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Ch. 15 Sound Milbank High School Sec. 15.1 Properties of Sound • Objectives – Demonstrate knowledge of the nature of sound waves and the properties sound shares with other waves. – Solve problems relating the frequency, wavelength, and velocity of sound. – Relate the physical properties of sound waves to the way we perceive sound. – Define the Doppler shift and identify some of its applications. Sound Waves • Are pressure variations that are transmitted through matter • Air molecules colliding • Longitudinal waves • Speed depends on air temperature – Warmer=faster Sound Waves • Can’t exist in a vacuum • Echoes – Reflected sound waves – Sonar Loudness • Depends on amplitude • We hear based upon pressure differences – Larger amplitudes create a higher pressure differences • Sound level – Decibels (dB) – 0 dB (most faintly heard), 2 x 10-5 Pa – Tenfold increase, 2 x 10-4 Pa is 20 dB – 110 is painful (most concerts) Pitch • Difference in frequency – Middle C: 262 Hz – E: 327 Hz • Human hearing – 20-16000 Hz – SIM – Pitch/Beat Freq – SIM – Pitch/Beating Freq Doppler Shift • Police siren as it drives by SIM • More sound waves reach you when the vehicle is moving towards you • Vehicle + waves = higher frequency • Waves – vehicle = lower frequency • Bats: – Insects flying away: lower frequency – Insects flying towards: higher frequency Sec. 15.2 The Physics of Music • Objectives – Describe the origin of sound. – Demonstrate an understanding of resonance, especially as applied to air columns. – Explain why there is a variation among instruments and among voices using terms timbre, resonance, fundamental, and harmonic. – Determine why beats occur. Sources of Sound • Produced by vibrating objects – Vocal cords – Brass instruments – Reed instruments – Stringed Instruments – Others Resonance • When one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion. • Sometimes occurs in a column or tube (trombone) • Length of air column determines the frequency Closed pipe resonance • One closed end (marimba, pipe organ, sea shell) • First sound heard at ¼ wavelengths • Additional resonance lengths found at half-wavelength intervals Open-end Resonance • Woodwinds, brass • Sounds loudest at half-wavelengths • Wavelengths half as long as in a closed ended tube • Leads to twice the frequency Sound Quality • Fundamental – Lowest frequency • Harmonics – Odd-numbered multiples of the fundamental frequencies • Dissonance – Unpleasant set of pitches • Consonance – Pleasant set of pitches Musical intervals • Octave – Frequencies in a 1:2 ratio – Ex: 1st note-440 Hz, octave higher would be 880 Hz – Also corresponds to harmonics Beat Notes • Beat – Oscillation of wave amplitude – Beat frequency is the difference between the two frequencies