1 - Matawan-Aberdeen Regional School District



Characteristics of sound waves

a. Source of sound is vibrations

b. Have properties of other waves: amplitude, period, frequency, wavespeed

c. Be able to use the wave equation: [pic]

d. Must have a medium: Not vacuum

e. Speed a function of the medium

i. Higher density objects usually transmit sound at a higher speed

f. Speed of sound in air increases with increasing temperature

g. The frequency of the sound does not change its speed

1. Beats

a. Interference between two sounds with different frequencies

b. Goes from constructive to destructive to constructive

c. Know how to calculate the beat frequency: [pic]

2. Sonar, Ultrasound, Bat navigation, Echoes

a. Uses time of travel of sound pulse

b. Distance: [pic] - Sound travels there and back – or: [pic]

3. Doppler shift, Doppler effect

a. Frequency sounds different to observer if observer is moving or source is moving

b. An observer moving with the source of the sound will not hear any change

c. If getting closer – frequency increases

d. If getting further apart – frequency decreases

4. Perception

a. Frequency – Pitch

b. Intensity (Amplitude squared) – Loudness

5. Standing wave vs. traveling wave

6. Musical instruments

a. Instruments have resonances – all other frequencies die out quickly

b. A mode is a specific resonance – specific standing wave pattern

c. Lowest mode is called fundamental – also called first harmonic

d. Other resonances have frequencies that are multiples of the first harmonic

e. Nodes or antinodes relate to the displacement of air from its neutral position

f. String instruments – all integer multiples-both ends nodes

i. [pic]

g. Open tube resonators – all integer multiples-both ends antinodes

i. [pic]

h. Closed tube resonators – only odd integer multiples – node and antinode

i. [pic]

i. Octave- Multiple of factor of 2

7. How hearing works

a. Outer-Collects sounds

b. Middle ear - amplifies

c. Inner ear – converts sounds to electrical pulses

Practice Problems

1) Which of the following is a false statement?

A) Sound waves are longitudinal pressure waves. B) Sound can travel through a vacuum.

C) Light travels very much faster than sound. D) The transverse waves on a vibrating string are different from sound waves. E) "Pitch" (in music) and frequency have approximately the same meaning.

2) In order to produce beats, the two sound waves should have

A) the same amplitude. B) slightly different amplitudes. C) the same frequency. D) slightly different frequencies.

3) Compared to the velocity of a 400 Hz sound, the velocity of a 200 Hz sound through air is

A) twice as great. B) the same. C) one-half as great. D) none of the above

4) Which of the following increases as a sound becomes louder?

A) frequency B) wavelength C) amplitude D) period E) velocity

5) A music tuner uses a 550-Hz tuning fork to tune the frequency of a musical instrument. If the tuner hears a beat frequency of 2 Hz, what is the frequency of the instrument?

A) It must be 552 Hz. B) It must be 548 Hz. C) It could be either 552 Hz or 548 Hz. D) It is neither 552 Hz or 548 Hz.

6) The Doppler shift explains

A) why the siren on a police car changes its pitch as it races past us. B) how sonar works.

C) why a sound grows quieter as we move away from the source. D) the phenomenon of beats.

E) why it is that our hearing is best near 3000 Hz.

7) What is the length of the shortest pipe closed on one end that will have a fundamental frequency of 60 Hz on a day when the velocity of sound is 340 m/s? (Hint – start by determining the wavelength)

A) 1.24 m B) 1.42 m C) 2.14 m D) 4.12 m

8) Explain why the ratio of the frequency when you hit a tube and remove your hand quickly to when you hit a tube and leave your hand against the end is a factor of two.

9) Draw the standing wave pattern for the first, second, and third lowest modes for

a) a vibrating string

b) an open tube resonator

c) a closed tube resonator

10) The speed of sound in air is 340 m/s. If you stand on a cliff and clap your hands, how long does it take for the sound to echo back to you, if the nearest cliff is 6.8km away?

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Answers: 1.B 2.D 3.B 4.C 5.C 6.A 7.D 8.Remove quickly – open tube. Leave hand in place – closed tube. 10. 40s

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