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9.3 What is a sound wave? • Sound waves are pressure waves with alternating high and low pressure regions. • When they are pushed by the vibrations, it creates a layer of higher pressure which results in a traveling vibration of pressure. 9.3 What is a sound wave? • At the same temperature, higher pressure contains more molecules per unit of volume than lower pressure. 9.3 How we hear sound • The parts of the ear work together: 1. When the eardrum vibrates, three small bones transmit the vibrations to the cochlea. 2. The vibrations make waves inside the cochlea, which vibrates nerves in the spiral. 3. Each part of the spiral is sensitive to a different frequency. Hearing Loss & Deafness • Tinnitus is one of the most common types of hear loss. Results from long term exposure to loud noises. • The loud noise damages the hair cells and nerve ending in the cochlea. • Damaged hair doesn’t grow back so to any part of the inner ear usually means permanent hearing loss. Protecting Your Hearing • Loud sounds can be blocked out by earplugs. • Listen to things at a lower volume • Put a distance between yourself and the loud noises. • When doubling the distance between yourself and the loud sound will lessen the sound’s intensity by ¼. 9.3 The wavelength of sound • We usually think about different sounds in terms of frequency, but wavelength is also important. • Musical instruments use the wavelength of a sound to create different frequencies. 9.3 The wavelength of sound • The wavelength of sound in air is similar to the size of everyday objects. 9.3 The frequency of sound • The pitch of a sound is how you hear and interpret its frequency. • A low-frequency sound has a low pitch. • A high-frequency sound has a high pitch. Each person is saying “Hello”. 9.3 The frequency of sound • Humans can generally hear frequencies between 20 Hz and 20,000 Hz. • Most sound has more than one frequency. • Almost all the sounds you hear contain many frequencies at the same time. 9.3 The frequency of sound • Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level. • According to this curve, a 40 dB sound at 2,000 Hz sounds just as loud as an 80 dB sound at 50 Hz. 9.3 The loudness of sound • The loudness of a sound is measured in decibels (dB). • The decibel is a unit used to express relative differences in the loudness of sounds. 9.3 The loudness of sound • Most sounds fall between 0 and 100 on the decibel scale, making it a very convenient number to understand and use. 9.3 The Doppler effect • When the object is moving, the frequency will not be the same to all listeners. • The shift in frequency caused by motion is called the Doppler effect. • You hear the Doppler effect when you hear a police or fire siren coming toward you, then going away from you. Reflection of Sound Waves • An echo is a reflected sound wave • The strength of the reflected sound wave depends on the reflecting surface • Sound waves reflect best off of smooth, hard surfaces • Example: a shout in a gym will echo better than in an auditorium because the walls in an auditorium are designed to absorb sound, not reflect it Echolocation • Reflected sound waves can be used for many different things • Echolocation is the process of using reflected sound waves (echos) to find objects • Some animals use echos to find food, they can tell how far away something is by how long it takes sound waves to echo back to their ears Examples: bats, beluga whales • People use sonar to locate objects underwater, helps navigators on ships avoid icebergs, also helps to map ocean floor Echolocation continued • Ultrasonography is a medical procedure that uses echos to “see” inside a patient’s body without doing surgery. • Echos reflect off the patient’s organs and are then changed into images that can be seen on a screen • Used to see gallbladders, kidneys, babies, and other organs Interference of Sound Waves • Orchestras and bands make use of constructive interference when several instruments are playing, this causes the combined amplitude to increase resulting in a louder sound 9.3 The speed of sound • Objects that move faster than sound are called supersonic. • If you were on the ground watching a supersonic plane fly toward you, there would be silence. • The sound would be behind the plane, racing to catch up. 9.3 The speed of sound • A supersonic jet “squishes” the sound waves so that a cone-shaped shock wave forms where the waves “pile up” ahead of the plane. • In front of the shock wave there is total silence. • Passenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr. Resonance • Resonance happens when an object vibrating at or near a resonant frequency of a second object causes the second object to vibrate. • If you strike a tuning fork and hold it near a guitar string, the guitar string will start to vibrate