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Old February 14th, 2010, 02:38 AM   #1
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Active vs Passive mics.

Why do some external mics require batteries and other don't?
What are the benefits of those that do use batteries in them?
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Old February 14th, 2010, 08:46 AM   #2
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Generally we're talking dynamic mics and condenser mics - we don't normally talk of active or passive mics.

A dynamic microphone is in essence a small loudspeaker used in reverse. A small diaphragm with a coil of wire in the magnetic field of a small magnet (remember your school physics lessons?) The sound energy from the voice or musical instrument physically moves the diaphragm (hence the term dynamic), wobbles the coil of wire, and electrical energy emerges from the bottom. This electrical energy has been sourced from the acoustic energy - one of the reasons mics are technically called transducers. So somebody shouts, the coil wobbles and some pretty weak voltages, measured in millivolts, emerge. If instead of a shout, you wanted to pickup the sound of a pin dropping, then the energy from the pin hitting the floor is not really going to move the diaphragm - and the consequence is virtually nothing will be produced in terms of electrical output. If you replace the big mechanical diaphragm with a much, much lighter one -made from mylar - it's very, very light, and will wobble when the pin falling sound comes along. The output is generated by having this piece of mylar (or other similar material) very close to a plate with a voltage applied to it. As it wobbles, the charge varies a little - and a small amplifier can detect this and convert it into something bigger. Charge is called capacitance, and capacitors can also be called condensers - hence the term for this kind of microphone. The voltage needed to run the amplifiers can be quite small, maybe 1.5V or thereabouts - this can easily be found from a small battery or two. The actual charge on the backplate can be put there in the factory - this doesn't need any topping up for the lifetime of the microphone. However - the charged plate doesn't like damp - one of the reasons condenser mics are a bit unpredictable when humidity is high - the dampness can short out the charge! Larger diaphragms need 48V to polarise them, hence why this kind of microphone doesn't have battery facilities - and need remote powering.

The practical upshot of all this is that condenser mics, with their internal pre-amplifiers are more sensitive and respond better to quiet and harmonically complex sound sources. Their downside is that they tend to be more delicate, hate being banged around, and need power. Dynamic mics tend to be pretty bullet proof - don't need external power, but are a bit 'deaf' - working best on loud sound sources. The larger, more stiff diaphragm also means that their response at the higher frequencies is poorer.

Both have uses - and sound quite different. Small diaphragm condensers can be physically tiny, and have very wide flat frequency response. Large diaphragm condensers tend to have a 'character' that work best on certain sound sources.

You could make a dynamic shotgun mic, but the kind of distance they're used at would mean they wouldn't be much use - a quiet sound source at 10 feet is going to be very much like the pin drop, energy wise.

The other answer to your question is simpler. A condenser mic needs power. Some cameras can supply it, some can't. So one that can work from it's own battery is pretty essential for a camcorder that has no power facility.
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Old February 14th, 2010, 08:25 PM   #3
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paul covered it. I'll only add that dynamic mics are usually only used for voice on stage where the source is close to the mic. Its lack of sensitivity is an advantage because it can with stand someone shouting into it, you don't have to worry about batteries dieing, and dynamics are very durable, withstanding drops. In any other situation you'll use a condenser mic that's either battery or phantom powered, which ever is more convenient.
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Old February 15th, 2010, 02:39 AM   #4
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Pete, it's pretty common for dynamics to mic guitar amps and drum kits as well. Otherwise, I'd agree with both of what these guys said.
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Old February 15th, 2010, 10:44 AM   #5
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In my tenure as a FOH mixer in the 70s-80s, normally the only condensers on-stage were the drum overheads and this included bands with horn sections.
An occasional acoustic guitar would usually have a pick-up and I would send that though an active DI. There were variations occasionally, a small diaphragm condenser would be used on chimes and instruments of that nature.
Of course there were no 'affordable' LD condensers available at that time.
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Old February 15th, 2010, 12:53 PM   #6
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With or without battery

Originally Posted by Jim Lee View Post
Why do some external mics require batteries and other don't?
What are the benefits of those that do use batteries in them?
These are excellent responses and have saved Jim quite a lot of research time.

Just in case a narrower answer was expected, I would just mention as an example the difference between the Rode NTG-1 condenser mic RØDE Microphones - NTG-1 which requires a 48v phantom power supply and the very similar Rode NTG-2 condenser mic RØDE Microphones - NTG-2 which can use either a 48v phantom power supply or an internal AA battery when phantom power is not available.

There are similar mics from other manufacturers of course, but these two are among the mics very commonly used with video cameras by dvinfo contributors.
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