Chapter 15

The Nature of Sound What is Sound??? Sound is a Longitudinal Wave traveling through matter.

Longitudinal Waves Matter vibrates in the same direction as the wave travels.

Longitudinal Waves Compression Rarefaction

λ

Sound from a Tuning Fork

Speed of Sound Sound is transmitted through matter. The Velocity of Sound depends on the matter that carries it.

Sound travels at a velocity of 332m/s in air at 0  C. Sound travels faster through warm air than through cold air. The velocity of sound increases about 0.6m/s for each degree in temperature. Sound travels much faster through liquids and solids than through gases. At 20  C sound travels at 344m/s.

Comparing Media MediaSpeed of Sound Air at 0°C331m/s Air at 20°C343m/s Water at 25°C1493m/s Sea Water at 25°C1533m/s Iron at 25°C5130m/s Rubber at 25°C1550m/s

Human Hearing Frequency of Sound 20 Hz to 20,000 Hz. Sound above 20,000 Hz - Ultrasonic Sound less than 20 Hz – Subsonic (Infrasonic)

Frequency is Pitch

Detection of Pressure Waves

Ear Drum

Intensity and Loudness Intensity of Sound Depends on the amplitude of the wave. Loudness Describes a person’s response to sound intensity.

Loudness is measured in Decibels(dB) For every 10dB change the sound doubles!! 70dB is twice 60dB 80dB is four times 60dB

Faintest Sound Heard0dB Whisper15dB Rustling Leaves20dB Purring Cat25dB Average Home50dB Vacuum Cleaner75dB Noisy Restaurant 80dB Power Mower100dB Chain Saw115dB Painful dB Jet Plane Taking Off150dB

Interference Constructive Interference Occurs when the compressions and rarefactions of two or more waves come together. Louder Sound

Interference Destructive Interference Occurs when a compression of one wave arrives at the same time as a rarefaction of another wave. Quieter Sound

Interference Beats The result of compressions and rarefactions of two slightly different frequencies reaching your ears together. Beats

f 1 = 512Hz f 2 = 514Hz Beats = f 1 - f 2 Beats = 2Hz (beats/s) = 514Hz - 512Hz

The Doppler Effect The change in wave frequency caused by the motion of the sound source or the motion of the observer.

The Doppler Effect Shorter Wavelength Higher Frequency

The Doppler Effect Longer Wavelength Lower Frequency

Speed of Sound

Greater than the Speed of Sound

Homework #15-1 PP: 1-4 Page: 352 Section Review Page:355 Due: 3/12/03

Resonance A resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object.

Harmonics Vibrations which occur at a particular frequency is known as a harmonic.

First Harmonic The lowest possible frequency at which a string could vibrate to form a standing wave pattern is known as the fundamental frequency or the first harmonic.

First Harmonic

Second Harmonic

Third Harmonic

Resonance in Air Columns Closed Air Column λ = 4L L λ = 4 / 3 Lλ = 4 / 5 L

Resonance in Air Columns Open Air Column λ = 2L L λ = Lλ = 2 / 3 L

Example A tuning fork is placed above an open-pipe resonator in which the length can be changed. The loudest sound is heard at a length of 67cm and the next loudest was heard at 100.5cm. If the temperature of the air is 20°C what is the frequency of the tuning fork?

Example 67cm 100.5cm ( )= 33.5cm 33.5cm = ½λ 233.5cm = λ 67cm = λ

Example λ = 67cm = 0.67m = 343m/s v = λf f = v/λ f = 512Hz f = 343m/s 0.67m

Homework #15-2 PP: 5-9 Page: 362 Due: 3/17/03

Music to Your Ears A back and forth motion is set up in a string, resulting in a regular vibration. The vibration is called a standing wave the location of the crests and troughs are always in the same place.

In a wind instrument, holes are opened and closed, changing the length of the vibrating column of air. This changes the size of the standing wave.

Noise Sound with no regular pattern or definite pitch.

Tone Quality The differences among sounds of the same pitch and loudness.

Music Musical Sounds Based on a series of notes called a musical scale.

The Sound Spectrum: Fundamental and Harmonics

Open Air Column λ = 2L L λ = L λ = 2 / 3 L f 1 = v/λ f 1 = v/2L f 2 = v/L f 2 = 2f 1 f 3 = v/ 2 / 3 L f 3 = 3f 1

Fundamental Frequency First Overtone Second Overtone Third Overtone 262Hz 524Hz 786Hz 1048Hz

Closed Air Column λ = 4L L λ = 4 / 3 Lλ = 4 / 5 L f 1 = v/4L f 2 = v/ 4 / 3 L f 2 = 3f 1 f 3 = v/ 4 / 5 L f 3 = 5f 1

Fundamental Frequency First Overtone Second Overtone Third Overtone 256Hz 768Hz 1280Hz 1792Hz

Harmony Notes that sound pleasing together. The ratio of the frequencies of tones that are in harmony are small whole numbers.  Notes that are one octave apart. Middle C and C 524/262 = 2/1  Notes E and C 330/262 = 5/4

Dissonance and Consonance Dissonance combination of pitches that sound unpleasant. Consonance combination of pitches that sound pleasant.

Musical Intervals Octave: Two notes that have a ratio of 1:2. Example: 440Hz 880Hz one octave higher. 220Hz one octave lower.

Interference Constructive Interference Occurs when the compressions and rarefactions of two or more waves come together. Louder Sound

Interference Destructive Interference Occurs when a compression of one wave arrives at the same time as a rarefaction of another wave. Quieter Sound

Interference Beats The result of compressions and rarefactions of two slightly different frequencies reaching your ears together. Beats

f 1 = 512Hz f 2 = 514Hz Beats = f 1 - f 2 Beats = 2Hz (beats/s) = 514Hz - 512Hz

Homework #15-3 Practice Problem:10 Section Review Page: 367 Due: 3/18/03

Homework #15-4 Study Guide Due: 3/19/03 Test: 3/20/03