decibels & mics

Question 1

define pressure in terms of sound

Answer 1

pressure is measurable change in air pressure caused by a wave

Question 2

define intensity in terms of sound

Answer 2

energy directed at a given area per second

Question 3

give the relationship between intensity and pressure

Answer 3

intensity = pressure squared
I = P²

Question 4

pressure drops __________ to distance sound travels

Answer 4

proportionally

Question 5

give the log formula to translate change in intensity to a decibel

Answer 5

dB IL = 10 (log (R2/R1))

IL = intensity level
R2 = intensity of sound we wish to measure
R1 = reference level of intensity

Question 6

give the formula to translate change in pressure to a decibel scale

Answer 6

dB SPL = 20 (log (P2/P1))

SPL = sound pressure level
P2 = measured pressure
P1 = reference pressure

Question 7

how many dB is a doubling or halving of intensity?

Answer 7

±3.01 dB

Question 8

how many dB is a doubling or halving of pressure?

Answer 8

±6.02 dB

Question 9

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⁻

Answer 9

repeat that shit

Question 10

dB is __________

Answer 10

dimensionless

Question 11

+10dB always means…

Answer 11

plus one power in base 10

Question 12

give the values of
0dB SPL ?
0dB SIL ?

Answer 12

0dB SPL = 2 x 10⁻⁵ N/m²

0dB SIL = 1 x 10⁻¹² W/m²

Question 13

define transducer

Answer 13

something that converts energy from one form to another

Question 14

give 5 factors affecting mic sound

Answer 14

polar response
frequency response
sensitivity
distortion
self noise - from electronics inside

Question 15

describe how a dynamic mic works

Answer 15

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

Question 16

describe how a condenser mic works

Answer 16

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

Question 17

describe an electret condenser

Answer 17

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

Question 18

what is white noise

Answer 18

a signal with equal amounts of ray across the audio spectrum
like white light

Question 19

what is pink noise

Answer 19

filtered white noise that has the same energy across all octaves

Question 20

give 2 ways you can build noise out of mic cables

Answer 20

1. 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
2. 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

Question 21

what is mic sensitivity?

Answer 21

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

Question 22

give pros and cons of dynamic mics

Answer 22

pros
- useful for close by, loud sounds
- bass mid-range freqs
- tough, inexpensive
- no 48V

cons
- not good with high frequencies
- restricted sensitivity

Question 23

give pros and cons of condenser mics

Answer 23

pros
- high sensitivity
- high frequency response
- wide range of applications

cons
- delicate
- more expensive
- not good in high-humidity
- 48V