SOUND WAVES

Sound Waves

Speed of Sound • Depends on the material of the vibrating medium

• Sound can vibrate water, wood (speaker enclosures,

pianos), metal, plastic, etc.

• Sound speed in dry air is 330 meters/second at 0o C

• Faster in warm air, slower in cold

• Water 4 times faster, steel 15 times faster

Speed of Sound

• Medium velocity m/sec

air (20 C) 343

air (0 C) 331

water (25 C) 1493

sea water 1533

diamond 12000

iron 5130

copper 3560

glass 5640

II. PROPERTIES OF SOUND

Pitch - description of how high or low the

sound seems to a person - The property of sound that varies with

variation in the frequency of vibration

Loudness - how loud or soft a sound is perceived to be.

- how much energy a sound wave carries

High frequency means more vibrations

hitting the ear.

Pitch is how high or how low a sound

seems to be.

Healthy humans can hear from 20 Hz to

20,000 Hz

We are most sensitive from 440 Hz to

7,000 Hz.

Frequency and pitch

Sound Waves

Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of

sound the pitch.

Loudness of Sound in Decibels

SOUND LOUDNESS (dbs) HEARING DAMAGE

Average Home 40-50

Loud Music 90-100 After long exposure

Rock Concert 115-120 Progressive

Jet Engine 120-170 Pain

Speed of sound

1) 332 m/s in air at 0 C.

2) Changes by 0.6 m/s for every

Celsius degree from 0 C.

3) Subsonic – slower

4) Supersonic – faster than sound

(Mach 1 = speed of sound)

5) Sonic boom (pressure cone)

The ratio of the speed of a moving

body to the speed of sound

INFRASOUND - sounds with frequencies below the

normal human range of hearing. Sounds in the 20-200 Hz range

they are felt rather than heard

(earthquakes, heavy machinery).

ULTRASOUND - sound waves with frequencies above the

normal human range of hearing. Sounds in the range from 20-100kHz

Ultrasonic sound has a frequency

greater than 20,000 Hz.

a) Dogs (up to 35,000 Hz)

b) Bats (over 100,000 Hz)

c) Medical diagnosis

Supersonic Flight SUPERSONIC

(of speed) greater than the speed of sound in a given medium (especially air)

Having frequencies above those of audible sound

An explosive sound caused by the shock wave of an

aeroplane travelling faster than the speed of sound

SONIC BOOM

Interference • the result of two or more sound

• waves overlapping

Different sounds that you hear include (A) noise, (B) pure tones, and (C) musical notes.

Standing waves in these open tubes have an antinode

at the open end, where air is free to vibrate.

Doppler Effect

- is the apparent change in the frequency of a sound caused by the motion of either the listener or the

source of the sound.

Sounds from Moving Sources

• A moving source of sound or a moving observer experiences an apparent shift of frequency called the Doppler Effect.

• If the source is moving as fast or faster than the speed of sound, the sound waves pile up into a shock wave called a sonic boom.

• A sonic boom sounds very much like the pressure wave from an explosion

Doppler Effect 4 cases

• Source moving toward receiver

• Source moving away from receiver

• Receiver (observer) moving towards source

• Receiver (observer) moving away from source.

Wave Reflection

• When a sound wave reflects from a

surface we generate an echo

• Wave reflection from surfaces depends on

the characteristics of the surface

• Smooth hard surfaces reflect best

• Rough soft surfaces reflect poorly

• Energy not reflected is absorbed or

transmitted through the material

Wave Reflection

• Think of arrows pointing in the direction of the

wave motion

• We can trace the path of these arrows

Angles Equal

Wave Reflection

Acoustics of room

design is very

interesting. Need some

reflections to “liven” the

room. Too many

reflections and the sound

gets mushy. Look in a

concert hall or

auditorium to see the

different sound

treatments

Wave Refraction • If there is a change in the characteristics of a

medium, waves are bent

• This occurs because different parts of the wave

front travel at different speeds

• Think of a marching around a curved track

• The inside people have to move more slowly

than the outside people to keep the lines

straight

PROPERTIES OF SOUND WAVE

Reflection

ECHO – reflected sound wave

REVERBERATION – Multiple echo

Wave Refraction

Wave Reflection & Refraction

• The combination of reflection and

refraction enables imaging

• Ultrasonic medical imaging

• Naval SONAR for detecting submarines

• Bats catch mosquitoes

Diffraction of Sound

the bending of waves around small*

obstacles and the spreading out of

waves beyond small* openings.

* small compared to the wavelength

Natural Frequencies

• Objects have “natural” frequencies based

on their size and structure

• Guitar strings are an example

• Timpani heads

• Air columns

Forced Vibrations

• Can externally impose a vibration on an object

• Guitars and violins and pianos

• Set the wood into motion at the frequency

of the string

• This provides a larger surface to interact with the air

• Harp vs. Piano

Resonance • When the forced vibration matches a natural

frequency we get a “resonance” condition

• Think about a swing on a playground

• You go high when you pump the swing at its

natural vibration frequency

• Sympathetic vibrations in tuning forks

• Famous Tacoma Narrows bridge collapse

Resonance - the ability of an

object to vibrate by absorbing

energy at its natural frequency.

5. Uses of sound

a. Acoustics – the study of sound.

Soft materials dampen sound; hard

materials reflect it (echoes and

reverberations).

b. SONAR – Sound Navigation and

Ranging (echolocation).

c. Ultrasound imaging

d. Kidney stones & gallstones.