Jim Lesurf wrote:

Hard to be sure. However the above seems plausible. In general the LF
roll-away at frequencies below 50-100 Hz region does tend to be
dominated by how easily the speaker can produce pressure variations
in the room. Ideally, an infinite baffle speaker in an airtight room
with no leaks and firm walls would allow a response down to dc.! :-)
In practice, though, the LF response will depend on how much air
pressure change the room will experience for a given cone movement
once you are below the lowest room resonance.

Well, I don't have an ideal environment, or speakers, so I think I'm going
to the Behringer digital parametric EQ shop... :-)


FWIW my experience here is limited, but it is that there are two
problems which may combine if you are unlucky. One is the basic room
pressure resonance which will probably be somewhere below 100Hz. The
other is the 'mechanical' resonance of the speaker cone
mass/compliance which will also be somewhere below 100Hz. If the two
are at a similar frequency they may produce a particularly severe
'boom', and the bass may fall away rapidly below this. Which is 'more
significant' will depend on the details of your situation.

The way I see it, the only thing measurements will tell me is whether sound
I like has an even response, or a bumpy one. :-)


With the sub I use in my living room I can measure a change in the
peak frequency and level if I open/close the room door.

That's interesting - I'll bear that in mind for when I start doing bass EQ.


IIUC commercial subwoofers employ an amp with a correction network to
try and iron out some of the speaker effects and give a more extended
response. You'd need to experiment to determine what was required for
a given system.

That's pretty-much the plan. Now that the bass is separate from the rest, I
think my next step is to get the parametric gizmo and try doing some EQ on
the low end. I'd like to see how much improvement can be had from the
present set up, before I look into building new bass cabinets. (Before
someone tells me that I might not need it by then, I would like to mention
that I am already aware of this possibility.)


Not sure. The amount of output (sensitivity) will vary in a different
way to the actual drive levels. With HF speakers you need to avoid LF
as the excursions might cause problems. With LF speakers you avoid HF
as it can excite cone resonances. So the degree of attenuation
required depends on different factors as well. However I'm not a
speaker designer so don't know much about the details of practical
designs.

Okay, Fairy Nuff. :-)


Yes. The snag, though, is the phenomenon I tend to call "lost in
space". This is where there are many variables you can fiddle with,
or which alter out of your control. This means as you change some
things you sometimes don't know what is really happening, or if some
other slight change would be desirable, etc. This is worse when
working entirely 'by ear' as your reactions will vary according to
how you feel and your 'recent' experiences, and affected by the
unreliability of memory.

I appreaciate the somewhat non-scientific approach you're talking about, and
I admit that that's kinda what I'm doing. However, it's not so much about
the small incremental changes, it's the overall change that results from the
whole tri-amping thing. Using the active crossover to split the bass from
the rest of the system is just the first stage. I fully intend to get rid of
the passive crossovers, put the bass drivers into better boxes in an
isobaric configuration, equalise the limitations of the bass system and room
out of the equation as much as I can, and pile on the watts to be sure that
straining amplifiers aren't a factor.

Once it's got to that stage, *then* I ask myself if it's an improvement over
what I started with, establish what limitations there may be, and look into
ways of improving it further if I think that's warranted. I know I'll be
relying on memory of the previous configuration, but I have had these
speakers most of the last 25 years - they're the one thing in my system that
has never changed, so I reckon I'm pretty familiar with their sound. In
other words, aside from the memory aspect, which I think is mitigated, the
thing that I'm seeking to test is the difference between the old set up and
the tri-amped one. It'll be interesting to note the effects of the changes
that are made along the way to get to that point, but it's really the end
result that I want to check out.


Afraid I don't know any figures for the valve amp you mentioned.
However if you look in reviews, values when quoted are often of the
order of 0.5 Ohms or more. i.e. somewhat larger than the value for
your Cyrus,

I never did do an A-B comparison when I got the Cyrus - I just changed out
the valve amp and got on with it. My impression at the time was that the
fidelity of each amp was very similar - very difficult to separate them,
other than the Cyrus being capable of producing more clean volume. I don't
know what effect a higher o/p impedance would likely have.


Maybe that depends on what is causing the change in frequency
response.

Yes. Indeed, there are some good reasons for feeling that an active
crossover and 'direct drive' of the speaker units can be optimum. But
this then may well require the crossover to also do some other
alterations to cater for any inherent variations in sensitivity with
frequency of the speaker units. However it is difficult to say
without more specific measured data.

It strikes me that the basic principle is sound. I can't claim that the
drivers have a flat response in their working bandwidths, but I'm not aware
of the Kef passive crossovers having stuff in them to smooth out any
howlers. I stand to be corrected, but I gather they're basically dividing
networks.


My concern here is that people may buy relatively expensive active
crossovers and bi- or tru-amp, but then get result which could have
been obtained more easily and cheaply with a slightly better amp and
a tweaked speaker or room acoustic. Hence I am inclined to be wary of
this without suitable measurements, etc.

Cost is a factor, but, while I'm not willing to pay audiophile prices, I'm
willing to spend a bit to get a marked improvement over what I see as the
current system's shortcomings, which are primarily messy, uneven bass with
too much roll-off, and not enough clarity when played up loud. There's also
a curiosity factor, in that I want to see what difference is to be had by
eliminating the crossovers and driving the speakers directly.


--
Wally
www.artbywally.com/FiatPandaRally/index.htm
www.wally.myby.co.uk