chapter 15 sounds. 15.1- properties and detection of sound importance of sound
TRANSCRIPT
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Chapter 15
Sounds
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15.1- Properties and Detection of Sound
Importance of Sound
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• Place your hand on your throat. • Speak to someone next to you for 30 seconds.• Sing to someone next to you for 5 seconds.• What do you feel?
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• Movement forward compresses air particles- increases pressure
• Movement backwards separates air particles- decreases pressure
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• Sound waves- longitudinal waves with pressure variation that is transmitted through matter (cannot move in a vacuum)– Speed of sound depends on temperature• 0.6 m/s per 1oC• 343 m/s @ room temperature (20oC) @ sea level
– Speeds increase in liquids and solids
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• Echoes- reflected sounds off hard surfaces
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Detection of Pressure Waves
• Human ear takes vibrations in the air and transmits them into electrical impulses
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Perceiving Sound
• Pitch- depends on the frequency of the vibration
• Human Ear can hear– 20 Hz-16,000 Hz– 20 Hz-10,000 Hz (older people)– 20 Hz-8,000 Hz (age 70-cannot understand
speech)
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• Find the wavelength in air at 20oC of an 18 Hz sound wave, which is one of the lowest frequencies that is detectable by the human ear.
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• Loudness- perceived by our sense of hearing, depends primarily on the amplitude of the pressure wave– 1 billionth of an atmosphere or 2x10-5 Pa to 20 Pa
(pain)– Sound level-logarithmic scale measured in
decibels (dB).• 10 dB increase is about 2x as loud
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The Doppler Effect
• Doppler Effect- frequency shift
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• Fd=fs(v-vd/v-vs)
– v=velocity of the sound wave– vd=velocity of the detector
– vs=velocity of the sound source
– fd=frequency received by the detector
– fs=waves frequency
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Setting up Parameters
• + from source to detector• - from detector to source
• The velocity of sound is always positive!
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• You are in an auto traveling at 25.0 m/s toward a pole mounted warning siren. If the siren’s frequency is 365 Hz, what frequency do you hear? Use 343 m/s as the speed of sound.
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• A sound source plays middle C (262 Hz). How fast would the source have to go to raise the pitch to C sharp (271 Hz)? Use 343 m/s as the speed of sound.
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15.2- The Physics of Music
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Resonance in Air Columns
• Closed pipe resonator- a resonating tube with one end closed to air– High pressure reflects back on high pressure
• Open pipe resonator- resonating tube with both ends open– Low pressure reflects back on high pressure
– Increased amplitude from constructive interference causes the sound to get louder
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Open Closed
• Flutes• Saxophones
• Clarinets• Sea Shells
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Resonance on Strings
• Each end is clamped and therefore has a node on each end.
• Speed of the wave depends on the tension and mass per unit length.
• Must attach to a sounding board (which must resonate as many frequencies) to intensify sound
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Sound Quality
• Tuning fork- uses simple harmonic motion which can be uninteresting
• Instruments and Voices- use superposition to blend many frequencies which seems more pleasing to hear– Timbre, tone color, tone quality
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The sound spectrum: fundamental and harmonic
• Fundamental- lowest frequency (f1)– Closed pipe- f1=λ/4– Open pipe- f1=v/2L
• Harmonics- multiples of the lowest frequency– Closed pipe- odd multiples– Open pipe- even multiples
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Consonance and Dissonance
• Dissonance- unpleasant set of pitches• Consonance- please set of pitches (pitches
with small whole number ratios)– Ex: 1:2, 2:3, 3:4
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Musical Intervals
• Octave- 2 notes with frequencies related 1:2– Ex: 440 Hz: 880 Hz– Ex: Fundamental: 1st Harmonic: 2nd Harmonic
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Beats
• Beat- oscillation of wave amplitude
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Sound Reproduction
• Stereo system- 20-20,000 Hz frequencies are played with less than 3 dB difference so all notes can be heard
• Telephone- 300- 3000 Hz• Noise- mixture of many frequencies (some say
has a calming effect)