Math Flashcards
Image height to furthest viewer
Height=Furthest distance/4,6,8
Decibel formula for distance
DB=20*log(D1/D2)
D1=original distance
D2=new distance
Increase in distance will yield a negative number
Decibel formula for Voltage
dB=20*log(V1/V2)
V1=original voltage
V2=new voltage
Adding voltage will increase dB
Decibel formula for Power
dB=10*log(P1/P2)
Formula for Current Ohm’s Law
I=V/R
Current or Amps=Volts/Resistance
Power Formula
P=I*V
Power or watts = Current or Amps / Volts
Series circuit impedance formula
Zt = Z1+Z2+Z3……
Parallel circuit impedance formula where all speakers have the same impedance
Zt = Z1/n
Total impedance is the impedance of one speaker divided by the number of speakers
Parallel circuit impedance with loudspeakers of different impedance
Zt = 1/(1/Z1+1/Z2+1/Z3……)
Series and Parallel circuit impedance calculation
First calculate each series branch then use the parallel formula
Needed Acoustic Gain NAG
How much gain do you need for listeners to hear
NAG= 20*log(D0/EAD)
D=distance from source to listener
EAD= Equivalent Acoustic Distance or the furthest you can go from the source without needing amplification or reinforcement
PAG or Potential Acoustic Gain
Gain before feedback
PAG= 20log[(D0D1)/(D2*D3)]
D0 distance from talker to furthest listener
D1 distance from microphone to closest speaker
D2 distance from furthest lister to their closest speaker
D3 distance from talker to their mic
Audio system stability formula
Is NAG less than PAG?
NAG < PAG - 10*log(NOM)-FSM
NOM number of open mics
FSM feedback stability margin
Conduit capacity
ID=square root of (Od1+Od2+Od3../FP
ID = inside diaper of conduit
Od = outside diameter of each cable
FP = permissible fill percentage
Permissible Fill Percentage in conduit
One cable is 53%
Two cables is 31%
Three cables or more is 40%
Just remember 5-3-4
Jam ratio
Applicable to 3 cables only
Jam=ID/[(Od1+2+3)/3]
Heat Load formula
BTU = Watts * 3.4
Power amplifier heat load
BTU = watts * 3.4 * (1-efficiency)
Power amplifier heat load (constant voltage)
Watts = watt tap setting * # of speakers * 1.5
Room mode calculation - Frequencies where room mode will be present
Hz = 3* velocity of sound/Rooms smallest dimension
Loudspeaker coverage diameter
Diameter = 2 x ceiling to listener’s ears x tan(half of the loudspeaker’s angle of coverage)
Loudspakers spacing for ceiling mounted speakers
Edge to edge = 2*radius of the speaker’s coverage
Minimal overlap = radius of speaker’s coverage x sqrt of 2
Centre to centre = radius of speaker’s coverage
Digital video bandwidth
bit rate (bits/second) = horz. pixels * vert. pixels * bit dept * frames/second
