The Myth of Microphone Reach

   Shure  is  often  asked "How far away will my microphone pick up?'" or
   "Which  microphone  has the best reach?" Both questions are based on a
   misunderstanding  of  how  microphones work. This bulletin attempts to
   debunk the myth of microphone reach.

Myth: A microphone reaches out to capture sound.

   Fact:  A  microphone  only  responds to sound waves that travel to its
   location. A microphone measures local rapid variations in air pressure
   and provides an electrical output that mirrors these variations. These
   rapid  air  pressure variations are sensed as sound if they are within
   the  hearing  frequency  range of 20 - 20,000 Hertz. The microphone is
   stimulated  only  by  the  sound waves that travel to its location. It
   cannot "reach" out and capture the sound wave from a distance.

Myth: A directional microphone enhances sound waves which approach from the
      front.

   Fact:  A  directional microphone merely rejects sounds from directions
   other  than  the  front. An omnidirectional microphone "hears" equally
   well  in  all  directions.  It  does  not  reject  any  sound as it is
   insensitive   to   the   direction   of  the  passing  sound  wave.  A
   unidirectional  microphone  "hears" well in certain directions and not
   so well in other directions. As an example, a cardioid microphone does
   not  reject  sound  waves  which  approach from the front; is slightly
   "deaf"  to  sounds  approaching  from  the  left or right; and is very
   "deaf"  to  sounds approaching from the rear. So a cardioid microphone
   appears  to  enhance  sound  waves  from  the front, by being far less
   sensitive to sound waves from the left, right, and rear.

Myth: A microphone has a reach specification that can be measured in feet or
      meters.

   Fact: A microphone's effectiveness at different distances is primarily
   dependent  on  the background noise level. Let's use an example. For a
   nature  film,  you want to record the call of a wolf in the wilderness
   of Canada.
   The  recording  conditions  are  superb. There is no wind; the closest
   town  is  100  miles  away;  your  equipment is a state-of-the-art DAT
   machine  (Digital  Audio  Tape)  with  a  quiet mic preamplifier. Your
   microphone  is  an  omnidirectional dynamic. You spot a wolf about 1/2
   mile  away  and start the recorder. The wolf howls for two minutes and
   you obtain a fantastic recording. [If you believe in microphone reach,
   your  microphone  has a reach of at least 1/2 mile.] The next week you
   are  back  in  New York City. A wolf escapes from the Bronx Zoo, takes
   the  subway  to  Manhattan,  and  is now howling on 6th Avenue at 59th
   Street, which happens to be 1/2 mile from your studio. You immediately
   activate   your   DAT  recorder  and  hang  the  same  omnidirectional
   microphone  outside  your  window to record the wolf, but much to your
   surprise all you hear is traffic and wind noise when you play back the
   tape.  The wolf cannot be heard even though you saw it howling through
   your  binoculars.  Did the reach of the microphone somehow change? No,
   only the ambient noise conditions changed.

Myth: A shotgun microphone is like a zoom lens on a camera

   Fact:  Using  a shotgun mic is like taking a photo with the lens aimed
   down a cardboard tube. The image being photographed is not brought any
   closer,   but  the  unwanted  images  to  the  sides  are  reduced  or
   eliminated.  A  shotgun  mic  seems  to bring the desired sound source
   closer  because  the  unwanted  sounds  from  the  side  and  rear are
   attenuated.  It  is  tempting to compare light with sound. Light waves
   and  sound  waves do have similarities, but many more differences. The
   most important difference is in wavelength. The wavelength of light is
   measured in millionths of an inch. The wavelength of sound is measured
   in inches and feet. For a zoom lens to be effective, its diameter must
   be  hundreds  of thousands times larger than the wavelength. A typical
   camera  zoom  lens  is approximately 3 inches in diameter. But a "zoom
   lens"  for  sound  waves (using the same ratio) would have to be tens,
   even  hundreds,  of  miles in diameter. There is an acoustical device,
   called  a  parabolic reflector, that can be combined with a microphone
   for  distant  pickup.  However,  it  too  will  be limited by unwanted
   ambient noise, and to be effective at lower frequencies, the parabolic
   reflector  must  be six to eight feet in diameter. It is the extremely
   long  wavelengths that make sound so difficult to control, manipulate,
   and focus.

   http://www.shure.com/support/technotes/app-micreach.html