smaller-than-optimal enclosure, crossover design, any advice appreciated

[punintended]

So I'm designing a 2-way monitor using Aura NS6 and Vifa BC25SC06.

The Aura is only a 6" driver and software suggested that it would only produce decent bass with a huge enclosure (vented F3=35 with a 46 L enclosure), so I'm thinking about using a smaller sealed box. The idea is that I will sacrifice bass response for smaller size and add a subwoofer to make up for this.

Qtc=.707 with an enclosure of 14.2 L, but I was hoping to go even smaller... I was planning a 10L enclosure (my notes say Qtc=.75), but then I found PE had a 9L pre-built cabinet on sale (probably cheaper and definitely better than I could manage myself).

My questions are:
Is this design (sacrificing low-end response and adding a sub) a valid approach?
This driver's T/S parameters seem to indicate that it would prefer a vented enclosure, but if I'm not going for optimal low-frequency response, does this matter?
I understand that higher Qtc values aren't optimal, but are there any unexpected consequences I should keep in mind when I'm designing the crossover?
These are going to an on-wall placement, so do I not need to worry about baffle step compensation? In the XO I designed for a 10L box (which I'll edit for the smaller size), adding some helped flatten out a hump in the woofer's response, but apart from that, is it necessary for my use?

And then to my XO - here's my current plan (10L, so it'll need revision for the smaller box), if you could tell me if I'm on the right track or not, that would be great. PCD gives a pretty flat response in the simulation, but I'm no expert...

The woofer has a breakup peak at about 4600Hz, so to avoid that, I shoot for 1 octave down, 2300Hz.
The tweeter is resonant at 1350Hz, so shoot for 1 octave up, 2700.

That's a pretty tight range, so what I did was put a 2nd order low-pass on the woofer at 2200Hz and a 3rd order high-pass on the tweeter at 2400Hz. I figure the higher slope will help my results.

I'm sure I have more questions, but I have to run right now - I'll post again later if I think of anything else.

[WernerM]

Check Don Radicks thread on the AURBACS here for a design using the exact driver combination you are thinking of. Also I'm sure he will pipe in this evening for assistance.

[donradick]

Don't know if you see this, measured T/S params.
Basically, Fs and Qt are larger than stated by the manufacturer, so you need a larger
box for comparable bass:

Thiele-Small parameters:

Fs = 68.14 Hz
Re = 6.00 ohms[dc]
Le = 296.86 uH
L2 = 697.22 uH
R2 = 13.45 ohms
Qt = 0.64
Qes = 0.68
Qms = 10.62
Mms = 11.47 grams
Rms = 0.462366 kg/s
Cms = 0.000476 m/N
Vas = 14.10 liters
Sd= 145.27 cm^2
Bl = 6.586313 Tm
ETA = 0.63 %
Lp(2.83V/1m) = 91.36 dB

I haven't modeled it, but I'm pretty sure you'll need about 14 Liters for 80 Hz.
Other than that, your plan sounds ok. If you want to usa a different tweeter, you're on
your own. The NS6 is pretty easy to work with, but you're right about the breakup
spike at 5-6K.

regards,

[Whitneyville1]

Don, using your T/S numbers and Speaker Workshop/WinISP PRO, I show 26.2 liters sealed "optimum" for an FB of ~78Hz, so for 80Hz, it might take a bit more real estate. The low-end roll-off is very slow and gentle, so if you aren't nutzy-coo-coo about "flat" responses and 83 Hz is "close-enough", don't lose sleep over it in 14 liters. In 9L sealed, I get Fb~108Hz, F3~85Hz, with a 1dB "bulge" from about 250Hz to 108Hz.

[donradick]

Don, using your T/S numbers and Speaker Workshop/WinISP PRO, I show 26.2 liters sealed "optimum" for an FB of ~78Hz, so for 80Hz, it might take a bit more real estate. The low-end roll-off is very slow and gentle, so if you aren't nutzy-coo-coo about "flat" responses and 83 Hz is "close-enough", don't lose sleep over it in 14 liters. In 9L sealed, I get Fb~108Hz, F3~85Hz, with a 1dB "bulge" from about 250Hz to 108Hz.

Thanks Dude - as stated, I had not done a range of modeling.
Your results look about what I thought, but I am pretty amazed you got an F3 of 85Hz in 9L. Make a very nice surround speaker that way.

[mdocod]

So I'm designing a 2-way monitor using Aura NS6 and Vifa BC25SC06.

The Aura is only a 6" driver and software suggested that it would only produce decent bass with a huge enclosure

There is an enormous amount of misconception about what you saw in simulation. You can cram down the box size, and still use a vented alignment. Yes, the response for equal voltage input will have a roll-off to it that doesn't look all "flat-n-purdy," but the ability for the system to produce bass is still in play. The NS6-8 is Xmax limited to ~100dB whether it's in a 10L or 50L vented enclosure if used down into the ~50hz range. The difference is that, in the 50L enclosure, you don't need any EQ or "extra" amplification to reach this goal, and the larger enclosure will also dig a bit deeper.

(vented F3=35 with a 46 L enclosure), so I'm thinking about using a smaller sealed box. The idea is that I will sacrifice bass response[/quote]

Remember, when you look at these simulations, the simulated response assumes equal input voltage. Don't ignore EQ. The voice coil can often handle the increased power required to put the "response" back to where you want it. The NS6 is a great example of a driver that runs out of Xmax long before it runs out of power handling in what many people would call "optimum" enclosures.

for smaller size and add a subwoofer to make up for this.

