Sidebar to current drive discussions: microphone behavior
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 13th August 2004, 06:50 PM #1 RHosch   diyAudio Member   Join Date: Sep 2003 Location: SouthEast Sidebar to current drive discussions: microphone behavior I'll preface my extended set of questions by clearly stating that electronics is not my forte. I probably know enough to injure myself, and precious little beyond. It was briefly mentioned in the tansconductance thread that since the force produced by a dynamic field-coil motor is proportional to the current, and not necessarily to the voltage, then the acceleration would be proportional to the current as well. It was also mentioned (IIRC) that the velocity is proportional to the voltage, and suggested that we care about the motion of the cone and not necessarily the velocity or acceleration independently. Barring the obvious relationships between voltage and current, and between veocity, acceleration, and position, I have to question if that is really the case... and to further ponder just what we should care about. The natural place to answer that question, it would seem, is in the behavior of microphones and the recording process. This question is undoubtedly complicated by the issue of whether we should consider (for our simplified purposes) sound to be pressure waves, velocity waves, or both. Dynamic microphones, being miniature versions of field-coils, should follow the same proportionality between diaphram acceleration and current, which implies that they should also have a current output proportional to the pressure on the diaphram. An omni-directional dynamic microphone combined with a closed box loudspeaker using current drive would appear to me to be a natural reproduction solution. But is it? Here enters my ignorance... if the microphone's current output is what we want to record and preserve, as being proportional to the signal we wished to record, is that what we actually measure/sample/record? i.e., does an ADC look at voltage or current? Does it matter? I'm not sure what the impedance curve of a typical dynamic microphone is... if it measures flat, is that because it has no resonant peak in the audible range, or because of electrical damping of that peak similar to a voltage drive amplifier? In my mind, at least, it gets more confusing when contemplating velocity transducers - ribbon mics and dipolar speakers. If a ribbon mic has a current output proportional to the air velocity, would we want to reproduce that on a closed box current drive system that produces a pressure output proprotional to current? Or, if the current and voltage output of microphones is directly proportional (which may well be the case I suppose), is there any fundamental mismatch between a pressure transducer microphone and a dipolar loudspeaker? In all of my rambling thoughts, the consistent theme is my confusion over the very basics of how typical microphones and sampling stages (in digital recording) work. The fundamental question is really what is it that we should be attempting to reproduce accurately: the velocity of the microphone diaphram, the acceleration, the position? And is that in any way dependent on whether it is an omni or figure-8 mic, or whether we are using closed box or dipolar speakers, or whether we are using voltage or current drive? It's too easy to get bogged down in the details of crossover slopes, philosophies of speaker directivity, driver layouts, etc., and I have realized that I should probably get the basics right first.
 13th August 2004, 07:02 PM #2 BillFitzmaurice   Banned   Join Date: May 2004 Location: New Hampshire It's a far less daunting task that you may think. Dynamic microphones and loudspeakers, for instance, operate virtually in precisely the same manner, all that differs is that one turns sound into an electrical waveform and the other performs the inverse. Ribbon mics and speakers also are closely related in how the function. In fact you can use a dynamic speaker as a mic, and vis-versa. That scenario was first successfully employed by Alexander Graham Bell circa 1876.
 13th August 2004, 07:11 PM #3 RHosch   diyAudio Member   Join Date: Sep 2003 Location: SouthEast Well, I understand the broad picture, but as they say the Devil is in the details. For instance, it seemed obvious that dynamic microphones and speakers were simply scaled inverses of one another, at least in a general sense. I'm sure I've read as much several times through the years in any case. But the recent thread(s) on current drive had driven me to take a closer look at those details. If the response of a speaker (an underdamped speaker, at least) to current drive and voltage drive are significantly different, I ponder whether the current and voltage outputs of microphones can also be significantly different (proportionately). Do dynamic microphones have resonances in the audible range that could cause current and voltage non-proportionalities? If so, are these damped electrically as is done in the inverse case of speaker voltage drive? If the current output is proportional to the "sound" (whatever that happens to be defined as...) do ADC's sample the signal voltage or use an appropriate impedance to effectively look at signal current? Likewise ribbon mics and speakers are very similar in principle. Are there any extraneous considerations to be made when reproducing a ribbon mic recording through a dynamic microphone, or vice versa?
 13th August 2004, 07:19 PM #4 BillFitzmaurice   Banned   Join Date: May 2004 Location: New Hampshire Mics do have impedance and resonance issues just as speakers do, for the same reasons, and with the same results: they aren't perfectly flat in response. The same applies to ribbon or condensor mics versus dynamic. All have positive and negative attributes; what you end up using is determined by your needs- and your budget.
moamps
diyAudio Member

Join Date: Oct 2002
Location: Croatia
Quote:
 Originally posted by RHosch For instance, it seemed obvious that dynamic microphones and speakers were simply scaled inverses of one another, at least in a general sense.
Only in general sense. (converter type)
Quote:
 .... I ponder whether the current and voltage outputs of microphones can also be significantly different (proportionately).
No. This depends of load impedance (usually resistive).

Quote:
 Do dynamic microphones have resonances in the audible range that could cause current and voltage non-proportionalities? ]
" .....We generally want the sensitivity of a microphone to be independent of frequency, or "flat." If the microphone is sensitive to pressure................ It is not hard to see that any oscillator is stiffness controlled at frequencies much less than its natural frequency. The response is essentially level for frequencies less than a tenth of the resonant frequency. This is the reason that the resonant frequency of the diaphragm of a microphone is made greater than the maximum frequency at which the microphone will be used. ..........A microphone whose output depends on the velocity of the response, such as a moving-coil or dynamic microphone, or a ribbon microphone, is made flat by operating near the resonant frequency with a low Q. In this case, v = f/r is independent of frequency. .........."

Good book also about microphones is
John Borwick: Microphones

Regards
Milan

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