Microphone Mythbusters, Vol. 8 – Why Won't My Mic Work?
by Gino Sigismondi
Why Won’t My Mic Work? Gino Sigismondi shares three vital specifications that are necessary for it to work.
Greetings! Things have been really hopping here this summer, so it’s been a while since my last post. This month’s topic comes once again courtesy of Michael Pettersen, Director of Shure Applications Engineering. I can personally vouch for the numerous times we get asked this question. Enjoy…
Shure Applications Engineering is frequently asked “Will my microphone (wired or wireless) model ‘X’ work with my what-cha-ma-call-it?” Or “Why does my mic not work as expected with my new i-thing-a-ma-bob?” While we would love to have the microphone input specifications of every device in the world that needs a microphone, it is an impossible task. So to select the proper microphone, it is essential to have three vital specifications for the microphone input of the customer’s device. Typically, these specifications will be provided in the Owner’s Manual for the device…we hope.
VITAL MICROPHONE INPUT SPECIFICATION #1
Called ‘Input Sensitivity’ or ‘Nominal Input Level’, this specification indicates how strong of a signal the microphone must supply to satisfy the microphone input of the device. This specification might be given in millivolts (mV), or volts (V), or in a minus dB form (-dBV, -dBm, -dBu, -dBs).
There is a wide variation of microphone signal levels available, depending on the model. If a microphone is selected whose signal level is too low for the device, the audio will be noisy and low in level. If a microphone is selected whose signal level is too great for the device, the audio will be distorted and unintelligible. Proper matching of the microphone’s signal level to the device’s required input level is imperative.
VITAL MICROPHONE INPUT SPECIFICATION #2
Called ‘Input Impedance’ or ‘Actual Input Impedance’, this specification is important as it determines the proper impedance range of the chosen microphone. This specification will be given in ohms. Contrary to popular audio mythology, the impedance of a microphone does not need to exactly match the input impedance of the device.
In the Shure product line, there are different impedances available depending on the microphone model. If a microphone is selected whose impedance is lower than, or equal to, the device’s input impedance, the microphone will work as long as it provides the proper signal level – see #1 above. If a microphone is selected whose impedance is greater than the device’s input impedance, the microphone will not deliver its full signal level to the device and the audio will be noisy and low in level.
VITAL MICROPHONE INPUT SPECIFICATION #3a, #3b, and #3c
The final requirements are:
- a) The type of microphone input connector on the device,
- b) How many connection points are inside the connector
- c) What is the function of each connection point
This specification will be the name of the connector, such as: XLR female, 3.5 mm mini-phone jack, TRS 1/4 inch female phone jack, screw terminals, TINI QG connector.
Each of these connectors has at least two connection points and most have three (or more) connection points. Common circuit functions include: ground, audio send, audio return, and DC bias. It is imperative that the circuit function of each connection point be known so that the proper microphone wiring can be determined.
In the Shure product line, there are different wiring schemes available depending on the microphone model. If the microphone connections are not properly matched to the device’s input connector, there may be no audio, or funny sounding audio, or the microphone might be damaged if there is an unexpected voltage appearing on the device’s connection points.
And finally, just because two connectors mate does not mean that each is wired the same.
CONCLUSION
There are many variables that affect whether a particular microphone (wired or wireless) will work properly with a particular device. Without the unique audio input specifications of that device, it is impossible to accurately answer the question,
“Which mic will work with my brand new gizmo?”