Sound Waves

What is Sound?

Sound is the movement of longitudinal
waves (vibrations) that travel through
the air

Needs to produce sound

1. Source

A vibration of some kind
(Wave)

2. Medium

Something for the Waves
to travel through

For example: Air

3. Receiver

Something or someone to
"hear" the sound

Speed

The speed of sound varies
depending on how the fast
the particles are able to
vibrate

Vibrate easily in solids
due to the particles being
close together allowing for
the fastest travel time (speed)

Don't vibrate at all
in vacuums

Vibrate the least amount
in gasses

Vibrate second fastest in
liquids

Formula

V = 331.4 + (0.606 x T)

331.4 in m/s

0.606 in m/s degrees celcius

T = temp in celcius

V = Speed in m/s

Sound Intensity

The amount of sound energy
passing through a certain a
unit area in a second

Measured in Decibel Units

Logarithmic Scale

20 dB is 10 times
greater then 10 dB

30 dB is 100 times
greater then 10 dB

Formula

dB = 10 x log (I2/I1)

I2 is the amount of
sound energy

I1 is the threshold
of hearing

1 x 10^-12 W/m^2

Frequency Levels

The human ear can hear sounds
that have a frequency that is
between 16 Hz - 20,000 Hz

Infrasound

A sound that's frequency is below
the hearing range of humans

Below 16 Hz

Ultrasound

A sound that's frequency is above
the hearing range of humans

Above 20,000 Hz

Sound Characteristics

Loudness

The amplitude of a
sound wave

The higher the amplitude, the
louder the sound

Sound Quality

Pure Tone

Simple Sound

Overtone

Rich Sound

No Pattern

Noise

Pitch

The frequency of the sound wave

The higher the frequency, the
higher the pitch

Octave

Doubling the frequency of
a sound wave

Mechanical Resonance

When 2 different objects
vibrate at the same frequency

Results into a louder sound

Beat Frequency

When two object vibrate
at very close frequencies

The waves of both the objects
will interfere and go from loud
to soft to loud to soft

This pattern is known as beat
frequency

Air Columns

Open-Air Column

Has both open ends

Both open ends are antinodes

Resonant Lengths

Closed-Air Column

One open end and
one closed end

Closed end has a node

Open end has an antinode

Resonant Lengths

An Air Column is a tube
where sound waves
reflect causing a standing
wave

Due to resonance, air columns
amplify the sound that are causing
the standing wave

Stringed Instruments

The vibrating of the string
causes a standing wave

The fundamental frequency is
the natural lowest frequency
of the string in one segment

When a string vibrates in
more then one segment, its
called an overtone

Harmonic

A harmonic is a multiple
of the fundamental frequency
of the string

1st Harmonic

Length

1/2 x Lambda

Frequency

f = v/2L

2nd Harmonic

Length

2/2 x Lambda

Frequency

f = 2v/2L

2 times the fundamental
frequency

3rd Harmonic

Length

3/2 x Lambda

Frequency

3 times the fundamental
frequency

f = 3v/2L

Factors Affecting
Vibrating Strings

Length

Shorter length produces
a higher frequency

Diameter

A higher diameter produces
a lower frequency

Tension

A higher tension produces
a higher frequency

Density

A higher density produces
a lower frequency