Last week, UCI hosted the Meyer Sound seminar on
Fundamentals of System Design, Implementation & Optimization, led by Steve
Bush. For all of us involved, this was an excellent opportunity to return to
the basics of system design and ear training for live sound. Steve started at
the ground level and took us through the process of critical decision-making,
at a pace that allowed everyone in the room to stay on board.
The first day was spent reinforcing the basics. Steve set up
combinations of UPJ arrays, UPJ & M1D subs, & M1D sub arrays, and
demonstrated the results both aurally and visually in SIM. We did a lot of
listening to comb filtering and polarity reversal with pink noise to get our
ears acquainted with those sounds. Steve drilled us on wavelength and period
recognition, asking questions like:
Q: If Period = 1/Frequency &
Frequency = 1/Period, what is the period of 100 Hz?
A: 10ms.
Q: What happens if two 100hz sine
waves are sent to the same speaker with one delayed 5ms?
A. Complete cancellation
The underlying point of the exercise was to reinforce the
importance of loudspeaker placement when designing systems, allowing us to
speedily approximate the basic interaction between two speakers at any distance
from each other. Just remember: 100hz is a mini cooper, 1kHz is a hoagie, and
10kHz is your pinkie finger.
Steve heavily reinforced the causes and effects of phase in
all stages of system design. He set up a situation in which one source of pink
noise was split and sent to a single speaker, and one feed was delayed an
unknown amount. By examining a combination of the frequency and phase traces in
SIM, we were able to see correlation between the frequency of the first dip in
the combing and the 180º phase cancellation point. We would then double that
frequency to determine the frequency of the next peak, then find the period of
that peak to determine the delay between the two sources. For instance, if the
dip is at 500, the peak would be at 1k — therefore, there would be 1 ms of
delay present.
To demonstrate some of these
effects, Steve set up a fairly typical tuning situation with a single UPJ and
an M1D sub. Typically when tuning these speakers, you typically insert some
form of high pass filter on the UPJ & low pass on the M1D sub. As Steve
demonstrated, however, these filters introduce a shift in the phase curve of
each cabinet — specifically, these 2nd order filters used within the
Galileo 616 processor create a 90 degree phase shift at the cut frequency for each speaker. In other words, if the two
filters are set at the same frequency, they create a 180º phase difference at
this crossover point. The solution? Steve merely flipped the polarity on the
sub, “turning the nulls into peaks and the peaks into nulls.” This shifted the
cancellation point of the two speakers upwards in frequency, to a range that
the subwoofer does not cover — in other words, there was no perceivable
cancellation!
We also practiced arraying 2 UPJs, but in a different style
than has been done here at UCI recently. Steve decided to treat one of the two
cabinets as the main source, with the other acting as a kind of a fill that
expands the width of coverage of the first. The measurement process went as
follows:
1.
Capture the response of the main cabinet on-axis
in SIM.
2.
Shift the captured trace down 6dB.
3.
Move the mic toward the coverage limit of the
horn, until the live trace matches the -6dB one. This is the “-6dB point” of
the horn.
4.
Mute the main speaker and bring in the fill.
5.
Adjust the placement of the fill until it
matches the -6dB response of the main speaker. This means that the coverage of
the speakers crossover at the -6dB point of each.
6.
Unmute the main speaker and tune the delay of
the fill until the responses match.
7.
EQ the fill as necessary
Effectively, this creates the illusion of a single UPJ that
has the extended coverage of an additional UPJ horn in one direction. Very
cool, and definitely something we will want to experiment with.
One of the most interesting topics we covered was the phase
response of a single loudspeaker within itself. Steve definitely blew a few
minds in the room when he talked about the slow response of the woofer of a
cabinet when compared to the tweeter. He also covered the use of all-pass
filters in tuning. All pass filters provide no change in the frequency response
of a signal, but merely introduce a delay in the signal. Steve explained that
Meyer tunes the phase response of its cabinets by splitting the input signal
into frequency ranges and inserting all-pass filters in such a way that the
higher frequency ranges are delayed to arrive more in time with lower
ranges — effectively evening out the
phase response of the cabinet. In the case of the UPJ that he used for the
demonstration, the phase deviation is no more than 540º. Steve also mentioned
that all-pass filters can be useful tools when matching the phase response of
two speakers made by different manufacturers — but he also warned us that
misuse of the all-pass filters can result in significant problems in other
areas of tuning.
Steve touched on numerous methods
of arraying subwoofers to allow directional control of the low frequencies. A
sub-gradient array consists of two subs facing forward in a single-file line,
with 3.5’ of separation. The rear sub is delayed to the front one, and then the
polarity is reversed on the rear sub. This effectively creates strong
cancellation in the rear, allowing the overall volume to be pushed without
affecting those behind the array as much. An endfire array is a similar setup,
except that the front sub is delayed to the back one, and there is no polarity
flip. This creates constructive interference in the front, and destructive in
the back. Endfire arrays can also consist of more than two subs in a line.
Steve also showed examples of arrays-of-arrays with subs — for instance, a line
of endfire arrays across the front of a stage.
He cautioned us on the use of these sub arrays, though, as even the
smallest problem (like an XLR wired with reversed polarity) can completely
destroy the effect.
One of the goals for the seminar was to practice our
newfound skills on the rep system of the Claire Trevor Theater. The rep system
includes:
•
Two line arrays, each with (8) M1Ds & (1)
MID sub, on either side of the proscenium
•
(2) arrayed UPQs serving as the center
•
(2) MM-4XPDs at the rear of house serving as
fills for the FOH mix position and the handicap seating
Steve decided to leave one of the arrays set up in the rep
position, and experiment with the other one for comparison. We lowered the
array to try new splay angles and heights and listened to how each set of boxes
interacted with the room. It was a rare and unique opportunity to go through
the steps of tuning a line array while Steve talked us through what we were
hearing.
This seminar was an incredible experience for all of us at
UCI. Steve was a wonderful teacher who gave some of the best-worded examples and
definitions for fundamental concepts I have heard to date. He did a fantastic
job of translating the results of SIM measurements into aural examples, and he
explained everything in a manner that everyone, at every pace, could grasp. We
really can’t wait to try out what we learned in the fall! Thank you to everyone
involved and we hope to do this again soon!
^Patricia & Matt
