Chapter 13Sound

Sound is a compressional wave created by a disturbance or vibration

that compresses molecules.

When the tuning fork pushes toward the direction of wave motion, the air molecules are compressed, when it pushes away, the air molecules are spread out.

Frequency is how many compressions are created in a second (Hertz), and is also called pitch.

The speed of sound depends on the density of the medium. Molecules are closer together in denser materials, so there is less distance the disturbance needs to move through to reach the next molecule.

  vmedium (m/s)air (0°) 331water 1490iron 5130

Sound waves propagate in 3d spherically

If a sound source is moving waves get pushed together and spread apart parallel to the motion.

This causes a change in frequency

Sound Intensity

Sound intensity is the rate at which sound energy flows through a unit of area.

Intensity =E/tarea

P

= area = P

4r2

Units = W/m2

Loudness is a function of the log of the intensity. For something to sound twice as loud, it would have to be 10x as intense.

Loudness is measured in decibels (dB), with sound being twice as loud every 10 dB.

vacuum 70 dBalarm clock 80 dB

All things have natural vibrating frequencies

If one tuning fork is struck and the other tuning fork is the same frequency, it will start to vibrate. This is called resonance.

HarmonicsStanding waves can be created in a vibrating string.

,...3,2,12

nL

vnfn

fn = frequency of harmonic numbern = harmonic number

v = wave speed

L = length of vibrating string

Standing waves in an air column

Open at both ends

,...3,2,12

nL

vnfn

Closed at one end

,...5,3,14

nL

vnfn