By David Gatti
in a home theatre surround sound system has become much of a headache
for even the most dedicated enthusiasts.
users have an elementary knowledge of loudspeakers, crossover design,
the physics of sound, and room acoustics.
Furthermore, the standards and approaches used by equipment manufactures
vary widely in regard to bass-management crossover frequencies,
overload margins, how & when bass is routed between small/large/subwoofer
speakers, different DVD / Blu-ray / HTPC / SACD / DVD-A requirements,
etc, etc, etc.
Its so complex, with so little clear information provided to the
user, that it all usually ends up being a guessing game. It's
not surprising then, that the results will almost certainly be less
the whole system needs to be simplified, and I believe the best
way to achieve that (in a high end system) is to use -:
full-range main speakers (requiring no bass management).
- centre and
surrounds big enough that they can be set to "large" (no
- 2 x dedicated
LFE subwoofers (no filtering)
- sealed enclosures
for all channels. for minimal group delay and excursion limiting.
Note : "Bass
management" refers to the process of filtering the low frequencies
from a loudspeaker, and routing those frequencies to a subwoofer.
Now, with some
new speaker designs in the works, I'll attempt to explain how subwoofers
and main speakers interact, and the simplest (not necessarily cheapest)
way to get the best low frequency performance. I
refer you to Rob Elliots articles
on phase & group delay for some background reading.
To begin with, lets compare the frequency response, and group delay
for a variety of configurations.
Below is the
response of a sealed main speaker with a natural 2nd order rolloff
Notice the 12dB/octave
rolloff (left graph), smooth phase-transition from +180 to 0 degrees
(dotted line), and in the 2nd graph, peak 4mS group delay at 50Hz..
to a vented main speaker (below), with a 4th order Butterworth rolloff
The response falls at 24dB/octave below 80Hz, the phase wraps through
360 degrees, and the group delay more than doubles to 8ms.
Now lets add
a subwoofer to the above main speakers with a few different filters,
and see what the result is.
Assume the subwoofer
has -3dB at 20Hz, whether it's sealed or vented.
First, a vented
subwoofer with 4th order lowpass at 80Hz. Mains are vented with
an additional 2nd order highpass filter, giving it a 6th order rolloff
Nice flat frequency
response, although the phase wraps through 630 degrees and the group
delay skyrockets to 36mS
How bad is 36mS?
Sound travels at 330m/s. distance = velocity x time = 330m/s x 36mS
= 12 metres!
low bass frequencies will be effectively 12 metres further away!
Now you know why vented subs are sometimes characterised as slow.
Lets try to
improve on it and use sealed main speakers. The rolloff of the mains
is now 4th order, effectively creating a 4th order Linkwitz-Riley
crossover at 80Hz.
Not much change.
Slightly less output in the 50-150Hz region, and virtually identical
Lets seal the
subwoofer enclosure and see what happens.
Nice flat frequency
response, and the phase shift is now improved 20% from 630 to 500
degrees (but stil poor), and group delay almost halved to 19mS.
This is probably
the most commonly used setup in the marketplace because it has a
good balance of compromises. The steep subwoofer rollof allows a
lot of flexibility in regard to it's placement, and most speakers
have a poor phase-response anyway.
Perhaps we could
improve the phase/group delay performance.
How about using
2nd order crossover at 80Hz. In other worlds, a sealed main enclosure
with no high-pass filter, and a 2nd order lowpass filter on the
bad cancellation around 80 Hz due to the sub and mains being almost
180 degrees out of phase.
the subwoofer in the above scenario, to put both the sub and mains
back in phase.
with phase still shifting through 315 degrees, group delay at 17mS,
but the 12dB dip replaced with a 3 dB hump. That could be improved
by reducing the sub gain by 3dB, and applying boost of 3 dB at 20
Now, the optimum
(theoretical) response. This is achieved by using no high-pass filter
on the main (sealed) speakers, and a 1st order lowpass filter on
The active subwoofer also has 2dB of gain applied. Reponse is +/-
0.75 dB, phase-shift an excellent 135 degrees, and group delay only
a catch though, and here the slow rolloff of the subwoofer means
that 2 are preferred and placement is more critical to prevent localisation.
power handling, I would like to protect my mains bass-mid driver
a bit more by giving it a 1st order lowpass filter at 80Hz and see
what happens (below).
Very bad dip.
However, that can be corrected by overlapping the responses more.
I think is the optimum compromise for frequency response, phase
and group delay in a specific real-world situation.
It is achieved
using 1st order lowpass on the subwoofer, subwoofer gain and overlapping
the sub and main speaker responses.
The main speakers
are sealed (with a -3dB at 65hz), and a further 6dB/octave filter
is applied at 65Hz with a simple, series, line-level capacitor between
pre & power amps.
has 2dB gain and a 1st order filter at 150Hz. Note: Due to baffle-step
diffraction (i.e. 4 - 6dB rising output from 100-400Hz) , the actual
filtering may be 12dB electrical to achieve an approximate 6dB acoustic
The result -
response +/- 1dB, phase shift only 135 degrees and group delay below
15mS, almost identical to the "optimum" setup above.