decibels & mics Flashcards
define pressure in terms of sound
pressure is measurable change in air pressure caused by a wave
define intensity in terms of sound
energy directed at a given area per second
give the relationship between intensity and pressure
intensity = pressure squared
I = P²
pressure drops __________ to distance sound travels
proportionally
give the log formula to translate change in intensity to a decibel
dB IL = 10 (log (R2/R1))
IL = intensity level
R2 = intensity of sound we wish to measure
R1 = reference level of intensity
give the formula to translate change in pressure to a decibel scale
dB SPL = 20 (log (P2/P1))
SPL = sound pressure level
P2 = measured pressure
P1 = reference pressure
how many dB is a doubling or halving of intensity?
±3.01 dB
how many dB is a doubling or halving of pressure?
±6.02 dB
both intensity and pressure are being measured as a ratio compared to the threshold of hearing, in terms of minimum energy needed to create audible sound and minimum pressure needed to create audible sound⁻
repeat that shit
dB is __________
dimensionless
+10dB always means…
plus one power in base 10
give the values of
0dB SPL ?
0dB SIL ?
0dB SPL = 2 x 10⁻⁵ N/m²
0dB SIL = 1 x 10⁻¹² W/m²
define transducer
something that converts energy from one form to another
give 5 factors affecting mic sound
polar response
frequency response
sensitivity
distortion
self noise - from electronics inside
describe how a dynamic mic works
sound wave causes a thin metallic diaphragm to move a coil it is attached to which is attached to a magnet that creates a magnetic field around that coil
the coil moving within the magnetic field causes current to flow
current is caused by the diaphragm’s movement
describe how a condenser mic works
metallic diaphragm is mounted to a metal rigid back plate.
a battery is connected to both pieces of metal, creating charge between them, amount of charge is determined by the V of the battery, area of diaphragm and back plate and the distance between the two.
the diaphragm moves in response to sound waves hitting it, causing distance between it and the back plate to change. amount of current is proportional to the displacement of the diaphragm.
current must be amplified before it leaves the microphone
describe an electret condenser
a variant of a condenser mic
uses an extra bit of material that can be the diaphragm itself, mounted onto the back plate or somewhere else
this bit of material has the charge for the diaphragm permanently embedded in it, a good material for this is plastic.
disadvantage - the charge runs out after several years of using them and they cease to work
what is white noise
a signal with equal amounts of ray across the audio spectrum
like white light
what is pink noise
filtered white noise that has the same energy across all octaves
give 2 ways you can build noise out of mic cables
- shielding the cable with a metallic shield that deflects radio waves.
stops the cable becoming an antenna, mic preamp is similar to a radio receiver - balance the cables, using two cables that are out of phase, when they reach the amplifier one is swapped phase so they are received in phase.
noise in the cables is in phase to begin with so when one cable is swapped phase, the noise is phase cancelled
what is mic sensitivity?
measure of how much electrical signal comes out the mic for a given level of audio input
less sensitive mic = more amplification needed to get a signal to a useful level
give pros and cons of dynamic mics
pros
- useful for close by, loud sounds
- bass mid-range freqs
- tough, inexpensive
- no 48V
cons
- not good with high frequencies
- restricted sensitivity
give pros and cons of condenser mics
pros
- high sensitivity
- high frequency response
- wide range of applications
cons
- delicate
- more expensive
- not good in high-humidity
- 48V
