Midrange question and odd design software results

[amandal]

I have a scan speak 13m/8640-00 5" woofer that I want to use as a midrange. The box calculators I've checked call for a ported box in the neighborhood of 0.08 cu. ft. I can't imagine that a port would even fit in such a box.

Using it as a midrange, I assume the porting would be unnecessary as porting affects (only?) low end extension. If it should be ported, would it matter if its being ported into a 3 cu. ft. sealed cabinet? My only other 3-way experience was with a sealed-back midrange which obviously gave me no chance to tinker.

If mids are usually sealed off, and since its a midrange you therefore aren't concerned about the cabinet size's effect on bass response, does it really matter what size box it goes in? Does it only serve to keep the woofer's backwaves from exiting through the mid? Does it even need its own box?

[Pete Schumacher ®]

> I have a scan speak 13m/8640-00 5"
> woofer that I want to use as a midrange. The
> box calculators I've checked call for a
> ported box in the neighborhood of 0.08 cu.
> ft. I can't imagine that a port would even
> fit in such a box.

> Using it as a midrange, I assume the porting
> would be unnecessary as porting affects
> (only?) low end extension. If it should be
> ported, would it matter if its being ported
> into a 3 cu. ft. sealed cabinet? My only
> other 3-way experience was with a
> sealed-back midrange which obviously gave me
> no chance to tinker.

> If mids are usually sealed off, and since
> its a midrange you therefore aren't
> concerned about the cabinet size's effect on
> bass response, does it really matter what
> size box it goes in? Does it only serve to
> keep the woofer's backwaves from exiting
> through the mid? Does it even need its own
> box?

If the enclosure is too small, you will affect it's response. If the rear wave is not properly absorbed, it will return and transmit through the cone, out of phase with the front wave causing ragged response.

Ideally, you could simply have a tube from the back of the midrange to the back of the cabinet and have it open to the back. Fill the tube with fiber to dampen resonance modes and you'll have a fine midrange. A tapered and sealed chamber, stuffed, will also be good.

[madrok]

I agree about the open back, or stuffed, roomy sealed box. BUT, if you are trying to extend the response of the midrange into the region below 200Hz, where the response is partially determined by the cabinet, then you have to start thinking about bass alignments.

[dlr]

Pete and Madrok have made good points, but as I've had several years experience with the 13m, I'll add my $0.02 worth.

> I have a scan speak 13m/8640-00 5"
> woofer that I want to use as a midrange. The
> box calculators I've checked call for a
> ported box in the neighborhood of 0.08 cu.
> ft. I can't imagine that a port would even
> fit in such a box.

I would not try to use the box as the highpass. The 13m is limited in its low end extension for one. 300Hz seems to be the lowest reasonable response. You're also going to to get nothing more than about 85db sensitivity when done. This is not unusual for a number of midrange drivers, however.

> Using it as a midrange, I assume the porting
> would be unnecessary as porting affects
> (only?) low end extension. If it should be
> ported, would it matter if its being ported
> into a 3 cu. ft. sealed cabinet? My only
> other 3-way experience was with a
> sealed-back midrange which obviously gave me
> no chance to tinker.

It should not be ported. A highpass crossover should be utilized, if for nothing more than to keep low end frequencies from heating the coil.

> If mids are usually sealed off, and since
> its a midrange you therefore aren't
> concerned about the cabinet size's effect on
> bass response, does it really matter what
> size box it goes in? Does it only serve to
> keep the woofer's backwaves from exiting
> through the mid? Does it even need its own
> box?

I used it in a 6L box, reasonably stuffed. Then a highpass crossover was used to tailor the driver/box response to achieve the target. An oversized box used in this manner is helpful. It provides more low end response of lower Q. This helps to reduce the size of highpass caps needed and provides more rear wave damping.

Keep in mind that with a crossover, the driver Fs will come into play. The 13m's I've had all had an Fs well above the stated level. Than is, rather than 65Hz, I measured 95-105.

Due to this and the rather high peak around 10K, the crossover for this driver won't be simple, nor cheap. But properly done it's an excellent driver.

dlr

[Pete Schumacher ®]

> I agree about the open back, or stuffed,
> roomy sealed box. BUT, if you are trying to
> extend the response of the midrange into the
> region below 200Hz, where the response is
> partially determined by the cabinet, then
> you have to start thinking about bass
> alignments.

Not to augment bass, just to swallow up the rear wave and dampen resonance. Makes the XO easier to implement as well.

[wg_ski]

> Not to augment bass, just to swallow up the
> rear wave and dampen resonance. Makes the XO
> easier to implement as well.

So what you end up with is a very low Q alignment (as close to driver Qts as possible). This starts to roll off in the low midrange, quite possibly above the desired x/o point. So when you implement the highpass the Q will be higher than what you would otherwise use. This will flatten it and bring the response closer to target. Just watch the excursion and the minimum impedance, as there is no "free" acoustic output.

[Pete Schumacher ®]

> So what you end up with is a very low Q
> alignment (as close to driver Qts as
> possible). This starts to roll off in the
> low midrange, quite possibly above the
> desired x/o point. So when you implement the
> highpass the Q will be higher than what you
> would otherwise use. This will flatten it
> and bring the response closer to target.
> Just watch the excursion and the minimum
> impedance, as there is no "free"
> acoustic output.

Yes, a low Q alignment that eases the circuit requirements. Excursion limits always apply, whether or not the driver is sealed or open back. Actually, having the open back driver extends the F10 point, since the open back allows the shallowest possible roll-off. SPL requirements will determine excursion limits, and with that in mind, the XO point should be chosen to allow that end-result. Typically, a small mid-bass unit will do well from 300Hz and up where the excursion requirements are drastically reduced.

