BSS Audio FDS 310 User Manual page 23

If the mid output is not enough, even when the crossovers MID RANGE
control is set at maximum (+6dB), check the mid-range power amplifiers
sensitivity. If this cannot be altered, set the MID RANGE control at 0dB, then
decrease the LOW RANGE control, until the two frequency bands fuse
together to give an essentially straight line on the R.T.A display. If the
response appears uneven around the LOW-to-MID crossover frequency, the
drivers polarity may be reversed. Refer to section 24 to remedy this.
• Repeat the procedure again for the HIGH RANGE control. Again, look for
the response around the MID-to-HIGH crossover frequency. If uneven, refer to
section 19. If the high frequency output is not enough, even when the HIGH
RANGE control is set at maximum (+6dB), check the high end amplifier(s).
Since HF drive-units are generally 6 to 12dB more sensitive than mid and low
drive-units, this condition suggests an error in the wiring. When looking for a
'flat' response, do not forget that the crossover cannot correct for the drivers
rolloff at low and high frequencies. Concentrate instead on looking for a flat
response in upper regions of the low drivers frequency range, and the lower
regions of the high end drivers frequency range (Unless Horn EQ is applied,
then refer to section 14). You may wish to repeat the analyser readings with
the microphone placed in a different location. If so, be aware that at locations
of greater than 15 feet or off-axis to the drive-units, the rooms acoustic will
affect the results. In general, the human hearing mechanism will give
preference to the direct sound balance.
Using an SPL meter: If you do not have access to a Real Time Analyser, a
conventional SPL meter can be used instead, provided you have a pink noise
source. In general, tone or sine-wave generators are NOT acceptable in place
of pink noise, unless they produce a pulsed 'warble' effect. Otherwise major
standing-waves can build up, leading to an exaggerated response at spot
frequencies. Like the RTAs microphone, an SPL meter should be mounted on a
microphone stand, on-axis to the speaker stack. Set the meter to read 'C'
weighted (flat). When setting up, measure just one frequency band at once.
This means muting the low frequency band (Using the MUTE switch, see
section 18) before moving on to the mid (or high) frequency band. Then mute
the low and mid, before moving onto the high setting. Overall, your aim is to
ensure that the indicated SPLs of the low, mid and high ranges are the same,
all 90dB for example. Again, fine tuning should be done by ear.
CSPL
Using your ears: Many skilled sound engineers will be confident about
adjusting the LOW, MID and HIGH RANGE controls by ear alone. This
method is quick, and here are some tips to enhance accuracy:
• For a system principally intended for music, use a selection of prerecorded
music you know well. You should have a good idea of what it is supposed to
sound like. For a speech only system, there is nothing better than using an
experienced public speaker. If you have to use recorded speech, look out for
comparable dynamics; recordings from FM radio can be heavily compressed.
• If the sound system is principally intended to reproduce recorded music, we
suggest you use quality recordings of vinyl disc on IEC type II chrome (CrO )
2
tape.
• If the sound system is principally intended to reproduce live music,
prerecorder music from CDs will provide more representative dynamics. If a
CD player is unavailable, recordings from CDs onto IEC type II chrome (CrO )
2
tape are the next best thing.
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