1
Q
How are air particles, moving slightly back and fourth, able to carry music from a loudspeaker to our ears?
A
- Air molecules crowded together represent areas of compression, in which the air pressure is slightly greater than the prevailing atmospheric pressure
- the sparse areas represent rarefactions, in which the pressure is slightly less than the atmospheric pressure
- any given molecule, because of elasticity, after an initial displacement, will return toward its original position
- it will move a certain distance to the right and then the same distance to the left of it’s undisplayed position as the sound wave progresses uniformly to the right
- sound exists because of the transfer of momentum from one particle to another
2
Q
what is the speed of sound?
A
- 1,130 ft/sec (344m/sec)
3
Q
what dictates the speed of sound?
A
- sound will propagate at a certain speed that depends on the medium and other factors
- the denser the molecular structure, the easier it is for the molecules to transfer sound energy
- humidity also affects the speed of sound, the more humid the air, the faster the speed
4
Q
what is the relationship of long and short wavelengths within the world?
A
- long wavelengths (or low frequencies) bend around objects that are smaller than themselves
- short wavelengths (high frequencies) reflect off of, or are absorbed by those same objects
5
Q
why are low frequencies described as non directional?
A
- wavelengths of low frequencies (which can be more than a meter long) are so large that the listeners head is much smaller than the wavelength
- as a result, there is very little difference between the sound received by the left and right ears, and as we know, the brain uses these different cues to calculate the direction in which sound is coming from
6
Q
what is a frequency?
A
- frequency is measured in Hertz (Hz) and is the number of wavelengths in a given second
- you can think of frequency as the horizontal component of sound
7
Q
what is the equation for wavelength?
A
Wavelength = speed / frequency
8
Q
what is a critical listening environment?
A
- a space where an engineer / producer can monitor sound through speakers as purely as possible
- this is achieved by a room being treated acoustically, altering the shape, size and building materials
9
Q
what is reverb?
A
- sound in a room will repeatedly bounce off surfaces such as the floor, walls, ceiling, windows or tables
- when these reflections mix together, a phenomenon known as reverberation is created
- the original sound source can stop but the reflections continue, decreasing in amplitude until they reach zero
10
Q
What is RT60?
A
- the time required for the sound to fade away or decay in a space is known as reverb or RT60
- reverberation time of a room or space is defined as the time it takes for sound to decay by 60dB
- for example, if the sound in a room took 10 seconds to decay from 100dB to 40dB the reverberation time would be 10 seconds
11
Q
in order to dissipate, what form of energy do sound waves need to turn into?
A
heat
12
Q
how can you measure RT60?
A
- in order to measure RT60, the room must first be energised with noise
- this can be done by popping a balloon or using a speaker or other sound source such as a snare drum
- multiple measurements will be used to define the RT60 of a space
13
Q
what is the desired RT60 for a control room?
A
250ms
14
Q
what is the desired RT60 for a live room?
A
between 250ms - 900ms
15
Q
what are the suggested room dimensions for a critical listening environment?
A
- width: 17ft
- length: 23ft
- height: 10ft
16
Q
what is absorption?
A
- acoustic panels that absorb sound waves and convert their energy to heat
- sound absorbers are one of these types: porous absorbers, panel absorbers and volume or resonance absorbers
- porous absorbers are most effective at higher frequencies whereas panel and volume absorbers are most effective at lower frequencies
17
Q
what are diffusers?
A
- acoustic panels that reflect sound waves and spread out and scatter sound waves creating a more natural dispersion
18
Q
what are bass traps?
A
- acoustic panels that absorb low frequency content specifically and turn the energy into heat stopping a build up of low frequency content
- tend to be placed in corners and along front and back walls behind speakers
19
Q
how do we hear sound?
A
- vibrating air molecules cause the human ear drum to vibrate, which the brain interprets as sound
20
Q
what type of wave are sound waves?
A
Longitudinal
21
Q
what is a longitudinal wave?
A
- a wave in which the particles of a medium vibrate in the same direction as the wave is travelling (go back and fourth)
22
Q
what is a transverse wave?
A
- vibrate at 90 degrees to the direction of the wave (go up and down)
23
Q
what is a waveform?
A
- a waveform is a graphical representation of a sound wave as it moves through a medium over time
- any waveform has four fundamental characteristics
- wavelength
- amplitude
- Frequency
- Velocity
24
Q
what is a wavelength?
A
- the wavelength of a wave is the length in meters from the start to the end of one full cycle of the waveform
- e.g. form crest to crest
25
what is amplitude?
- the maximum displacement of a wave from the centreline to the peak, not peak to peak
26
what is velocity?
