Once the subs
were moved into position I adjusted the phase using my TEST
CD. (here: www.soundoctor.com/testcd
) When you play the 80 Hz tone (which comes through one channel
only) since the crossover sends that to the sub as well, the
80 Hz comes out of both the mains and the sub at essentially
the same level. You then reverse the wiring to the main speaker.
Now the "mains" and the "sub" are purposefully
out of polarity. You then carefully adjust the polarity switch
and the phase control on the sub until you hear the null. It's
clean and sharp ! Then you put the wiring back on the main and
you're all set. Then do the same thing to the other channel.
The net result
of all this is astonishing -- everything about the setup is
a win-win situation:
you are now NOT putting in 20 Hz - 80 Hz into the mains you
are not using up the LF cone movement with bass, so the LF cone
is able to play its higher freqs (up to IT'S crossover point)
much more cleanly. You get an apparent 6dB more dynamic range.
you are not putting bass into that same driver you are not Doppler
modulating everything between 80 and 600, or whatever the next
crossover point is. This means cleaner mids. By far.
are not sucking current out of your main power amp, so there
is more current reserve to play those highs louder...
the cones aren't moving as far at the low freqs the driver itself
is not generating as much back EMF therefore the damping factor
and all of its issues are greatly negated.
below 80 are now NOT causing transient intermodulation distortion
with the higher freqs (and vice versa). Cleaner still.
speaker cable you are now using likes it better, since there
is much less Low Freq current going to the mains.
I should point
out that these are v1 F112's and the JL Audio ARO in this version
sub has one band of pull-down EQ. HOWEVER - and this is very
important - because the room acoustics is SO GOOD, the ARO function
was completely unnecessary. You can do Ethan Winers' test of
playing a sine wave and walking across the room right in front
of your main chair from completely left to completely right,
and in MOST INSTANCES you will get a null or more than one null
that might be 10-40 dB down - a real "black hole".
In this room there is only a variation of about 1 dB or so,
anywhere in the area. Stunning! This is completely because of
the correct physical room acoustics and construction.
The whole end
result speaks of all those lovely adjectives that magazine writers
love to bandy about: the imaging is palpable, holographic, with
the correct warmth AND HEIGHT (this was one of the first things
I noticed) ; you get the sense you can look behind the layers
for example look "behind" the lead female vocalist
and detect the band "behind" her. It's uncanny and
reads like all those superlatives that the magazine writers
use when they are astounded.
All of this is
a direct result of IMPULSE RESPONSE, the holy grail of audio.
People have been taught to think (or not taught to think otherwise)
about frequency response when in fact frequency response is
the LEAST important issue; phase response and certainly impulse
response are to be considered first. Under the mathematical
umbrella of impulse response IS CONTAINED both phase response
and frequency response.
Imagine for a
moment a great recording of a kick drum. (That in itself is
a fantasy, but more on that later) If you play this back on
a relatively poor system, yes, your brain knows it's a kick
drum; you intellectually 'know' it's a kick drum, i.e. "I'm
playing a rock n roll song and I expect a kick drum there..."
but the first time you are almost frightened into thinking someone
has actually placed a kick drum IN your living room and is playing
it, it's a revelation.
A kick drum has
a fundamental around 50-60 Hz, and also a SUB-harmonic an octave
lower, which makes it about 25-30 Hz. Since it is an IMPULSE
signal, it essentially has an odd harmonic train that extends
quite high (theoretically infinitely...) but in our case I'll
suggest at least to 4 or 5 kHz. That means that in order to
CORRECTLY reproduce this kick drum hit, which is a positive
pressure wave leaving your speaker cones and traveling toward
your body, ALL the drivers in your system MUST be perfectly
aligned in TIME (that means phase). If the drivers are OUT of
time (for example the sub is 8.5 msec late) you will psychologically
still know this is a kick drum hit but chances are you won't
have that magic feeling that there's actually a kick drum IN
THE ROOM with you.
So we can extrapolate
this cleanliness of impulse response through the entire system.
At LOW freqs, this is mainly a function of the subwoofer tied
into the main system and how well it's matched up; it should
have the same (but opposite) slope in both the frequency and
phase domain, which is why we use a 24 dB/octave Linkwitz-Riley
crossover. The acoustic summing of these wavefronts, from both
the mains and the subs, therefore results in flat response in
both the frequency and phase domains at the listening position
-- which we can call the sweet spot.
It is also possible
to get a great result in one domain with poor or atrocious
results in another domain. All too often, people may adjust
a system for whatever they consider to be reasonable or correct
frequency response while completely ignoring phase
response. And each of these issues is a multi-edged
sword: the response may be different in either domain off vs
on axis of the speakers... and so on. And the problem is slightly
different still when discussing different SIZED rooms or venues,
where there are differing standing wave issues, and differing
...to read on
about all the technical reasons for all of this, go to my SUBS