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audio specialisms > acoustics > Flashcards

acoustics Flashcards

1
Q

define wavelength

A

distance across a space covered by one complete cycle of a waveform

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2
Q

how many deg in a phase

A

a complete cycle has 360° of phase counting from the start of the cycle.
eg from 0 amplitude to the first peak is 90°, then to the 0 point again is 180°, to the trough is 270° then 360° to complete the cycle

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3
Q

what is the speed of sound in air? (m/s)

A

344 m/s

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4
Q

define sound

A

a longitudinal-wave disturbance in any compressible substance

longitudinal meaning with a direction parallel to the thing that made it happen

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5
Q

define frequency

what is the perceivable freq range of humans?

moving an octave does what to frequency?

A

number of times per second a wave completes a cycle, repetition rate of a vibration
measured in Hz or kHz

humans can perceive 20Hz to 20kHz, approx 10 octaves

moving an octave doubles or halves frequency

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6
Q

how do you calculate the value of a normalised (amplitude between 1 and -1) sine wave with peak value of +1 and trough value of -1?

A

sin(degrees of phase)

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7
Q

give the equation linking wavelength, speed and freq

A

W=S/F

a wave with freq 100Hz travelling through air has a wavelength of = 344/100 = 3.44 m

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8
Q

compression a rarefaction are…

A

equal and opposite

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9
Q

define displacement amplitude

A

distance each bit of air moves to either side of its normal position during its vibration

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10
Q

define pressure amplitude

A

max increase of air pressure in a sound wave compression which is measured by microphones

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11
Q

what’s the freq of concert pitch?

A

A2 = 440Hz

therefore A3 = 880Hz and A1 = 220Hz

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12
Q

define waveform

how are waveforms of musical instruments produced?

A

a waveform is a graphical representation of pressure as a function of time

fluctuations in pressure in the air are measured by a microphone and converted to an equivalent electrical voltage and displayed on an oscilloscope

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13
Q

define periodic sound

A

perfectly steady sound, repeats same vibrational sound for a long time, eg synths

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14
Q

define transient sound

A

a sound that is not sustained, dies away quickly, eg drum kick
has attack and decay

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15
Q

define sound propagation

A

sound travelling through a compressible media like air or water

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16
Q

how does absorption of sound happen?

A

particles of a soft or flexible surface can easily be made to vibrate by a sound wave so the energy is drained into these surfaces

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17
Q

how does reflection of sound happen?

A

hard materials with densely packed particles are not easily made to vibrate so energy just bounces off these surfaces

18
Q

what is the law of reflection?

A

the angle of incidence is equal to the angle of reflection to the normal of the surface
i=r

19
Q

for diffuse reflection to occur, the irregular surface must have…

A

irregularities that are roughly on the same scale as the wavelength of the sound wave

irregularities that 1/4 of of wavelength will begin to cause diffusion

20
Q

a sound directed at concave wall, how does distance from the wall affect where the sound can reach?

A

if the source is far form the wall, the sound reflects right back to the source
if the source moves a bit closer to the wall, the sound spreads out when it reflects

21
Q

define refraction

A

change of wave direction due to difference in speed from one place to another

22
Q

define diffraction

A

a behaviour of waves in which they will spread outward, filling regions rather than leaving a shadow

eg, sounds can be heard from around a corner

23
Q

diffraction
when sound comes thru an opening narrow compared to its wavelength how does the wave behave?

when it goes thru an opening that is wide compared to its wavelength how does it behave?

A

spreads out equally in every direction

remains largely confined to a beam continuing in the original direction

24
Q

describe the inverse square law

A

sound spreading from a source into unobstructed air, the sound intensity decays at a rate inversely proportional to the square of the distance travelled thru the unobstructed space

if sound travels double the distance, the intensity decreases by a factor of 4, distance 2d gives intensity i/4

25
Q

define intensity

A

measurement of energy acting on a given areag

26
Q

give the 5 classifications of instruments according to curt sachs

A
  • idiophones - entire instrument moves to produce sound, eg cymbal
  • membranophones - vibrating membrane stretched over an opening, eg drums
  • chordophones - sound produced by vibrating string, eg piano
  • aerophones - sound produced by vibrating column of air, eg brass and wind
  • electrophones - instruments requiring voltage
27
Q

define sympathetic vibration

A

when an object cannot produce sound on its own and uses a larger surface area to create loudness and pass their vibrational energy into the air

eg guitar strings using the body to create sound

28
Q

describe the natural modes of an ideal string

A

they form a pattern of standing waves with nodes and antinodes
fundamental
first harmonic
second harmonic and so on

29
Q

define harmonic series

A

a series of musical tones whose frequencies are integer multiples of the fundamental frequency, wavelength is twice the length of the string
harmonics are considerably lower in volume than fundamental
eg, 110Hz, 220Hz, 330Hz 440Hz…

30
Q

define resonance

A

resonance is the large amplitude vibration that is caused when an object is vibrated sympathetically by some source at a frequency that closely matches the object’s natural oscillation frequency

31
Q

how does dissipition affect resonance?

A

weak dissipation gives strong resonance

32
Q

describe the law of superposition

A

if multiple sine waves happen simultaneously their waves can be combined to form a new wave shape

alternatively they can cancel each other out

33
Q

when a wave on a fixed string hits a fixed point, what happens?

A

the wave changes direction

inverts phase: the reflected wave is an 180° phase change of what the wave would’ve been if it continued past the barrier/fixed point

superimposes: so the reflected wave interferes constructively with the incident wave (they are the same) and the resultant wave is larger sum of these two due to law of superposition

34
Q

define standing wave

A

a wave formed when an object vibrates at a frequency whose wavelength is related to the dimensions of that object

so the nodes and antinodes are at fixed points along the object, these points cannot move so the whole pattern appears to standing still, hence a standing wave

35
Q

which harmonic frequencies will propagate on a string?

A

the harmonics that “fit” or are related to the length of the string

the ends of the string are fixed so only harmonics that fit in between those fixed points can propagate

36
Q

in air columns, which part is analogous to the fixed points on a string (nodes) and antinodes?

A

nodes are the closed end of a tube

and antinodes would be the open end of a tube because the air is free to undergo longitudinal motion

37
Q

in an open ended air column (both ends are open) describe how a sound wave would reflect

A

blowing into the tube introduces pressure in the tube, when it reaches the end it experiences a change in air density and reflects and flips phase too

38
Q

in an open end (both ends open) air column, what is the wavelength of the fundamental compared to column length ?

A

fundamental’s wavelength is double that of the column length

39
Q

in a closed end air column (one end is closed one open) which harmonics will be present?

A

fundamental, whose wavelength is 4 times the length of the tube
and odd harmonics , because the closed end is confined to being a node, it prevents from positive harmonics forming

40
Q

define node and antinode

A

node - 0 amplitude, medium does not move

antinode - point of max vibration (peaks and troughs)

41
Q

how does change in temp affect speed of sound?

A

±6m/s change in speed for each 1deg rise or drop in temp

audio specialisms (9 decks)

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