Chapter 18 Waves and Sound

18 – 1 Wave Characteristics A wave is rhythmic disturbance that carries energy through matter or space. Two major types: transverse and compression. Both must travel through a medium - a material through which a wave can transfer energy. Transverse wave - the matter/medium moves at right angles to the direction the wave travels. Water waves and a rope moving up and down make a transverse wave. Compressional wave - matter vibrates in the same direction as the wave travels. Sound and a slinky travel in compression waves.

18 – 1 Wave Characteristics Parts of a Transverse wave – Crests - the highest points of a wave. Troughs - the lowest points of a wave. Wavelength - l (Greek letter lambda) the distance between a point on one wave and the identical point on the next wave, such as from crest to crest, or trough to trough. Amplitude - the distance from the crest (or trough) of a wave to the rest position of the medium.

Parts of a Transverse wave (continued) - Waves transfer “energy” The amplitude corresponds to the amount of energy carried by the wave. Waves that carry great amounts of energy have large heights or amplitudes, and waves that carry less energy have smaller amplitudes.

18 – 1 Wave Characteristics Wave frequency - the number of wave crests that pass one place each second. Unit: hertz (Hz) - 1 Hz = 1 wave per second As frequency increases (higher pitch), wavelength decreases (shorter wavelengths), as frequency decreases (lower pitch), wavelength increases (longer wavelengths). Radio stations emit a particular frequency; this frequency determines their call numbers. FM stations emit at MegaHz (MHz) and AM stations emit at kiloHz (kHz).

18 – 1 Wave Characteristics Wave velocity - v - describes how fast the wave moves forward. Wave velocity = wavelength x frequency v = l x f ____________________________________ Solve the following: A wave is generated in a wave pool. The wavelength is 3.2 m. The frequency of the wave is 0.60 Hz. What is the velocity of the wave? v = l x f = (3.20 m)(0.60 Hz) = 1.9 m/s *Solve practice problems on p. 461

18 – 2 Nature of Sound Compressional waves - matter vibrates in the same direction as the wave travels. Sound and a slinky travel in compression waves. Compression - the area in which the coils are squeezed together Rarefraction - the area in which the coils are spread apart. Wavelength - the distance from the center of one compression to the center of the next compression. Frequency - the number of compressions that pass a point in one second. Your voice causes compressions and rarefractions among the particles in air to create a sound wave.

18 – 2 Nature of Sound Frequency and pitch - Pitch is the highness or lowness of sound; pitch depends on frequency. The higher the frequency, the higher the pitch; the lower the frequency, the lower the pitch. A healthy human ear can hear frequencies from about 20 Hz to 20,000 Hz. Infrasonic below 20 Hz Ultrasonic above 20,000 Hz

18 – 2 Nature of Sound Intensity and loudness - Intensity - depends on the amount of energy in each wave, which corresponds to the wave’s amplitude. Intensity increases as amplitude increases; measured in decibels (dB). Loudness - human perception of sound intensity. The higher the intensity and amplitude, the louder the sound. People vary in sensitivity to different frequencies. The faintest sound heard by most humans is 0 dB. Intensities above 120 dB may cause pain and permanent hearing loss. Rock concerts are usually at this loudness. *Look at Table 18-1 on p. 468.

18 – 2 Nature of Sound Doppler effect - the apparent change in frequency and pitch of a moving object. The object can be moving or the observer can be moving for the effect to occur.

18 – 2 Nature of Sound Noise pollution - sounds that are loud, annoying, or harmful to the ear. They can come from machinery, engines, or amplified music. Noise pollution can cause damage by shattering glass and cracking plaster. The most damaging effects of noise pollution is damage to the human ear. It can cause permanent hearing loss.

18 – 3 Musical Sounds Music vs. noise Music is specific pitches and sound quality following regular patterns. Noise has no set pattern and no definite pitch. Stringed instruments generate a sound when the strings create waves that move back and forth between the ends of the string. Changing the length of the string and the diameter can change the sound/pitch. Resonance - when another object vibrates at the same frequency as the source of sound. (The air inside a guitar vibrates at the same frequency as the string of the guitar)

18 – 3 Musical Sounds Sound quality describes the differences among sounds of the same pitch and loudness but from different sources. Overtones have frequencies that are multiples of the fundamental frequency. Occurs in stringed instruments and wind instruments. The various overtones compose musical chords.

18 – 3 Musical Sounds Interference - the ability of two or more waves to combine and form a new wave. The compressions overlap to make a much larger compression. Constructive interference - when the compressions arrive at the same place at the same time. Destructive interference - when the compression of one wave and the rarefraction of another arrive at the same time. They cancel out each other causing a decrease in loudness. If instruments are not in tune - producing the same frequencies - then the sounds create a “beat” and an unpleasant sound.

18 – 3 Musical Sounds Acoustics - the study of sound; it is used to reduce excess “reverberations”. Reverberations ( echoes ) - the effect produced by sound waves reflecting from objects/walls.