Acoustics Flashcards by Mark Stephenson

How is sound created?

Sound is caused by something emitting energy in the form of a vibration

Everything that vibrates creates a sound

Sound through the air are vibrations of air molecules a pressure variation - sound pressure perceived by the ear as sound

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What is ultra and infra sound?

Infrasound are sounds Below 20Hz
Ultrasound are sounds above 20,000Hz

Human ears perceives frequencies between 20-20,000Hz

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What is a sound wave?

It is a disturbance that travels through a medium from one location to another. The waves are made up of compressions (peaks) and rarefactions (troughs)

Example - tuning fork
As fork moves outward, air molecules in the air are pushed closer together. This is high pressure - compression

As fork moves inwards, air molecules pushed further apart. This is low pressure - rarefaction

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What are the two types of waves? Describe them

LONGITUDINAL (sound)
Particles move/vibrate in same direction as the wave energy. It runs PARALLEL to the wave every source. (Hand pushing/pulling)

TRANSVERSE (water)
Particles move/vibrate perpendicular (90 degrees) to direction of wave energy (dropping stone into water causing ripples - stone downwards, ripple outward)

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What is a cycle?

One complete vibration (rotation) made up of one compression and one rarefaction

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What is the period?

The time (in seconds) required for one cycle to complete

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Describe a wavelength

The measure of distance between 2 identical parts of a wave ie peak to peak or trough to trough

It is the DISTANCE it takes for one complete cycle (1 compression and 1 rarefaction) and is measured in METRES

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What is the difference between a periodic, aperiodic and sine wave?

Periodic is a tone which repeats itself it is pure tone.
Sine wave is a pure tone, 1 single frequency repeating. It is the simplest sound
Aperiodic is random, non repeating complex

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What is the speed of sound?

340 metres per second

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What is frequency?

The rate at which something occurs over a particular period of time.
It is measured in HERTZ

It is subjectively experienced s as PITCH

Ie how many wavelengths (complete cycles of a wave) pass a certain point per second.

Less vibrations = low frequency due to longer (extended) wavelength
More vibrations = higher frequency as particles pushed closer together (compressed)

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What is pitch?

It is the subjective experience/sensation of a wavelength and it’s frequency

Sounds are classes as high or low pitch but, unlike frequency, it cannot be measured as it varies from one person to another.

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What is intensity?

Intensity (amplitude) is the distance from normal pressure (where particles normally sit) to the point of maximum displacement - the larger the amplitude/displacement the LOUDER the sound
Therefore amplitude is subjectively experienced as LOUDNESS

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What is sound decay?

Sounds naturally dissipate over time as they lose their energy. This is known as decay.

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Equation to measure sound?

V
—-
F W

Velocity we know is 340m/s
Frequency will be in hertz
Wavelength in meters

Velocity divided by F or W gives the other
F x W = velocity

Ie v divided f (@1kHz) is:
340 divided by 1000 = 0.34 metres wavelength

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Transmission of sounds - describe 8 types

DIFFRACTION - ability of sound waves to BEND around objects. Longer the wavelength (lower frequency) the easier it is

DIFFUSION - scattering of sound, more even

ECHOES - delayed return of sound, a effective wave with an audio gap

ABSORPTION - gets absorbed ie by soft furnishings

RESONANCE - continuation of a sound due to reflection

HEAD SHADOW - region of reduced amplitude of a sound as it is obstructed by the head - this is an important part of Localisation

REFLECTION - of sound not absorbed or transmitted when it strikes a surface it is reflected. Direct when strikes, indirect when reflected

REVERBERATION - when reflection occurs but causes persistent sounds due to multiple simultaneously occurring reflected sound waves

Reverberation TIME - use a sound source louder than 60 dB.
Switch off sound source - time taken for sound to drop by 60 dB is measured

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What is Phase?

Relates to the position of a particle on a waveform.
Where 0 degrees means both waves exactly in phase then
If waves are 360 degrees to one another they are in sync
If waves are 180 to one another there are exactly opposite and therefore out of phase

What is interference?

When 2 waves overlap it can be construction or destructive interference

2 sounds in phase = combined their stronger ie constructive

2 sounds out of phase combine b cancel one another out ie destructive - no sound

What is the inverse square law?

As the sound doubles the distance from its source, it loses 6dB every time it doubles

‘Intensity’ decreases proportionally to the square of the distance from the source

Original intensity divided by the distance squared

If sound is 65dBSPL it will be 59dBSPL at 2m, 53 at 4m, 47m at 8m and so on

What is distortion?

