mic frequency response

[veenife]

Hey, in this graph im attaching here is showing that if one do a 1 to 1 input impedance to output impedance of a dynamic microphone and an mic pre amp the low and high frequency response of the mic is reduced but the middle frequencies stay intact....

I would like to understand why JUST low and high frequencies get affected but not the mid ones????

**broken link removed**

 

[FvM]

I don't know what "10:1" or "1:1" load actually means. I guess 1:1 means termination with the nominal microphone impedance, e.g. 600 ohms and 10:1 means respectively higher load impedance.

If it's a basic dynamic microphone without electronic circuits (e.g. high frequency enhancement, bass cut, etc.), I would guess that the high frequency drop is caused by voice coil inductance and the low frequency drop by damping the voice coil self resonance.

 

[veenife]

... the graph is from a "sm57", this mic has an output impedance of 310 ohms... and its a moving-coil dynamic microphone...

yes.... 1:1 means equal output to input impedance

i guess i understand the part of the high frequencies loss due to the inductor (moving coil) ....

but ...

1 - could you explain me better the reason why that happens with the low frequencies???

2 - the same thing happens with ribbon microphones... how does it work then as ribbon mic have no inductor in it????

 

[FvM]

Ribbon microphones use a transformer which involves leakage inductance.

For the low frequency behaviour, you have to analyze the mechanical behaviour of the transducer. It's a mass-spring system, the resonance sets the lower cutoff frequency. The resonator quality factor, which depends among other parameters on the electrical load, affects the transition shape. That's what you see in the diagram.

 

[veenife]

i would like to get more deep in the subject, in the net i find no information about that related to microphones.... but to speakers... how much can i relate the theories of impedance and frequency response from a speaker compared to a mocing coil microphone???

this website has a lot of info on that but all about speakers....

**broken link removed**

 

[Audioguru]

A microphone is almost never loaded with a matched impedance because it reduces its output level. A "600 ohm" mic is loaded with a preamp that has an input impedance of 2.2k to 10k ohms. The graph shows the small effect on frequency response with different loads. A low resistance load reduces the low frequency resonance and reduces high frequency ringing of the mic.

A speaker has a very strong low frequency resonance and it is damped by the extremely low output impedance of a modern amplifier that is about 0.04 ohms or less.
The enclosure for a speaker has its dimensions and whether it has a port or is sealed dependent on the spec's of the speaker.

 

[FvM]

I agree with Audioguru. The usual operation of dynamic microphones is with a high load impedance, transducer impedance doesn't affect the frequency response in this case. That's why microphone users and even audio equipment designers usually don't think about frequency dependend microphone impedance.

 

[veenife]

Okay thats good to know, but I am at this point more interested to know how exactly the `resonant frequency` of the coil is affecting the frequency response of the microphone...

"FvM" said that the low frequencies are lost because the resonant frequency of the coil is damped.... I want to understand how it happens...,???

i assume it could be similar to a speaker where some of the signal is reflected back down its own cable (back-emf) and so damping the resonant frequency... is that correct?

 

[Audioguru]

The resonance of a dynamic mic or speaker is not electrical, instead it is mechanical. The weight of the mic diaphragm and coil or speaker cone and coil and its springy suspension create the low frequency resonance.

Yes, when the back EMF is loaded with a low resistance then the resonance is reduced.

Try it with a speaker. Without a load then when you tap its cone it makes a resonant "BONG" sound like a bongo drum. Then short the terminals like an amplifier with a very low output impedance and tap its cone. It sounds like a well damped "THUD".

The air inside the enclosure for a speaker and the air inside its port tube affects the frequency and amount of a speaker's resonance.