Physics: Sound

sound

Sound – a longitudinal wave produced by vibration and travels through a medium

Categories of Sound Waves1. Audible Waves – can be heard by humans, 20 -20 kHz2. Infrasonic Waves – cannot be heard by humans, below audible 3. Ultrasonic Waves – cannot be heard by humans, above audible

Speed of Sound

• Medium Speed (m/s) Air 331 Water 1490 Iron 5130 v = fλEffect of Temperature to Speed in Air

v = (331)(T/273 K)1/2

1. The coldest recorded temperature of air on earth, -128.6o F, occurred on July 21, 1983, at Votsok, a Russian station in Antartica. What is the speed of sound in air at this temperature?

2. A stone is dropped from rest into a well. The sound of the splash is heard exactly 2.0s later. Find the depth of the well if the temperature is 10o C.

Energy and Intensity of Sound

• Intensity (I) – rate at which energy flows through the surface divided by the surface area

I = E/t in W/m2

Intensity Level – transformation of intensity to a logarithmic scale /relative intensity

β = 10 log (I/Io) in decibels (dB)

Io= 1.0 x 10-12 W/m2

Intensity Levels of Different Sources

Source of Sound β (dB) Nearby Jet Airplane 150

Siren 120Busy Traffic 80

Normal conversation 50Mosquito Buzzing 40

Whisper 30Rustling of Leaves 10

Reflection of Sound

• Echo – is a reflected sound. To humans, echo can be distinguished from the original sound when it reaches the listener at least 0.10 s after the original sound is heard

1. A dolphin located in seawater at a temperature of 298 K emits a sound directed toward the bottom of the ocean 150 m below. How much time passes before it hears an echo?

2. A group of hikers hears an echo 3.0 s after shouting. If the temperature is 22o C, how far away is the mountain that reflected the sound wave?

The Doppler Effect

Stationary source Source in relative motion

Doppler Effect –the change in frequency heard by an observer whenever there is relative motion between

the observer and the source of sound

The observed frequency fo

fo = fs (v + vo)/(v – vs)

fs= frequency when there is no relative motion

v= speed of soundvo = speed of observer

(+) if observer is moving towards source ( - ) if observer is moving away from source

vs= speed of source

(+) if moving towards the observer (- ) if moving away from the observer

1. An airplane traveling at 172 m/s emits a sound of frequency 5.0 kHz. At what frequency does the stationary listener hear the sound (a) as the plane approaches (b) After it passes?

2. A bat flying at 5.0 m/s is chasing an insect flying in the same direction. If the bat emits a chirp of 40 kHz and receives back an echo at 40.4 kHz, what is the speed of the insect? Take the speed of sound to be 340 m/s.

Extreme Doppler Effect

Shock wave ( vs=mach 1) Sonic Boom (vs>mach 1)

Interference of Sound Waves

1. Resonance – the build up of larger amplitudes for sound waves

2. Beats – superimposed sound waves of slightly different frequencies

Quality of Sound

1. Quality or Timbre –the combined harmonics and intensity of sound

2. Pitch – psychological reaction to frequency