Sound

I. Sound is a longitudinal, mechanical wave. * A.Molecules move parallel to the direction of the waves velocity. B.Areas of high pressure and low pressure C.“compression” and “rarefaction”- molecules are compressed and than move apart

II. Requires a vibrating object A.Guitar string B.Stereo speaker C.Voice: vocal cords *

III. Speed of sound A.As sound travels through air, at 20˚C (68˚F) and sea level pressure, v is about 343 m/sv = f B.As the temp goes up, the velocity increases- Why?? C.As the density of the medium goes up, the velocity increases- Why?? D.Travels much slower than light Count time from when you see the flash of lightning to when you hear the thunder- divide by 5 = miles to lightning III. Speed of sound A.As sound travels through air, at 20˚C (68˚F) and sea level pressure, v is about 343 m/sv = f B.As the temp goes up, the velocity increases- Why?? C.As the density of the medium goes up, the velocity increases- Why?? D.Travels much slower than light Count time from when you see the flash of lightning to when you hear the thunder- divide by 5 = miles to lightning

The velocity of a wave depends on the medium through which is travels. If you know the medium, you can find the velocity by Bulk modulus- fluids Elastic modulus- solids

Sound Wave Behavior Reflect: an echo Refract: changes direction when the medium changes Diffract: curves around barriers and through openings

You stand at the edge of a canyon and yell, “Hello!”. If you hear the echo 3 seconds later, how wide was the canyon? v sound = 343 m/s d = vt The time for the sound wave to strike the opposite canyon wall is ½ the total time. d = (343 m/s) (1.5 s) d = m

If you drop a rock at the top of a 40 meter high cliff, how long will it be until you hear the sound when it hits the ground? Total time = time for the rock to fall + time for the sound to travel back to you. d = v o t + ½ at 2 Time for the rock to fall: v o = 0, a = g d = ½ at 2 Time for the sound: a = 0, v = 343 m/s d = vt

What kind of sound wave is produced when the source of the sound is moving?

A “shock wave” is produced from these overlapping waves. It produces a loud “sonic boom”. Sonic booms occur when the source of sound exceeds the speed of sound * Sonic Booms captured on video Sonic Booms captured on video

IV. Reflection A.Echo B.Sonar: invented in a reflected sound wave is used underwater instead of light because light is more easily absorbed by water, so sound will travel much farther. C.Ultrasound D.Autofocus cameras

V. Pitch A.Determined by the frequency B.Hi frequency = high pitch C.Musical notes- if you double the frequency you go up by one OCTAVE Example: 400 Hz, 200 Hz, 800 Hz A.Range of hearing humans 20 Hz up to about 20,000 Hz dogsup to about 50,000 Hz catsup to about 70,000 Hz

The Doppler Shift A.A detected change in the frequency of a wave as the source of the wave moves B.Police siren, car horn, weather, stars Police sirencar hornPolice sirencar horn

Wave Amplitude For a sound wave, the wave amplitude corresponds to the VOLUME. Loudness is measured in decibels, dB Where zero decibels is the threshold of human hearing and 120 dB is the point at which sound becomes painful and hearing can be damaged.

ResonanceResonance- the tendency of an object to vibrate with a greater amplitude at certain frequencies Resonance  One simple example is pushing a child on a swing.  If two objects are vibrating with the same frequency, they are said to be “in resonance”  Examples: two tuning forks- if they are “in resonance”, the vibration of one will produce vibration in the other even if they are not touching.

Beats  A “beat frequency” is produced when two objects are vibrating at nearly the same frequency. beat frequencybeat frequency  Used for tuning orchestral instruments Beat frequency = f 1 – f 2

Resonance All rigid objects have a “natural” frequency or group of frequencies at which they will vibrate with greater amplitude. These frequencies are based on many factors like mass, density, shape, elasticity, etc. When exposed to an external source of their natural resonate frequency, they will begin to vibrate in response.

Resonance Even very large objects can have a resonant frequency at which they will vibrate in all different modes. Broughton Suspension Bridge was a suspended-deck suspension bridge built in 1826 to span the River Irwell between Broughton and Pendleton, now in Greater Manchester, England. It was one of the first suspension bridges constructed in Europe. On 12 April 1831 the bridge collapsed, reportedly owing to a mechanical resonance induced by troops marching over the bridge in step. [1] A bolt in one of the stay-chains snapped, causing the bridge to collapse at one end, throwing about forty of the men into the river. As a result of the incident the British Military issued an order that troops should "break step" when crossing a bridge. Wikipediasuspended-deck suspension bridgeRiver IrwellBroughtonPendletonGreater Manchestermechanical resonance [1]British Military Millennium bridge Millennium bridge  Tacoma Narrows bridge Tacoma Narrows bridge Tacoma Narrows bridge

Resonance For musical instruments, the resonant frequency of the instrument can be changed by adjusting the length of the chamber or string. The same string will vibrate at different resonant frequencies shown by “standing waves” along the string. Standing Waves along a string Standing Waves along a string

Resonators  All musical instruments create standing wave forms within them.  Wind instruments: waves of air molecules inside the cavities  Stringed instruments have vibrating strings, but the majority of sound is produced when that vibration is spread to a resonating box, often called the “sound board” or “sound box”

Standing Waves

Standing Waves in an “Open Pipe” resonator The standing wave always has a node at each end of the pipe or string. The “fundamental frequency”- the lowest note, is produced when only ½ of a wave is being generated.  Length of pipe = ½ of a wavelength

Harmonics  Other frequencies, called “harmonics” are produced AT THE SAME TIME as the fundamental frequency.  2 nd Harmonic  Length = one wavelength  The frequency (pitch) is higher, the wavelength is smaller.

 3 rd Harmonic  Length = 1 ½ wavelengths

Transverse waves along a string- example: a guitar string Resonance (Open Pipe)

 3 rd Harmonic Example: If the standing wave above has a frequency of 4 Hz in an open pipe of length 1.2 m, what is the velocity of the wave? v = f What is the wavelength, if the length of the pipe is 1.2 m? There are 1 ½ wavelengths in the pipe, therefore L = 1.5 There are 1 ½ wavelengths in the pipe, therefore L = 1.5 = 0.8 m = 0.8 m v = (0.8 m)(4 Hz) = 3.2 m/s

Closed Pipe Resonators  Node at open end. Antinode at closed end.  Fundamental frequency:  Length = ¼ of a wavelength

Closed Pipe Resonators 2 nd Harmonic  Length = ¾ of a wavelength

For the same length, which type of organ pipe will produce a lower note, an open pipe or a closed pipe? A closed pipe!