Page 215 - Master Handbook of Acoustics
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When the noise is topped, the sound in the room decays into silence. The microphone, at its selected
  position, picks up this decay, which is recorded for later analysis.
      The signal-to-noise ratio determines the length of the reverberatory decay available for study. As

  noted, it is often not possible to realize the entire 60-dB decay used in the definition of RT . It is
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  quite possible, however, to obtain 45- to 50-dB decays with the equipment shown in Fig. 11-5 by the
  simple expedient of double filtering. For example, to obtain an RT  measurement at 500 Hz, the
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  octave filter centered on 500 Hz in the sound-level meter is used both in recording and in later
  playback.
      The analysis procedure outlined in Fig. 11-5B uses recorder playback and a sound-level meter,
  with the addition of a graphic-level display. The line output of the recorder is connected to the input

  of the sound-level meter circuit through a 40-dB attenuating pad. To do this, the microphone of the
  sound-level meter is removed and a special fitting is screwed in its place. The output of the sound-
  level meter is connected directly to the graphic-level display input, completing the equipment
  interconnection. The appropriate octave filters are switched in as the decay recording is played back.






  Reverberation and Normal Modes

  The natural resonances of a room are revealed in its normal modes, as described in Chap. 13. It is
  necessary to somewhat anticipate this topic to understand the relationship of these natural resonances
  and the reverberation of the room. For now, simply stated, most rooms have preferred resonance
  frequencies where sound energy at those frequencies is accentuated.

      The Sabine equation or alternatives are widely used to characterize a room’s reverberation time.
  However, single-number calculations or measurement cannot completely describe the behavior of
  room reverberation, particularly when considering the effects of normal modes of rooms. Room
  modes pose particular difficulties when characterizing the reverberation time of small rooms.
      Consider an untreated, small studio room. Starting with a sine-wave oscillator set to about 20 Hz

  below the first axial mode, the acoustics of the room do not load the loudspeaker and a relatively
  weak sound is produced with the amplifier gain turned up full (even assuming the use of a good
  subwoofer). As the oscillator frequency is adjusted upward, however, the sound becomes very loud
  as the (1, 0, 0) mode (24.18 Hz in this example) is energized, as shown in Fig. 11-6. Adjusting the
  oscillator upward, we go through a weak valley, but at the frequency of the (0, 1, 0) mode (35.27 Hz),

  there is high-level sound once more. Similar peaks are found at the (1, 1, 0) tangential mode (42.76
  Hz), the (2, 0, 0) axial mode (48.37 Hz), and the (0, 0, 1) axial mode (56.43 Hz).
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