- the velocity is the speed and direction of a sound wave
- this changes depending on the medium in which sound travels through
27
what is reflection?
- reflections help define the sonic characteristics of a room
- the reflections convey significant information about a room such as, size, shape, and boundary composition
- when sound strikes a boundary surface (walls, ceilings, floors) some sound energy is transmitted or absorbed by the surface and some is reflected
- the reflected energy is alway less than the incident energy
- surfaces made of heavy materials are usually more reflective than lighter materials that tend to absorb or transmit sound
- sound may undergo many reflections as it bounces around a room
- the energy lost at each reflection results in the eventual demise of that sound
28
what are specular reflections?
- the essential mechanism of a reflection from a flat surface is simple
- spherical wavefronts strike the wall and the reflected wavefronts are returned towards the source
- this is called specular reflection and behaves the same as light reflections form a mirror
- specular reflections are types of reflections that bounce off in a predictable manor
- sound follows the same rule as light, the angle of incidence is equal to the angle of reflection
29
what are flutter echoes?
- parallel walls present an acoustical problem
- if the distance between the walls is large enough so the time between reflections is outside the haas fusion zone, a flutter echo is created as sound bounces back and fourth from one wall to another
- because of the regularity of these reflections, the ear is very sensitive to the effect
- in fact, even if the time delays are otherwise in the fusion zone, the effect may still be audible as an echo
- this echo can be very prominent in an otherwise diffuse sound field and is highly undesirable
30
what is the dissipation of sound energy?
- a sound wave travelling in the air strikes a concrete block wall covered with an acoustical material, what happens to the energy it contains?
- as a sound wave travels through air, there is first a small heat loss from air absorption that is appreciable only at higher audio frequencies
- when the sound wave hits a wall, there is a reflected component returned to the air from the surface of the acoustic material
- some of the sound penetrates the acoustical material
- the direction of travel of the sound is refracted downward because the acoustical material is denser than air
- there is heat lost by the frictional resistance the acoustical material offers to the vibration of air particles
- as the sound strikes the surface of the concrete blocks, two things happen, a component is reflected, and the wave is also bent strongly downward as it enters the much denser concrete blocks
31
what are absorption coefficients?
- used to rate a materials effectiveness in absorbing sound
- absorption coefficients vary with the angle as which sound impinges on the material
- in an established diffuse sound field in a room, sound is travelling in every imaginable direction
- in many calculations, we need sound absorption coefficients that are averaged over all possible angles of incidence
- the random incidence absorption coefficient is a coefficient that is averaged over all incidence angles
- this is usually referred to as the absorption coefficient of a material
- the absorption coefficient is a measure of the efficiency of a surface or material in absorbing sound
- for example, if 55% of the incident sound energy is absorbed at some frequency, the absorption coefficient is said to be 0.55 at the frequency
- a perfect sound absorber will absorb 100% of incident sound, thus 1.0
32
what is the absorption calculator?
- the sound absorption provided by a particular area of material is obtained by multiplying it's absorption units, by the surface are of the material exposed to sound
- therefore = where A = absorption units x and S = Surface area in ft2 or m2 = absorption coefficient
- sound absorption is measured in Sabines
- e.g. a square foot of carpet has an absorption coefficient of 0.55, 20ft of carpet would therefore provide 11 sabines of absorption
33
what is a sabine?
- the concept of a unit for absorption was first suggested by Wallace Sabine
- he defined the open window unit as the absorption of 1 square foot of open window
- the unit was renamed the sabin after sabine, and it is now define as 'the absorption due to the uni area of a totally absorbent surface'
- sabines may be calculated with either imperial or metric units
- one square foot of 100% absorbing material has a value of one imperial sabin
34
absorption and frequency
- the absorption coefficient of a material varies with frequency
- coefficients are typically published at the size standard frequencies of 125, 250, 500, 1000, 2000 and 4000 Hz
- in some cases, a materials absorption is given as single number rating known as the Noise Reduction Coefficient
- the noise reduction coefficient is the average of coefficients for 250, 500, 1000 and 2000 (125 and 4000 are not used)
- it is important to remember that NRC is an average value, and also accounts for absorption at middle frequencies
- therefore NRC is most useful for speech applications
- when considering wider-band music, individual coefficients, at a wider range of frequencies should be used
35
how thick should a porous absorber be to absorb a sound wave?
- at least a quarter of the wavelength thick of the frequency in interest
- for example, for a frequency of 1KHz, the minimum absorber thickness should be about 3.4in
36
what is an impulse response?
