1.5 Audio Principles Flashcards
Logarithms
A logarithm of a number is how many times the number 1Omust be multiplied by itself to get a certain value. For example, the log of 10,000 is
4 and the log of 0.0001 is -4.
Decibel
Decibel (dB) describes a base-ten logarithmic relationship of a power ratio between two numbers .
Decibel Formula for Power
dB= 10 * log (P 1 IP,)
Where:
• dB= The change in decibels
• P 1 = The new or measured power measurement
• P, = The original or reference power measurement
Decibel Formula for Distance
dB= 20 *log (01 I D2)
Where:
• dB= The change in decibels
• 0 1 = The original or reference distance
• D 2 =The new or measured distance
Decibel Formula for Voltage
dB = 20 * log ( V 1 I V,)
Where :
• dB= The change i n decibels
• V 1 = The new or measured voltage
• V, = The original or reference voltage
1Oand 20 Log Formulas
In AV, the “10 log” formula is for power calculations only. The “20 log” formula is for voltage, pressure, and distance calculations. Just remember: 1O for power, 20 for everything else.
Reference Level (Decibels)
- OdB SPL = 0.00002 Pa
- 0 dBV = 1 V
- 0 dBu = 0.775 V
- OdBW= 1 W
- O dBm = 0.001 mW
Sound pressure level should always fall between
O and 140 dB SPL
Microphone level, which is typically measured in dBu, should be
60
to-50 dBu, well below the zero reference of 0.775 volts for the dBu.
Line level on the other hand, should be between
O and +4 dBu for pro
audio.
Consumer audio level is
-10 dBV (0.316 V)
SPL (Sound Pressure Level)
Sound pressure level (SPL} is a measurement of all the
acoustic energy present in an environment. It is typically
expressed in decibels (dB SPL) .
SPL Meter Classes
0- lab reference standard
1- engineering-grade accuracy
2- is for general purpose
3- intended for noise surveys
A Weighting ( dB SPL A wtd)
hearing conservation and noise ordinance enforcement.
C Weighting ( dB SPL C wtd)
More uniform response over the entire frequency
range.
Flat Weighting
no filtering. Sometimes referred to as Z (zero) weighting.
loudspeaker polar
pattern directivity
how much area
each of your selected loudspeakers will cover
Loudspeaker Coverage Area Formula
D = 2 * (H - h) *tan (CL I 2)
Where:
• D is the diameter of the coverage area I
• H is the ceiling height I
• h is the height of the listeners’ ears
• CL is the loudspeaker’s angle of coverage in dergees
Current and Voltage ( Ohm’s Law)
I= VI R I Where: l • I= Current I • V= Voltage • R= Resistance
Ohms Law
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Sensitivity Specification
Sens iti vity specifications are a way to determine a
device’s ability to convert one form of energy into
another form of energy . It is used to define the device’s efficiency in converting from one form to another.
Constant Voltage
- Constant voltage (25V, 70V , 1OOV) is a method of
distributing power to loudspeakers over a large area
with less loss than a typical connection would provide . This can also be called a “high impedance system.”
2 . Also called a high-impedance system, this is a
method of distribution over a large area with less loss
than a typical connection would provide ,
Specifying a Power Amplifier for
Direct Connection Audio
- Determine SPL level at the listener
- Add 1OdB for voice or 20 dB for music
- Find loss over distance in dB (loudspeaker to listener)
- Determine power (watts) required at the loudspeaker using the EPR
formula - Round the result up to an amplifier value that can be readily
purchased
Specifying a Power Amplifier for
Distributed Audio
- Determine SPL level at the listener
- Add 1OdB for voice or 20 dB for music
- Find loss over distance in dB
- Determine watts required at the loudspeaker
- Select appropriate tap value for loudspeaker
- Repeat steps 1-5 for each loudspeaker
- Sum the tap settings from all loudspeakers
- Increase the total tap settings by a factor of 1.5
- Round the result up to an amplifier value that can be readily purchased
Lavalier Microphone
Lavalier is a small microphone designed to be worn
either around the neck or clipped to apparel.
Boundary microphones
are mounted directly against a hard surface, such
as a conference table, wall, or ceiling.
Omnidirectional
Sound pickup is uniform in all directions.
Supercardioid
Provides better directionality than the hypercardioid. It has less rear pickup than the hypercardioid
Hypercardioid
Is a variant of the cardioid. More directional than
the regular cardioid because it rejects more sound from the side. The tradeoff is that some sound will be picked up directly at the rear of the
microphone.
