues throughout the medium; each par5cle vibrates at the same frequency. And of course the frequency at which each par5cle vibrates is the same as the frequency of the original source of the sound wave. Subsequently, a guitar string vibra5ng at 500 Hz will set the air par5cles in the room vibra5ng at the same frequency of 500 Hz, which carries a sound signal to the ear of a listener, which is detected as a 500 Hz sound wave. •  The ears of a human (and other animals) are sensi5ve detectors capable of detec5ng the fluctua5ons in air pressure that impinge upon the eardrum. The human ear is capable of detec5ng sound waves with a wide range of frequencies, ranging between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound. Humans are not alone in their ability to detect a wide range of frequencies. Dogs can detect frequencies as low as approximately 50 Hz and as high as 45 000 Hz. Cats can detect frequencies as low as approximately 45 Hz and as high as 85 000 Hz. Bats, being nocturnal creature, must rely on sound echoloca5on for naviga5on and hun5ng. Bats can detect frequencies as high as 120 000 Hz. Dolphins can detect frequencies as high as 200 000 Hz. While dogs, cats, bats, and dolphins have an unusual ability to detect ultrasound, an elephant possesses the unusual ability to detect infrasound, having an audible range from approximately 5 Hz to approximately 10 000 Hz. V = f λ Vs = 343 m/s f → λ→ 21.4m 10.9 m 5.4 m 2.7 m 1.4 m 0.7m 0.34m 0.17m .086m .043m .021m •  The sensa5on of a frequency is commonly referred to as the pitch of a sound. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave. When two sounds with a frequency difference of greater than 7 Hz are played simultaneously, most people are capable of detec5ng the presence of a complex wave pa[ern resul5ng from the interference and superposi5on of the two sound waves. Certain sound waves when played (and heard) simultaneously will produce a par5cularly pleasant sensa5on when heard, are said to be consonant. Such sound waves form the basis of intervals in music. For example, any two sounds whose frequencies make a 2:1 ra5o are said to be separated by an octave and result in a par5cularly pleasing sensa5on when heard. That is, two sound waves sound good when played together if one sound has twice the frequency of the other. Similarly two sounds with a frequency ra5o of 5:4 are said to be separated by an interval of a third; such sound waves also sound good when played together. Examples of other sound wave intervals and their respec5ve frequency ra5os are listed in the table below. Musical intervals
Interval Frequency Ra7o Examples Octave 2:1 512 Hz and 256 Hz Third 5:4 320 Hz and 256 Hz Fourth 4:3 342 Hz and 256 Hz Fi`h 3:2 384 Hz and 256 Hz