The technical specifications of the microphone are an important basis for selecting and using the microphone. Only by understanding its exact meaning can we properly handle the distance between the microphone and the sound source, master the conversion relationship between sound pressure and voltage, and accurately convert the sound pressure level of the sound field of the microphone into the level of the input end of the mixer. Thereby determining the input gain of the mixer, and obtaining the linear signal with the highest signal to noise ratio and the lowest distortion.
So which indicators of the microphone are closely related to the input of the mixer? At the same time, how to determine the working level of the mixer?
The meaning of a few important indicators audio applications are midi, recording, mixing, music production, recording, audio, vst, mixing, musical instruments, guitars, microphones, headphones, sound cards to provide the most comprehensive audio information and information exchange ,Learning platform!
There are nearly 10 technical indicators for broadcast microphones. For sound sources and mixers, the most important ones are the following: maximum input sound pressure level, sensitivity and maximum output level. The maximum input sound pressure level is a measure of the maximum sound pressure level that the microphone can withstand 0.5% total harmonic distortion. Its relationship with sound pressure is defined as:
0dB SPL=2×10-5Pa
The maximum input sound pressure level of a professional microphone is generally set higher, and as long as it is properly spaced from the sound source, no audible distortion is produced. So here we focus on the latter two indicators that directly affect the working level of the mixer.
1. Sensitivity Sensitivity is the ratio of the output voltage of the microphone to the input sound pressure under unit sound pressure excitation. The unit is mV/Pa. In order to be consistent with the measurement of the level in the circuit, the sensitivity can also be expressed in decibels. Early decibels are expressed in units of dBm and dBV:
0dBm=1mW/Pa, that is, the 1mW power output brought to the 600Ω load by 1Pa input sound pressure is defined as 0dB;
0dBV = 1V / μbar, the 1V voltage output generated at 1μbar input sound pressure is defined as 0dB.
The current decibel is expressed in units of dBμ:
0dBμ=0.775V/Pa, that is, the voltage output of the microphone 0.775V under the input sound pressure of 1Pa is defined as 0dB (so the level measurement of the microphone sound pressure-voltage conversion is unified to 0dBμ=0.775V commonly used in the circuit. Reference unit).
Obviously, regardless of the sensitivity, we can convert it to dBμ, provided that the line input is unified to the unit of Pa.
For example, the sensitivity of the NEUMANN U89 microphone is 8mV/Pa, which can be directly from 20lg[(0.008V/Pa)÷(0.775V/Pa)]
It is found that its sensitivity is about -40dBμ.
Another example: the sensitivity of the AKG C414 microphone is -60dBV, from 0dBV=1V/μbar=10V/Pa
First find the output voltage X of -60dBV at 1Pa sound pressure:
20lg[(XV/Pa)÷(10V/Pa)]=-60
It is found that X = 0.01 (V), that is, its sensitivity is 10 mV / Pa. Then by the formula 20lg [(0.01V / Pa) ÷ (0.775V / Pa)]
The sensitivity is about -37dBμ.
2. The output level of the maximum output level microphone at the maximum input sound pressure level is the maximum output level. Just as the maximum input sound pressure level defines the distance between the microphone and the sound source, the maximum output level directly determines the input operating level of the mixer. For a given microphone, its maximum output level can be calculated as long as its sensitivity and highest sound pressure level are known.
The NEUMANN U89 microphone is still taken as an example: its sensitivity is 8mV/Pa, and the maximum input sound pressure level is 134dB SPL (at 10dB input attenuation). First convert the input sound pressure level into sound pressure, and then determine the maximum output level by sensitivity: because 0dB SPL= 2×10-5Pa, the maximum input sound pressure X can be 134dB SPL=20lgX/2×10-5Pa
It is found that X = 100 (Pa). Known by the sensitivity of 8mV/Pa, the maximum output is 800mV at 100Pa, and the output level is:
20lg[(0.8V/Pa)÷(0.775V/Pa)]
It is about 0dBμ. This is the theoretical maximum output level of the U89 microphone. In fact, the normal use does not generally select the 10dB attenuation file (to reduce the input noise), so as long as the microphone is placed at the right distance, the maximum sound pressure level before U89 will generally not exceed the 124dB SPL in its normal gear, so it The maximum output level is typically well below 0dBμ.
Determination of the working level of the second mixer Technically, the working level of the mixer is specified by the maximum signal level above which the maximum signal level is not exceeded by the maximum dynamic headroom set by the manufacturer (the actual working level is also obtained). According to the needs of the program can be determined). Here, the maximum headroom refers to the maximum level under the total harmonic distortion indicator and a range of levels above 0 dBμ, as shown.
In this way, as long as the maximum sound pressure level of the sound source and the sensitivity of the microphone are known at the microphone end, the corresponding maximum output level can be calculated; the difference between this level and the upper limit of the dynamic headroom of the mixer is The input stage of the mixer, that is, the input gain of the microphone amplifier (referred to as the microphone); the signal level at this gain is the working level of the mixer.
Take the recorded sound source as the flower soprano, using the U89 microphone and the AMEK RemBandt mixer as an example. Explain that if the maximum sound pressure level of the sound source at 70cm from the microphone is 112dB SPL (peak, A-weighted), the microphone The maximum input sound pressure level that can be withstood is 124dB, the sensitivity is 8mV/Pa, and the formula is 112dB SPL=20lgX/2×10-5
It can be seen that the 112dB sound pressure level is equivalent to 8Pa sound pressure, and it can have a voltage output of 8Pa×8mV/Pa=64mV, that is, there is a level output:
20lg[(0.064V/Pa)÷(0.775V/Pa)]=-22dBμ
This level is connected to the 70dB mic gain, and the dynamic headroom of the vocal stage is 12dBμ after the total harmonic distortion is 0.017% (the dynamic headroom of the mixer line amplifier stage is 28dBμ), and The upper limit of +12dBμ has a 34dB margin (+12dB-(-22dB)=34dB), which is the input gain that the mixer should have. This determines the operating level of the mixer.
In practice, to ensure that the total harmonic distortion of the mixer is less than 0.017% at the maximum signal level, the above-mentioned speech gain should generally be reduced by a few decibels. The magnitude of the reduction is determined by the following method: firstly, the channel, the monitor and the total output three-part push-pull attenuator are placed in the 0dB working position, and then the volume table of the input and output at this time is observed, and the indication is determined in the normal area, and the determination is made. The number of decibels in which it is reduced.
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