Properties and Detection of Sound

1. State that sound waves are a longitudinal pressure wave.
2. State that the speed of sound depends on the medium it is transmitted through.

   high density: slows a wave down because the media the wave passes through has more inertia

   high stiffness (unit of force per unit of deformation): speeds a wave up

   Media	Stiffness (bulk modulus of elasticity, N/m^2)	Density (kg/m^2)	Speed of Sound (m/s)
   air	5 (10^4)	1.2	340
   water	2 (10^9)	1000	1500
   steel	2 (10^11)	7850	6100

    

3. Explain why the wavelength of a sound is altered when it is transmitted through different media because the frequency is not. Since frequency (number of peaks per second) is constant, wave length is directly proportional to the speed of sound in a material as shown:

   l = (1/f) v

   where f = a constant

4. Explain the difference between hearing by bone transmitted sound vs. air transmitted sound.

   Historical Relevance: Although Beethoven was considered deaf, the problem with his hearing was primarily due to a loss of air transmission hearing. He affixed a metal rod to his piano so that he could press his head against it and still hear by bone conduction. Nevertheless, this would have distorted the sound he was able to hear. One can only wonder how this affected his compositions.

5. Explain why a noise transmitted in water sounds different (somewhat lower pitched) than the same noise transmitted in air. The wavelength of a sound traveling in water will be longer than a wavelength traveling in air.
6. Explain why audible sound could not be transmitted through outer space. (see Stokes' law of sound attenuation.)

   extreme low density of space: While there is some matter in outer space, the density is near zero causing sound to be almost instantly attenuated (reduced to essentially zero loudness).

   extremely low frequency mechanical waves are attenuated less: these extremely low frequency waves can travel longer distances but are far below human hearing levels.

7. Solve problems using the speed of sound. v = l f

   given any two of the quantities v, l, or f solve for the third

    

Homefun (formative/summative assessment): Read sections 14.1, practice problems 1, 3, 5 page 405

Formative Assessment: Physics Investigation

Follow up Questions

 

Perceiving Sound

8. State that with respect to air transmitted sounds, pitch depends on the frequency of the sound.

9. State that the range of human hearing is roughly from 20 to 16,000 Hz.

10. State that ultrasound is any sound with a frequency above the frequency range of human hearing.

11. State that loudness is measured using a nonlinear scale with a unit called the decibel (dB). Note that the perception of loudness also depends on the specific frequency.

    (sound level in dB) = 10 log (P1/Po)

    where

    Po = the power output of the most faintly heard sound

    P1 = the power output of the measured sound

12. Demonstrate mathematically that adding 2 sound of equal power, in other words doubling the total sound power, will raise the level of loudness by only 3 dB, a barely perceptible increase.

    Relevance: Quieting a loud noise is a very difficult process requiring a high level of engineering expertise.

     

13. State that a 10 dB increase in sound loudness is typically perceived as a doubling of loudness.

14. State that the pain level is at about 125 dB while while sustained noise levels at only about 90 dB can produce permanent hearing loss. Generally, permanent hearing damage is a function of the loudness and length of time of a sound. Very loud sounds do damage in much less time.

Relevance: Music playing in ear phones can easily exceed noise levels known to produce permanent hearing loss. Maximum loudness levels at rock concerts can reach 150 dB.

 

15. State that hearing protection generally comes in two forms ear plugs and ear muffs and explain why they can only reduce sound loudness by about 45 dB when used in combination. Hearing protection typically only reduces air transmitted sounds.

Homefun (formative/summative assessment) .

Doppler Effect

16. Describe the Doppler effect for both light and sound. In the diagrams shown below, visualize the waves as moving outward from the red dot at the center.

    f = f0[ |v - vr| / |v - vs| ] where: v = velocity of wave in the medium (air for sound) vr = velocity of the receiver of the wave vs= velocity of the source of the wave f0= frequency of the wave when vr= vs=0

    For moving source, wave length changes--shorter when moving toward, longer when moving away

    For moving receiver, velocity changes--faster when moving toward, longer when moving away

17. Calculate frequencies using the Doppler effect equation.

Relevance: The Doppler effect is used in numerous applications for measuring speed. These include radar systems used by traffic police, blood flow measurements and the measurement of the movements of stars and galaxies.

Homefun (formative/summative assessment): problems 6, 7, 8, page 409

Resonance

24. Explain the concept of resonance and state the conditions necessary for resonance to occur.
    • Energy is continuously added to an object in sync with one of its natural frequency and at a rate that is faster than the vibrational energy is converted into heat.
    • The amplitude of the object's vibration increases until the object breaks or begins emitting or loosing energy in some manner.

24. Define the term damping and state how it affects resonance. Damping is the process of doing work so that a vibration's mechanical energy is converted into heat. If the damping is high enough, an object will not resonate. The damping force that does work acts in the opposite direction of velocity.
25. Give examples of resonant systems.

    mechanical

    • swing--for large angles, the resonant frequency changes, hence limiting the amplitude
    • pendulum--for large angles, the resonant frequency changes, hence limiting the amplitude
    • spring and mass
    • pipe organ
    • bells

    electrical

    • antennae
    • LC circuit

26. Explain a simple way to experimentally evaluate an object's resonant frequency if it has little or no internal damping. Hit it with a hammer or in other words, give it a pulse of energy and analyze the frequencies of the resulting vibrations.
27. Note that fundamental frequency and first harmonic are interchangeable terms. An object's natural frequencies are the same thing as its harmonics.
28. Solve problems involving the fundamental and higher harmonic modes.

• stretched strings

• open pipes

• closed pipes

Relevance: Virtually all musical instruments work on the principles of resonance. Antennas are often designed to resonate as are the tuning and broadcasting electronics attached to them. In machine design, systems are often designed to avoid resonance. since the resulting vibrations can cause mechanical failures..

Homefun (formative/summative assessment): problems 6, 7, 8, page 409

Review

Formative Assessments:

1. Work review problems at the board

2. Work practice problems.

Metacognition Problem Solving Question: Can I still work the problems done in class, several hours or days later? Some amount of repetition on the exact same problems is necessary to lock in learning. It is often better to thoroughly understand a single example of a problem type than to work example after example understanding none of them completely.

Relevance: Good test preparation is essential to performance in physics class.

Homefun (formative/summative assessment): turn in on the day stapled to the back of the test.

Summative Assessment: Unit exam objectives 1-23