Chapter 15 Properties of Sound Pitch and Loudness Sound Intensity Level Doppler Effect

Sound Sound is a longitudinal mechanical wave. Sound is a longitudinal mechanical wave. Compressions(High Pressure) Compressions(High Pressure) Rarefactions(Low Pressure) Rarefactions(Low Pressure)

Speed of Sound Depends on the medium. Depends on the medium. The more elastic the medium the faster sound will travel through it. The more elastic the medium the faster sound will travel through it. Speed in metals>speed in water>speed in air Speed in metals>speed in water>speed in air Sound can’t travel through vacuum. Sound can’t travel through vacuum.

Speed of Sound in Air v = T ( in meters/sec) T is the temperature in 0 C. T is the temperature in 0 C. In higher humidity, sound will travel faster. In higher humidity, sound will travel faster.

Properties of Sound Share general properties of other waves Share general properties of other waves Reflection (Echoes) – Echolocation, SONAR Reflection (Echoes) – Echolocation, SONAR Refraction Refraction Interference – “dead spots” at nodes Interference – “dead spots” at nodes Diffraction – hearing someone around a corner Diffraction – hearing someone around a corner

Doppler Effect The change in a wave's perceived frequency due to the motion of either the sound source or the observer. The change in a wave's perceived frequency due to the motion of either the sound source or the observer. All waves Doppler shift All waves Doppler shift Applications – Radar (baseballs, cars), distances of galaxies, heart beats during ultrasounds Applications – Radar (baseballs, cars), distances of galaxies, heart beats during ultrasounds Austrian physicist Christian Doppler ( ). Austrian physicist Christian Doppler ( ). Demonstrations Demonstrations Doppler Ball Doppler Ball Train sound clip Train sound clip Train sound clip Train sound clip Simulations Simulations physlet animation physlet animation physlet animation physlet animation

Doppler Effect

When a sound source moves towards the observer, the detected frequency is higher. When a sound source moves towards the observer, the detected frequency is higher. When the sound source moves away from the observer, the detected frequency is lower. When the sound source moves away from the observer, the detected frequency is lower. Can occur if the observer is moving and the source is stationary. Can occur if the observer is moving and the source is stationary.

The Doppler Effect

Doppler Effect f d = Detected Frequency f : frequency of source f : frequency of source v : speed of sound v : speed of sound v o : speed of observer v o : speed of observer (+)when observer moving towards source v s : speed of source v s : speed of source (-)when source is moving towards observer

Example 1 An ambulance is approaching a stationary observer. The siren of the ambulance emits a frequency of 480Hz and the speed of the ambulance is 50km/h(=13.88m/s). What frequency will the stationary observer hear when the ambulance is approaching? Assume T=20 o C. An ambulance is approaching a stationary observer. The siren of the ambulance emits a frequency of 480Hz and the speed of the ambulance is 50km/h(=13.88m/s). What frequency will the stationary observer hear when the ambulance is approaching? Assume T=20 o C. Answer: f d = 500Hz Answer: f d = 500Hz

Shock Waves Shock Waves When the speed of a moving object is greater than the speed of the wave, the wave fronts pile up and produce a high pressure ridge similar to the wave created by the bow of a ship. When the speed of a moving object is greater than the speed of the wave, the wave fronts pile up and produce a high pressure ridge similar to the wave created by the bow of a ship. physlet animation physlet animation

Sonic Boom Sonic Boom When the pressure ridge of a bow wave of a jet passes over an observer on the ground, the observer experiences a sonic boom.

Pitch: the frequency of a sound wave. Musical notes have a given pitch. Musical notes have a given pitch. The note C has a frequency of 262 Hz. The note C has a frequency of 262 Hz. When two notes differ by a ratio of 2:1 they are one octave apart. Different notes have different ratios of frequencies When two notes differ by a ratio of 2:1 they are one octave apart. Different notes have different ratios of frequencies What would be the next higher C and next lower C? What would be the next higher C and next lower C? Ans: 524 Hz, 131 Hz Ans: 524 Hz, 131 Hz

Sound Intensity (I) Energy transmitted by a wave in a given time over a given area. I = (Power transmitted by the Wave)/Area SI unit - Watts/m 2 SI unit - Watts/m 2 Threshold of human hearing: I o =1 x Watts/m 2 Threshold of human hearing: I o =1 x Watts/m 2 Threshold of Pain: Threshold of Pain: I P = 1 Watts/m 2

Loudness Related to intensity but it’s a sensation in the mind of a human being Related to intensity but it’s a sensation in the mind of a human being Presumably because of the wide range of intensities detected, what we perceive as loudness is not directly proportional to the intensity. Presumably because of the wide range of intensities detected, what we perceive as loudness is not directly proportional to the intensity. 10 dB increase is heard as being twice as loud. 10 dB increase is heard as being twice as loud.

Sound Level A measure of our perception of the loudness of the sound. A measure of our perception of the loudness of the sound. Based on logarithmic scale Based on logarithmic scale Unit: decibel(dB) Unit: decibel(dB)

Sound Level, (decibel) β – measured in decibels β – measured in decibels The decibel compares the sound intensity (I), to I o (1 x Watts/m 2 ), the threshold of human hearing. The decibel compares the sound intensity (I), to I o (1 x Watts/m 2 ), the threshold of human hearing. An increase in 10 decibel is 10x as intense, 20 dB increase is 100x as intense. An increase in 10 decibel is 10x as intense, 20 dB increase is 100x as intense. Remember: log(10 x )= x Remember: log(10 x )= x x = log y is the same as y = 10 x x = log y is the same as y = 10 x

Examples of Sound Intensity Levels jet plane taking off140 dB jet plane taking off140 dB air raid siren125 dB air raid siren125 dB threshold of pain120 dB threshold of pain120 dB loud rock music115 dB loud rock music115 dB ear damage starts 85 dB ear damage starts 85 dB busy traffic 70 dB busy traffic 70 dB normal conversation 60 dB normal conversation 60 dB quiet library 40 dB quiet library 40 dB soft whisper 20 dB soft whisper 20 dB threshold of human hearing 0 dB threshold of human hearing 0 dB

Example 2 What is the sound level (decibel) of a sound of W/m 2 intensity ? What is the sound level (decibel) of a sound of W/m 2 intensity ? Answer: 80dB Answer: 80dB

Example 3 How many times louder is a quiet library sound (40 dB) compared to a soft whisper (20 dB)? How many times louder is a quiet library sound (40 dB) compared to a soft whisper (20 dB)? Hint 1: Property of logs: log(A/B) = logA –logB Hint 1: Property of logs: log(A/B) = logA –logB Hint 2: Take the difference in dB and then find (I 2 /I 1 ) Hint 2: Take the difference in dB and then find (I 2 /I 1 ) Answer: (I 2 /I 1 )= 100/1 Answer: (I 2 /I 1 )= 100/1