Table of Contents
Advertisement
TECH NOTES
System Calibration
Because it is difficult to deduce exact levels from the LEDs, we
recommend calibrating your system based on electrical input levels
to the 3D. Using a 500 Hz test tone, an input level of 0 dBV (1 Vrms)
produces 124 dB-SPL. Figuring from that reference, the Perform
Mode threshold of 105 dB-SPL is produced with a 19 dB lower input
of -19 dBV (112 mVrms), and 85 dB-SPL is produced from an input
of -39 BV (11.2 mVrms). The sensitivity of the 3D-L mixer (discontin-
ued hardwired version) with balanced drive is 3.5 dB lower than
these numbers at full gain, so the respective input levels will be 3.5
dB (1.5x) higher. Turning down the Monitor gain control from its full
clockwise position reduces these sensitivities further, increasing the
required drive levels. In the middle of the Monitor gain control's
rotation, the sensitivity reduction is about 17 dB, requiring 17 dB
higher (7x) drive levels.
Limiter & LED Behavior
The specified limiter threshold is 105 dBA (85 dBA for the monitor
input in Full Ambient mode). The two yellow "Loud" LEDs, while
related to these thresholds, should be considered as indicators of
general loudness and not as direct indicators of limiter action. Each
LED first starts to flicker yellow 16 dB below the limiter threshold of
its operating mode, reaching what could be called "bright" at the
limiter threshold, whether limiter switch is on or off. About 6 dB
beyond limiter threshold, they become brighter, up to what could
be called "maximum brightness." Thus, the yellow Monitor and
Ambient LEDs light well below limiter threshold to indicate signal
presence at listening levels, but go to bright illumination to indicate
"loud," with average signal levels in the general range of 105 dBA
(Perform) or 85 dBA (Full Ambient), regardless of Mode Switch
position. Note: 105 dBA is not considered safe for extended
listening.
The 3D system's limiters and "Loud" LEDs are slow, average
responding, so short duration peaks can be much louder than
indicated, as can bass frequencies, due to reduced limiting action in
that frequency range. The SPL indications also correspond to a
sound as it would be measured outside the ear, before entering the
canal. Due primarily to the effects of the ear canal, for frequencies
from 1.5 to 5 kHz, sound levels at the eardrum are relatively higher
than at other frequencies (with or without the 3D).
NOTE: Use of the limiter, while strongly recommended, does not
guarantee safe monitoring. The intention of the limiter is to provide
a musically pleasing limiting function while guarding against
occasional excessive levels. For safe monitoring, levels should not
consistently enter the limiting region.
13
Advertisement
Table of Contents
