Topic Three - Waves and their Spectra, Filtering and Distortion Flashcards
What is the atmospheric pressure value?
101.3 kPa
What causes sound waves?
Sound waves are produced by the vibration of an object
What objects can be made to vibrate?
Any object with:
- Inertia: a force is required to set it moving
- Elasticity: It has the ability to return to its original state
What is frequency?
Frequency is how many cycles of a wave their are per unit of time.
Frequency is measured in Hertz (Hz)
What is the period of a wave?
The amount of time taken to complete one full cycle of a wave.
Can be calculated:
Period = 1/frequency
What determines an object’s vibration frequency?
The object’s:
- Mass
- Stiffness
What is velocity?
How fast a signal travels through a medium.
It is a vector quantity and therefore describes both direction and magnitude (e.g., 60 km/h North).
The magnitude of velocity is speed (a scalar) denoted as c.
What is the speed of sound in air (15 degrees Celsius)?
340m/s
The speed of sound traveling through air varies with the air temperature (warmer = faster).
Equation
C(air) = [331.5 + (0.6 x air temp.)] m/s
What is the fastest medium sound travels through and why?
- Solid
- Liquid
- Gas
Because the molecules in solids are more dense and the molecules are more tightly packed, the energy transfer is faster.
What is the speed of sound in water?
1500 m/s
What is the wavelength?
A measure of how far the wave travels in one period of a cycle
Equation:
wavelength (λ) = velocity/frequency
What are the equations to calculate wavelength, velocity and frequency?
λ = c/f
f = c/λ
c = f.λ
λ = wavelength
f = frequency
c = speed
What is amplitude?
How can we break up a sound wave to see the different frequencies?
Through a spectral analysis known as a fast Fourier transform.
This allows us to look at the wave form in the frequency domain on the x-axis and amplitude/phase on the y-axis.
There is also a inverse fast Fourier transform which allows us to obtain an original wave form using the magnitude and phase spectra.
What is additive synthesis?
A way to make complex sound waves through adding multiple sine waves of different frequencies.
What comprises a square wave?
Fundamental frequency + odd harmonics
What comprises a triangle wave?
Fundamental frequency + odd harmonics
What comprises a saw-tooth wave?
Fundamental frequency + all harmonics
True/false: the more higher harmonics added, the rounder the wave?
False, the waves get sharper as the higher harmonics are added
What is a sound spectrogram?
- Shows frequency on the y-axis and time on the x-axis
- Shows amplitude with a colour scale
What gives a sound its timbre?
The unique harmonic structure
What is a complex aperiodic waveform?
A waveform made up of components that:
- Are not harmonically related
- Do not repeat over time
- Can be classified as either continuous or transient
What is narrow-band noise?
How does the spectral content of continuous wave forms differ from wave forms that have a finite length?
The spectral content of a finite wave is broadened. Wave forms that start and end (transient waves) have spectral splatter.
Spectral splatter is the spreading of energy across a wide range of frequencies that occurs when you suddenly start or stop a wave form.
What is a tone burst?
- A brief sinusoid
- Because it starts and stops, the bandwidth is broader than that of a simple sinusoid.
What is a click?
A type of noise that has a very wide bandwidth due to their short duration.
What is windowing?
Windowing is how we remove the spectral splatter from transient wave forms.
It ensures that the sound is gradually turned on and does not contain any clicks. This is relevant to audiological testing as we want to ensure there are no click or extra frequencies present in the stimulus that can impact the client’s responses.
Windowing increases the frequency specificity.
It also allows us to get an accurate idea of what the frequency content is.
What is the bandwidth of a wave?
True/false: The bandwidth (and frequency specificity) of a wave are inversely proportional to their duration.
True - The shorter the duration of the wave the larger the bandwidth
What does filtering do?
A filter is anything that modifies the amplitude or phase spectrum of a signal.
This means you can remove spectral content/frequencies and accentuate others.
Applications include:
- improving signal-to-noise ratio
- Compensating for room acoustics or speaker acoustics
- Compensating for hearing loss
What is the corner frequency of a filter?
The point at which the signal is altered by 3dB - half of the signal’s energy is removed (relates to the logarithmic scale)
What is the filter order?
The slope that cuts the signal away or how abruptly the signal gets filtered.
We can measure the slope by the decibel drop per octave.
What does an all-pass filter do?
Doesn’t remove anything
What does a low-pass filter do?
Only lets in low frequency sounds - removes the high frequencies
What does a high-pass filter do?
Only lets in high frequency sounds - removes the low frequencies
What does a band-pass filter do?
Cuts out some high frequencies and some low frequencies.
Keeps only a pass band in the middle.
What does a band stop filter do?
Cuts out a narrow range in the middle called a stop band.
What happens if you over filter?
- It alters the wave shape
- Over low-pass filtering smooths the wave out
- Over high-pass filtering distorts the wave
What is a linear system?
What is distortion?
What is clipping?
Clipping occurs when the amplitude of a signal exceeds the limits of the system it is passing through.
When clipping occurs additional frequency components are added. Some energy of the fundamental frequency is transferred into additional harmonic components.
True or false: clipping always cuts off the top and the bottom of a wave.
False. You can have BOTH symmetrical and asymmetrical clipping.
Symmetrical clipping occurs when the wave is clipped equally on the positive and negative point on the waveform (top and bottom). This results in additional harmonics in the odd harmonics only.
Asymmetrical clipping occurs when the wave is clipped in one direction more than the other. This results in additional odd and even harmonics.
What is a transfer curve?
A function that relates an output waveform to an input waveform.
A straight line represents a 1:1 correspondence - linearity
In what things is linearity good and bad?
Linearity is good in:
- Audiometers
- Electronic devices
- Loudspeakers, microphones, sound systems
linearity Bad:
- Hearing aids and cochlear-implant processors
- Cochlear
How can we measure distortion?
Through the Total Harmonic Distortion (THD)
