Sound of Music Flashcards
Sound sources
- all require a medium
- causes vibrations (oscillations)
Periodic oscillation
motion that repeats itself at regular time intervals
Oscillating periodic motion
Harmonic motion
Time period
the time it takes for one complete cycle to occur (return to its original position)
Frequency (pitch)
number of waves per second (number of cycles per unit time)
Amplitude (loudness)
maximum displacement either side of its equilibrium
Equilibrium position
- where the object comes to rest after oscillations
- all forces are equal so no acceleration
Phase
the stage an oscillation has reached in its cycle
In-phase
two oscillations in step; at the same point at the same time (on top of each other)
Out of Phase
two oscillations reaching peaks and troughs at different times (opposite)
Phase difference
the time difference between out of phase waves
Antiphase
have a phase difference of 180° or π/2
Quadrature
waves/oscillations with a phase difference of 90° or π/4
Wavefront
the leading edge of a single wave
Sound waves
travel by spreading out in all directions from the vibrating source
What do waves transfer?
energy from the source to other places without transferring matter
Transverse waves
oscillations occur at 90° to the direction of wave propagation (direction of travel) (direction of energy transfer)
Longitudinal waves
oscillations occur parallel to the direction of wave propagation (direction of travel) (direction of energy transfer) create compressions and rarefactions
Are sound waves transverse or longitudinal?
longitudinal
Wavelength
distance from peak to peak/trough to trough
Superposition
combination of two waves as they arrive at the same place at the same time (two waves pass through each other often reflections without being disturbed so the amplitudes, generally, sum together)
Coherent
waves of the same frequency/wavelength and in phase with each other
Constructive superposition
when two waves that are in phase meet and their amplitudes sum together
Destructive superposition
when two waves that are out of phase meet and their amplitudes subtract/cancel each other out
Standing waves
- When waves of a single frequency reflect back and forth on top of each other
- A wave in which some points don’t/hardly move and others are at maximum amplitude
Node
- a point on standing waves where the amplitude is zero
- where the wave doesn’t move very much
Antinode
- a point on standing waves where the maximum amplitude occurs
- where the wave has the biggest difference
How is sound created within tube instruments?
due to a standing wave being set up within the tube. Standing waves are set up because sound waves are reflected from either end
Fundamental frequency
- the lowest frequency that can be created from a given tube L= λ/2
- the most basic frequency, every other frequency is one harmonic up
Stringed instruments
produce a transverse standing wave - due to superposition of waves travelling along string and reflecting at support
Noise
unwanted sound
Analogue storage and retrieval systems
vinyl and tape
CD’s method of storage
digital recording - storing the sound as binary code
How is the digital information coded onto a CD
as a series of small raised areas creating pits and lands on the CD. Light (specifically LASER) is shone on the CD it reads the data due to the waves superposing
Superposition in a CD player
If the wave hits a flat surface all the light will be reflected in phase with no phase difference so constructive superposition will occur telling the reader the light is ‘on’. If the wave hits half a bump some of the light will travel further so there will be a phase difference causing destructive superposition to occur so the light will send the reader ‘off’
Monochromatic light
light of a single frequency
CD’s have a coated surface
this reduces damage to them but as light passes through the transparent material refraction may occur so this needs to be taken into account
Refraction
the change in direction of light as it reaches a boundary of two materials with different densities causing a change in speed
Converging lenses (convex)
Fatter in centre than edge will bring light together at focal point
Diverging lenses (concave)
fatter at edge, thin in middle will spread light out
Focal length
distance from lens to focal point
Total internal reflection
when light approaches a boundary at an angle greater than the critical angle it will be reflected internally
Polarised light
light goes in all directions so this is when light has been blocked going in one direction so the intensity of light is reduced
Polarising filter
a filter which blocks the light going in one direction
Electromagnetic waves
are transverse
Polarised
the oscillations only occur in one plane (plane polarised)
Unpolarised
light going in all directions
Light blocked
when two polarising filters are placed at 90° to each other
How can you observe and analyse the light spectrum?
Using a spectrum analyser - separates out the different frequencies present
Diffraction grating
allows you to spread the colours over a wider range of angles so they’re more easily distinguished (spread white light)
Colour and Refraction
- different wavelength appear as different colours
- refraction is linked to wave speed
Light particle
Photon
Wavicle
light can be modelled as both a wave and a particle, this is called wave-particle duality. Sometimes we need to use properties of particles to describe waves and other times we use properties of waves
Photon
concentrated packet of electromagnetic energy
Line spectra
light that contains only a few distinct colours separated by gaps rather than a continuous spectrum - light was emitted by atoms in an electrical discharge tube
Electrical discharge tube
extra energy from the electrical supply is transferred to electrons. The electrons come to lose this energy and emit light.
One photon
absorbs one electron
Energy levels
there are different energy levels in an atom and when given energy the electrons move to higher energy levels and drop back down. When they drop back down they emit photons (light). Photons emitted correspond to electrons moving between levels.
Laser light
- emits light of one frequency
- light is emitted in a narrow beam not every direction
- emission is coherent (waves in phase)
LASER
Light Amplification by Stimulated Emission of Radiation
How do we get laser light?
Electrons will need to be stimulated to a higher energy level, then as they drop back down to their original energy level they will emit a photon. Sometimes the excited electron will be disturbed by a photon that’s already been emitted, this will cause the excited electron to drop back down and emit a photon. The two photons will be in phase and have the same frequency and direction.
What is a photodiode used for?
to transfer energy from light to an electrical signal - used in a CD player
Describe a speaker
- there is a permanent magnet at the rear of the speaker which has a magnetic field
- the cone has a coil of wire with many turns
- the amp sends weak a.c to the coil creating a second magnetic field around the wire
- the two magnetic fields interact and attract and repel the cone creating the oscillating movement
How can we make a magnetic field stronger?
Have more turns of the coil of wire
1st Harmonic
2nd Harmonic
L=2 λ/2
L=3 λ/2
every harmonic add half a wave
How do you create sound from a beer/wine bottle?
by blowing on it as you create a different pressure causing particles to move
Close ended tube at one end
node at one end- closed (particles can’t move)
antinode at the other- open
Open ended at both end
antinode at both ends
Why does a guitar have a sound box?
it allows a larger volume of air to oscillate making it louder
What happens when in phase light passes through a double slit?
The waves will diffract and at certain points constructive superposition will occur creating dark and bright bands on a screen (bright and dark fringes)
CD
has grooves which are called pits and lands. When the light is on just a pit or a land constructive superposition occurs but if on both a pit and a land then destructive superposition occurs because of a path difference
Path difference
one wave (of light) travels further than another wave
CD pit
must be λ/4 because the path difference must be λ/2 and it goes there and back
Monochromatic light
single frequency
Emission spectra
colour shows where photons would be
Absorbance spectra
blocks out where the photons would be absorbed
How is LASER light amplified and concentrated?
-trap between mirrors to let the photons bounce between them stimulating more electrons to drop making more photons. Make a hole in one which allows any photons that hit them to come out in a stream of light
