Sound

The Wave Model of Sound Sound travels in longitudinal waves. Sound waves can only move through matter, not empty space. Several things change the speed of sound--the kind of medium, temperature, elasticity, and density of the medium. The higher the temperature, the faster sound travels. If the medium is very elastic, sound travels faster. If the medium is very dense, sound travels faster.

The Sound Barrier When a plane travels slower than the speed of sound, its wave crests do not overlap. When a jet travels faster than sound, it moves past its wave crests. The wave crests overlap, causing constructive interference. Where the wave crests overlap a cone-shaped shock wave forms. This is the sonic boom you hear. Wave crests shock wave

Properties of Sound - Intensity The more energy a sound has, the more amplitude the sound wave and the louder the sound. The loudness of sound is called intensity. Sound intensity is measured in units called decibels (dB). Sounds louder than 120 dB can cause pain and permanent hearing loss.

Properties of Sound - Frequency The more sound waves pass a point each second, the higher the sound’s frequency, and the higher the sound’s pitch. Pitch describes how high or low a sound is. Frequency is measured in units called Hertz (Hz). The range of human hearing is from low sounds of 20 Hz to high sounds of 20,000 Hz. Many animals can hear sounds that humans cannot hear.

The Doppler Effect The Doppler Effect is a change in the pitch and frequency of a sound caused when either the sound source or the listener moves. When an ambulance comes toward you, its sound waves are bunched together, but when the ambulance passes you, its sound waves spread out. Bunched together waves produce a high pitched sound, but spread out waves make a low pitched sound.

Sound Quality Sound quality, or timbre makes the different instruments in a orchestra sound different even when they are playing the same note. Musical instruments make sounds of several different frequencies at the same time. The blending of these frequencies is timbre.

Sound Wave Interactions - Reflection and Diffraction When sound waves hit a hard flat surface, they are reflected back. This causes an echo. To prevent echoes, theaters have heavy curtains on the walls to absorb sound. Why can you hear sounds around a corner? Sound waves are diffracted around edges of barriers and fan out in all directions. Low frequency sounds diffract more than high frequency sounds.

Sound Wave Interactions - Refraction and Interference Sound waves travel faster in warm air than in cool air. You can hear sounds better at night than in the daytime because as air cools at night, the sound waves are refracted back towards the earth. Sound waves can interfere with each other. In some places in a stadium or auditorium, it can be harder to hear than in others. This is because of “dead spots” produced by destructive interference.

Sound Wave Interactions - Resonance Many objects have a natural frequency that can be matched by other things. Wind can make windows vibrate. If wind blows at the natural frequency of the concrete in a bridge, the bridge can vibrate so much it collapses! The vibration of an object at its natural frequency is called resonance. A tuning fork vibrates when a musical instrument plays at its natural frequency.

How We Hear Sound waves travel through your ear canal to the eardrum, which vibrates when the sound waves hit it. The eardrum pushes on the hammer, anvil, and stirrup, making them vibrate. The stirrup pushes on the oval window of the cochlea. Inside the cochlea, hair cells of the same frequency are made to vibrate, and they change the vibrations into nerve signals. The nerve signals travel by the auditory nerve to your brain, which interprets the sound. Oval window ear canal eardrum Auditory nerve semicircular canals cochlea Stirrup anvil hammer Eustachian tube

Sounds You Hear - Music Musical sounds are pleasing. They have pitch, intensity, rhythm, harmony, melody, and quality. Rhythm is a repeating pattern of beats or accents. Each piece of music has a different series of pitches, or melody, accompanied by other notes played together (harmony). Musical instruments produce several sounds at once, their main tone, and higher frequency tones, called overtones. The combination of overtones makes each instrument sound different. There are three main groups of musical instruments: wind, string, and percussion.

Sounds You Hear - Noise Noise is sound that is annoying and unwanted. Sound waves from music are regular and predictable but sound waves from noise are random and irregular. Sounds between 60-100 dB can be annoying, but sounds above 100 dB can damage your hearing. Sounds above 120dB can cause pain. Rustling leaves 10 dB Whispering 10-20 dB Talking 60-70 dB Vacuum cleaner 75-85 dB Jet engine 170 dB