 # Define sound waves A sound wave is a pressure variation ( changing pressure) that is transmitted through matter Sound Waves.

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Define sound waves A sound wave is a pressure variation ( changing pressure) that is transmitted through matter Sound Waves

source vibrates Air molecules collide Oscillations in air pressure are produced. Pressure changes is transferred away from the source in all directions HOW DO SOUND WAVES TRAVEL IN AIR

Frequency : number of oscillations in pressure in each second How are sound waves similar to other waves Wavelength : distance between two successive high pressure or low pressure regions Compression Rarefaction

Sound waves are Longitudinal waves. Why? Particles in the medium move parallel to the direction of wave motion Sound waves are mechanical waves. Why? Sound waves need a medium ( solid, liquid or gas) to travel

How much can you remember? 1. Pressure variation that is transmitted through matter 2. Number of oscillations in pressure per second 3. Distance between two high or two low pressure regions 4.Type of wave where particles in the medium move parallel to the direction of wave motion 5.Waves that need a medium to travel 6.High pressure region 7.Low pressure region

Sound waves cannot travel in a Vacuum/spaceVacuum/ (no particles to collide) Speed of sound depends on: Medium Temperature Speed of sound Sound travels fastest through solids, then liquids, then gases. ( particles in solid are close together)

Speed of sound 2. Depends on temperature If temperature increases by 1 0, speed of sound increases by 0.6m/s What is the speed of sound at a temperature of 35 0 C? Change in temperature = 35 0 - 20 0 = 15 0 For 1 0 increase in temperature, speed increases by 0.6m/s For 15 0 increase in temperature, speed increases by 0.6m/s x 15 =9m/s Speed = 343m/s + 9m/s = 352m/s Speed of sound at room temperature( 20 0 C) is 343m/s

At 20 0 speed is 343m/s At 35 0 speed is 343 + x m/s What is the speed of sound at a temperature of 35 0 C? Speed of sound at room temperature( 20 0 C) is 343m/s Change in temperature = 35 -20 = 15 0 For 1 0 increase in temperature, speed increases by 0.6m/s For 15 0 increase in temperature, speed increases by 0.6m/s x 15 =9m/s Speed = 343m/s + 9m/s = 352m/s

At 20 0 speed is 343m/s At 15 0 speed is 343 + x m/s What is the speed of sound at a temperature of 15 0 C? Speed of sound at room temperature( 20 0 C) is 343m/s Change in temperature = 15 - 20 = -5 0 For 1 0 increase in temperature, speed increases by 0.6m/s For -5 0 change in temperature, speed changes by 0.6m/s x- 5 =-3m/s Speed = 343m/s - 3m/s = 340m/s

At 20 0 speed is 343m/s At 20+ x 0 speed is 335 m/s At what temperature is the speed of sound 335m/s? Speed of sound at room temperature( 20 0 C) is 343m/s Change in speed = 335 - 343 = -8m/s For 1 0 increase in temperature, speed increases by 0.6m/s For x 0 change in temperature, speed changes by - 8m/s X= -8 x 1 = -13.3 0 C 0.6 Temperature = 20 – 13.3 = 6.7 0 C

Equations used to calculate velocity of a sound wave Calculations: Ali stands 100m away from Rashid. When Ali claps his hands, Rashid hears it 0.3s later. What is the speed of the wave? What is the velocity of a wave having a wavelength of 2m and a frequency of 100Hz?

Echoes Used to find the distance between the source and the reflected object by determining the time taken for the echo to return to the source. Are reflected sound waves Uses of echoes V = 2 x d …. Equation used with echoes t If you shout across a canyon and hear your echo 0.80s later, how wide is the canyon?

Uses of echoes - used by bats - cameras - ships that use sonar

recap Frequency Low high

Physical characteristics of sound frequency Pitch amplitude loudness wavelength We detect / perceive the sound as pitch

Perceiving Sound A B C D Which wave shows: a.High frequency b.High amplitude c.Low frequency d.Low amplitude e.High pitch f.Low pitch g.Loud sound h.Soft sound A D B C A B D C How do we perceive - high frequencies low frequencies - high amplitudes low amplitudes?

