CTS Audio 2 Flashcards
Describe the differences in how dBA, dBC, and dB unweighted measure SPL.
- Unweighted takes an average of the SPL across all the frequencies.
- dBA is a sharp upside down curve, weighing low frequencies and very high frequencies much less than those around 1k
- dBC is a similar shape as dBA but far less severe.
When would you use dBA and dBC? Why?
dBA works well for spoken word reinforcement, dBC for loud concerts. This is because at low volumes we hear low frequencies very poorly (speech) but at high volumes the difference isn’t so bad. The weighting tries to more accurately capture how we hear sound.
Where’s the dB cutoff where you would switch from dBA to dBC?
Below 80 dB use dBA, above 80 use dBC
How many dB above ambient noise should you aim for speech reinforcement to be at?
25 dB S/N
ANSI standards recommend background SPL in not huge rooms and classrooms to be what?
35 dBA
What’s a general good aim for speech reinforcement dB levels?
65-75 dBA
ANSI Standards recommend audio coverage within a room standard to _______ dB.
6 dB
ANSI Standards recommends you measure uniformity of audio levels how?
In six octave bands from 250 Hz to 8 kHz.
Recommended ELECTRONIC signal to noise level?
60 dB. 60 dB signal above combined noise in components.
Recommended ACOUSTIC signal to noise level?
25 dB above background
What is EAD?
Equivalent Acoustic Distance. How far away can you move away from a source and keep the volume the same?
What is stability in an audio system?
Can you turn the mics up without feedback? Gain before feedback.
What is PAG?
Potential Acoustic Gain. Gain before feedback.
What is NAG?
Needed Acoustic Gain. A standard you set in an audio system.
At what point does a reflection become an echo?
50 - 80 ms. Bad for intelligibility!
What’s the dB used for power?
dBW or dBm
What’s the dB used for voltage?
dBV or dBu
Consumer electronics use what dB?
dBV
ProAV electronics use what dB?
dBu
What’s the dB used for Pressure (Pascals)?
dB SPL
What’s the decibel formula for Power?
dB = 10 * log (P1/Pr). P1 = New or measured power measurement, Pr = original or reference power measurement
What’s the decibel formula for Distance, Pressure, and Voltage?
dB = 20 * log (D1/D2) or dB = 20 * log (V1/VR)
dB = 20 * log (x1/x2) is for measuring what?
Voltage, Pressure, and Distance
dB = 10 * log (x1/x2) is for measuring what?
Power
Mic sensitivity is expressed as what?
dB of voltage per pascal of sound pressure
What does -54.5 dBV/PA (1.85 mV) 1 Pa = 94 dB SPL on a mic spec sheet tell you?
with a reference level of 94 dB SPL into the microphone, it will produce a voltage level of -54.5 dBV (0.00185 V) out.
1 Pa = _______ dB SPL?
1 Pa = 94 dB SPL
What is a polar plot in a mic spec?
A graphic representation of the directionality of a mic, often at difference frequencies.
A frequency response graph in a mic spec shows what?
Level of decibels at a given frequency.
What is impedance?
Impedance is the total opposition to current flow in an AC circuit
What is inductive reactance?
Forces that oppose changes in CURRENT in an AC circuit.
What is capacitive reactance?
Forces that oppose changes in VOLTAGE in an AC circuit.
Impedance is measured in what?
Ohms. “z”
In the early days of telephone and mics, what was important about impedance?
Needed to match an output impedance with an input impedance for maximum power transfer.
How do modern audio systems use maximum voltage transfer?
device’s output impedance should be one-tenth or less of the input impedance the device is being plugged into.
How do you effectively match impedance?
Device output is 1/10th or less the input impedance of device being plugged into.
What impedance should a professional mic have?
200 ohms or less
What impedance should a device have that will have a professional mic plugged into it?
2,000 ohms or more
A mic with impedance of 200 ohms or less is what?
A low impedance mic, or professional mic.
A mic with impedance of 25,000 ohms or more is what?
A high impedance mic, or consumer mic.
An electret is what (in condenser mic)?
A prepolarized material.
-58 dBu would be a typical signal level for a(n)…
Low impedance (pro) mic
