Chapter 15 – Sound

15.1 Properties and Detection of Sound

If a tree falls in a forest and no one is around to hear it, does it make a sound? Three aspects of any sound: 1. There must be a source for a sound (a vibrating object) 2. The energy is transferred from the source in the form of longitudinal sound waves (through a medium) 3. The sound is detected by an ear or an instrument

A vibrating tuning fork causes alternating regions of high and low air pressure Sound wave – a pressure variation that is transmitted through matter Properties and Detection of Sound

Regions of high air pressure Regions of low air pressure 15.1 Properties and Detection of Sound

Two aspects of sound are immediately evident to a human listener. - Pitch - Loudness These are both subjective sensations in the consciousness of the listener Each corresponds to a physically measurable quantity 15.1 Properties and Detection of Sound

Pitch – refers to the “highness” (i.e. – a piccolo) or “lowness” (i.e. – a bass drum) of a sound wave Lower frequency – lower pitch Higher frequency – higher pitch The human ear responds to frequencies from about 20 Hz to 20,000 (20 kHz) As humans age, they lose the ability to hear higher pitched sounds Dogs can hear sounds as high as 50,000 Hz, bats as high as 100,000 Hz, dolphins as high as 200,000 Hz 15.1 Properties and Detection of Sound

Loudness – Sound intensity as sensed by the ear and interpreted by the brain Intensity – the energy transported by a wave per unit time across unit area (measured in decibels, dB) Loudness can be measured by considering the intensity of a sound wave The human ear is sensitive to pressure variations in sound waves (amplitude of the wave) 15.1 Properties and Detection of Sound

Typically, a 10 dB increase is twice the loudness 15.1 Properties and Detection of Sound A decibel scale Threshold of pain (about 120 dB)

15.1 Properties and Detection of Sound

Doppler Effect – The change in frequency of sound caused by the movement of either the source, the detector, or both. Link to applet of passing ambulance Listen to an ambulance and the Doppler Effect Watch a fire engine video and the Doppler Effect Doppler Car 1 Doppler Car Properties and Detection of Sound

2. If you shout across a canyon and hear the echo 0.95 s later, how wide is the canyon? Assume the air temperature to be 20ºC. 1.A Phillies fan on a warm summer day (28ºC) sits in the bleacher 162 m away from home plate. a)What is the speed of sound in air at 28ºC b)How long after seeing the ball hit the bat does the fan hear the crack of the bat?

Doppler Effect – The change in frequency of sound caused by the movement of either the source, the detector, or both Properties and Detection of Sound

Doppler Effect Equation f d = frequency received by the detector f s = frequency of the wave emitted by the source v = velocity of the sound wave v s = velocity of the sound’s source v d = velocity of the detector 15.1 Properties and Detection of Sound Define the positive direction to be from the source to the detector

A trumpet player plays a sound with a frequency of 524 Hz while traveling in a convertible at 24.6 m/s. If the car is coming toward you, what frequency would you hear? Assume that the temperature is 20°C Properties and Detection of Sound

A local fire station has a roof-mounted siren 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 20ºC Properties and Detection of Sound