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Sound: When a person speaks, vibrations of the vocal cords produce sound waves. Sound waves usually travel faster through solids than through liquids or gases. Since they require a medium to travel through, sound waves will not travel through a vacuum. The loudness of a sound, often referred to as the intensity, is dependent upon the amplitude of the wave. As amplitude increases, loudness increases. The intensity of a sound is expressed in units called decibels. The intensity of a sound is related to the pressure on the eardrum. A sound of 120 decibels is intense enough to cause pain in the ear. The softest sound that can be heard is 0 decibels, while normal talking is about 65 decibels. The pitch of a sound refers to its highness or lowness. The pitch of a sound depends on frequency. The higher the frequency, the higher the pitch. The pitch of a sound changes when the sound or the listener moves. When you listen to a siren on an approaching vehicle, the pitch of the sound appears to increase as the vehicle approaches (pitch decreases for vehicle going away). However, the pitch of the sound does not change. Instead, the number of vibrations that reach your ear is changed when the source of the sound moves. Therefore, the pitch appears to be higher or lower depending on whether the sound is moving toward or away from you. This rise and fall of pitch due to relative motion between the observer and the source of the sound is called the doppler effect.
Sound = vibrations of matter (sound and other longitudinal waves require particle of matter to travel through). The closer the molecules are together the louder the sound. Speed of sound in air at 0° C = 331 m/s or about 1100 ft/s (speed varies with temperature, as temperature increases the speed increases)
For every
degree above 0° C multiply by 0.6 / t Sound travels about 12 times as fast as a car at 60 miles/hour Musical instruments: 3 major groups; wind (open or closed pipe), stringed, or percussion. Wind instruments use vibrations in columns of air to produce sounds. Percussion instruments produce sounds when hit with a hammer, stick, or hand. Stringed instruments produce sound when a string is bowed, plucked, or struck. Stringed instruments pitch depends on thickness, tension, and length of string. Pitch decreases with increased length and thickness, and increases with increased tension. Music = regular pattern of vibrations (noise = set of irregular patterns) Octaves: higher octave = double frequency, lower octave = half the frequency 440 Hz = A (880 Hz = one octave above, 220 Hz is one octave below)
Wire or
string: V =
√
(t / (m/L))
f = (√(t/m))
/ (2 L/m)
Harmonics: Drawing of 1st harmonic:
2nd harmonic (1st overtone) · higher frequencies than fundamental · overtones = notes that go together well · combination of overtones and fundamental produces the quality of a tone · overtones that sound well together are said to be in harmony
Drawing of 2nd harmonic:
Drawing of 3rd harmonic:
Tacoma Narrows Bridge: wind hitting natural vibrating frequency Resonance: occurs when one vibration reinforces another because they have the same natural frequency. · Have you ever pushed a moving swing? The right way to push is to time the push so that your frequency is at the natural frequency of the swing. Thus, your energy adds to that of the swing. In short, the two frequencies are in resonance. · A common example of resonance is the sound made by blowing into a soda bottle. · A less common, but more spectacular example, is the shattering of a glass by an opera singer. To shatter glass you must produce frequencies for that glass (produce 1 tone and its harmonics only = a high note (high frequency) causes molecules to vibrate faster). · If a driving force, such as vibrating air, is applied, the oscillator will move in simple harmonic motion with a frequency equal to the driving frequency. If the driving frequency is equal to the natural frequency, the amplitude, in theory, goes to infinity. This is resonance. · If the driving force has no natural frequency of its own, as for example when a piano key is struck, the oscillator will select the frequency which fits its form. In the case of the piano string, the frequency is such that a whole number of wavelengths fit between the string supports, as there must be nodes at the fixed points.
Demonstration of tuning forks and tubes: · A tuning fork will cause resonance in a tube open to the air at one end. A node will be at the end closed by the water and an antinode will be at the air end. Thus, a quarter of a wavelength fits into the tube. A correction is needed to account for the small amount of vibrating air just above the tube. The resulting formula for wavelength is given as: 4 (1 + .4d) where d is the tubes inside diameter. The velocity of sound then will be obtained by multiplying the wavelength by the tuning fork frequency.
Tap a long steel rod on a hard surface: you are exciting the resonate frequency for length of device (a longitudinal wave where ends are free to move). If you hold it in the middle of the rod you can get fundamental resonance but holding it elsewhere and kill wave or possibly find another node). Two tuning forks of the same frequency will resonate if one is vibrated. Doppler effect: change in frequency in sound you hear due to change in motion (yourself) or change in motion of object causing sound (race car sound coming toward you = higher pitch or frequency, lower pitch going away from you. Echo: · depends on λ and roughness of surface · lower sounds have longer λ · all λ travel at same speed · for trees to echo, the distance apart must be ≤ λ · to get reflection roughness of surface must compare to λ Sample Definitions: · wave rhythmic disturbances that transfers energy through space or matter (has kinetic energy)j · medium material through which waves travel · crest point of maximum displacement · trough point of maximum displacement · node point on standing wave where amplitude is zero · antinode point on standing wave with maximum amplitude · wavelength (λ) linear distance between any two corresponding points on consecutive waves · amplitude (A) is the distance from resting position to antinode (is dependent on the energy that creates the wave) · frequency (f) refers to the number of waves that pass a given point in one second · vibration back and forth motion of a particle of a wave · Hertz (Hz) 1 vibration (or cycle) per second (frequency of vibration) a tuning fork with a frequency of 256 Hz will vibrate 256 times in one second · period (T) the time it takes for 1 complete vibration · velocity - (v) speed at which wave travels (includes direction) · transverse (light/wave waves) matter moves a right angles to direction of propagation · compressional - (longitudinal sound) matter vibrates in same direction as wave motion · area of compression molecules crowded together · areas of rarefaction molecules spread out · loudness (intensity) is dependent on amplitude, as amplitude increases loudness increases · pitch refers to highness or lowness depends on frequency (high frequency = higher pitch) · Doppler effect rise and fall of pitch due to relative motion between observer and sound of sound · overtones higher frequencies than fundamental · harmony overtones that sound well together are said to be in harmony · resonance occurs when one vibration reinforces another because they have the same natural frequency
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Page Last Updated: Friday March 02, 2007 Webmaster: Larry Jones Pickens County School District |