ch13_cfrn


 * Chapter 13: Sound **

13-1: Sound Waves

The Production of Sound Waves
 * **Compression**- The region of a longitudinal wave in which the density and pressure are greater than normal
 * **Rarefaction**- The region of a longitudinal wave in which the density and pressure are less than normal
 * Sound waves are longitudinal

Characteristics of Sound Waves
 * Sound waves that the average human ear can hear are called **audible** sound waves.
 * They have frequencies between 20 and 20 000 Hz.
 * Sound waves with frequenices less than 20 Hz are called **infrasonic** waves.
 * Those above 20 000 Hz are called **ultrasonic** waves.
 * Frequency determines pitch.
 * Pitch- The perceived highness or low-ness of a sound, depending on the frequency of the sound waves.
 * Ultrasonic waves can produce images.
 * Speed of sound depends on the medium.
 * Sound waves propagate in three dimensions.
 * The circles that represent the centers of compression are called **wave fronts**.
 * The radial lines perpendicular to the wave fronts are called **rays**.
 * Any small portion of a spherical wave that is far from the source can be considered a **plane wave**.

The Doppler Effect
 * Relative motion creates a change in frequency.
 * **Doppler effect**- A frequency shift that is the result of relative motion between the source of waves and an observer.
 * The Doppler effect occurs whenever there is **relative motion** between the source of waves and an observer.

13-2: Sound Intensity and Resonance

Sound Intensity Forced Vibrations and Resonance
 * Intensity is the rate of energy flow through a given area.
 * **Intensity**- The rate at which energy flows through a unit area perpendicular to the direction of wave motion.
 * Because power, P, is deined as the rate of energy transfer, intensity can also be described in terms of power.
 * The SI unit for power is the watt, thus intensity has units of watts per square meter (W/m²).
 * Internsity of a spherical wave.
 * Intensity= (P/4**π**r²)
 * Intensity and frequency determine which sounds are audible.
 * **Threshold of hearing**- The softest sounds that can be heard by the average human ear occur at a frequency of about 1000 Hz and an intensity of 1.0 X 10¯¹² W/m².
 * **Threshold of pain-** The loudest sounds that the human ear can tolerate have an intensity of about 1.0 W/m².
 * Relative intensity is measure in decibles.
 * **Decibel level**- Relative intensity, determined by relating the intensity of a sound wave to the intensity at the threshold of hearing.
 * Vibration at the natural frequency produces resonance.
 * **Resonance**- A condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system.
 * The human ear transmits vibrations that cause nerve impluses.

13:3 Harmonic

Standing Waves On a Vibrating String Standing waves in an air column
 * **Fundamental frequency**- The lowest frequency of vibration of a standing wave.
 * Harmonics are integral multiples of the fundamental frequency.
 * **Harmonic series-** A series of frequencies that includes the fundamental frequency and integral multiples of the fundamental frequency.
 * Harmonic series of standing waves on a vibrating string:
 * Fn=n(V/2L) n= 1,2,3,...
 * If both ends of a pipe are open, all harmonics are present
 * Harmonic series of a pipe open at both ends:
 * Fn=n(V/2L) n= 1,2,3,...
 * If one end of a pipe is closed, only odd harmonics are present
 * Harmonic series of a pipe closed at one end:
 * Fn=n(V/4L) n= 1,3,5,...
 * Harmonics account for sound quality, or timbre.
 * **Timbre**- The quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities.
 * Notes that each consist of repeating patterns are said to be **periodic.**
 * Fundamental frequency determines pitch.
 * The frequency of the thirteenth note is exactly twice that of the first note, and togeather the 13 notes constitute an **octave**.

Beats
 * When two waves of **slightly** different frequencies interfere, the interference pattern varies in such way that a listener hears an alternation between loudness and softness.
 * **Beat**- Interference of waves of slightly different frequencies traveling in the same direction, perceived as a variation in loudness.
 * Sound waves at slightly different frequencies produce beats.
 * When the two waves are exactly opposite one another, they are said to be **out of phase**.
 * The number of beats per second corresponds to the difference between frequencies.
 * The expansion of the universe.
 * The eruption of the universe is often referred to as the big bang.
 * Experimental verification.


 * Practice Problems**:

1) Calculate the intensity of the sound waves from an electric guitar's amplifier at a distance of 5.0m when its power output is equal to each of the following values: a. 0.25 W b. 0.50 W c. 2.0 W 2) If the intensity of a person's voice is 4.6 X 10^ -7 W/m² at a distance of 2.0 m, how much sound power deos that person generate?

3) The power output of a tuba is 0.35 W. At what distance is the sound intensity of the tube 1.2 X 10^ -3 W/m²?

4) What is the fundamental frequency of a 0.20 m long organ pipe that is closed at one end, when the speed of sound in the pipe is 352 m/s?

5) What is the fundamental frequency of a guitar string when the speed of waves on the string is 115 m/s and the effective string lengths are as follows: a. 70.0 cm b. 50.0 cm c. 40.0 cm

Answers: 1. a. 8.0 X 10^ -4 W/m², b. 1.6 X 10^ -3 W/m², c. 6.4 X 10^ -3 W/m² 2. 2.3 X 10^ -5 W 3. 4.8 m 4. 440 Hz 5. a. 82.1 Hz, b. 115 Hz, c. 144 Hz


 * __Holt, Rinehart and Holt Physics__. New York: Holt, Rinehart & Winston, 2001.**