SOUND CHAPTER 12

All Sound Has 3 Aspects… 1.Source 2.Energy 3.Detected Sound is Longitudinal Waves (Compression Waves) Sound must have a medium. Sound can’t travel in the absence of 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 1atm Material Speed (m/s) Air Air(0°C) Helium Hydrogen Water Seawater Iron and Steel Glass Aluminum Hardwood approx 5000 approx 4500 approx 5100 approx 4000

Loudness : Energy in a wave... Loosely Amplitude Pitch or Frequency : # of Vibrations per second –High – Flute –Low – Tuba Humans can hear a range of frequencies 20Hz to 20,000Hz – Humans20Hz to 20,000Hz – Humans Above 20,000 Hz – Bats Ultra SonicAbove 20,000 Hz – Bats Ultra Sonic Below 20Hz – Elephants Infra SonicBelow 20Hz – Elephants Infra Sonic

Sound Intensity (B) The unit for loudness is the Bel -- named after Alexander Gram Bell. The unit was too big so we commonly refer to a 1/10 of a bel dB = decibel The dB scale is logarithmic- it begins at 0 – the quietest sound that can be heard Doubling sound intensity corresponds to an increase in dB of 3

10x intensity gives an increase in dB of x intensity corresponds to dB increase of 20 a 50 dB is 100 times more intense than a 30 dB Pg. 351 has various sound levels space shuttle 170dB see chart THE EAR see model

Source of the sound TABLE 12-2 Intensity of Various Sounds Intensity Level (dB) Intensity W/m 2 Jet Plane at 30m Threshold of Pain Loud Indoor rock concert Siren at 30m Auto Interior, moving at 90km/h Busy Street Traffic Ordinary conversation, 50cm Quiet Radio Whisper Rustle of Leaves Threshold of hearing x x x x x x x x

Musical Instrument Pitch is determined by the lowest frequency. (fundamental) F 1= ½λ F 2= λ F 3= 3/2λ

Overhead Picture- Guitar Changing the length of a vibrating string (column of air) changes the frequency (Pitch)Changing the length of a vibrating string (column of air) changes the frequency (Pitch) The velocity is the same on each string so F is inversely proportional to lengthThe velocity is the same on each string so F is inversely proportional to length

Wind Instruments TromboneTrumpetTuba Pressing valves changes L of air column

Bugle – no moving parts, octave increments Sounding Board

Moving air in a column is a musical instrument. The frequencies that persist are the ones that correspond to standing waves -Regarding Molecules of air Nodes represent NO motion of air molecules Antinodes represent the most displacement.

Tubes, Open and Closed OPENFluteClosedClarinet We can explain BOTH in terms of… 1.Displacement of air 2.Pressure

OVERHEAD Figure Standing waves in open-open tube

Overhead Figure Standing Waves in open- closed tube

Quality of Sound Quality of sound : timbre – tone color Quality is determined by overtones Quality happens by superposition of waves ConstructiveDestructive Interference

Oscilloscope Sum of ALL THREE 1 2 3

Interference of Sound Waves 12-7 Fig Fig Beats Fig Doppler Effect Fig Example probs: Pg. 368 Red Shift Shock Waves Fig 12-24

Fig OVERHEAD Fig Overhead Fig 12-17, OVERHEAD Fig Overhead Fig Overhead