Chapter 15: SoundBarry Latham

Physics: Principles & ProblemsGlencoe Science, 2005

15.1:Properties and Detection of Sound

Sound Waves- pressure variation transmitted through matter

Speed of Sound- l=v/fPhET “Wave Interference”

Detection of Pressure WavesHuman Ear

Tympanic membrane vibrates at the same frequency as the pressure variation

Three tiny bones transfer motion to cochlea

Tiny hairs in cochlea are sensitive to vibration

Nerve cells send impulses to the brain

Ear Animation.swf

Perceiving SoundPitch- human perception of frequency

Approximately 20Hz-16,000Hz is the audible rangeLoudness-human perception of amplitude

Sound level- logarithmic scale of pressure variations

Decibel (dB)- unit of loudness 10x higher pressure represents an increase in 20dB Humans perceive an increase of 10dB as being twice as

loud

The Doppler EffectDoppler Shift- change in frequency due to

motion between the sound source and receiverPhysics Physlet I.18.4 (Doppler Effect.swf)Motion only depends on the objects getting closer

or further away. It doesn’t matter who does which.

Blue Shift- objects getting closer Source stationary, receiver moving towards source Receiver stationary, source moving towards receiver

Red Shift- objects getting further apart Source stationary, receiver moving away from source Receiver stationary, source moving away from receiver

Doppler Effectfd=fs(v-vd)/(v-vs)

fs=frequency of source(Hz)fd=detector (your ear?)v=velocity of sound (m/s)vs=velocity of source (m/s)vd=velocity of detector (m/s)

Takes into account direction, so be cautious with positive and negative signs

Super Mario Sound Barrier.swf

15.2 The Physics of MusicSources of Sound

Air rushing from lungs through vocal cords

Lips vibrating in the mouthpiece of a brass instrument

A wooden reed vibrating in a clarinet (woodwind)

Strings vibrating in a guitar, piano, harp, etc.

Resonance in an Air ColumnClosed-pipe resonator- tuning fork

above a hollow tubeThe tube can be adjusted so that it

resonates with the tuning forks frequency

Loudest at the resonance point(s)- amplitudes are added together

Standing Pressure WaveOpen-pipe resonator- similar to closed-pipe, but

not sealed at the other endSound wave reflects off of open end and is

inverted

Closed vs Open Pipe Resonators

Resonance FrequenciesClosed-pipe resonators- the shortest pipe length

that will resonate is l/4=L1

All additional resonances occur with the addition of l/2 to the pipe length

l/4=L1, 3l/4=L2, 5l/4=L3, 7l/4=L4, etc.

Open-pipe resonators- the shortest pipe length that will resonate is l/2=L1

All additional resonances occur with the addition of l/2 to the pipe length

l/2=L1, l=L2, 3l/2=L3, 2l=L4, etc.

Resonance tubes.swf

Resonant FrequenciesClosed

Pipe

Open Pipe

Resonance Tubes.swf

Resonance on StringsStrings act as open-pipe resonatorsFirst resonance is at l=L1, though, not l/2

l=L1, 3l/2=L2, 2l=L3, etc.

Closed-Pipe HarmonicsFor l1 in a closed-pipe instrument, that frequency is

the fundamental frequency, f1

Each additional odd-numbered resonance point is the next harmonicl3f3, third harmonicl5f5, fifth harmonicl7f7, seventh harmonic

Open-Pipe HarmonicsFor l1 in an open-pipe or stringed instrument, that

frequency is the fundamental frequency, f1

Each additional resonance point is the next harmonicl2f2, second harmonicl3f3, third harmonicl4f4, fourth harmonic

Consonance, Dissonance & BeatsDissonance- an unpleasant set of pitches played at

the same timeConsonance- a pleasant set of pitches played at the

same time (a chord)Both experiences are culturally influencedPhysics Physlets E.18.1, E.18.2

Beats- oscillation of amplitude due to two frequencies being slightly differentfbeat=|fA-fB|Used to tune instruments- when the beats slow down

and disappear, the instrument is in tune