Whether the boxes are built big or small, you could still need a sub to hit a desired extension and SPL, it's also possible, that if your listening requirements are not all hat "loud" or "far-field" that the setup may reach your goals through EQ and amplification well enough.

Qtc=.707 with an enclosure of 14.2 L, but I was hoping to go even smaller... I was planning a 10L enclosure (my notes say Qtc=.75), but then I found PE had a 9L pre-built cabinet on sale (probably cheaper and definitely better than I could manage myself).

You could add a 2" flared port kit from PE to this box (come in from the top or bottom if possible, to get port length) and gain an easy 3-6dB through the usable bottom end on average.

My questions are:
Is this design (sacrificing low-end response and adding a sub) a valid approach?

Yes, and, more importantly, in the case of the NS6, you aren't sacrificing as much as looking at a simulated response is telling you.

This driver's T/S parameters seem to indicate that it would prefer a vented enclosure, but if I'm not going for optimal low-frequency response, does this matter?

Nope, doesn't matter at all. In fact, the only time this consideration is "important" is when driver specifications are very favouring of sealed or IB configurations. Any driver with enough motor strength in relation to it's moving mass to "work well" in vented boxes, is also perfectly fine in a sealed box, albeit, with typically fast falling "response" through the bottom end that will either need more amplification, EQ, and or, a crossover to a sub.

I understand that higher Qtc values aren't optimal, but are there any unexpected consequences I should keep in mind when I'm designing the crossover?

In a 2-way, not significantly... There is no component in the network that will be effected by the "move" of the impedance spike caused by high Q alignments. Try not to worry about it too much. There may be some implications on the sound "quality" in that range (distortion figures), but there is nothing you can do in the crossover to solve this.

These are going to an on-wall placement, so do I not need to worry about baffle step compensation?

You should still take into account diffraction of the box edges, however, don't "correct" for the step loss much if any (leave the 6dB loss in place for transition to 4pi, it won't actually be "lost" much anyways, just the diffraction above the first major transition where the baffle shape and size and driver locations are in play heavily.

In the XO I designed for a 10L box (which I'll edit for the smaller size), adding some helped flatten out a hump in the woofer's response, but apart from that, is it necessary for my use?

Which hump? If you're talking about a low frequency "hump" (like below 500hz,) you should just ignore it. Often, a hump in response in the 100-200hz range is desirable anyways, as it helps overcome some common room cancellations any-ways. Furthermore, ears aren't that sensitive down there, you can have large swings in response with little consequence.

And then to my XO - here's my current plan (10L, so it'll need revision for the smaller box), if you could tell me if I'm on the right track or not, that would be great. PCD gives a pretty flat response in the simulation, but I'm no expert...

The woofer has a breakup peak at about 4600Hz, so to avoid that, I shoot for 1 octave down, 2300Hz.
The tweeter is resonant at 1350Hz, so shoot for 1 octave up, 2700.

That's a pretty tight range, so what I did was put a 2nd order low-pass on the woofer at 2200Hz and a 3rd order high-pass on the tweeter at 2400Hz. I figure the higher slope will help my results.

The goal is flat response, symmetrical slopes, in-phase relationships through the x-over transitions as best as possible, and knocking down driver problem areas. In this case, you have drivers that actually overlap VERY WELL considering their budget. You have crossover options everywhere from 2-3K hz. I gravitate towards lower x-overs, even in light of potential tweeter distortion, because I like the increase in "point-source" behaviour, which allows speakers to play the "disappearing act" better in my experience.

Try not to concern yourself directly with what "order" each leg of the network is. The goals I stated above, are often reached with mixed order crossovers. Make sure you are at least using simulated minimum phase data, and driver origin (physical) differences in the network design. Then follow the response and phase until you have a combination of components that produce an in phase, ideally symmetrical crossover, without causing any impedance dips that are "too" low. (how much is "too" low would depend on the systems goal impedance minimum).

I'm sure I have more questions, but I have to run right now - I'll post again later if I think of anything else.

Finally, I'll say, for a smaller enclosure, if your amplifier doesn't mind a 4 ohm load, madisound has the 4 ohm version of this speaker, whose characteristics are a bit more friendly to the smaller box. Also, it would save on crossover component costs. (lower impedance woofer, smaller inductors in the low pass).

Regards,
Eric

[punintended]

All right, thanks for the help guys. Here's what I have so far, modeled for the 9L sealed box.
This is without BSC (on-wall placing), and feel free to ignore the splice below 100 Hz (response modeler was acting strange, plus with the subwoofer I'm using this with I'm not too concerned about bass response).
I switched the high-pass to a 2nd order for phase matching.
So I guess I'll maybe do a little more tinkering tonight and then see what I can do with the component values available.
Any advice?

[greywarden]

It looks good, except the impedance drops kind of low for an 8 ohm design, so try putting the series resistor in the L-pad section "Before" the tweeter crossover. Warning though, it will affect the tweeter's phase, so you may have to tweek it a little after that. Also, I noticed your woofer FRD seems to only go to 5-6kHz before it's a flat line... Idk if that will affect the top end response much, but you may need to get a measured FRD file, unless that's what it is.

[WernerM]

When splicing box response, set the dB to the correct level, I would try to set the box splice to about 82.5dB so that it is a nice smooth transition.

[mdocod]

Trace as much of the available response and impedance data for each driver as you have available. Stopping within an octave of the intended pass band will result in poor phase simulation. You should trace data for the woofer and tweeter out as far as it goes, in fact, consider splicing in a 12dB/octave roll-off to extend the charted responses and natural rolloff another few octaves above/below where the response "drops off the chart" for each driver if you have the ability to do so. This will result in a further improvement of simulated phase data.