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Waves and Sound I. Wave Characteristics A wave is a transfer of ENERGY from one point to another without the transfer of material between the points WAVES “MOVE” ENERGY, NOT MATTER A. Definition Amplitude : the height of the wave, measured in meters. Wavelength (λ) : the distance between adjacent crests, measured in meters. Period : the time it takes for one complete wave to pass a given point, measured in seconds. Frequency : the number of complete waves that pass a point in one second, measured in inverse seconds, or Hertz (Hz). Speed : the horizontal speed of a point on a wave as it propagates, measured in meters / second. trough crest
B. Some basic equations relating wave properties
T
T =
f
v = λυ
frequency (Greek letter ν) period speed Be careful interpreting this equation. It would seem that changing wavelength would change wave speed but it does not. This has to do with how wavelength and frequency are related. Doubling the wavelength does not double the speed because it has the effect of reducing the the frequency by half. SPEED REMAINS THE SAME. WAVE SPEED DEPENDS ON THE MEDIA IN WHICH THE WAVE TRAVELS.
C. ELECTROMAGNETIC WAVES - transverse waves that DO NOT require a medium through which to travel. They have an electric field and a magnetic field that propagate in the same direction but the fields are perpendicular to each other. They can be POLARIZED. This means they can be filtered so that only one direction of the electric field passes a polarizing filter.
III. Adding Waves-Interference The nodes and antinodes of waves that are IN-PHASE “line up” When added, a wave with a larger amplitude results The nodes and antinodes of waves that are out of PHASE are “off set” When added, they cancel
V. SOUND
A. Overview SOUND is produced only when a media is present to transmit the waves. The more ELASTIC the media, the FASTER the sound wave travels. Sound is caused by the vibration of matter. The medium (air generally) transmits the vibrations by longitudinal waves. The faster (more frequent) the vibrations the higher the pitch. Very low frequency sounds are called infrasonic. Very high frequency sounds are called ultrasonic. SOUND WILL NOT TRAVEL IN A VACUUM. B. How it travels Sound waves are MECHANICAL waves. Sound waves are PRESSURE waves. They cause the particles of the carrying medium (air) to compress. A region of compressed air leaves a region of space with fewer air particles- rarefied air. Particles rush into the rarefied space. A vibrating object causes these pulses of compressed and rarefied air (waves) to travel in all directions. When the pulses reach you ear, you interpret the sound.
tuning forks vibrating with different frequencies. dark regions are compressed air. “thin” regions are rarefied air. The distance between compressed regions are the wavelength of the sound wave. The number of them hitting your ear per given time is the frequency. The more of them per given time, the higher the pitch increasing frequency
D. Doppler effect The Doppler effect applies to electromagnetic radiation (light) as well as sound waves. The Doppler effect is observed for MOVING objects emitting light or sound waves. How you perceive the effect depends upon your location relative to the wave source. light source moving away from observer. Shift to longer frequency…RED SHIFT light source moving toward observer. Shift to shorter frequency…BLUE SHIFT
LIGHT
SOUND
sound source moving toward observer. High pitch. sound source moving away from observer. Low pitch.
VI. SOME PROPERTIES OF SOUND
A. LOUDNESS
The energy of a sound wave is proportional to the amplitude of the wave. The higher the amplitude, the more energy it carries:
E ∝ A
2 E is energy A is amplitude
LOUDNESS, CONTINUED
LOUDNESS, or intensity, is reported using the DECIBEL scale. The decibel scale is a LOG scale (remember pH?). The decibel scale is based on a value called the THRESHOLD OF HEARING (TOH). This is the lowest intensity sound that most people can hear. TOH = 1 x 10-12^ Watts/m^2 Sound 10x more intense than TOH has a decibel value of 10 Sound 100x more intense than TOH has a decibel value of 20 Sound 1000x more intense than TOH has a decibel value of 30
dB = 10 log
I
I
o dB is decibels (1 Bel = 10 dB) I is intensity of the sound Io is the TOH
B. FORCED VIBRATIONS
Forced vibrations occurs when a vibrating object causes another object nearby to vibrate. image from Cliffnotes C. NATURAL FREQUENCY An object that is struck, thumped, whatever, might begin to vibrate. The frequencies at which the object vibrates after being struck is the natural frequency of the object. Natural frequency is responsible for the characteristic sounds an object makes. For example, if you closed your eyes and I dropped a pencil or a basketball on the floor, you’d be able to tell which one based on experience. Because of their composition and shape, their natural frequencies differ.
