Sound Pitch Loudness Sound Intensity Level Doppler Effect

Sound Pitch Loudness Sound Intensity Level Doppler Effect Physics Mrs. Coyle

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

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

Speed of Sound in Air v = 331 + 0. 6 T ( in meters/sec) T is the temperature in 0 C. In higher humidity, sound will travel faster.

Properties of Sound Reflection (Echo) Refraction Interference Diffraction

Pitch: the frequency of a sound wave. Musical notes have a given pitch. The note C has a frequency of 327 Hz. When two notes differ by a ratio of 2: 1 they are one octave apart. What would be the next higher C?

Bow (Shock) Waves When the speed of a moving sound source is greater than the speed of the wave, a pressure ridge builds similar to the wave created by the bow of a ship. physlet animation

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.

Doppler Effect The change in a wave's perceived frequency due to the motion of either the sound source or the observer. It is applicable to any type of wave. Austrian physicist Christian Doppler (1803 -1853). train sound clip Simulations physlet animation http: //www. walter-fendt. de/ph 14 e/dopplereff. htm

The Doppler Effect http: //www. physicsclassroom. com/Class/sound/u 11 l 3 b. cfm

Doppler Effect Detected Frequency, (f. D) f. D = f ( v±vo ) v±vs f : frequency of source v : speed of sound vo : speed of observer (+)when observer moving towards source vs : speed of source (-)when source is moving towards

Example 1 An ambulance is approaching a stationary observer. The siren of the ambulance emits a frequency of 480 Hz and the speed of the ambulance is 50 km/h(=13. 88 m/s). What frequency will the stationary observer hear when the ambulance is approaching? Assume T=20 o. C. Answer: fd = 500 Hz

Sound Intensity (I) I = (Power transmitted by the Wave)/Area Threshold of human hearing: Io =1 x 10 -12 Watts/m 2 Threshold of Pain: IP = 1 Watts/m 2

Sound (Intensity) Level A measure of our perception of the loudness of the sound. Unit: decibel(d. B)

Sound (Intensity) Level, (decibel) b= 10 log I , unit for b is d. B (decibel) I 0 The decibel compares the sound intensity (I), to Io (1 x 10 -12 Watts/m 2) , the threshold of human hearing. Remember: log(10 x )= x

Examples of Sound Intensity Levels jet plane taking off air raid siren threshold of pain loud rock music ear damage starts busy traffic normal conversation quiet library soft whisper threshold of human hearing 140 d. B 125 d. B 120 d. B 115 d. B 85 d. B 70 d. B 60 d. B 40 d. B 20 d. B

Example 2 What is the sound level (decibel) of a sound of 10 -4 W/m 2 intensity ? Answer: 80 d. B

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