Which two waves have the same loudness? How do the diagrams show this? Which two waves have the same pitch? How do the diagrams show this?

Sound level Loudness is not directly proportional to the pressure variations in a sound wave alone. The ear’s sensitivity depends on both pitch and amplitude Sound level is the logarithmic scale that measures the amplitude of sound that humans can hear The unit used to measure sound level is the decibel (dB).

Eg. Compare the sound pressure level of 20dB to sound pressure of 80dB ( By how much does sound pressure increase) 80dB – 20dB = 60dB (20 + 20 + 20) sound pressure increase by 10 x 10 x 10 = 1000 x ( or 10 3 ) The sound pressure increases by a factor of 10 for every 20-dB increase in sound level. 20 40 60 80 10 xx

If sound pressure at 0dB is 2 x 10 -5 Pa, what is the sound pressure at 40dB? Sound pressure at 40 dB = 2 x 10 -5 x 10 2 = 2 x 10 -3 Pa 1.Find the change in sound level 2.Determine by how much sound pressure increases 3.Determine sound pressure 40dB – 0 dB = 40 dB ( 20 + 20) sound pressure increase by 10 x 10 = 100 ( or 10 2 )

Sound levels The increase in sound level = 70dB – 30dB = 40dB sound pressure level for heavy traffic is 10 x 10= 100 times more than the whisper The sound pressure increases by a factor of 10 for every 20-dB increase in sound level. Eg. Compare the sound pressure of a whisper ( 30dB) to heavy traffic ( 70dB) For every 10dB increase in sound level, the sound is twice as loud If sound pressure at 30dB is 2 x 10 -3 Pa, what is the sound pressure at 70dB? 2 x 10 -3 Pa x 100 = 2 x 10 -1 Pa We hear the heavy traffic as (2 x 2 x 2 x2 = 16 times louder than the whisper For every 10dB increase in sound level, the sound is twice as loud We hear the heavy traffic as (2 x 2 x 2 x2 = 16 times louder than the whisper

Doppler effect Is the change in frequency of sound caused by moving sound source or a moving observer The pitch is higher when the sound moves toward you, then it drops to a lower pitch as the source moves away.

DOPPLER EFECT: source moves Frequency is higher (wavelength decreases) as the source moves towards the sound detector Pitch is higher Frequency is lower ( wavelength increases) as the source moves away from the sound detector Pitch is lower

Stationary source Detector moves The Doppler shift results from the relative velocity of the sound waves and the detector. As the detector moves towards the stationary source, the relative velocity is larger – number of wavecrest reaching the detector each second increases.( frequency increase) As the detector moves away from the source, the relative velocity is smaller, -number of wave crests reaching the detector each second decreases

Vd = velocity of the detector V = velocity of the sound wave, Vs = velocity of the source Fs = wave’s frequency fd = frequency perceived by a detector calculations Moving detector Moving source Note: when source moves towards a detector: V s is positive when detector moves towards a source: V d is negative

V d = 0m/s V s = + Vs = - Vd = - Vd = + Vs = 0m/s Sound source is moving Sound detector is moving

Both source and detectors move Vd = + Vs = + Vs = - Vd = -

Example problem A fire station has a siren on the roof that has a frequency of 975 Hz. If you are on your bike moving away from the station at a speed of 6.00 m/s, what will be the frequency of the sound waves reaching your ear? Assume that the air temperature is 20°C. (therefore speed=343m/s Source is stationary- V s =0m/s Or use this equation:

Applications of Doppler Effect The Doppler effect occurs in all wave motion, both mechanical and electromagnetic. Radar detectors - to measure the speed of baseballs and automobiles. Astronomers observe light from distant galaxies -to measure their speeds and infer their distances. Ultrasound –to detect the speed of the moving heart wall in a fetus Bats -detect and catch flying insects. When an insect is flying faster than a bat, the reflected frequency is lower, but when the bat is catching up to the insect, the reflected frequency is higher.

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