Sounds/ noises (unwanted) that were not present in the original sound ie additional sounds at the output that were not present at the input

What are decibels?

It is a measurement of sound pressure. It tells us how LOUD a sound is,but it is not an absolute measurement (intensity is subjectively experienced as loudness)

How do we measure a sound using decibels?

We compare the ratio (of pressure) of one sound (the sound we hear) to a reference sound (the quietest sound humans can hear 0 on dBHL and 0 on dBSPL)

Therefore hearing tests actually test the sensitivity to pressure changes.

How do we measure pressure?

PASCALS! Unit of measurement for pressure. Pascal (Pa)

Humans quietest sound heard is
0.00002Pa (or 20 micropascals)

Loudest sound humans hear is
20 pascals (or 20,000,000 micro pascals)

1 micro pascal = 1,000,000 pascals

How do we convert pascals to dBSPL?

dBSPL = 20 x log (pressure divided by reference pressure)

Examples - what is 2000 microPa in dBSPL?
20 x log (2000 divided by 20 quietest sound = 100 count the zeros - 2)
Therefore,
20 x 2 = 40
2000 microPa = 40 dBSPL

What is MAF and MAP?

Minimum audible field and
Minimum audible pressure

MAF - minimum level of hearing detected binaural without headphones (free field). Normal threshold of hearing.

MAP - minimum level of hearing detected monoaurally via headphones. More sound needed monoaurally as no summation of sound.
How much sound needed to be noticed.

MAP scale used for correction factors when converting dBSPL to dBHL.

Explain bels and decibels

Both unit of measurement of intensity (loudness) Logarithmic scale converts pascals to bels however bels scales is 1-12 which is too small a scale for sensitive ears Therefore each bel divided into 10 creating 12 decibels each containing 10 bels so hearing scale now 0-120 dBSPL

What is the dynamic range?

0 dBHL (threshold) to 120 dBHL which is true for 1-2kHz Lower or higher frequency the range is narrowed Sounds above 90dB are damaging to inner ear Over 120dB causes irreversible damage

What is RETSPL?

Reference equivalent threshold sound pressure level As we’re do now hear all frequencies equally, different frequencies require different sound pressures for them all to be heard equally. So average normal hearing is plotted as a threshold with correction values: 1kHz 7dB 2kHz. 11dB 4kHz. 10.5 dB The RETSPL now gives us a flat starting point (1k is really 7 and 4kHz is 10.5 etc)

What is Audiometric zero?

Where normal hearing should be able to hear at a level of 0dBHL across all frequencies. It is not then really zero but the average level a person should be able to hear at.

What is Localisation?

The ability to pinpoint source and location of sound using input from the ears, as well as the cognitive process

Describe horizontal Localisation

It is dependent on BOTH ears hearing. One ear (closest to sound source) will hear sounds louder (intensity) and quicker (time) compared with the other ear. It will NOT work if HL in one ear or only one HA worn.

What 3 elements does the brain compare to achieve horizontal Localisation? Describe them.

ITD - INTERAURAL TIME DIFFERENCE sounds coming from the side arrive at one ear (closest ear) earlier and the other ear later. Difference in arrival time is the ITD. Normally around 6 millionth of a second. IID - INTERAURAL INTENSITY DIFFERENCE sounds dissipate over distance. Therefore sound is heard louder in the closest ear to sound source. IPD - INTERAURAL PHASE DIFFERENCE relates to sound wave being at different parts (or phases) in each of the ears ie closest ear in compression but by time gets to other ear it’s in rarefaction Phase difference used by brain to identify direction of sound. Tends to work better with low frequency as wavelength longer useful for sounds below 1500Hz ie wavelengths with longer width than head.

What is the head shadow (Localisation)

The head acts as a barrier when sound is presented from one side. The ear in shadow of head can have a 6dB less intensity difference to other ear. Higher frequencies less able to diffract around head so furthest ear lacks high frequencies heard at closest ear.

Describe vertical Localisation

The contours and folds of the pinna create ECHOES determined by the size and shape of the pinna. This only applies to HIGH frequencies which can bounce around where as low freq tend to diffract. Sounds arriving at TM is combination of DIRECT and REFLECTED sounds. Brain uses this info to determine position of sound. Pinna echoes are the difference between direct and reflected sounds. Sounds from above have a FASTER echo. Sounds from below are SLOWER.

Describe front Localisation

Pinna faces forward at an angle. High frequency boosted by natural resonance of pinna when they arrive from the front. From the rear, low frequency diffract around and into ear. High frequency above 3kHz less able to do so. Brain uses this ‘loss’ of high frequency to identify sound from rear.