Crossover too near resonance?

[niacin]

Crossover too near resonance?

Everything I have read says you have to keep the crossover frequency far above the driver’s resonant frequency, like 1-2 octaves.

But in Vance Dickason’s speaker cookbook he mentions an article by Joe D’Appolito in Speaker Builder 4/84,
(Which I don’t have, has anybody read the article?) Which apparently says that a second-order Butterworth network “...can be used in conjunction with the 6dB/octave acoustic high-pass response [of the driver] to derive a combined 18dB/octave Butterworth response... “

In other words, you put the crossover frequency right on top of the driver’s resonant frequency, instead of far above it.

First, wait a minute, is that a misprint? I thought the acoustic high-pass response of a driver was 12 db/octave, not 6dB /octave?

Second, if that technique really worked, wouldn’t everybody use it? If you could really cross over your tweeter AT resonance, instead of an octave or two above resonance, you would get a lot more extension out of the tweeter. If you could really cross over your mids AT resonance, instead of an octave or two higher, you would get a lot more extension out of the mids.

It seems too good to be true. Has anybody tried this, or read the article, or if not, what are the drawbacks to this technique?


Thanks,

Niacin

[kelticwizard]

Originally posted by niacin

First, wait a minute, is that a misprint? I thought the acoustic high-pass response of a driver was 12 db/octave, not 6dB /octave?
Niacin

I haven't read Dickason's book, so I cannot say what he means.

However, I would point out one thing. My understanding is that a speaker with a crossover slope of 18 dB/octave should be 3 dB down at the crossover point.

As the chart below shows, a speaker in a box to yield a combo of Qtc of 0.5, (or a speaker in an open baffle with a Qts of 0.5), will be 3 dB down at 1.4 times resonance. Which is half an octave above resonance.

An octave below that, at 0.7 times resonance, the speaker will still be only about 9 dB down. So from the crossover point to an octave below it, a speaker/box combo with a Qtc of 0.5-or slightly above-will have a rolloff of 6 dB/octave.

Below that, the rolloff rate begins to increase up to 12 dB/octave. However, once the speaker hits -12 dB compared to the midband, it is pretty much "out of the loop" anyway, as far as being heard. At any rate, the conversion from -6 dB rolloff rate to -12 dB rolloff rate is a gradual one, so by the time a speaker with a Qtc of 0.5 really is declining at the rate of 12 dB/octave, it is far, far below the midband and cannot be heard.

I can't tell you step-by-step how to build this kind of electrical/acoustic filter, only that from the crossover point to an octave below it, a box with a Qtc of 0.5 or so will have a slope of 6 dB/octave.

[kelticwizard]

This thread here deals with something similar to this:
Q of box re: hi-pass x/o

[roddyama]

Originally posted by niacin
Crossover too near resonance?

Everything I have read says you have to keep the crossover frequency far above the driver’s resonant frequency, like 1-2 octaves.

This is WRT the mid-range and tweeter drivers.

Originally posted by niacin
But in Vance Dickason’s speaker cookbook he mentions an article by Joe D’Appolito in Speaker Builder 4/84,
(Which I don’t have, has anybody read the article?) Which apparently says that a second-order Butterworth network “...can be used in conjunction with the 6dB/octave acoustic high-pass response [of the driver] to derive a combined 18dB/octave Butterworth response... “

This is an effective way of getting a faster roll-off with a far less complex x-over when using a mid-bass driver. This is usually done in the 50Hz to 150Hz range and than crossing over to a sub below that.

As KW indicates, this also allows you to run a lower Q closed box alignment on the mid-bass to maintain good transient response while still having the benefit of a fast roll-off in the stop band.

[sreten]

6dB per octave high pass is not a general high pass characteristic
of drivers though if it was the the statement would be true.

A much more realistic statement would be
" the "2nd order high pass function of a driver, e.g. a sealed
midrange driver can be combined with a 2nd order hign pass
electrical filter to form an overall acoustic 4th order high pass
function.
Using active filters this is easy to arrange however using
passive filters the impedance of the driver at resonance
complicates matters considerably.
If the driver is a tweeter power handling issues often
become a major factor preventing this approach "

What is common is combining a 6dB/12dB low pass roll-off
of a mid or bass mid unit with a 6dB/ 12dB electrical c/o to
form acoustic 12DB/18dB/24dB low pass alignments.

I suspect in original context "high pass" should be "low pass".

sreten.

[Jay]

Originally posted by sreten
I suspect in original context "high pass" should be "low pass".

Sorry I didn't follow the thread. This statement catched my eyes. I do find those two terms used interchangeably in many writings.

[johnnyx]

Jeff Macaulay did this in one of his designs, with a Morel tweeter.
He combined a 2nd order filter with that of the tweeter to give a 4th order response, crossing over at the tweeter's resonant frequency. The filter's Q was chosen so that when combined with the tweeter's Q, it gave the desired 4th order Q. He said it worked well.
I may depend on which tweeter you use, because it might be too distorted, depending on the construction of the tweeter. Power handling may be an issue too.

[gary f]

The problem is not power handling but limited excursion. If you cross a common tweeter (Fs= 1000 hz, x max = 0.25mm) at 1000Hz, the tweeter will exceed it's linear excursion before anything else, whatever the slope used.
Surely, the power handling may be OK but not the excursion, even at moderate levels.

There are ways to calculate the excursion for a given frequency, at a given spl, for a specific tweeter of known dome area. This is the way to determine the correct highpass function.
F

[sreten]

OK,

I meant and should have said "excursion limited power handling".

sreten.

[niacin]

I think I get it... if you try to cross over at resonance using a passive crossover it will be very tricky due to the impedanace peaks, phase shifts, etc. , that's why it is not a common design.

But, if you cross over at resonance using an active crossover it will be easier.

Here is what I want to do. I want to use a sealed box with a Q of 1.0 (for maximum SPL and power handling) and a f3 of about 100Hz. That will give me ( I think) a rolloff of 12dB/octave below cutoff. Then I want to add a single-pole active highpass filter, 6db/octave, so I end up with a 18db/octave Butterworth response.

My question is: what frequency do I want to set the active crossover to, and what is my final f3?

I don't have the fancy software to model active/passive responses together, can anybody help me?

thanks, niacin