Sound Waves

Can you hear me now?

Wave Review

• Sound is a Longitudinal Wave- it moves back and forth like a spring.

• Sound is a Mechanical Wave- it needs a medium, like water or air to travel through.

• Sound is a Pressure Wave- it creates a difference in pressure in the medium.

Tuning Forks

• Tuning Forks create sound by vibrating tines that push air away, creating sound waves.

Pressure Waves

• As air is pushed by the tines, it creates areas of compression (higher pressure).

• In between areas of compression are areas of rarefaction (lower pressure).

• This creates the “high” part of the wave, and the “low” part.

Density of Air Molecules

Pressure of a Sound Wave

Now do you hear me?

• When the high/low pressure wave hits the eardrum, it causes it to vibrate, and your brain translates these into sounds, letting you “hear”.

Wave Terminology Review

• How fast the tuning fork tines are moving back and forth is the frequency (f) that the sound wave is being created at.

• Frequency (f) = # of vibrations / second

• Frequency (f)- Hertz (Hz)

Wave Terminology Review

• The distance between high pressure areas of the sound wave is the wave’s Period (T)

Getting Musical

• The sensation (what we “hear”) of a sound wave’s frequency is referred to as it’s pitch.

• A higher frequency (faster vibrating tuning fork), creates a higher pitch- a treble note.

• Lower frequency (slower vibrating tuning fork), creates a lower pitch- a bass note.

Octaves

• Any two notes (pitch) whose frequencies are a 2:1 ratio (one is twice as fast as the other), are separated by an ocatve.

• Putting notes together with specific mathematical ratios creates scales, chords, and harmonies.

Breaking down the music scale

• Interval Frequency Ratio Examples• Octave 2:1 512 Hz and 256

Hz

• Third 5:4 320 Hz and 256 Hz

• Fourth 4:3 342 Hz and 256 Hz

• Fifth 3:2 384 Hz and 256 Hz

Characteristics of Sound Waves

• Because of the nature of air molecules, sound waves travel outward in every direction at once.

• The high/low pressure waves don’t just go in straight lines (imagine a rock being dropped in a pond).

Decibels

• The larger the amplitude (amount of high and low pressure created) of the sound wave, the more energy it carries.

• Greater energy creates a louder sound.• Sound intensity is measured in Decibels

(dB).• Clapping your hands harder requires more

energy, the sound wave carries more energy, the sound is louder.

Source IntensityIntensity

Level# of Times

Greater Than TOH

Threshold of Hearing (TOH) 1*10-12 W/m2 0 dB 100

Rustling Leaves 1*10-11 W/m2 10 dB 101

Whisper 1*10-10 W/m2 20 dB 102

Normal Conversation 1*10-6 W/m2 60 dB 106

Busy Street Traffic 1*10-5 W/m2 70 dB 107

Vacuum Cleaner 1*10-4 W/m2 80 dB 108

Large Orchestra 6.3*10-3 W/m2 98 dB 109.8

Walkman at Maximum Level 1*10-2 W/m2 100 dB 1010

Front Rows of Rock Concert 1*10-1 W/m2 110 dB 1011

Threshold of Pain 1*101 W/m2 130 dB 1013

Military Jet Takeoff 1*102 W/m2 140 dB 1014

Instant Perforation of Eardrum 1*104 W/m2 160 dB 1016

 

Sound Wave Intensity

• Sound waves carry a certain amount of power (Watts), and they move outward in a circle from their source.

• Putting these two together gives:

24 r

PIntensity

Sound Wave Intensity

• What is the intensity of the sound waves produced by a trumpet at a distance of 3.2m when the power output of the trumpet is 0.20 W? Assume that the sound waves are spherical.

Practice

• Pg. 415 Practice A