- an impulse response is created by playing a sound in a space
- this impulse could be a short, percussive sound
- this impulse produces a snapshot of the space's characteristic acoustics, by charging and activating the room acoustically this is captured / recorded which in turn, creates the impulse response of a space in audio form
- since the impulse introduces a lot of energy into the room over the full frequency spectrum and then dies quickly, it is a very useful tool to be able to record and analyse what happens in the room
37
what is a direct sound?
- this has high sound pressure but short duration
- this is the time it takes for the sound to reach the received (listener or microphone)
38
what is the initial time delay gap?
- this shows the time between the direct sound and the first reflections and tells us how far away the direct sound source is from the listener
- the ITDG provides information to the brain on the size of the room
- a larger gap mean a bigger room
39
what are early reflections?
- early reflections are the first ones we hear and are distinguishable
- you may only have a few in a simple rectangle room but there can be more if the room is more complex
- early reflections let us know, how big the room is and, is what we describe as the reverb of a space, measured with the RT60 calculation
40
what are standing waves?
- also known as stationary waves, are a combination of sound waves reflecting off a rooms boundaries moving in opposite directions, each having the same amplitude and frequency
- the phenomenon is the rest of interference, that is, when waves are superimposed, their energies are either added together or cancelled out
- standing waves are usually low frequency waves below 300Hz (this cut off is known as the Schroder frequency and this changes depending on rooms dimensions)
- above 300Hz, the waves tend not to reflect directly back
- a standing wave is called as such because, unlike regular waves, it does not look like its travelling from one side to another, rather, it looks like it's moving up and down while standing in place
41
what are nodes?
- points along a standing wave where the wave cancels out
42
what are antinodes?
- points along a standing wave with maximum amplitude with constructive interference
43
what is the standing wave ratio?
- the standing wave ratio is the ratio of the amplitude at the antinode (maximum) of the standing wave to the amplitude at the node (minimum)
- this difference would be measured in SPL
44
what are room modes?
- room modes are caused by sound reflecting off of various room surfaces
- modal activity occurs at frequencies with are directly related to the dimensions of the room
- occurs when the distance between 2 surfaces is the same as half the wavelength of that particular frequency
- room modes are areas of the frequency range that are prone to resonance
- the different directions a sound wave travels in a space are called modes
- whilst the number is infinite, we can break them down in a subset of pathways being, axial, tangential and oblique
- these room modes can caused both peaks and nulls in frequency response
- when two or more waves meet and are in phase with each other at a specific frequency (constructing standing wave), you will have a peak in response
- when they meet and are out of phase with each other (destructive standing wave), they cancel each other out and you end up with a dip or null in response
45
what are axial room modes?
- axial modes involve just two parallel surfaces - opposite walls, or the floor and ceiling
- in other words, an axial mode consist of waves resonating only along one dimension such as the length, width or height of the room
- axial modes are the strongest and many times, the only ones that are considered
46
what are tangential room modes?
- tangential room modes involve two sets of parallel surfaces - all four walls, or two walls the ceiling and floor
- in other words, tangential modes involve two dimensions, the length and width, length and height or width and height of a room and are about half as strong as axial room modes
47
what are oblique room modes?
- oblique room modes involve all six surfaces - four walls, the ceiling and the floor
- they are about one quarter as strong as the axial modes, and half as strong as the tangential modes
- oblique modes are the ones that most often become overly excited as they are is more of them usually
48
what is the room mode calculations?
- by dividing the speed of sound 1,130 ft per second, by the longest measured dimension in a space and doubled, this will give you the 1st modal response
- for example, if you got 50Hz for the first too mode of the resonant calculation, then the second mode would be 100Hz, third would one 150Hz
- all these additional modal frequencies will also resonate because of the ability of a sound wave to continue travelling back and fourth between walls
49
what are the three types of acoustic treatment?
- porous
- membrane
- resonance
50
what are porous absorbers?
- made of porous material
- mineral wool, textiles, clothing, carpets and curtain types of foam plastic all fall into this category
- the sound-absorbing effect stems from the fact that the sound energy can penetrate the material on hitting the surface
- here, the sound energy is converted into heat energy, so that only a small part is reflected in the form of the sound energy
- in other words, the material has absorbed some of the sound
- porous absorbers are most effective from frequencies from about 150Hz upwards
51
what is the broadband porous absorber?
