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Old 14th February 2013, 12:29 PM   #1
xrk971 is offline xrk971  United States
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Default Acoustical Cross-Over FAST on a Budget

I wanted to get some feedback on an idea I have for building a FAST system on a budget that uses no electrical cross-overs, but rather, relies purely on the acoustical band-pass and high-pass features of the enclosures to effect the combined cross-over response.

Basically, I am running the fullrange in a sealed cabinet to get a nice tight and flat response. Here, the Vifa TC9FD will be used in a 5 liter cabinet.

This is coupled with a sub-woofer using a Aura NS6 driver in a 4th order double-chambered band-pass cabinet with 7 liter front + 24 liter rear chamber with a 2 in x 2 in x 8 in long vent.

The sub-woofer will be wired 180 deg out of phase in order to match the FR driver's phase.

My question is this, can I simply wire the two drivers in parallel (effective 4 ohm load) and rely on the acoustical properties of the cabinets to effect the cross-over function and skip the electrical cross-over completely? This keeps system nice and simple and doesn't introduce any phase-altering reactive components. Does this cause problems by wasting power in the incorrect bandwidth or will the power self-distribute nicely? I think, bi-amping could probably solve many of the problems, if any.

Attached is the calculated frequency response for the sub and FR driver - as you can see, I think they will add up nicely. I adjusted a bit of extra SPL for the sub to give some BSC for the system.

More info on 4th order bandpass bass reflex system design: http://www.diysubwoofers.org/bnd/4thorder.htm
Attached Images
File Type: png TC9FD-NS6-FAST-SPL.png (46.6 KB, 501 views)

Last edited by xrk971; 14th February 2013 at 10:37 PM.
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Old 14th February 2013, 03:39 PM   #2
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I think this is somewhat along the lines of what Geddes does, except lower in frequency so phase is less important (he spreads around the room).

What is the acoustic roll off of the NS6 in that enclosure, I'm not familiar with bandpass. And where is the NS6 acoustic position relative to the Vifa's acoustic center?

I'd build these in separate enclosures and wire them in phase and measure the spl as you slide the Vifa enclosure forwards and backwards. Flip phase and do it again.

What does the NS6's impedance do in that enclosure? The natural inductance of the NS6 will eventually make power sharing a non-issue, but likely only above 500hz or so. In general I don't think power sharing would be an issue.
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Old 14th February 2013, 03:41 PM   #3
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Quote:
Originally Posted by xrk971 View Post
This keeps system nice and simple and doesn't introduce any phase-altering reactive components.
Just wanted to say, a couple sealed boxes keeps the system simple, and your bandpass box and sealed box are altering phase just as much as caps and coils do. Not sure how avoiding those is an advantage. Nothing wrong with your approach either though, I like it.
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Old 14th February 2013, 03:49 PM   #4
chrisb is offline chrisb  Canada
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I think you answered your own question: "I think, bi-amping could probably solve many of the problems, if any" This affords the opportunity to achieve line level filters with very few and much lower cost parts than is the case with speaker level passive XOs, while maintaining the "purity" of lack of "reactive components" connected to the amps - except of course the drivers themselves - as a review of their polar impedance plots when driven with musical signal rather than simple tone sweeps will reveal
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Old 14th February 2013, 07:55 PM   #5
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by tuxedocivic View Post
I think this is somewhat along the lines of what Geddes does, except lower in frequency so phase is less important (he spreads around the room).

What is the acoustic roll off of the NS6 in that enclosure, I'm not familiar with bandpass. And where is the NS6 acoustic position relative to the Vifa's acoustic center?

I'd build these in separate enclosures and wire them in phase and measure the spl as you slide the Vifa enclosure forwards and backwards. Flip phase and do it again.

What does the NS6's impedance do in that enclosure? The natural inductance of the NS6 will eventually make power sharing a non-issue, but likely only above 500hz or so. In general I don't think power sharing would be an issue.
What do you mean by acoustic roll off of NS6? The actual driver direct radiation plus port emission? It all goes through the port as this is a driver inside a sealed box mounted between the back and front volumes. I am hoping not much high freq leaks out of the 2x2 in port. I may add some damping or stuffing to make sure only bass comes out.

The acoustic centers - I assume you mean the physical locations of the radiating sources? Well the full range will be placed ideally at ear height on stands or shelves and the bass will be in a separate cabinet on floor. My understanding is that 100 Hz and below is very non directional so it is not critical, but it will probably be at about 2 ft of the floor and aimed either at listener or against wall in back. I will play with it.

