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 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) 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)