Chapter 2

Impulsive Sounds, Alone and in Sequence

Picture of a Transverse Wavel

WavelengthWavelength

AmplitudeAmplitude

AA

CrestCrest

TroughTrough

WAVE DESCRIPTION

Transverse and Longitudinal Waves

Wave Motion

Click on the phrase above and then select either transverse or longitudinal

waves

Sound is a longitudinal wave disturbance

Wavelength is the distance between compression.

Sound is a Longitudinal Wave

The tuning fork moves air molecules back and forth in the direction the sound wave travels

- meters or feet

• Distance between adjacent crests in a transverse wave

• Distance between compressions in a longitudinal wave

• Distance a wave travels during one vibration

• Units

Wavelength ()

• Time required to make one vibration.

• Time required to generate one wave.

• Time required for the wave to travel one wavelength.

Period ()

The number of vibrations per unit of time made by the vibrating source.

Units - cycles per second

1/s

Hertz (Hz)

Frequency (f)

• What is the frequency of the second hand of a clock?

Frequency = 1cycle/60 secFrequency = 1cycle/60 sec Period = 60 secPeriod = 60 sec

• What is the frequency of US Presidential elections?

Frequency = 1 election/4 yrsFrequency = 1 election/4 yrs Period = 4 yrsPeriod = 4 yrs

Examples of Frequency

T 1T

f

T

f

T

fT

f Tf fTf

In symbolic form

oror

f

• Demo - Drop Different Sounding Objects

• Rap on Table

• Finger Snap

• Tuning Fork

NATURAL FREQUENCY

• Objects have natural frequencies at which they vibrate.

• The natural frequency depends on elasticity and shape.

A Selection of Natural Frequencies

• Resonance occurs when successive impulses are applied to a vibrating object at the object’s natural frequency.

• Result - increased amplitude• Examples:

Swinging

Marching on a bridge

RESONANCE

Tacoma Narrows Bridge

• Demo - Tuning Fork Touching a Table

• Sound is intensified because of the larger

surface area that can vibrate the air.

• The surface is forced to vibrate at the

frequency of the tuning fork. (It is not a

resonance phenomenon.)

• Examples: Musical sounding boards

FORCED VIBRATIONS

Equally Tempered ScaleIn most acoustics texts the octaves are numbered from the left side of the piano keyboard. The note labeled C4 is middle C.

Octaves

• If the note has the same pitch name, then the frequency is related by a power of 2. C4 = 256 Hz A4 = 440 Hz

C5 = 512 Hz A5 = 880 Hz

Repetition Rates

• Consider a drummer playing with his left hand (o) and his right hand (x). He might play

-------o-------o-------o-------o-------o-------o-------o-------o

• Next he plays a flam

-------o-x-----o-x-----o-x-----o-x-----o-x-----o-x-----o-x-----

Or,

-------o---x----o---x----o---x----o---x----o---x----o---x----o

Repetition Rates

• We would have no trouble identifying this as the same frequency as the original

• But put the beat of the other hand at the exact midpoint and the listener hears twice the repetition rate.

Electronic Experiment

FrequencyGenerator

DelayD

Shaper2

Shaper1

SumAudioAmpl.