a range of a range of compressioncompression wave wavefrequencies to which thefrequencies to which thehuman earhuman ear is sensitive is sensitive

Sounds are produced bySounds are produced byvibrating mattervibrating matter

1. reedsreeds

2. stringsstrings

3. membranesmembranes

4. air columnsair columns

Sound is a Sound is a mechanical wavemechanical wave (longitudinal). (longitudinal). It will It will notnot travel through a vacuum. travel through a vacuum.

• PITCH = The impression about the frequency of Sound

• high pitched – high frequency (ex: piccolo)• low pitched – low frequency (ex: fog horn)

• The frequency range for normal human hearing is . between 20 Hz and 20,000 Hz . (for younger people, older people lose the higher frequencies)

• Infrasonic or subsonic = Sounds below 20 Hz • Ultrasonic = frequencies above 20,000 Hz

Range of Some Common SoundsRange of Some Common Sounds

Intensity Range for Some Common SoundsIntensity Range for Some Common Sounds

Compression waves travel through air or along springs.

These waves travel with

areas of compression and rarefaction.

The medium does not travel from one place to another, but the pulse that travels.

Any matter will transmit sound, whether it is a solid, liquid or a gas.

However; sound cannot travel through a vacuum.

Sound wave requires medium

DING

No Sound

VACUUM

The velocity of sound in air The velocity of sound in air dependsdepends

on the air temperature. The speed on the air temperature. The speed ofof

sound in dry air is sound in dry air is 331.5 m/s331.5 m/s at at 0 0 ººCC.. This speedThis speed

increasesincreaseswith with

temperature: temperature: about about 0.6 m/s0.6 m/sfor every 1 for every 1 ººC C increase in increase in

temperature.temperature.

Speed of Sound • The speed of sound in dry air at 00 C is about

330 m/sec. (1200 km / hr) [or ~ .000,001 x the speed of light of 300,000 km / sec]

• So air at room temperature (~20oC) is ~340 m/sec.

• QUESTION : . How far away was the strike if there is a 3 second delay between the lightning flash and the sound of the thunder?

• 340 m/sec x 3 sec = 1020 m, over 1 km (~2/3 mile) away

Sound generally travels Sound generally travels fastestfastest

in solids and slowest in in solids and slowest in gases,gases,

but there are some but there are some exceptions.exceptions.

Medium Velocity (m/s) Medium Velocity (m/s)Medium Velocity (m/s) Medium Velocity (m/s)

Air 330 Carbon dioxide 260Air 330 Carbon dioxide 260

Helium 930 Hydrogen 1270Helium 930 Hydrogen 1270

Oxygen 320 Water 1460Oxygen 320 Water 1460

Sea water 1520 Mercury 1450Sea water 1520 Mercury 1450

Glass 5500 Granite 5950Glass 5500 Granite 5950

Lead 1230 Pine wood 3320Lead 1230 Pine wood 3320

Copper 3800 Aluminium 5100Copper 3800 Aluminium 5100

• The speed of sound in a material does NOT depend on its density (mass per unit volume [g/cm3 ]).

• The speed of sound in a material DOES depend on the elasticity of a material.

• Elasticity = the ability of a material to change shape in response to an applied force, then resume its original shape when the force is removed.

• Steel is elastic, putty is inelastic.

• Sound travels 15 times faster in steel than in air and about 4 times faster in water than in air.

Speed of Sound

LOUDNESS • The intensity of a sound is proportional to the

square of the amplitude of the sound wave. (i = ka2)

• Loudness is measured in decibels (dB)

• 1 10 100 1000

• The decibel scale is logarithmic, increasing by factors of 10

VIBRATINGLOUDSPEAKER

AMPLITUDE

MICROPHONE

OSCILLOSCOPE

LOUDNESS • TABLE of Loudness Levels• SOURCE OF SOUND LEVEL (dB)• Jet Engine (from 30 m) 140 • Threshold of pain 120 • Loud rock music 115 • Old subway train 100 • Average factory 90 • Busy street traffic 70 • Normal speech 60 • (Shshshshhh!) A library 40 • Close Whisper 20 • Normal breathing 10 • Hearing threshold 0

Forced Vibration• Sounding boards are used to augment (increase) the volume

(amplitude) of a vibrating object (like a string).

STRINGS

SOUNDING BOARD

Natural Frequency

• Everything vibrates, from planets and stars to atoms and almost everything in between.

• A NATURAL FREQUENCY is one at which minimum energy is required to produce forced vibrations

• and also requires the least amount of energy to continue this vibration

Resonance

Resonance – when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude of the vibrations occurs.

• For example, a swing, or the hollow box parts of musical instruments are designed to work best with resonance.

• In order to resonate, an object must be elastic enough to return to its original position and have enough force applied to keep it moving (vibrating)

A resonant air column isA resonant air column issimply a standing simply a standing

longitudinallongitudinalwave system, much likewave system, much like

standing waves on a standing waves on a string.string. closed-pipe resonatorclosed-pipe resonator tube in which one end is tube in which one end is

openopenand the other end is closedand the other end is closed

open-pipe resonatoropen-pipe resonatortube in which both endstube in which both ends

are openare open

A A closed pipeclosed pipe resonates when resonates when the the length length

of the air columnof the air column is approximately is approximatelyan an odd numberodd number of of quarterquarter

wavelengths long.wavelengths long.

l = {(1,3,5,7,…)/4} *

With a slight correction for tube diameter,With a slight correction for tube diameter,we find that the resonant wavelength of awe find that the resonant wavelength of a

closed pipe is given by the formula:closed pipe is given by the formula:

= 4 (l + 0.4d),= 4 (l + 0.4d),

where where is the wavelength of sound, is the wavelength of sound,l is the length of the closed pipe,l is the length of the closed pipe,and d is the diameter of the pipe.and d is the diameter of the pipe.

An An open pipeopen pipe resonates when resonates when the the lengthlength

of the air columnof the air column is approximately is approximatelyan an even numbereven number of of quarterquarter

wavelengths long.wavelengths long.

l = {(2,4,6,8,…)/4} *

With a slight correction for tube diameter,With a slight correction for tube diameter,we find that the resonant wavelength of anwe find that the resonant wavelength of an

open pipe is given by the formula:open pipe is given by the formula:

= 2 (l + 0.8d),= 2 (l + 0.8d),

where where is the wavelength of sound, is the wavelength of sound,l is the length of the closed pipe,l is the length of the closed pipe,and d is the diameter of the pipe.and d is the diameter of the pipe.

Interference • Sound waves interfere with each other in the same

way as all waves.• Constructive interference - augmentation

• Destructive interference - cancellation

Beats • BEATS - A periodic variation in the loudness of

sound. . (faint then loud, faint then loud and so on … )

• What is the frequency when a 262 Hz and a 266 Hz tuning fork are sounded together ?

• The 262 Hz and 266 Hz forks will produce 4 beats per sec. and the tone heard will be half- way between at 264 Hz as the ear averages the frequencies.