Cardioid
Pickup is from the front of the
microphone only (one direction) in a cardioid pattern. It rejects sounds coming from the side but the most rejection is at the rear of the microphone. The term cardioid refers to the heart shaped polar
plot.
Bi-directional
Pickup is equal in opposite directions with little or no pickup from the sides. This is sometimes also referred to as a ‘‘figure -eighf’ pattern, referring to the shape of its polar plot.
Polar Plot
A polar plot is a graphical representation of the
relationship between a device’s directionality and its
output.
Frequency Response
Frequency response is a shortened way of expressing a device’s amplitude response versus frequency characteristic . A frequency response is usually presented as a graph or plot of a device’s output on the vertical axis versus the frequency on the horizontal axis .
Dynamic Microphone
A dynamic microphone is a pressure-sensitive microphone of moving coil design that transduces sound into electricity using electromagnetic principles .
Condenser Microphone
A condenser microphone transduces sound into
electricity using electrostatic principles.
Phantom Power
Phantom power is a direct current (DC) power source available in various voltages
Electret Microphone
An electret microphone is a type of condenser
microphone. It has prepolarized material.
called “electret, n which is applied to the icrophone’s diaphragm or backplate . This provides a permanent, fixed charge for one side of the capacitor configuration. This permanent charge eliminates the need for the higher voltage required for powering the typical condenser microphone, so it can be powered using small batteries and normal phantom power .
Mic Level
Mic level is a very low level signal. It is only a few
millivolts of electrical energy.
Preamplifier
A microphone preamplifier , also known as a mic pre ,
amplifies a microphone level signal to line level for
routing and processing .
Line Level
Line level is the strength of an audio signal. Line level 1s
used for all routing and processing between
components.
Line level in a professional audio
1.23 volts, +4 dBu
Line Level in Consumer audio
0.316 volts (-10 dBV)
Mic Level Voltage
0.001 - 0.003 volts (-60 to -50 dBu)
Sensitivity Specification
Sensitivity specifications are a way to determine a
device’s ability to convert one form of energy into
another form of energy . It is used to define the device’s efficiency in converting from one form to another.
1 Pa( pascal )=
94 dB SPL
Microphone Sensitivity is measured in?
decibels of voltage per pascal of
sound pressure. For example, a microphone may have a specification that reads, -54.5 dBV/PA (1.85 mV)
PAG
Potential Acoustic Gain. Gain before feedback is frequently referred to as potential acoustic gain (PAG).
NAG
Needed Acoustic Gain
typical speech reinforcement system SPL
65-70 dB SPL
signal-to-noise (S/N) ratio for voice reinforcement?
25 dB
electronic S/N ratio
This is the electronic S/N ratio of the electronic components. The signal level in the system should
be at least 60 dB above the combined noise inherent in all the electronics in the signal path. Should be 60 dB
SIN Ratio
Signal to noise ratio is the ratio , measured in decibels, between the audio or video signal, and the noise accompanying the signal. The higher the S/N ratio, the better the quality of the sound or picture.
Stable Audio
Stability applies only to systems using microphones that are being
reinforced in that same room. Can you turn the loudspeakers’ volume up
loud enough for the signal to be heard without causing feedback?
Speech loudness level
Average speech levels should be in the 65 to 70 dB range if persons with normal hearing are to understand it without effort. typically, a 15 dB
speech-to-noise ratio may suffice in sports activities were there is
considerable spectator noise.
Normal face-to-face speech communication SPL
is about 60 to 65 dB SPL,but
most speech reinforcement systems operate between 70 - 75 dB SPL. If
the level of amplified speech is increased beyond that range to about 85 to
90 dB SPL, overall intelligibility won’t increase much, and most listeners will
complain of excessive levels.
direct to reflected ratio
It takes the level of the direct sound - all the sound directly from the source to the listener - and compares it to the level of the reflected sound - all the sound that has arrived to the listener by an indirect pathway and determines the intelligibility of the system.
In order to work properly, a sound system must do three things:
- It must be loud enough
- It must be intelligible
- It must remain stable
EAD
Equivalent Acoustic Distance, is the farthest
distance one can go from the source without the need for sound amplification or reinforcement to maintain good speech intelligibility . It is a design parameter dependent on the level of the presenter and the noise level in the room .
NOM
Number of Open Microphones - Each time the number of open microphones is doubled, there is a 3 dB loss before feedback. In speech applications, microphones assigned to an auto mix bus in an automatic microphone mixer will have an equivalent open microphone of 1 .
FSM
Feedback Stability Margin - FSM refers to how close the system is to actual feedback. Any
system on the edge of feedback will produce a ringing behavior prior to actual feedback. The FSM in a carefully equalized system is typically 6 dB.
