Using the Decibel
Chapter 8 Interfacing Electrical and Acoustic
8.12 Handling the Acoustic Input and Output of the System
Knowing the expected performer’s maximum LP allows for an intelligent choice of microphone sensitivities. The choice of microphones can be affected by:
1. Appearance.
2. Sensitivity.
3. Reliability.
4. Circuit types—wired, wireless, dynamic con-denser, etc.
5. Directivity characteristics.
6. Freedom from wind noise, extraneous electrical field pickup, and handling noise.
For our purpose of interface, sensitivity and imped-ance need explanation.
Figure 8-31. Complex impedance Nyquist plot.
Figure 8-32. How the plots in Fig. 8-33through 8-37 were made.
20
10
0
−10
−200 10 20 30 40 Resistance–ohms
Reactance–ohms
+j = XL
−j = XC
20kHz
10kHz
1kHz 58Hz 20Hz
Osc. Pwr.
Amp. Analyzer
Load M1
10K S1
1. Close S1 and set M to 0.1V.
2. Open S1 attach test load and measure.
3. Adjust analyzer input sensitivity until you read test load value.
4. Measure desired load.
138 Chapter 8
Figure 8-33. A resistive input (10Ω, 5% resistor).
Figure 8-34. A single cone in an enclosure box. Cursor at the lowest value magnitude 8.4Ω. Manufacturer rating is 8Ω.
Figure 8-35. A two way system. Cursor at the lowest value 5.3Ω. Rating chosen by manufacturer is 8Ω.
Phase
Magnitude and phase response Nyquist plot
Magnitude
Magnitude
Phase
Magnitude and phase response Nyquist plot
Magnitude
Magnitude
Magnitude and phase response Nyquist plot
Phase
Magnitude
Magnitude
Interfacing Electrical and Acoustic Systems 139
8.12.1 The EIA Microphone Rating
The EIA has chosen to rate microphone sensitivity as:
(8-111) where,
SV = 20log EO – Test LP + 74,
RMR is the center value of the impedance range, typically 38Ω , 150Ω , 600Ω , Table 8-2,
EO is the open circuit voltage at Test LP , LP is the Test LP (usually either 94dB or 74dB),
*50dB = 10log (1 ⁄ 0.001 + (94 − 74).
To obtain GM from other ratings use:
(8-112) then insert SV into the GM equation (typical Test LP is either 94dB or 74dB).
GM provides the microphone’s output level in dBm (theoretical) for an input LP = 0dB. This allows the output of the microphone to be added directly to the performer’s LPMAX for the total elec-trical output of the microphone at the mixer’s input.
Low sensitivity microphones, suitable for “rock”
concerts, can have an output level, when used by quiet talkers, too near the noise floor—for example, Figure 8-36. A smaller two way system. Cursor at the lowest point 3.7Ω. Rating chosen by manufacturer is 4Ω.
Figure 8-37. A “rock” performer’s microphone specified to operate into an “open” circuit.
Phase
Magnitude Magnitude
Magnitude and phase response Nyquist plot
Magnitude
Phase
Magnitude
Magnitude and phase response Nyquist plot
GM = SV–10logRMR–50dB*
SV = 20logEO–Test Lp+74
Table 8-2. RMR defined
Ranges (Ω) Values Used (Ω)
20–80 38
80–300 150
300–1250 600
1250–4500 2400
4500–20,000 9600
20,000–70,000 40,000
1dyn/cm2 = 1μbar = 0.1Pa = 0.1N/m2
140 Chapter 8 in conference rooms, by ministers, anyone speaking softly. Conversely a very high sensitivity micro-phone choice for the high level performer can result in input overload at the mixer’s input amplifier.
This is an “available input power” figure. An other-wise noiseless device has a thermal noise floor of
−132dBm (for a spectrum from 20Hz–20,000Hz).
The GM figure allows an instant estimate of SNR at the very beginning of the system; however, the acoustic SNR at the microphone is a separate case.
8.12.2 The Mixer Output
The audio engineer needs to know the source voltage and source impedance of the device. This is depicted in Fig. 8-38.
The source voltage can be found with the following equation:
(8-114)
8.12.3 Available Input Power
From this we can find the available input power in dBm to the input of the device following the mixer
(8-115) Each subsequent device is handled in exactly the same manner. We only care about AIP from the previous device and the AIP at the output of the device itself, in order to specify the gain or loss the device occasions, Fig. 8-39.
