Technical index of the microphone is an important basis for the selection and use of microphone. Only understand the exact meaning of it, can reasonably dispose of microphone and the sound source distance, conversion between the sound pressure and voltage, and the level of sound pressure level microphone sound field accurately converted into the mixer input, so as to determine the input gain of the mixer, the highest signal-to-noise ratio, linear signal minimum distortion. Then the phone which indicators and mixer input closely related? At the same time, how to determine the level of the mixer?
One, the meaning of several important indexes
Technical indicators broadcast microphone has nearly 10, for the sound source and the mixer, the most important is the following 3: maximum input sound pressure level, sensitivity and maximum output level. Maximum input sound pressure level is reached the maximum sound pressure level measuring total harmonic distortion of 0.5% microphone can bear, it and defining the relationship between sound pressure for:
0dBSPL=2 * 10-5Pa
Maximum input sound pressure level professional microphone in general is higher, as long as it sound source distance properly, will not produce audible distortion. So here we focus on directly affect the mixer operating level after two kinds of index.
The sensitivity of 1
Sensitivity is the ratio of the output voltage and the input pressure in unit pressure excitation, the unit is mV/Pa. For the uniform metric and circuit level, sensitivity can decibel value representation. Early DB to unit of dBm and dBV:
0dBm=1mW/Pa, namely the 1Pa input pressure under 600 load 1mW power output is defined from 0dB;
0dBV=1V/ bar, the 1V voltage output is defined in 1 μ bar input sound pressure is 0dB
Now the DB are represented by unit dB:
0dB =0.775V/Pa, the 1Pa input sound pressure microphone 0.775V voltage output is defined as 0dB (so the microphone sound pressure level the converted voltage measurement, unified to the circuit in the 0dB μ =0.775V the reference unit)
Obviously, no matter how sensitivity said, we can convert it to dB μ, premise is the line of the input to Pa this unit
For example: NEUMANNU89 microphone sensitivity is 8mV/Pa, can be directly by the
20lg[(0.008V/Pa) / (0.775V/Pa)]
The sensitivity is about -40dB μ
Another example: the sensitivity of AKGC414 receiver for -60dBV, by
0dBV=1V/ μ bar=10V/P
To obtain the output voltage X: 1Pa pressure -60dBV
20lg[(XV/Pa) / (10V/Pa)]=-60
The X=0.01 (V), the sensitivity of 10mV/Pa. Then the type
20lg[(0.01V/Pa) / (0.775V/Pa)]
But the sensitivity is about -37dB
2 of the maximum output level
The microphone output level at maximum input sound pressure level under the maximum output level. As the maximum input sound pressure level defines the microphone and the sound source distance, maximum output level directly determines the input level mixer. For a given microphone, as long as is known for its sensitivity and maximum sound pressure level, it can calculate the maximum output level of it.
Still by NEUMANNU89 microphone as an example: the sensitivity is 8mV/Pa, the maximum input level of 134dBSPL (input attenuation profile in 10dB). First the input sound pressure level is converted into sound, then the sensitivity obtained maximum output level: because 0dBSPL=2 * 10-5Pa, the maximum input pressure X by
134dBSPL=20lgX/2 * 10-5Pa
The X=100 (Pa). The sensitivity of 8mV/Pa, 100Pa maximum output is 800mV, into the output level is:
20lg[(0.8V/Pa) / (0.775V/Pa)]
About 0dB μ. This is the maximum output level U89 microphone theory. In fact, when in normal use generally do not choose the 10dB attenuation profile (to reduce the input noise), so long as the microphone placed the appropriate distance, maximum sound pressure level U89 is generally not more than the normal gear 124dBSPL, the maximum output level is generally much lower than the 0dB
Two, to determine the operating level of a mixer
Technically, the working level mixer is set to maximum headroom upper bound on the maximum signal level does not exceed the set of manufacturers (actual work level was determined according to the required to program). Here, the maximum dynamic margin refers to the total harmonic distortion index on the maximum level of 0dB μ a level range, as shown below
So, as long as the maximum sound pressure level and microphone microphone end known sound source possible sensitivity, we can calculate the corresponding maximum output level; the difference in the level and the mixer headroom upper limit, is the mixer input level, namely the microphone amplifier (referred to as words) input gain; the working level signal level this gain is mixer.
The sound source for Coloratura Soprano, using U89 receiver and AMEKRemBandt mixer as an example, this explanation: if the maximum sound pressure level at 70cm distance microphone sound source is the 112dBSPL (peak, A weighted sound pressure level), the maximum input microphone can bear is 124dB, sensitivity is 8mV/Pa, from the formula
112dBSPL=20lgX/2 * 10-5
The sound pressure level 112dB is equivalent to 8Pa pressure, voltage output which can be 8Pa × 8mV/Pa=64mV, namely a level output:
20lg[(0.064V/Pa) / (0.775V/Pa)]=-22dB
Dynamic headroom this level is connected to the 70dB amplifier gain, word level in the total harmonic distortion is 0.017% when the mixer for 12dB μ (dynamic margin after the mixer circuit amplifier stage is 28dB μ), and +12dB upper limit is 34dB margin (+12dB- (-22dB) =34dB), which is mixer should input gain. This will determine the working level mixe
In actual operation, to ensure that the mixer at maximum signal level the total harmonic distortion of less than 0.017%, the general should put the words gain and reduce several db. Reduce the amplitude is determined by the following method: firstly, monitor and total transport channel