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Acoustics > Acoustics > Flashcards

Acoustics Flashcards

1
Q

define acoustics

A

the study of the production, control, transmission, reception and effect of sound in a room.

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2
Q

how sound is transmitted in a room is determined by

A

the properties or qualities of a room or building

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3
Q

how are standing waves/ room modes formed

A
  • a sound source placed between two parallel surfaces
  • sound waves reach an obstacle
  • sound heads back towards point of origin
  • if the wavelength of the audio signal holds a simple mathematical relationship between the walls, the interference between wave fronts will cause compression and rare-fractions to coincide physically as sound bounces back and forth
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4
Q

what are ideal features of room shpes

A

non parallel surfaces are idea and also walls of different lengths.

cubes are bad

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5
Q

what is a node

A

where the wave crosses the zero point. the neutral point of a waveform

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6
Q

what is an anti node

A

either the point of maximum compression or rarefaction. the maximum point of amplitude

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7
Q

what are axial room modes

A

reflection of one pair of parallel surfaces, for a half cycle. bounces of two surfaces. the primary resonance is one that fits exactly once into one of the parallel surfaces of a room

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8
Q

what are tangential room modes

A

reflection between two pairs of parallel surfaces. They have less energy than axial modes. bounces of 2 pairs of parallel surfaces. this is when the primary resonance fits exactly once into two pairs of parallel surfaces. they have less energy therefore less noticeable and less volume than axial

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9
Q

what are oblique room modes

A

reflection between three pairs of parallel surfaces. lower energy/ amplitude than tangential

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10
Q

what happens when waves bounce of more surfaces

A

power is diminished. they loose about 3dB as they bounce of different surfaces/ more sets of boundaries

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11
Q

what does the spacing of room modes have an effect on

A

how spaced out room modes are effects the rooms response. low modal density rooms are worse, high modal density rooms are better.

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12
Q

what is modal density

A

the number of resonant frequencies within a given range

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13
Q

what are modal frequencies

A

they follow the harmonic series due to wavelength relasionships

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14
Q

what are the golden room ratios

A

height width length

1 1.14 1.39
1 1.28 1.54
1 1.6 2.33

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15
Q

how is a rooms reverb time measured

A

RT60 which is the time it takes for reverb to drop 60dB in level. as the frequency increases the RT60 stabalises

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16
Q

what does the RT60 depend on

A

a rooms dimensions and materials.

17
Q

what is acoustic absorbtion

A

the process by which a material, structure or object takes in sound energy when sound waves are encountered, as opposed to reflecting energy

18
Q

what happens to the absorbed energy

A

part of it is transformed into heat, and part is transmitted through the absorbing body.

the energy transformed into heat is said to have been lost

19
Q

what are porous absorbers

A

they include foam, fabric, carpets etc. as sound travels through the absorber acoustic energy is turned into heat

20
Q

how do porous absorbers function

A

the space between the fibres dictates the degree of absorption

if there not packed closely enough little energy will be lost as heat. if fibres are packed to closely sound may not penetrate the absorber sufficiently

21
Q

how does a porous absorbers distance from the wall affect absorbtion

A

distance between the wall and the thickness of materials will dictate the frequencies of absorption

porours absorbers mounted directly on boundaries work well with mid to high frequencies, but are poor at controlling low frequencies

22
Q

what does overuse of porous absorbers lead to

A

excessive absorption of high frequencies

23
Q

what are panale absorbers

A

they are similar to porous absorbers however you have plywood, followed by the absorber, then an air space

24
Q

what are the types of membrane absorbers

A

poly cylindrical absorbers/diffusers

helmholtz reonsators

perorated panel absorbers

25
Q

what creates a perfectly diffuse room

A

no measured frequency or level irregularities

a smooth reverb decay

same reverb decay for all frequencies

same reverb time at all points in room

26
Q

what types of diffusers are there

A

schroder diffusers

qrd diffusers

27
Q

what do wall depth frequency tables show

A

the expected low frequency cut off of a QRD based on the maximum wall depth

28
Q

what are some alternative ways to increase diffusion

A

the use of non parallel surfaces

the use of geometric irregularities

the use of concave surfaces or diffusion panels

the use of everyday objects like bookcase

29
Q

what is acoustic isolation

A

the aim is to convert as much sound energy into heat as possible

30
Q

airbourne sound transmission is generally higher in frequency above….

A

100Hz

31
Q

Structure bourne sound transmission is generally lower in frequency below…..

A

100Hz

32
Q

what is transmission loss

A

a mediums ability to attenuate sound passing through it can be specified by its transmission loss (TL)

33
Q

what is transmission loss defined as

A

the difference between the SPL on the source side and the SPL on the receiver side

34
Q

what is the effect of mass and frequency

A

general rule: the heavier the boundary the greater the TL

35
Q

what are ideal room shapes and design

A

polygonal shapes preffered

outer shell must be heavy, solid and high STC rating

inner shell provides ideal room acoustics

corners may be removed via the use of panels

flush mounted monitors

36
Q

what was the control room design pre 1960s

A

small control room

coloured

mono recordings

use of abosrbing materials resulted in RT60 of less than 0.2 secs

oppresive mixing environment

37
Q

studio designs in the 70s featured:

A

important studios in acoustics from tom hidley and don davis

control room expanded partially to accomodate large mixing console

attention paid to colouration (non parallel surfaces/ L-R symmetry)

different shapes/ materials

flush mounted monitors

Acoustics (1 decks)

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