Chapter 26Chapter 26

Sound & Music

Sound …Sound …...a longitudinal wave in air

caused by a vibrating object

Origin of SoundOrigin of Sound

Infrasonic frequencies < 20 Hz

Ultrasonicfrequencies > 20,000 Hz

Human hearing range frequencies between 20 Hz and 20,000 Hz

Audible FrequenciesAudible FrequenciesSound waves with frequencies between 20 Hz

and 20 kHz 200 Hz 500 Hz 1000 Hz 2000 Hz 5000 Hz

wiki SoundsUltrasound

Nature of Sound in AirNature of Sound in AirSound requires a medium.

• solid, liquid or gas

Sound waves have compression and rarefaction regions.

Speed of Sound in AirSpeed of Sound in Air331 meters/second

760 miles/hour

Mach 1

What is the approximate distance of a thunderstorm when you note a 3

second delay between the flash of the lightning and the sound of the

thunder?

Answer: 3 seconds 331 m/s

= 993 meters

AcousticsAcoustics... ...the study of sound properties.

When a sound wave strikes a surface it can be.…(a) reflected.(b) transmitted.(c) absorbed.(d) all of these.

Reflection of SoundReflection of Sounde.g. an echo

Reverberation - re-echoed sound, multiple reflections of sound waves from walls

Compare reflections from a hard wall with that from a carpet wall.

Refraction of SoundRefraction of Sound

Refraction - the bending of a wave

Sound waves bend toward cooler air.

Forced Vibrations...Forced Vibrations...…the setting up of vibrations in an object by a

vibrating force.

Examples of Forced Vibration:• A tuning fork touching a wood surface• Sounding boards for stringed instruments• Matching tuning fork boxes

During forced vibration sound is intensified because a larger surface area is available to

vibrate air molecules.

Natural Frequency...Natural Frequency...…the frequency at which an elastic object naturally tends to vibrate.

At this frequency, a minimum energy is required to produce a forced vibration.

The natural frequency of a body depends on its elasticity and its shape.

Natural Frequency ExamplesNatural Frequency Examples

Dropping Aluminum Rods

Ringing Small and Large Bells

Xylophone

Rubbing a Wine Glass

Mass on a Spring

ResonanceResonance...…is the result of forced vibrations in a body

when the applied frequency matches the natural frequency of the body.

The resulting vibration has a high amplitude and can destroy the body that is vibrating.

Examples of ResonanceExamples of Resonance• Breaking a wine glass using sound• Mass on a spring at resonance• A singing rod caused by forced vibration • A tuning fork exciting a guitar string• In 1940, the Tacoma Narrows Bridge

was destroyed by wind-generated resonance.

Resonance allows energy to be transferred to a

vibrating object efficiently if the energy is delivered at

the natural frequency of vibration.

Sound InterferenceSound InterferenceOverlapping crests of a wave will

result in an increased amplitude.

Overlapping a crest and a trough results in a decrease in amplitude.

Beats - the periodic variation in loudness of two sounds played together

The beat frequency is equal to the difference in the frequency of the two sounds.

What is the beat frequency when a 262 Hz and a 266 Hz tuning fork are sounded together?

400 Hz and 403 Hz400 Hz and 410 Hz

Radio Broadcasts• AM - Amplitude Modulation

• 535 kHz to 1605 kHz

• FM - Frequency Modulation• 88 MHz to 108 MHz

• Modulation - an impression of the sound wave on a higher frequency radio wave

Noise Versus MusicWhat is the difference between noise and

music?

Pitch...… is the "highness" or "lowness" of a tone.

Pitch corresponds to frequency.

Concert A on the Musical Scale has a frequency of 440 Hertz.

Major ScaleMajor Scale

Letter Frequency FrequencyNote Name (Hz) ratio Interval

do C 264 9/8 Wholere D 297 10/9 Wholemi E 330 16/15 Halffa F 352 9/8 Wholesol G 396 10/9 Wholela A 440 9/8 Wholeti B 495 16/15 Halfdo C 528

Sound Intensity and LoudnessSound Intensity and Loudness

Intensity of Sound refers to the amplitude of the

pressure variations in the sound wave

LoudnessLoudness•The physiological sensation directly related to the sound intensity

•Measured in bels

•(10 bels = 1 decibels)

LoudnessLoudness• A sound of 10 decibels is or 101 or 10 times as

intense as 0 decibels.

• 20 decibels is 102 or 100 times the intensity 0 decibels.

• How much more intense is sound at 100 dB than sound at 50 dB?

• Answer: 100000 times

Source of Sound Loudness (db)

Threshold of Hearing 0

Conversation 60

Ear Damage Begins 85

Amplified Music 110

Jet Airplane at 30 meters 140

Common Sound IntensitiesSource of Sound Intensity (W/m2) Sound Level (db)

Jet 30 m away 102 140

Air-raid siren, nearby 1 120

Disco music, amplified 10-1 110

Riveter 10-3 90

Busy street traffic 10-5 70

Conversation in home 10-6 60

Quiet radio in home 10-8 40

Whisper 10-10 20

Rustle of leaves 10-11 10

Threshold of hearing 10-12 0

Quality...…is the characteristic sound that allows us to distinguish between two musical instruments.

Partial Tones - one of the many frequencies present in a complex tone

Fundamental Frequency• the lowest frequency of vibration

• a.k.a. the first harmonic

Harmonic• a partial tone that is an integer

multiple of the fundamental frequency

Same Note - Different Instrument

Harmonics

• Harmonics on a Guitar String

• Harmonics in an Organ Pipe• Open on one end, close on the other

• Open on both ends

Fourier Analysis...…is a mathematical method that

will resolve any periodic wave form into a series of simple sine waves.

http://www.phys.unsw.edu.au/~jw/strings.html

COMPACT DISCSCOMPACT DISCS

Phonograph players give analog signals.

Digital signal is in binary code.

CD has flats and pits and is sampled

44,100 times per second.

1. The quality of a musical note can be changed by simply playing the note on a different instrument.

(a) True(b) False

2. The three characteristics of a musical tone are loudness,

quality, and timbre. (a) True

(b) False

3. Frequencies of sound that are too high for the human ear to hear are called...

(a) faster than the speed of sound

(b) supersonic

(c) infrasonic

(d) ultrasonic

(e) subsonic

(d) ultrasonic

4. For the same temperature for air, does sound travel faster in humid Galveston or in dry El Paso?

(a) Galveston

(b) El Paso

(c) same speed in either city

(a) Galveston

5. The bending of sound through air of uneven temperature is called(a) reflection

(b) refraction

(c) interference

(d) reverberation

(e) resonance

(b) refraction

6. Lightning is seen, then ten seconds later thunder is heard. Approximately, how far away in meters is the thunder cloud? (a) 10,500 m

(b) 5280 m

(c) 1050 m

(d) 3400 m

(e) 680 m

(d) 3400 m

7. A 250 Hz tuning fork and a 260 Hz tuning fork are vibrating near each other. How many beats per second are heard?(a) 255(b) 510(c) 10(d) 250(e) 260

(c) 10