Sound (physics)

Define Sound

Sound travels as longitudinal waves

Sound is a disturbance that travels as a series of compressions through a medium

Speed of sound

In the air, it is temperature ddependant

formula for speed of sound

v=331+0.6T

What happens when air temp increases

speed of sound increases

Explain what happens to the speed of sound as temp increases

as temp increases the air molecules are already moving faster , therefore are capable of carrying sound energy faster

Sound intensity

The intensity of a sound wave is a product of the amplitude

Intensity of a sound wave is perceived as loudness

Loudness is measured using the decibel scale

Decibel scale

The smallest detectable pressure by the average human ear is 2×10 -5 Pa = 0db

Every increase in pressure of 10 times is an increase of 20db

How do we detect sound waves

A sound wave is a pressure wave, we detect sound by detecting sound pressure with our ears

How loud is a decibel

The average listener describes an increase in volume of 10db and being twice as loud

Properties of sound waves

The frequency of a sound wave is related to the pitch of a sound

Pitch is defined as the degree of highness or lowness of a tone (the higher the frequency, the higher the pitch, vise versa)

The average human can pecieve sounds in the frequency range of 20-20000 Hz

The upper end decreases as we age

Doppler effect.

The perceived change in pitch of a moving sound sound due to the relative velocities of moving sound source or observer

When the sound source is stationary

it sends our waves that travel away in all directions at the speed of sound

When the sound source moves

The waves in the front of the sound source compress, becoming closer together and decrease the wavelength. Causes the perceived frequency to increase

Behind the source, the opposite happens, the wave get stretched apart and the wavelengths increases. Resulting in a lower frequency

which direction is positive

the direction of the detected sound wave

what happens when the source has a speed near or equal to the speed of sound

Sound waves ahead of the source cant escape because they are leaving at the same speed of the source.

This means waves ahead of the the source to pile up at the front

A ridge of high pressure builds up in front of the object called the sound barrier, immediately followed by a region of low pressure

When is an object supersonic

You must “break to sound barrier” to go above the speed of sound

Explain a mach cone

When the speed of the sound is exceeded, constructive interference occurs, causeing a Shockwave to follow

Sonic boom

to experience the passing of a shockwave. Will happen to anyone standing close enough

what happens when 2 waves meet in the same medium and time

interfere with one another

lead to the formation of a node or antinode

Beat frequency

when 2 sound waves close in frequency, they will interfere at regular intervals

when does resonance in air columns occur

standing waves in air columns can happen in an open tube or close tube.

Boundary conditions of an open tube standing waves

An antinode must be present on both ends

Example of open tube standing

A flute is a tube that that is open at both ends

When playing, you blow air across the mouth piece which creates vibrations

The waves created travel through the tube and reflect back from the open end

loudness

the perceived intensity of the sound wave

Decibel

A unit used to measure loudness

2 ways to create different harmonics in a closed/open tube

In an tube of fixed length we can create different harmonics by varying the frequency

In a tube of fixed frequency, we can vary the length of the tube

how do you determine the harmonic in an OPEN tube

the number of nodes

what eqautions can be used to find frequency in an open tube

Fn = nv/2L

F1=V/2L

Fn= n x F

Equations to find length on a tube with fixed frquency in an open tube

Ln= nv/2F

L1= V/ 2F

Ln= n x L

Boundary conditions of close tube sound sources

a node must be present at the closed end and a antinode must be present at the open end

Example of a closed tube sound source

When you play a clarinet the musician seals the mouth piece with their mouth. Closing one end of the instrument. Air is blown through a thin reed which vibrates, creating a source of vibration for the air column which travels along the tube and reflect back from the open end.

what is the wave length increasing by every harmonic for a closed tube standing wave

2/4 wavelengths (1/4 is the fundamental frequency)

when can you hear a sonic boom

anyone standing where the shockwave passes

open tube boundary condition

an antitnode must be present at both ends