Acoustics Flashcards
define acoustics
the study of the production, control, transmission, reception and effect of sound in a room.
how sound is transmitted in a room is determined by
the properties or qualities of a room or building
how are standing waves/ room modes formed
- 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
what are ideal features of room shpes
non parallel surfaces are idea and also walls of different lengths.
cubes are bad
what is a node
where the wave crosses the zero point. the neutral point of a waveform
what is an anti node
either the point of maximum compression or rarefaction. the maximum point of amplitude
what are axial room modes
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
what are tangential room modes
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
what are oblique room modes
reflection between three pairs of parallel surfaces. lower energy/ amplitude than tangential
what happens when waves bounce of more surfaces
power is diminished. they loose about 3dB as they bounce of different surfaces/ more sets of boundaries
what does the spacing of room modes have an effect on
how spaced out room modes are effects the rooms response. low modal density rooms are worse, high modal density rooms are better.
what is modal density
the number of resonant frequencies within a given range
what are modal frequencies
they follow the harmonic series due to wavelength relasionships
what are the golden room ratios
height width length
1 1.14 1.39
1 1.28 1.54
1 1.6 2.33
how is a rooms reverb time measured
RT60 which is the time it takes for reverb to drop 60dB in level. as the frequency increases the RT60 stabalises
what does the RT60 depend on
a rooms dimensions and materials.
what is acoustic absorbtion
the process by which a material, structure or object takes in sound energy when sound waves are encountered, as opposed to reflecting energy
what happens to the absorbed energy
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
what are porous absorbers
they include foam, fabric, carpets etc. as sound travels through the absorber acoustic energy is turned into heat
how do porous absorbers function
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
how does a porous absorbers distance from the wall affect absorbtion
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
what does overuse of porous absorbers lead to
excessive absorption of high frequencies
what are panale absorbers
they are similar to porous absorbers however you have plywood, followed by the absorber, then an air space
what are the types of membrane absorbers
poly cylindrical absorbers/diffusers
helmholtz reonsators
perorated panel absorbers
what creates a perfectly diffuse room
no measured frequency or level irregularities
a smooth reverb decay
same reverb decay for all frequencies
same reverb time at all points in room
what types of diffusers are there
schroder diffusers
qrd diffusers
what do wall depth frequency tables show
the expected low frequency cut off of a QRD based on the maximum wall depth
what are some alternative ways to increase diffusion
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
what is acoustic isolation
the aim is to convert as much sound energy into heat as possible
airbourne sound transmission is generally higher in frequency above….
100Hz
Structure bourne sound transmission is generally lower in frequency below…..
100Hz
what is transmission loss
a mediums ability to attenuate sound passing through it can be specified by its transmission loss (TL)
what is transmission loss defined as
the difference between the SPL on the source side and the SPL on the receiver side
what is the effect of mass and frequency
general rule: the heavier the boundary the greater the TL
what are ideal room shapes and design
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
what was the control room design pre 1960s
small control room
coloured
mono recordings
use of abosrbing materials resulted in RT60 of less than 0.2 secs
oppresive mixing environment
studio designs in the 70s featured:
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