At the output of the power amplifier we compute the actual power into the load rather than its AIP
(8-116)
or
(8-117) where,
P is the power in watts.
8.12.4 Open and Matched Circuits
Fig. 8-40 shows an open circuit and a matched circuit. For each circuit:
(8-118) Figure 8-38. Finding Rs from voltage measurements.
RS
Measure EO, close switch, measure EADD
RS 600 1.0 V
Figure 8-39. AIP of a system.
ES EIN RS+RIN
Interfacing Electrical and Acoustic Systems 141 where,
EL is the load voltage, Es is the source voltage, RL is the load resistance.
In the circuits of Fig. 8-40, assume the voltage E is 1V in both cases. The open circuit load voltage, Fig. 8-40A would be:
For the matched circuit, Fig. 8-40B:
The ratio of the two load voltages, converted to decibels, is:
8.12.5 When to Measure Z
Knowing the actual source, input, output, and load
|Z| is necessary if systems are to be installed prop-erly. The installer must be aware of the difference between the values normally specified by the
manu-facturers of equipment and the actual measured values needed by the installer for matching.
There is usually a rated Rs, RIN, ROUT, and RL as well as an actual value. Rs is the actual source impedance (often an electroacoustic transducer) and RIN is the actual input impedance of a system device.
ROUT is the actual output impedance of a system device (as distinguished from its output impedance rating) and RL is the load impedance (output imped-ance ratings are usually the desired RL). Rated input impedances are often the desired Rs.
The term “matching” may be read as “appro-priate match.” Normally, only in the case of passive devices is the appropriate value also the exact value.
There are two types of reactance—inductive and capacitive. Resistance also has two components—ac and dc. Impedance also can be seen as being composed of lumped parameters (circuit compo-nents all in one place) or distributed parameters such as 100 miles of telephone cable.
8.12.6 System Problems Located by Z Measurements
Problems detectable via Z measurements vary. They can detect such problems as reactive 70V trans-formers overloading power amplifiers, woofers in incorrectly ported enclosures, link circuits not matching passive devices to be inserted in them, and discovery of intermittent circuits.
8.12.7 Gain and Loss Blocks*
Gain blocks are available in increments as small as 30dB and as large as 100dB. Very useful gain incre-ments fall in the 40dB area. If a high SNR is to be maintained with variable gain blocks (e.g., mixers and power amplifiers with gain control) while a proper peaking factor and low distortion are preserved, the variable gain control must be prop-erly set. In the case of fixed gain blocks, variable or fixed loss blocks will need to be inserted as required.
Loss blocks can be attenuators, mixing networks, equalizers, and pads. A rule of thumb for a typical sound system is that after algebraically totaling all the gains and losses, you should have an overall gain figure of approximately 115dB. For example, assume a microphone is calculated to have a sensitivity level of −59dBm in a sound field with an LP of 94dB Figure 8-40. Comparison of open and matched circuit.
RS 130 Ω
100,000 Ω RL
RS 130 Ω
130 Ω RL
A. Open circuit
B. Closed circuit
EL 1.0 100,000 100,130
---⎝ ⎠
⎛ ⎞
= EL≅1V
EL 1.0 130 260
---⎝ ⎠
⎛ ⎞
= 0.5V
=
20 0.5 ---1
log = –6dB
*Gain describes what “level” change occurs at the system output upon insertion of the device into the system. If the level goes down it is a “loss”. If the level goes up it is a “gain”.
142 Chapter 8 r e c e i v e s a n a c o u s t i c i n p u t o f 8 8d B . T h e n 94 dB− 88 dB = 6dB, and −59dBm − 6dBm =
−65 dBm. If the sound system has a 100W output (+50dBm), then we need 115dB of gain to get from the level at the mixer input to the level at the loud-speaker at full power.
8.12.8 Typical Mixer Amplifier
A typical mixer amplifier has the following specifications:
We know we must allow 10dB as a meter lag factor, so the −65dBm program level out of the microphone should cause the output of the mixer to reach +8dBm. Therefore, we need to adjust the overall gain of the mixer to 73dB. (The attenuators in the mixer can be set back approximately 87 dB− 73dB = 14dB of working loss.) Suppose, for the moment, that we are going to connect the output of this mixer directly to the input of a power amplifier having the following characteristics: