Chapter 15

Sounds

15.1- Properties and Detection of Sound

Importance of Sound

• 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?

• Movement forward compresses air particles- increases pressure

• Movement backwards separates air particles- decreases pressure

• 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

• Echoes- reflected sounds off hard surfaces

Detection of Pressure Waves

• Human ear takes vibrations in the air and transmits them into electrical impulses

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)

• 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.

• 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

The Doppler Effect

• Doppler Effect- frequency shift

• 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

Setting up Parameters

• + from source to detector• - from detector to source

• The velocity of sound is always positive!

• 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.

• 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.

15.2- The Physics of Music

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

Open Closed

• Flutes• Saxophones

• Clarinets• Sea Shells

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

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

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

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

Musical Intervals

• Octave- 2 notes with frequencies related 1:2– Ex: 440 Hz: 880 Hz– Ex: Fundamental: 1st Harmonic: 2nd Harmonic

Beats

• Beat- oscillation of wave amplitude

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)