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Bass Decay
Index
Use the Bass Decay analyzer for a non-standard measure of how bass energy decays in a room.  Use only OmniMic "bass sweep" tracks to perform this test.

When a bass note is stopped within music, the sound in the room at some frequencies may still continue for some time.  That is because the sound reflects back and forth between walls, resonating and forming "modes" before eventually dying down.  This is not altogether bad and some reinforcement is normally desirable for natural sounding playback, but you would like to keep it under control and not have some notes sounding muddy or lingering much longer than others.

The top graph on the Bass Decay display shows the frequency response of the bass range, similar to that shown with the Frequency Response analyzer.  The bottom graph shows how long it takes the sound to decay at each frequency.  As shown on the legend to the right of the decay graph, the white area extends upward to indicate when the level drops no more than 5 decibels (dB).   The light blue indicates when the level has dropped between 5dB and 10dB, etc.

At the bottom is a check box labeled "adjust for response".   This affects whether the variations in relative bass strength are included when calculating the decay graph.
  • when the "adjust for response" box is unchecked, the bass decay shown does not take the frequency response into account.  All decibel levels are with respect to whatever the response level is at each frequency.  At frequencies where there is lower overall output (as shown on the upper graph), the decay may appear to be longer than actual because of background noise in the room that is inseparable from the low bass levels.
  • when the box is checked, the decibel levels are with respect to the blue line shown on the frequency response graph.  For instance, if the line is at 70dB SPL, then the white area of the Bass Decay graph shows how long it takes before the level dropped below 65dBSPL (that is, 70dBSPL minus 5dB), the light blue shows how long before the level dropped to 60dBSPL, etc. 

    Operating Notes:
     
  • When working with the "adjust for response" box checked, you would normally set the blue line (click inside the Bass Response graph) to be in the upper, most flat, region of the Bass Response curve.
  • The microphone should be placed out in the room, measuring at various listening positions.  You can try relocating subwoofers or main speakers, or listening chairs to find optimum locations for these.  For floor vibrations, spikes or pads below subwoofer boxes can also affect the bass decay (not always for the better).
  • Remember that the response is strongly position dependent.  Optimize the woofer placements and equalization for best overall results at all listening positions.  This is generally easier to accomplished if multiple subwoofers are used.
  • Slow decays should be less bothersome to the ears if response level is reduced at the problematic frequencies.   If you are using an equalizer in the room, you can reduce your system's response to accomplish this.  You can use the "adjust for response" checkbox to help find a trade off between tighter bass and flat response.  But in general, decay speed normally follows the overall frequency response curve.