Acoustics Flashcards
(37 cards)
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