- most common form of porous acoustic panel is the broadband absorber
- these are usually sound absorbing material (often a firm mineral wool such as rock wool), surrounded by a rigid frame and covered with fabric
- mineral wool absorbs a lot of acoustic energy (transducing it into microscopic amounts of heat)
- because this material is fairly similar to air, it reflects very little of the sound that strikes it
- this makes it excellent for controlling the RT-60 of a space through absorption
- broadband: simply refers to the bandwidth or effective workable range of the absorber
- technically speaking, you could even have a tune trap that is broadband, so long as it's effective range covers a couple of thousand Hertz
- a broadband trap or acoustic panel is simply anything that works over many frequencies, from bass to highs, and treats them with equal efficiency (although this varies across different products)
- a broadband absorbers main function in studio design applications should be to reduce the decay times of low and mid frequency modal content, as well as manage the RT-60 of space
- they are therefore suitably placed in the corners of the room and the reflecting points on the wall (between the speakers and the listening spot)
- they may also be used in a rehearsal or recording rooms to dampen decay times
- they should be dispersed evenly throughout the room in order to achieve the best results and a uniform recording area
- this application is kind of a general RT-60 lowering method
52
what is as air gap?
- an air gap between the ceiling or wall the absorber is mounted on, has an effect on the efficiency of the absorber
- the more different materials are used in sound proofing a space, the quicker the sound wave is broke down
- the optimum distance between a fixed ceiling and a suspended acoustic mineral wool ceiling is in practice 200-300 millimetres
- structures such a these are effective down to 200Hz
53
what are tuned bass absorbers?
- after broadband absorbers have been used, problems can still persist at low frequencies
- in these instances, tuned bass absorbers can be very useful
- these are designed to only work on specific low frequencies
- their design is such that they don't require the very large mass of a conventional bass trap, and leave higher frequencies untouched
54
what are membrane absorbers?
- in the bass frequency range, it is necessary to consider other types of absorbers, in particular the membrane absorbers which are proper bass absorbers
- the membrane absorber is a flat box, 100-200 millimetres deep, mounted on the wall with a thin sheet of plywood or similar on the front and with a light mineral wool filling the box cavity
- if you softly bang the front panel with your hand, you will hear a deep tone, a bit like a bass drum, though much weaker
- the tone you hear is the resonance frequency of the oscillating system which consists of the front panel with a certain mass combined with the spring that is formed by the trapped air
- the resonance frequency is also the frequency at which the membrane absorber absorbs, as sound energy causes the membrane to oscillate
- in other words, there is energy transformation, but this time from sound energy to mechanical oscillatory energy
- thus, the first significant characteristic of the membrane absorber is that it absorbs sound energy at low frequencies
- the second key characteristic of the membrane absorber is that it is a common feature, as it were, for our day to day life, as ordinary building components, such as doors, windows, wood floors and plaster walls all function as membrane absorbers
- the sound absorption coefficient is not staggering, perhaps 20-25%, but, as the components together constitute a significant area, the effect is significant
- for example, the large areas of glass in modern buildings lead to many problems with indoor climates, but as far as the acoustics are concerned, they ensure that the reverberation time does not increase outrageously for low frequency waves
- because they are membrane absorbers, the glass sections help to balance the rooms acoustics
- however, bear in mind, that membrane absorbers only work for bass tones, and therefore reflect higher frequency sounds
- windows can therefore produce uncomfortable reflections or echo effects which have to be counteracted in some other way
55
what is a resonance absorber?
- they work on the same principle as when you blow across the mouth of a beer bottle to produce a note
- the tone occurs when the oscillating system, which comprises the air in the bottleneck, oscillates on the spring from by the air in the bottle
- the mechanism is called a Helmholtz resonator, and you find it in many systems, among other things
- as with the membrane absorber, the resonance frequency you hear is the frequency which is absorbed from sound energy to mechanical oscillatory energy
56
what is a resonant bass trap?
- these traps are tuned to a certain frequency only e.g 100Hz and are all used to deal with specific problematic frequencies in a room
- normally installed after broadband absorbers have been installed
- placement usually in high pressure areas such as walls and corners
57
what is the rooms within rooms example?
- air gaps between absorbers and walls also help attenuate low frequencies
- for this reason, the 'room within a room' design for recording studios is still very popular
- the walls of the studio will be constructed to have absorptive properties, and absorbers will be affixed to the walls for even more absorption
58
what is the best way to split a room when considering acoustic treatment?
- its best to split the way a room behaves acoustically into two separate sections
- frequencies above approximately 250Hz, which contribute to the RT60 of a space through sound waves reflecting, and, frequency content below 250Hz causing room modes and in turn standing waves
- with that in mind its important to split sound proofing products across these two areas
- AMROC will give us the Schroeder frequency which is the cut off point between the two areas we have split the room into: reflective waves affecting the RT-60 of a room and room modes / standing waves
- by inputting the room dimensions and desired RT60, AMROC will give us the problem modal frequencies and the amount of absorption needed to achieve our new desired RT-60
- with this information we can plan acoustic treatment with a mix of broadband absorption, resonant, and membrane absorbers
Music production
flashcards
Decks in class (6)
# Cards
