equipment primarily in an audiovisual presenta- tion.   You   will   be   responsible   for   selecting, setting  up,  and  operating  this  equipment  in  a variety  of  situations.  These  could  range  from auditoriums   and   conference   rooms   to   class- rooms  and  offices.  This  section  presents  infor- mation  on  the  components  of  an  audio  system and a representative type of audio recorder. You will  gain  practical  experience  as  you  perform your  duties. AUDIO  SYSTEM  COMPONENTS Before  you  can  effectively  operate  any  type of  audio  equipment,  you  must  understand  the basic  principles  and  the  individual  components that  go  into  an  audio  system.  These  individual components   will   be   discussed   first.   They   are (1)   microphones,   (2)   amplifiers,   and   (3)   loud- speakers. Microphones The  function  of  a  microphone  is  to  change sound (mechanical) energy into electrical energy which  then  is  passed  on  to  the  amplifier.  The variations  in  sound  waves  are  converted  into corresponding   electrical   variations.   Since   the development  of  the  first  crude  telephone transmitter or microphone by Bell in 1875, many types  of  microphones  have  been  invented  but most  have  been  discarded  and  now  only  a  few are  used  to  any  extent.  In  a  microphone  two things   are   important:   sensitivity   and   fidelity. Sensitivity is the degree of electrical variation for a  given  intensity  of  sound  wave.  Fidelity  is  the ability  to  reproduce  the  audiofrequencies  in their   proper   relative   magnitude   without generating  other  frequencies  in  the  process.  In most  of  your  work,  the  second  requirement  is more   important   because   the   microphone   is primarily   used   so   much   in   narrations.   You should use a microphone that can reproduce the audiofrequencies  and  get  high-quality  sound.  If the sensitivity is low, the electrical output can be increased by using additional amplifiers. MICROPHONE    TYPES.—Two   types   of microphones are in current use in most facilities. They are the crystal and the dynamic. Crystal.—The    crystal    microphone    is    of medium  sensitivity  and  gives  good  fidelity  at speech  frequencies.  It  is  widely  used  in  public address work and radio other than broadcasting. Dynamic.—The   dynamic   microphone   con- sists  of  a  coil  attached  to  a  diaphragm  which holds  the  coil  suspended  in  the  field  of  a  cobalt permanent magnet. The speech waves cause the diaphragm  to  vibrate  and  move  the  coils  in  the field of the magnet and thus generate a voltage in  the  coil.  This  assembly  is  almost  universally used  for  high-quality  sound.  Thus,  the  dynamic microphone  is  especially  suited  for  use  in recording and in public address systems. G U I D E L I N E S    F O R    M I C R O P H O N E PLACEMENT.—In   recording   sound,   you   must have  the  proper  placement  of  the  microphone. If  it  is  necessary  to  record  speech  in  a  room that  is  too  “lively”  (as  in  most  classrooms  and offices),  the  microphone  should  be  brought  as close  as  permissible  to  the  speaker.  In  extreme cases,  any  sound-absorbing  materials  that  may be  available,  such  as  coats,  blankets,  or  burlap, could   be   suspended   at   the   sides   of   the microphone   about   3   feet   away.   Sound- absorbing  material  may  also  be  placed  on  the floor and ceiling to absorb, as much as possible, the   reflected   sound   coming   from   these   direc- tions. It is not necessary to absorb the reflected sound  coming  toward  the  back  of  a  directional microphone. Remember  that  in  sound  recording  you  can- not  always  place  the  microphone  in  the  most advantageous  position  for  ideal  sound  pickup. As  a  consequence,  the  microphone  position  is often  a  compromise,  Remember  to  consider  the position   of   both   temporary   and   permanent acoustical   materials   when   placing   the   micro- phone.   It   may   be   possible   to   improve   sound reproduction  by  simply  moving  the  position  of the  temporary  acoustical  materials.  The  follow- ing rules will prove helpful in microphone place- ment. Never  place  the  microphone  next  to reflecting  surfaces,  such  as  tabletops  and  bare walls. 7-35