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Sound What type of wave is sound?

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Honors Physics – Sound Notes

Sound

What type of wave is sound?





How do sound waves transmit energy?


How does sound radiate?




Draw a sound wave (transverse model):


What does sound need to be present?





Concept Question: In a science fiction movie, when a spaceship explodes, the vibrations from the sound nearly destroy a nearby spaceship. If you were the science consultant for the movie, what would your advice be for the producer?




Speed of a Sound Wave

What influences the speed of a sound wave?


Equation (speed of sound in a gas):

Practice Problem: What is the speed of sound in air at 10o C, -5 o C and room temperature (20 o C)?

Hearing Sounds – Frequency


Definition:


What is the range of audible frequencies?


Frequencies higher than the audible range are called…?


Examples:


Frequencies lower than the audible range are called…?


Examples:


Hearing Sounds – Amplitude

Definition:


What is the unit of measure for the loudness of a sound wave?


What is the relationship between sound intensity and distance?


Sources of Sound

What is resonance?


Define Octave:






Sources of Sound – String Instruments

What is the fundamental frequency?






Wavelength for the fundamental frequency:


Frequency for the fundamental frequency:


What happens to the pitch of a stringed instrument when the string length is shortened?





What affects the velocity of the wave on the string?




Equation of velocity of a wave on a string:

^ Concept Question: Why would different string instruments having the same length strings produce sounds of different pitches?




Concept Question: Vibrating string of string instruments is not very loud. What amplifies the sound produced by these vibrating strings and how is it done?





Practice Problem: The highest key on a piano corresponds to a frequency about 150 times that of the lowest key. If the string for the highest key is 5.0 cm long, how long would the string for the lowest note have to be if it had the same mass per unit length ratio and was under the same tension?


Sources of Sound – Wind Instruments


What produces sound in a wind instrument?


What are the features of an open pipe resonator?


Examples:


^ Open Pipe Resonator Properties:
Fundamental




2nd harmonic


3rd harmonic




How are properties of an open pipe related to the properties of a string instrument?

What are the features of a closed pipe resonator?




Examples:


^ Closed Pipe Resonator Properties:

Fundamental




3rd harmonic




5th harmonic

Practice Problem: An open pipe 25.0 cm long produces a second harmonic standing wave at 30o C. Determine the frequency of the sound in hertz. Draw a diagram of the second harmonic standing wave.







^ Practice Problem: How long do you need to make an organ pipe whose fundamental frequency is a C-sharp (273 Hz)? The pipe is closed on one end, and the speed of sound in air is 340 m/s.

a) Draw the fundamental.




b) Calculate the pipe length.




c) What is the wavelength and frequency of the 2nd overtone?




^ Practice Problem: Determine the frequency of the fourth harmonic produced by an organ pipe 0.125 m long which is open at both ends on a day when the speed of sound is 340 m/s. Draw the fourth harmonic wave in the open pipe.




Practice Problem: Determine the frequency of the fifth harmonic of the same organ pipe in the previous problem if it is closed at one end. Draw the fifth harmonic wave in the closed pipe.


Practice Problem: A pipe 0.500 m long is closed at one end. A guitar is placed near the open end of the tube and the string is plucked. The guitar string is 0.750 m long and has a mass of 0.0010 kg. The string vibrates in its second mode (second harmonic) and produces a third harmonic standing wave in the closed pipe. If the speed of sound is 340 m/s, determine the


  1. …frequency of the sound produced by the air column.




  1. …tension in the guitar string.



Sound Quality

What is the timbre or tone color of an instrument?


What is consonance?




What is dissonance?


Sound Quality – Discussion

What are some effects of ‘noise’? How can noise be controlled?


Interference of Sound Waves

In phase – Out of phase

What occurs when sound waves ‘in phase’ interfere?


What happens when sounds waves ‘out of phase’ interfere?


Practice Problem: Two loudspeakers are 1.0 m apart. A person is 4.0 m from one speaker. If both speakers emit a 1150 Hz sound at room temperature (20o C), how far must this person be from the second speaker in order to detect

  1. …constructive interference (in phase waves)?




  1. …destructive interference (out of phase waves)?



Interference of Sound Waves

Beats

Definition:




Equation:


Draw:


Practice Problem: A student strikes two tuning forks and hears 2 beats per second. He notes that 440 Hz is printed on one tuning fork. Determine the frequency of the other fork.





^ Practice Problem: A tuning fork produces a steady 400 Hz tone. When this tuning fork is struck and held near a vibrating guitar string, twenty beats are counted in five seconds. What are the possible frequencies produced by the guitar string?


Doppler Shift

Definition:


When source approaches observer, the pitch of the sound:


When source moves away from observer, the pitch of the sound:


Equation (moving toward):


Equation (moving away):


Practice Problem: A child whirls a toy at the end of a string with a velocity of 25 m/s. As the toy revolves, it makes a whistling sound of frequency 200 Hz. Assume the speed of sound in air is 340 m/s. What are the maximum and minimum frequencies heard by a stationary listener standing some distance away?


Practice Problem: A stationary source emits sound of frequency 600 Hz on a day the speed of sound is 340 m/s. An observer moves toward the sources of sound at 20.0 m/s. Determine the

  1. …frequency heard by the observer.




  1. …wavelength of the sound between the source and the observer.




  1. …frequency and wavelength for the observer moving away from the source.


Review Problem:

A
hollow tube of adjustable length, open at both ends, is held in midair as shown. A tuning fork with frequency 320 Hz vibrates at one end of the tube and causes the air in the tube to vibrate at its fundamental frequency. The speed of sound in the laboratory is 343 m/s.


a) Draw the fundamental standing wave inside the tube.





b) Determine the length of the tube that will support this fundamental frequency.






c) Determine the next higher frequency at which this air column would resonate. Draw the standing wave represented by this frequency. (Do not change the length of the tube.)





T
he tube is now submerged in a large, graduated cylinder filled with water. The tube is slowly raised out of the water and the same tuning fork, vibrating with frequency 320 Hz, is held a fixed distance from the top of the tube.


d) Determine the height h of the tube above the water when the air column resonates for the first time.


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