We need to stop reinventing the wheel
where bass is concerned. I’ve met
two industry veterans recently who’ve
espoused very antiquated ideas as if they
were new revelations of “purist” audio.
We abandoned these ideas more than
10 years ago because they don’t produce
smooth, tight bass for the whole audience.
There’s a better way to do it.
|Anthony Grimani (firstname.lastname@example.org) is president of Performance Media Industries, with offices in Novato and San Anselmo, CA.|
We don’t use full-range speakers
to play bass anymore. We don’t have
separate subwoofers for each main
channel or the LFE. That’s because in
the worst-case scenario, that approach
can lead to absolutely no bass in the main
seating area of a home cinema because of
inherent standing wave resonance issues.
Let’s review. Subwoofers showed up in
the early 1980s before home
theaters. Companies like Velodyne, M&K, or
Kinergetics Research made them to extend
the response of Hi-Fi speakers from 40
Hz down to 20 Hz and below. Later,
companies like Triad, Bose, and M&K
realized that speakers could be smaller
and more attractive when paired with
subwoofers. The early ’90s brought on
multi-channel home theater systems, like
THX, that used a common subwoofer for
all channels. By the 2000s, we realized how
important it was to place those subwoofers
correctly to minimize bass standing waves.
Now, thanks to the excellent and gamechanging
work of Todd Welti and Allan
Devantier at Harman International, we know
how many subwoofers to use and where to put
them for smooth response and high output.
The short and sweet version is that you
need at least four identical subwoofers. Eight, 12, 16, are OK, too, but
you need to have a multiple of four. Why? They go in the four corners
of the room, and each corner needs equal drive capacity. It’s true that
the midpoint layout typically produces the smoothest bass response by
cancelling standing waves, but that layout isn’t very efficient on sound
pressure levels, and it can be difficult to reach the prodigious output levels
required by movies (115dB SPL peak). Moving the subwoofers to the four
corners couples them to the room for higher output while still negating
most standing waves. It’s a good compromise.
The subwoofers are fed from the same output of the surround processor.
That feed should contain summed bass (typically below 80 Hz) from all the
main channels, plus the LFE channel low-pass filtered at between 80 Hz and
120 Hz. It has been confirmed many times that frequencies below 80 Hz are
not localizable, and many people cannot tell directionality as high as 120 Hz.
|Thanks to the game-changing work of Todd Welti and Allan Devantier at Harman International, we know how many subwoofers to use and where to put them for smooth response and high output in a home theater. The short version is that you need at least four identical subwoofers. Pictured are the Niles SW6.5 and the SW8.|
Now is the part where I interject my experience from the hundreds
of rooms I’ve commissioned. Hooking up the subwoofers requires a
manually controlled one-input, four-output audio processing box that
provides separate delay, gain, and digital parametric EQ (multiple filters)
for each subwoofer. Products that can perform this function are readily
available from pro audio companies like BSS, Ashly Audio, Symetrix,
DBX, and Peavey. All of these use high-quality digital conversion and
audio processing, so sound quality is not an issue.
Begin calibration by flattening out the overall response of the subwoofer
group. Make the same adjustments (delays, levels, and EQ) to each
subwoofer. It doesn’t have to be perfect, just a decent starting point. But do
more than just listen; employ a digital FFT analyzer or something similar
with a long window. Take many measurements across the audience. A
tool like LASSIE from Seagrave Instruments may be helpful. LASSIE
uses a system of microphones and LEDs located throughout the room
to provide visual feedback of the response. I find that, with bass response
issues, you need as many tools as possible to help visualize the results.
Next, play a polarity pulse. (Pulse generators are available from
the likes of Gold Line, or on test discs like the PMI 5.1 Audio
Toolkit.) Try delaying the back pair or front pair of subwoofers
together, 1 ms at a time. Listen for the pulse
to become richer and fuller. As it
does, measure the response across the
audience to see if it improves, or the
overall SPL gets louder. There may be
6 dB of in-band gain achieved just by
delaying groups’ subwoofers. However,
this may counteract standing wave
cancellation. Ensure that the frequency
response hasn’t deteriorated.
All that’s left is to tweak the individual
delays, gains, and EQ for each sub to coax the
best possible response and SPL out of the system.
Expect all this to take at least 3-4 hours.
It is theoretically possible to measure and
predict the correct settings for each subwoofer
without all the manual trial and error.
It requires a true impulse response and
some heavy know-how in the math
department. For now, the manual
method works just fine.
Chase Walton contributed to this column.