I recently had the opportunity to explore the topic of interfacing brand-X microphones to ICOM radios, first the IC-756 standard, then the IC-756PRO. Here's what I discovered.
The goal is to have a microphone drive the radio with an appropriate signal level. What really matters is that the two work as a pair. We want a matched system. Many ICOM radios have a reputation for requiring a stronger microphone signal than other radios. In fact, some third party microphone vendors sell a pre-amp specifically designed for ICOM. This issue became real for me when I replaced my Yaesu FT-920 with the ICOM IC-756.
When the Yaesu FT-920 was my primary HF radio, I used an Audio Technica (ATH-COM2) microphone/headphone (headset) combo. I really liked the headset because it had high quality earphones, a good sounding microphone, and it was lightweight. It was also moderately priced. When the ICOM IC-756 arrived, I was hoping to simply use the same headset with the new radio.
I quickly learned that the Audio Technica microphone did not drive the ICOM IC-756 nearly as well as the supplied HM-36 microphone. With the HM-36 microphone, the radio was fully driven with the microphone gain control at around 10:00 o'clock. The Audio Technica microphone could not generate an acceptable signal level even with the microphone gain near maximum. Something was very different.
In the end, it's this simple. The output signal that a microphone produces will be related to the microphone (element) technology. Two different classes of microphone are popular in this application area. One is the dynamic element, and the other is the electret condenser element. A dynamic element generates a signal which is approximately -55 dBV. An electret condenser element signal is approximately -35 dBV. That difference, 20 dB, is a 100X difference. Please note that these signal levels do vary quite a bit from microphone to microphone.
As best as I can tell, ICOM has designed their radios to expect the higher electret condenser level. I think that this is a reasonable assumption because their accessory microphones, the SM-8 and SM-20 are described as having electret condenser elements. In RF or electrically noisy environments, the stronger microphone level may improve RF feedback immunity through the microphone. In other words, the microphone and its cable may act as less of an (undesirable) antenna.
In checking my Audio Technica headset, I found that it was a dynamic element. The lack of signal drive finally made sense, I was using a microphone of the wrong type. I was 20 dB short of a load, so to speak.
Types of Microphones
The terms Omni-directional---uni-directional---
cardioid--& super cardiod refer to the polar pattern.
Basically--if you are looking at the microphone as it is laying on it's side,the ploar pattern shows how tight the pattern is and how much unwanted sound is rejected.
The super cardioid is the tightest and the cardioid is next.
Some of the cardioids are very close in pattern to the supercardioid mics.
If you are a vocalist and are concerned about quality,then I recommend the following mics:
1---Samson CO5 CL--Cardioid-condenser--40-16k--about $70. This is the best bang for the buck