Chapter 15 Sound

If a tree falls in the forest and no one is there to hear it, does it make a sound?

A sound wave is a pressure variation that istransmitted through matter.

Sound is a longitudinal and a mechanical wave

Chapter 15 Sound

The speed of sound in air at 0º C is 330 m/sand increases at .6 m/s for each degree increasein temperature.

In general, the speed of sound is faster in solidsand in liquids because the molecules are closertogether.

The frequency and the wavelength are relatedby the equation v = f.

Chapter 15 Sound

Loudness: The loudness of a sound depends primarily on the amplitude of the pressure wave.Loudness is measured in decibels.

Frequency: The number of cycles per second.The human ear can detect frequencies between20 and 20,000 Hz.

Frequency and loudness are not related.

Chapter 15 Sound

Find the frequency and the period of a sound wave moving in air at room temperature (20º C)with a wavelength of .8 m.

v = f or f = v/f = (330 m/s + (.6 m/s)(20))/.8 m

f = 342 m/s /.8 m= 427.5 Hz

Chapter 15 Sound

If a sound is made and hear the echo from a wall 3 seconds later, how far away is the wall?The temperature is 30º C.

d = vt

d = (330 m/s + (.6 m/s)(30) ( 1.5 s)

d = (348 m/s)(1.5 s) = 522 m

Chapter 15 Sound

Doppler Shift: A change in sound frequencydue to the relative motion of either the source or the detector.

Demonstration

Chapter 15 Sound

Chapter 15 Sound

Resonance: When the frequency of a soundmatches the natural frequency of an object.

Animation

Chapter 15 Sound

Chapter 15 Sound

Raising and lowering thetube changes the length ofthe air column. When theair column is in resonancewith the tuning fork, the sound is the loudest,

Chapter 15 Sound

Musical Instruments

•String -Vibration of a string resonate a sounding board.•Brass - Vibration of lips•Reed - Vibration of a column of air•Percussion - Vibration of an object, vibrates a column of

air.

Chapter 15 Sound

Columns of /4, 3/4, 5/4, … will all be in resonance with a tuning fork. Demonstration

Chapter 15 Sound

Columns of /2, , 3/2, 2, … will all be in resonance with a tuning fork.

Chapter 15 Sound

A tuning fork with a frequency of 392 Hzis found to cause resonances in an air columnSpaced by 44.3 cm. The air temperature is 27º C. Find the velocity of sound in air at thattemperature.

f = 392 Hzl = 44.3 cm

Resonances are spaced by one-halfwavelength so = 2l

V = f = f(2l) = (392 Hz)(.866m) = 347 m/s

Chapter 15 Sound

The frequency of a tuning fork is unknown. A student uses an air column at 27°C and findsresonances spaced by 39.2 cm. What is thefrequency of the tuning fork?

v = 347 m/sl = 39.2 cm

Resonances are spaced by one-halfwavelength so = 2l

F=v/ = 347m/s/.784 m = 443 Hz

Chapter 15 Sound

Higher frequencies which areOdd-number multiples of the fundamental frequency arecalled Harmonics.

Chapter 15 SoundThe ear consists of three basic parts -the outer ear, the middle ear, and the inner ear.Each part of the ear serves a specific purpose in the task of detecting and interpreting sound.The outer ear serves to collect and channel sound to the middle ear. The middle ear serves to transform the energy of a sound wave into the internal vibrationsof the bone structure of the middle ear and ultimatelytransform these vibrations into a compressional wave inthe inner ear. The inner ear serves to transform the energy of a compressional wave within the inner ear fluid into nerve impulses which can be transmitted to the brain.

Chapter 15 Sound

Chapter 15 Sound

Sound Quality•Chord Several pitches played together.•Dissonance An unpleasant set of pitches.•Consonance A pleasant set of pitches.•Octave The interval between two frequencies with a ratio of 2:1

Pythagoras noted that pleasing sounds occurred when the strings have lengths in smallwhole-number ratios, 1:2, 2:3, etc.

Chapter 15 Sound

Chapter 15 Sound

Chapter 15 Sound

Beat Notes When two frequencies are close together interfere to produce high and low sounds. Fbeat= |fa - fb|

Beats