U8: Physics of Hearing

Hearing is the perception of sound

● It has important applications beyond hearing

○ Sounds above 20,000 Hz are ultrasound, and those below 20 Hz are infrasound

● The perception of frequency is called pitch

○ Ex: notes from an instrument have different pitches

● The perception of intensity is loudness

○ Phon is the unit used to express loudness

● We call our perception of these combinations of frequencies and intensities tone quality, or timbre of the sound

Sound is a disturbance of matter that is transmitted from its source outward

● Sound is a wave

○ Some energy transfers into the air, but a small part goes into compressing and expanding the surrounding air

There is a relationship between the speed of sound, its frequency, and its wavelength:

● 𝑣𝑤 = 𝑓𝜆

○ The higher the frequency, the smaller the wavelength

○ The same relationship is given for all waves

● In air, the speed of sound is related to air temperature

○ 𝑣w = (331 𝑚/𝑠)√(𝑇/273 𝐾)

The sound of a motorcycle buzzing by is an example of the Doppler effect

● It is an alteration in the observed frequency of a sound due to motion of either the source or the observer

○ The actual change in frequency due to relative motion of source and observer is called a Doppler shift

Sound interference and resonance have the same properties as defined for all waves

● In air columns, the lowest-frequency resonance is called the fundamental

● The resonant frequencies of a tube are:

○ 𝑓n = 𝑛(𝑣𝑤/4𝐿), 𝑛 = 1,3,5. .. for a tube closed at one end

○ 𝑓n = 𝑛(𝑣𝑤/2𝐿), 𝑛 = 1,2,3. .. for a tube open at both ends