[amandal]

> If the enclosure is too small, you will
> affect it's response. If the rear wave is
> not properly absorbed, it will return and
> transmit through the cone, out of phase with
> the front wave causing ragged response.

> Ideally, you could simply have a tube from
> the back of the midrange to the back of the
> cabinet and have it open to the back. Fill
> the tube with fiber to dampen resonance
> modes and you'll have a fine midrange. A
> tapered and sealed chamber, stuffed, will
> also be good.

That sounds very interesting. It seems if you ran (lets say) a 6" diameter tube from the back of the mid, and continuing out through the back of the box. That would let you adjust its size as needed for tweaking, while still maintaining a static size for the larger box and its woofer. They could both be adjusted independantly that way.. hmm. Thanks. I'd actually worried about.. imagining suddenly needing a midrange box twice its original size, which would throw off the larger one.

[amandal]

> I agree about the open back, or stuffed,
> roomy sealed box. BUT, if you are trying to
> extend the response of the midrange into the
> region below 200Hz, where the response is
> partially determined by the cabinet, then
> you have to start thinking about bass
> alignments.

Nah, I'm still shopping for a crossover point.. and it will definitely be higher than 200hz. I'm actually kinda stuck here on this, as this will be the first 3-way I've attempted since back when I bought all my xovers pre-built. Any ideas for a some good xover points? Preliminary driver selection is Morel MW-1075 10" and Focal TD120 tweeters, in a 3.1 cu. ft. box (I just happen to have these all on hand), however, since I'm groveling for help as it is, I don't want to make everyone's work harder by adding a bunch of predetermined driver constraints.

[amandal]

> Pete and Madrok have made good points, but
> as I've had several years experience with
> the 13m, I'll add my $0.02 worth.

Hey thanks for your response. Even in a shamefully crude test box with all the wrong crossovers, I'm wow'd by the M13. Whatever I have to do, whichever previously built speakers must be cannibalized,.. its all on the table. Have you used it in anything interesting? Whats your favorite?

> I would not try to use the box as the
> highpass. The 13m is limited in its low end
> extension for one. 300Hz seems to be the
> lowest reasonable response. You're also
> going to to get nothing more than about 85db
> sensitivity when done. This is not unusual
> for a number of midrange drivers, however.

This is tentatively going with a morel MW 1075 and focal TC120TD.. any ideas for a good xover point? I mentioned this farther up the thread, but in a nutshell, this will be the first time I've been able to choose my xover points, and the driver selection isn't set in stone or anything, I just happen to have those loose. Theres also some vifa PL18 4 ohm available.

> It should not be ported. A highpass
> crossover should be utilized, if for nothing
> more than to keep low end frequencies from
> heating the coil.

> I used it in a 6L box, reasonably stuffed.
> Then a highpass crossover was used to tailor
> the driver/box response to achieve the
> target. An oversized box used in this manner
> is helpful. It provides more low end
> response of lower Q. This helps to reduce
> the size of highpass caps needed and
> provides more rear wave damping.

No problem on the oversized box.

> Keep in mind that with a crossover, the
> driver Fs will come into play. The 13m's
> I've had all had an Fs well above the stated
> level. Than is, rather than 65Hz, I measured
> 95-105.

Will it be an issue at the higher xover point (than the one you were about to recommend :-) ? I've heard of going >2 octaves for tweeters.. same for mids?

> Due to this and the rather high peak around
> 10K, the crossover for this driver won't be
> simple, nor cheap. But properly done it's an
> excellent driver.

> dlr

I'd have no idea what to do about a 10k peak, especially since, from my understanding, its some kinda ghost frequency outside the passband that will be 'haunting' the sound somehow? Some sort of harmonics phenomenon? How do you filter for something that you've already filtered for once (with an assumed lowpass to the mid a few khz at least below the 10k) ?

[dlr]

> Hey thanks for your response. Even in a
> shamefully crude test box with all the wrong
> crossovers, I'm wow'd by the M13. Whatever I
> have to do, whichever previously built
> speakers must be cannibalized,.. its all on
> the table. Have you used it in anything
> interesting? Whats your favorite?

I used it in a 6L box, but varied the tweeter several times over several years. I also experimented with different crossovers at times with the same tweeter.

The last one that I used that was my favorite before I switched to the 12m was with the SS 9300 and a 2nd L-R @2500. The woofer was always the SS 25w8565-01, Fc varying in the 300-400 Hz range. I think 350 was where I had it last.

> This is tentatively going with a morel MW
> 1075 and focal TC120TD.. any ideas for a
> good xover point? I mentioned this farther
> up the thread, but in a nutshell, this will
> be the first time I've been able to choose
> my xover points, and the driver selection
> isn't set in stone or anything, I just
> happen to have those loose. Theres also some
> vifa PL18 4 ohm available.

I have no experience at all with those drivers, but you should easily be able to do 300-400 with the Morel.

> Will it be an issue at the higher xover
> point (than the one you were about to
> recommend :-) ? I've heard of going >2
> octaves for tweeters.. same for mids?

It does depend on the Fc and the order, the higher each is the less the impact. I ran them 300-400 2nd only, so in both cases it had to be taken into account. 400 4th probably would not.

> I'd have no idea what to do about a 10k
> peak, especially since, from my
> understanding, its some kinda ghost
> frequency outside the passband that will be
> 'haunting' the sound somehow? Some sort of
> harmonics phenomenon? How do you filter for
> something that you've already filtered for
> once (with an assumed lowpass to the mid a
> few khz at least below the 10k) ?

There are two aspects. One is the direct filter reponse to tame the peak. The other is the harmonic distortion that can occur if the crossover is too close or too low an order.

The latter two are rather subjective. I liked 2500Hz 2nd L-R, but I have had it 4000 4th L-R and it was good as well.

dlr