Avoiding Wimpy Theater Sound
with a Proper Subs and Bass Setup
Anthony Grimani (firstname.lastname@example.org) is
president of Performance Media Industries,
with offices in Novato and San Anselmo,
I was chatting with an
integrator at CEDIA
who reads this column,
and we went through
various topics until the
turned to bass. I could tell
he was a little miffed about
something, so I asked him
what was up.
It turns out he had followed
the advice that I,
among others, have espoused
on good bass performance.
He had constructed
a friendly room with resilient
walls, positioned the
seats away from peaks and
nulls, arranged the subwoofers in a cross
pattern at the midpoints of each wall, and
even dialed in some EQ. He was ultimately disappointed with the way it
sounded; the frequency response was smooth and even across the room, but it
lacked the power, extension, and impact he was expecting.
Curious, I asked him to describe more about the setup. Everything
was fine until he got to the subwoofers. He had selected four active 12-
inch in-walls with small cabinets and remote rack-mount amps. They
were chosen for footprint (or lack thereof) and cost–about the same as
one large 18-inch. I stopped him at that point, because I knew what the
problem was. He simply didn’t have enough horsepower to produce the
thunderous bass levels he wanted. What’s more, the active limiting circuits
in the subwoofers were kicking in to taper off the level increase and reduce
the frequency extension. As he turned up the volume to get more impact,
the rest of the system would get louder, but the bass simply wouldn’t.
That’s a perfect recipe for wimpy sound.
The Four-Sub Layout
The practice of distributing subwoofers to four locations in cross patterns
in the room is now well established, following intense and thorough
research by the likes of Todd Welti and Allan Devantier. This layout
can help neutralize the room’s natural standing-wave resonances. The
benefits are smooth and consistent bass throughout the seating area while
the liability can be less room gain, and therefore less sensation of loudness.
Sometimes, however, the distributed solution actually rids the room of a
giant dip in the middle, so bass levels can in fact go up.
What Bass-Intensive Movies Require
We all know that movie soundtracks are bass hogs–several of Marvel’s
recent superhero movies come to mind. But I wonder if you know exactly
how demanding those soundtracks can be? Let’s start with the dedicated
LFE “0.1” channel. It alone can demand 115 decibels peak level from the
subwoofers, typically over a maximum range of 20 to 120 Hz (20 to 80
Hz is more common for home theaters, as the bass management low-pass
filter pulls out 80 to 120 Hz.) Assuming you’re using crossovers on all
the main channels, the subwoofers can theoretically see higher levels up
to 120 decibels if you have a bit of bass lift in the room EQ target curve.
So how many subwoofers are we
talking about? Well, let’s take a typical
THX-certified subwoofer that will play
105 decibels at the seating position in
a 3,000 cubic foot room. If you add
another one, you’ll get about four dB
of additional gain (varies depending
on summing conditions). To go from
109 dB to 113 dB, you’ll need two
more subwoofers. The jump to 117 dB
at the seating position will require four
more subwoofers. That’s a grand total
of eight THX subwoofers!
Sound mixes don’t have peak bass
levels in all channels simultaneously.
Even if they did, people will rarely
kick the volume up to reference level.
Every decibel below that is a decibel
that the subwoofers will never see.
That’s why it’s OK to go with a few smaller subwoofers. I usually tell
folks to consider about 110 to 112 dB a reasonable target. That’s still four
105-dB-capable subwoofers, but you need at least that many to position
them for maximum resonance neutralization. You won’t get quite as good
a sum spreading them out as you would with them stacked next to each
other, but four is still going to get you to around 110 dB in most cases at
the main seat in a 3,000 cubic foot room.
Dealing with Limiters
There are still these pesky limiters. Most manufacturers build them in so
users don’t disintegrate the voice coils by accident. Limiters are actually
good things when properly implemented. They keep the subwoofer from
producing high levels of distortion, wind, and port noise when stressed.
Limiters can confuse you, though, when you’re trying to measure the
maximum subwoofer output level.
For this exercise, I like to use a couple of chapters from the PMI/Gold
Line 5.1 Audio Toolkit DVD, one which produces 115 dB SPL at the
listening position, and another that is a mix of high-level bass noise, tones,
and actual program material all mixed together. A non-limited subwoofer
will usually bottom out with a host of pops, thumps, and snorts, allowing
you to easily measure the max SPL with a good meter. With a limited
subwoofer, you shouldn’t get all those noises. You will simply reach a
point where the SPL doesn’t go up in. Take your measurement at that
point and, if you’re below the desired 110 dB SPL, you’ll just have to try
other positioning options, or get some more subwoofers!
Chase Walton contributed to this column.