SOUNDCHAPTER 12

All Sound Has 3 All Sound Has 3 Aspects…Aspects…

1.1. SourceSource2.2. EnergyEnergy

3.3. DetectedDetectedSound is Longitudinal Waves Sound is Longitudinal Waves (Compression Waves)(Compression Waves)Sound must have a medium.Sound must have a medium.Sound can’t travel in the absence Sound can’t travel in the absence of a mediumof a medium

Sound travels differently in different media– Hear under water– Along train tracks– Localize

Speed of Sound– Air 0° 331m/s– Air 20° 343m/s– Water 1440m/s– Iron 5000m/s

Generally if density is increased, speed is

increased!

- What about air 0° + 20°C ??

Table 12-1 Speed of Sound in various materials at 20°C

and 1atmMaterialMaterial Speed (m/s)Speed (m/s)Air

Air(0°C)

Helium

Hydrogen

Water

Seawater

Iron and Steel

Glass

Aluminum

Hardwood

343

331

1005

1300

1440

1560

approx 5000

approx 4500

approx 5100

approx 4000

LoudnessLoudness : Energy in a wave... Loosely : Energy in a wave... Loosely AmplitudeAmplitudePitch or FrequencyPitch or Frequency : # of Vibrations per : # of Vibrations per secondsecond

– High – FluteHigh – Flute– Low – TubaLow – Tuba

Humans can hear a range of frequenciesHumans can hear a range of frequencies• 20Hz to 20,000Hz – Humans20Hz to 20,000Hz – Humans• Above 20,000 Hz – Bats Above 20,000 Hz – Bats Ultra SonicUltra Sonic• Below 20Hz – Elephants Below 20Hz – Elephants Infra SonicInfra Sonic

Sound Intensity (B)Sound Intensity (B)

The unit for loudness is the The unit for loudness is the BelBel -- named -- named after Alexander Gram Bell. The unit was after Alexander Gram Bell. The unit was too big so we commonly refer to a 1/10 of too big so we commonly refer to a 1/10 of a bela beldB = decibeldB = decibelThe dB scale is logarithmic- it begins at 0 The dB scale is logarithmic- it begins at 0 – the quietest sound that can be heard– the quietest sound that can be heardDoubling sound intensity corresponds to Doubling sound intensity corresponds to an increase in dB of 3an increase in dB of 3

10x intensity gives an increase in dB 10x intensity gives an increase in dB of 10of 10100x intensity corresponds to dB 100x intensity corresponds to dB increase of 20increase of 20a 50 dB is 100 times more intense a 50 dB is 100 times more intense than a 30 dBthan a 30 dB

Pg. 351 has various sound levelsPg. 351 has various sound levelsspace shuttle 170dBspace shuttle 170dB

see chartsee chartTHE EAR see modelTHE EAR see model

Source of the soundSource of the sound

TABLE 12-2 Intensity of TABLE 12-2 Intensity of Various SoundsVarious Sounds

Intensity Level Intensity Level (dB)(dB)

Intensity Intensity W/mW/m22

Jet Plane at 30mJet Plane at 30m

Threshold of PainThreshold of Pain

Loud Indoor rock concertLoud Indoor rock concert

Siren at 30mSiren at 30m

Auto Interior, moving at 90km/hAuto Interior, moving at 90km/h

Busy Street TrafficBusy Street Traffic

Ordinary conversation, 50cmOrdinary conversation, 50cm

Quiet RadioQuiet Radio

WhisperWhisper

Rustle of LeavesRustle of Leaves

Threshold of hearingThreshold of hearing

140140

120120

120120

100100

7575

7070

6565

4040

2020

1010

00

100

1

1

1 x 10-2

3 x 10-5

1 x 10-5

3 x 10-6

1 x 10-8

1 x 10-10

1 x 10-11

1 x 10-12

Musical InstrumentMusical Instrument

Pitch is determined by the Pitch is determined by the lowest frequency. lowest frequency. (fundamental)(fundamental)

FF1=1= ½ ½λλ

FF2=2= λλ

FF3=3=3/23/2λλ

Overhead Picture- Overhead Picture- GuitarGuitar

•Changing the length of a Changing the length of a vibrating string (column of air) vibrating string (column of air) changes the frequency (Pitch)changes the frequency (Pitch)

•The velocity is the same on each The velocity is the same on each string so F is inversely string so F is inversely proportional to lengthproportional to length

Wind InstrumentsWind Instruments

TromboneTrombone

TrumpetTrumpet

TubaTuba

Pressing valves changes L of air Pressing valves changes L of air columncolumn

Bugle – no moving parts, Bugle – no moving parts, octave incrementsoctave increments

Sounding BoardSounding Board

Moving air in a column is a musical Moving air in a column is a musical instrument. The frequencies that instrument. The frequencies that persist are the ones that persist are the ones that correspond to standing wavescorrespond to standing waves

-Regarding Molecules of air-Regarding Molecules of air

Nodes represent NO motion of air Nodes represent NO motion of air molecules Antinodes represent the molecules Antinodes represent the most displacement.most displacement.

Tubes, Tubes, Open and ClosedOpen and Closed

OPENOPEN

FluteFlute

ClosedClosed

ClarinetClarinetWe can explain BOTH in terms of…

1. Displacement of air

2. Pressure

OVERHEADOVERHEAD

Figure 12-12 Figure 12-12 Standing waves in Standing waves in

open-open tubeopen-open tube

OverheadOverhead

Figure 12-13 Figure 12-13 Standing Waves Standing Waves in open-closed in open-closed

tubetube

Quality of SoundQuality of Sound

Quality of sound : timbre – tone Quality of sound : timbre – tone colorcolor

Quality is determined by overtonesQuality is determined by overtones

Quality happens by superposition Quality happens by superposition of wavesof waves

ConstructiveConstructive

DestructiveDestructive

Interference Interference

OscilloscopeOscilloscope

Sum of ALL THREE

1

22

33

Interference of Sound Waves 12-7Fig 12-17Fig 12-18

BeatsFig 12-19

Doppler EffectFig 12-20

Example probs:12-1312-14

Pg. 368Red ShiftShock Waves

Fig 12-24

Fig 12-20Fig 12-20

OVERHEADOVERHEAD

Fig 12-21

Fig 12-21 OverheadOverhead

Fig 12-17, 12-18

Fig 12-17, 12-18

OVERHEADOVERHEAD

Fig 12-19Fig 12-19OverheadOverhead

Fig 12-24Fig 12-24OverheadOverhead