Manual-5
Fig. 2 In-Phase Axis Response Without Signal Delay
Fig. 3 Corrected In-Phase Axis Response with Delay on Low Driver
Signal Delay Adjustment Method One
If you can not get your hands on the equipment necessary to
electronically align the system, it is possible to set the delays
using only the horizontal displacement of the sound sources.
It is
very important to make certain that all drivers have correct polarity
before setting signal alignment delay
. First, let’s review the basic
information required for the task.
• The general equation for the speed of sound in dry air is:
331.4+0.6Tc m/s
Tc
= temperature in degrees Celsius,
m
= meters and
s
= seconds
• For those still having difficulty accepting the metric system, the
following approximation will do:
13.57 inches / millisecond at 72° Fahrenheit.
34.5 cm / millisecond at 22° Celsius.
• The AC 24 provides 10 ms of delay.
Each dot on the silkscreen
represents 1 ms or 13.57 inches (at 72 °F).
• This allows delay compensation for distances ranging from 0.0
to 135 inches (11.3 feet) [343 cm].
• The resolution of the adjustment is about 0.6 inch [1.5 cm].
Important Note: The horizontal location of a driver is deter-
mined by the front of the voice coil.
Method One, Step-by-Step
1. Identify the driver that is the furthest away from the front of
the stack. This driver gets zero delay. The horizontal displace-
ment of all other drivers is measured relative to this driver.
2. Take the distance,
measured in inches
, for each driver and
divide by 13.57. This gives the delay in milliseconds and pot
rotation in “fractional-dots.” See Figure 4.
Signal Delay Adjustment Method Two
OK, so you want to do this the hard way.
The following example
outlines one method for Signal Delay alignment of the system
shown in Figure 4. The procedure easily adapts to other configu-
rations.
Required tools:
RA 30 Realtime Analyzer
Cautions/considerations:
With 4
th
-order filters, it’s impor-
tant to accurately identify crossover frequency settings before
adjusting delay. A reference level must be set for each driver at
each crossover point. This eliminates errors due to non-flat driver
response and room acoustics.
When using a 1/3-octave realtime
analyzer, best results are achieved if crossover points are set to the
nearest 1/3-octave center.
Method Two, Step-by-Step
1. Initial AC 24 control settings:
a. Leave FREQUENCY controls as set in
Step One
.
b. MUTE all Outputs.
c. Set Input
and
Output LEVEL controls to unity (the mark
between the 8
th
and 9
th
dots).
d. Set DELAYS to zero.
e. Switch CD HORN EQs
off.
f. Set LIMITERS to 0 dB FS.
g. Set all rear INVERT switches to the
out
(non-inverting)
position (assuming all drivers are in phase).
2. Connect the RA 30 to the AC 24:
a. Connect RA 30 Pink Noise output to Crossover Input.
b. Connect AC 24 Sum Output to RA 30 Line Input.
c. Set RA 30 Source to Line In.
d. Set RA 30 to RTA Mode.
e. Set RA 30 RTA Scale to 3 dB.
f. Turn on RA 30 Pink Noise.
3. Adjust crossover frequencies to ISO centers:
a. For this crossover frequency calibration you are looking at
the line level Sum output and
NOT
the acoustic output.
b.
Amplifiers
off
.
c.
Un
MUTE the AC 24 Low Output.
i) Press RA 30 NORM
(Low Output passband should be green).
