Longitudinal (compression) waves made by vibrating matter Sound Waves

15.2 Sound Waves A sound wave is a wave of alternating high-pressure and low-pressure regions of air.

Sound waves need a medium to travel Sound travels faster through denser mediums Does sound travel faster through water or air?

15.2 The frequency of sound  We hear frequencies of sound as having different pitch.  A low frequency sound has a low pitch, like the rumble of a big truck.  A high-frequency sound has a high pitch, like a whistle or siren.  In speech, women have higher fundamental frequencies than men.

Measuring Sound  Sound wave frequency (pitch) is measured in Hertz (Hz)  Sound wave amplitude (loudness) is measured in Decibels (dB)

Common Sounds and their Loudness

15.1 Loudness Every increase of 20 dB, means the pressure wave is 10 times greater in amplitude. Logarithmic scale Linear scale Decibels (dB)Amplitude , , , ,000,000

15.1 Sensitivity of the ear  How we hear the loudness of sound is affected by the frequency of the sound as well as by the amplitude.  The human ear is most sensitive to sounds between 300 and 3,000 Hz.  The ear is less sensitive to sounds outside this range.  Most of the frequencies that make up speech are between 300 and 3,000 Hz.

15.1 How sound is created  The human voice is a complex sound that starts in the larynx, a small structure at the top of your windpipe.  The sound that starts in the larynx is changed by passing through openings in the throat and mouth.  Different sounds are made by changing both the vibrations in the larynx and the shape of the openings.

15.2 The wavelength of sound

15.2 The Doppler effect  The shift in frequency caused by motion is called the Doppler effect.  It occurs when a sound source is moving toward an person, or a person is moving toward the source of the sound

jY Doppler Effect Simulation

15.2 The speed of sound  The speed of sound in air is 340 meters per second (660 miles per hour) at one atmosphere of pressure and room temperature (21°C).  The speed of sound = Mach number

15.2 The speed of sound  We use the term supersonic to describe motion at speeds faster than the speed of sound.  The pressure change across the shock wave is what causes a very loud sound known as a sonic boom.  Supersonic booms break the sound barrier (exceed the speed of sound)

15.2 Sound waves and boundaries  Like other waves, sound waves can be reflected by surfaces and refracted as they pass from one material to another.  Sound waves reflect from hard surfaces.  Ex. Echo

15.3 Music  The pitch of a sound is how high or low we hear its frequency. Though pitch and frequency usually mean the same thing, the way we hear a pitch can be affected by the sounds we heard before and after.  Rhythm is a regular time pattern in a sound.  Music is a combination of sound and rhythm that we find pleasant.  Most of the music you listen to is created from a pattern of frequencies called a musical scale.

15.3 Consonance, dissonance, and beats  Harmony is the study of how sounds work together to create effects desired by the composer.  When we hear more than one frequency of sound and the combination sounds good, we call it consonance.  When the combination sounds bad or unsettling, we call it dissonance.

15.3 Consonance, dissonance, and beats  Consonance and dissonance are related to beats.  When frequencies are far enough apart that there are no beats, we get consonance.  When frequencies are too close together, we hear beats that are the cause of dissonance.  Beats occur when two frequencies are close, but not exactly the same.

15.3 Harmonics and instruments  The same note sounds different when played on different instruments because the sound from an instrument is not a single pure frequency.  The variation comes from the harmonics, multiples of the fundamental note.