You have a good point about measuring response with phase in both settings to see which is better.
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Old 14th February 2013, 07:59 PM   #6
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by tuxedocivic View Post
Just wanted to say, a couple sealed boxes keeps the system simple, and your bandpass box and sealed box are altering phase just as much as caps and coils do. Not sure how avoiding those is an advantage. Nothing wrong with your approach either though, I like it.
You are right about the boxes introducing their own phase alterations to the signal. Although the phase coherence of the critical mid range where the human ear is most sensitive will not be an issue as that comes from the single full range driver - true for all FAST systems I guess. I am just utilizing the natural bandpass of the acoustic enclosure for the sub to effect what a big fat expensive coil would be doing. Just could not justify spending $20 on a coil for a $9 driver! I will try to post the impedance plots of the full range and sub woofer.

So initial indications is that this can work and is not silly?


Thanks for feedback.

X

Update:
Below are the Phase and Group Delay plots from WinISD. The big phase variation is the sub-woofer and I think sliding it front to back will be key to making this work. I am concerned about the big group delay around the 40 Hz - but I guess that is what you get for trying to eek out a low freq from this 6 in driver.
Attached Images
File Type: png TC9FD-NS6-FAST-Phase.png (43.0 KB, 300 views)
File Type: png TC9FD-NS6-FAST-Group-Delay.png (42.6 KB, 299 views)

Last edited by xrk971; 14th February 2013 at 08:21 PM.
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Old 14th February 2013, 08:04 PM   #7
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Quote:
Originally Posted by xrk971 View Post
What do you mean by acoustic roll off of NS6? The actual driver direct radiation plus port emission? It all goes through the port as this is a driver inside a sealed box mounted between the back and front volumes. I am hoping not much high freq leaks out of the 2x2 in port. I may add some damping or stuffing to make sure only bass comes out.
Ya, what I mean is the acoustic slope of the low pass caused by the box. You said a 4th order bandpass. Does that mean the acoustic slope from 120hz or so, is 24db/oct?

Quote:
Originally Posted by xrk971 View Post

The acoustic centers - I assume you mean the physical locations of the radiating sources? Well the full range will be placed ideally at ear height on stands or shelves and the bass will be in a separate cabinet on floor. My understanding is that 100 Hz and below is very non directional so it is not critical, but it will probably be at about 2 ft of the floor and aimed either at listener or against wall in back. I will play with it.
Acoustic centers is what mean. The NS6 will have a longer path length to the design axis. That distace will likely dominate any phase requirements for flipping polarity.

I think this will work. I'd measure and see what happens at the cross over and slide the sub cab forward and back for max summation and you'll have a hit I think.
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Old 14th February 2013, 08:27 PM   #8
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by tuxedocivic View Post
Ya, what I mean is the acoustic slope of the low pass caused by the box. You said a 4th order bandpass. Does that mean the acoustic slope from 120hz or so, is 24db/oct?
It's called a "4th order band pass" enclosure on WinISD, but if I look at the freq response graph (post #1), I get (87 dB - 71 dB) from 40 Hz to 20 Hz, or 16 dB/oct.

And from 100 Hz to 200 Hz it falls from 86 dB to 73 dB or about more like 12 dB/oct on high frequency side.

As I said, this is a budget FAST system so the estimated costs are as follows:

qnty 2 Vifa TC9FD's $24 (I have them already)
qnty 2 Aura NS6 $18 (on sale at PE)
4x8 ft sheet of half in isocyanurate foam sheathing for cabinets. $10
qnty 2 20x30 in foam core for FR cabinets $2

Last edited by xrk971; 14th February 2013 at 08:33 PM.
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Old 16th February 2013, 12:09 AM   #9
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Is the group delay of 16 ms at 40 Hz for the sub going to be a problem?
If so what can I do to reduce this, or all ported subs look like this?
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Old 16th February 2013, 01:31 AM   #10
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People have such varying views on group delay. I like to think in terms of wavelength, right or wrong. 16ms is a little over half a wavelength of delay for a 40hz signal. Buts its also 40hz. How much musical content is there. And if its movies, well who cares if an explosion doesn't track the input signal exactly. This is a budget build so I'd say it doesn't matter. If it was a high end build I'd try and tackle it because that's when the little things pay off.
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