Chapter 15: Sound

1. Chapter 15: Sound Barry Latham Physics: Principles & Problems Glencoe Science, 2005

2. 15.1:Properties and Detection of Sound • Sound Waves- pressure variation transmitted through matter • Speed of Sound- l=v/f • Physics Physlet I.18.2

3. Detection of Pressure Waves • Human 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

4. Perceiving Sound • Pitch- human perception of frequency • Approximately 20Hz-16,000Hz is the audible range • Loudness-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

5. The Doppler Effect • Doppler Shift- change in frequency due to motion between the sound source and receiver • Physics PhysletI.18.4 • 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

6. Doppler Effect • fd=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 • Physics Physlets E.18.5, P.18.7

7. 15.2 The Physics of Music • Sources 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.

8. Resonance in an Air Column • Closed-pipe resonator- tuning fork above a hollow tube • The tube can be adjusted so that it resonates with the tuning forks frequency • Loudest at the resonance point(s)- amplitudes are added together

9. Standing Pressure Wave • Open-pipe resonator- similar to closed-pipe, but not sealed at the other end • Sound wave reflects off of open end and is inverted

10. Closed vs Open Pipe Resonators

11. Resonance Frequencies • Closed-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.

12. Resonant Frequencies • Closed Pipe • Open Pipe

13. Resonance on Strings • Strings act as open-pipe resonators • First resonance is at l=L1, though, not l/2 • l=L1, 3l/2=L2, 2l=L3, etc.

14. Closed-Pipe Harmonics • For l1 in a closed-pipe instrument, that frequency is the fundamental frequency, f1 • Each additional odd-numbered resonance point is the next harmonic • l3f3, third harmonic • l5f5, fifth harmonic • l7f7, seventh harmonic

15. Open-Pipe Harmonics • For l1 in an open-pipe or stringed instrument, that frequency is the fundamental frequency, f1 • Each additional resonance point is the next harmonic • l2f2, second harmonic • l3f3, third harmonic • l4f4, fourth harmonic

16. Consonance, Dissonance & Beats • Dissonance- an unpleasant set of pitches played at the same time • Consonance- a pleasant set of pitches played at the same time (a chord) • Both experiences are culturally influenced • Physics Physlets E.18.1, E.18.2 • Beats- oscillation of amplitude due to two frequencies being slightly different • fbeat=|fA-fB| • Used to tune instruments- when the beats slow down and disappear, the instrument is in tune • Physics Physets E.18.3