Music and Microphones for the Audiologist – Part 2

Marshall Chasin
May 29, 2012

This is the second in a series of blogs outlining some of what we know about optimal music recording.  When reading through these blogs one should be struck that we really know everything we need to know already when it comes to recording.  In some cases, what was learned in our first year audiology class provided  “obvious” recording cues (such as ensuring a wide bandwidth) and in other cases, the information and concepts have been learned but may have applied to other fields than music recording.

The second question concerning recording musical instruments is which microphone to use.

This may be a simple issue, and when it comes to the issues touched upon in the last blog, it is, but it’s worthwhile to review the two major types of microphones used for music (and speech) recording.  They are the condenser microphone and the dynamic microphone.

By far the simpler (and in most cases, the least expensive) of the two is the dynamic microphone.  These, like condenser microphones, can be either omni-directional or (uni-) directional.  Essentially a dynamic microphone consists of a coil of wire that is wound around a moveable, and permanent magnet.  Because the magnet is permanent, no external power source is required to maintain it.  And this is the first important feature of a dynamic microphone- there is no need for a power source.   Any microphone that requires a battery, a “hot pin” or “phantom power” is not a dynamic microphone.

A common misconception of a dynamic microphone is related to its dynamic range.  This type of microphone has a relatively poor “dynamic range”- it is the movement of the coil relative to the permanent magnet that is “dynamic” but for this to work the diaphragm needs to be either made of metal or have a sufficiently dense metallic coating.  This adds mass to the microphone diaphragm and this increases its inertia.

Similar to pushing a heavy car that is stuck in the snow (for the northerners amongst us) or stuck in the quick sand (for the southerners amongst us), it takes a while to get it moving (i.e., it exhibits inertia), and then once moving, it takes a superman to stop it.  In contrast, when trying to push a subcompact car, even a wimpy audiologist can get it moving, and similarly one can run around in front of the moving car and stop it on a dime.  BUT DON’T DO THAT… IT CAN BE DANGEROUS!

A more massive diaphragm takes longer to respond and continues to respond for a brief period after the cessation of the sound.  For speech however, this is not a problem.  The most rapid speech sound in any language of the world (an apical click or an affricate) is still slow enough for typical dynamic microphones to respond almost immediately to the changes in air pressure.  However, it is a problem for the proper transduction of percussive sounds such as those from a drum kit.

A rim shot or a percussive hit to a drum kit cymbal has a sudden increase in pressure.  Dynamic microphones will impart a delay in the rise time because of the mass of the diaphragm and this would limit the transduction of the higher frequency sound energy of the cymbal or rim shot to the recording device- it has inertia and takes a moment (albeit brief) to start moving.

In addition, a corollary of having a more massive diaphragm is that its output is limited to about 110-112 dB SPL.  Again, this is more than sufficient for any language of the world but can severely limit the dynamic range of music (and explosions so don’t use a dynamic microphone if you are a war correspondent,…, BUT DON’T DO THAT… IT CAN BE DANGEROUS!)

In contrast to a dynamic microphone, the condenser microphone does have a better dynamic range and can transduce sounds up to about 135 dB SPL.  This is directly related to a less massive diaphragm.

The way a condenser microphone works can be gleaned from its old name- a capacitor microphone.  As the name suggests, a capacitor is a structure with two parallel plates separated by some sort of dielectric material (such as air).  If the plates are brought closer together, there is an increased current flow.  The converse is also true when the plates are pulled apart- less current flow.

In the condenser microphone one plate (the back plate) is charged by a battery or external power source.  The other (front) plate is the diaphragm of the microphone and as it moves backward towards the rear charged plate and forward, away from the rear charged plate, the current flow is altered- energy is transduced.

All condenser microphones require a power source to charge the rear plate.  Some use the power from the recording device (called phantom power) and others have their own regulated supply.   Most modern day condenser microphones don’t require a highly regulated power supply, but this was not the case in the olden days.  In the “olden days” a condenser microphone would not work well unless it had its own on board dc power supply but with today’s technology, “phantom power” from a remote source is more than sufficient.

Because the diaphragm does not have to be metal or coated with metal ions, it is less massive than its dynamic microphone cousin so it can transduce a large dynamic range and can respond more rapidly to sudden percussive sounds such as rim shots or cymbals.

The only drawback of a condenser microphone is the requirement of a power supply and from personal experience, I can tell you that the power supply will die the moment you want to record something important.  Nevertheless, this is the only real drawback.

A bit of trivia that comes out of this discussion is that accelerometers (which measure acceleration) are also based on the same principle as a condenser microphone.  As one accelerates, the front plate is pushed back towards the rear plate, and the converse is true when it decelerates.  If the velocity is constant however (zero acceleration) then there is no movement towards or away from the rear plate- this approach is ideal for accelerometers but will not work as a speedometer.

In the next blog, we will touch on the cable that connects the microphone to the recording device.  It’s not as boring as it sounds and we will even go into the history of why an XLR cable is called, an XLR cable!

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