ABS said:
With as narrow a baffle as possible (6") there is no room to offset the driver side-to-side -- see pic.
Be wary trying to get too much bass out of the FR125, you will. When modeling go for a higher F3 with a better F10 and a more gradula roll-off.
Option 3 - Add a tweeter to the front FR125 (if needed) in conjunction with 1 and 2.
Room dependent... as a bipole (given some toe-in) at least 2 ft out from the wall. If just a 0.5 then it needs to be out at least the width of the cabinet. With no baffle-step, any room gain will be more obvoius.
They will throw a different soundstage. (ignoring the issue of how your amp behaves into the different impedances above & below the XO point.
That is where i'd put it.
Yes. They both load the box the same way.
How your amp behaves determines series or parallel wiring... keep in mind that if you series wire you need to use a shunt cap to roll off the back driver for use as a 0.5.
dave
Dave:
Thanks for the response - just a few more quick questions.
I am thinking that placing the drivers somewhat off-center will result in possible improvements to the baffle diffraction. Since this is going to be a .5 setup, the dispersion off the baffle from the front firing driver is going to be important in the upper frequencies - do you concur with this logic?
In regard to getting too much bass out of the FR - is it your thought that room effect will effectively increase perceived bass between the F3 and the F10?
What is the shunt cap you mention on the rear driver? I was expecting to run the bottom end of the rear firing driver with the same tuning as the front driver and then to use an inductor to roll-off the upper frequencies creating a 1st order low pass passive filter. Are you suggesting I should go 2nd order instead with both an inductor and a capacitor? Also, I am expecting to need that filter regardless of parallel or serial wiring, no?
Thank you.
Andrew
Thanks for the response - just a few more quick questions.
I am thinking that placing the drivers somewhat off-center will result in possible improvements to the baffle diffraction. Since this is going to be a .5 setup, the dispersion off the baffle from the front firing driver is going to be important in the upper frequencies - do you concur with this logic?
In regard to getting too much bass out of the FR - is it your thought that room effect will effectively increase perceived bass between the F3 and the F10?
What is the shunt cap you mention on the rear driver? I was expecting to run the bottom end of the rear firing driver with the same tuning as the front driver and then to use an inductor to roll-off the upper frequencies creating a 1st order low pass passive filter. Are you suggesting I should go 2nd order instead with both an inductor and a capacitor? Also, I am expecting to need that filter regardless of parallel or serial wiring, no?
Thank you.
Andrew
Since my amps aren't 4 ohm stable I am now thinking that I might have some issues running two 8 ohm FR drivers in either parallel or serial since it will result in either a 16 ohm or 4 ohm load.
Would it make more sense for me to go with a rear mounted WR125ST driver which I could wire in parallel with the front mounted FR driver? Of course my concern is whether or not that would that still provide the same baffle step correction or if the two drivers need to be identical to acheive the desired result?
Would it make more sense for me to go with a rear mounted WR125ST driver which I could wire in parallel with the front mounted FR driver? Of course my concern is whether or not that would that still provide the same baffle step correction or if the two drivers need to be identical to acheive the desired result?
ABS said:
Yes... just keep in mind that the narowist possible baffle means there is no room to offset the driver,
Yes... and it is very easy to get heavy bass out of the FR that is over-the-top.
If you wire in parallel you use an inductor to roll off the 0.5 woofer, if you wire in series that doesn't work, you have to use a shunt capaitor across the 0.5 driver.
dave
DSP_Geek said:
Unless on a perfect shaped baffle the baffle loss response has a ripple in it. If the 0.5 driver is on the back it perfectly compensates for this ripple (since the back driver has the same ripple but in the other direction -- assumes a cabinet symmetrical fore & aft), a filter or a front mounted driver can only compensate for the gross levels above & below the baffle-step region.
When you are rolling off a 0.5 woofer you introduce a LP pole at about the baffle-step frequency. With this pole in place the response of the driver rotates thru 90 degrees of phase while the driver going up to meet the tweeter (or midrange) does not, so the 2 drivers are partially out of phase. With the driver mounted on the back, the phase roll is in the shadow of the cabinet (ie the part of its response that only sees 2pi steriradians) so it does not interfere with the main driver's response.
dave
planet10 said:
I can understand how the back-radiation from the rear driver will be the dual of the front driver's output, but won't there be problems with phase delay due to the depth of the cabinet? For example, assume a 15" wide box, also 15" deep: the baffle step will occur around 300 Hz, but the depth of the box will introduce a 120 degree phase shift from the back speaker at that frequency, and 180 degrees at 450 Hz. Those phase shifts will mess up the vector sum of the outputs something awful, one would think. Mirage seems to get around this by having wide but shallow cabinets for their dipoles, causing the baffle step to occur fairly low down while minimizing back-to-front delay. I looked at placing 0.5 drivers on the back panel for my design, but finally the delay was just too much to deal with and I ended up putting them on the front. It might be you're not seeing nulls as deep as might be predicted since you're also getting reverberant energy in that part of the spectrum. I can see the back driver in your design adding some useful room energy right when the front driver radiates into half-space as opposed to full-space.
As for the LP pole in the 0.5 driver, doesn't that go away since baffle step is a minimum-phase phenomenon, where phase can be predicted from the shape of the response curve? In that case flattening the amplitude response will also clean up the baffle step phase response. Even if we accept the LP phase argument (which I think is open to question), baffle steps tend to occur from 250 to 400 Hz, thus placing the LP corner frequency at about 175 to 285 Hz. Most mid-tweeter crossover frequencies are about 10 times that, so the 1st order function is about 20 dB down at that point and thus its contribution doesn't much affect the phase of the dominant driver being crossed out.
Cheers,
Francois.
DSP_Geek said:
at 450 Hz the output of the back driver should be pretty much invisible to the front driver... the delay at the BS is theorectically an issue unless the box is wide & shallow.
Now i don't really build x.5 way system, but true bipoles, and in pratical use the benefits seem to outweigh the drawbacks (cabs we do are usually 1 unit across & 1.6 units deep -- a worse case than what you have described)
I've seen quite a few people converted from discarding bipoles on an intellectual basis, to being fans after living with them for a while (me included).
I don't think so, because we are talking about response above the BS frequency.
dave
planet10 said:
Turns out it's not quite so simple as that. If the baffle step is 2 dB down from maximum, that's about 0.8 in amplitude (as opposed to power), which means that 0.2 of the amplitude is diffracting around the cabinet (simplistic analysis, but bear with me). That 0.2 will return to the front, which is the point of the exercise of having a 0.5 driver on the back, but instead of supplementing the 0.8 amplitude of the front driver the summation is destructive and the sum becomes 0.6, or about -4 dB instead of 0 dB. The time delay will cause a series of ripples between the range where the path length adds minimal phase shift, and the point where the back driver radiates into half-space (and even then reflections from the back wall may hold a surprise or two).
Bipoles, or dipoles?
In any event, bipoles have a number of useful advantages if one has the room to let the back wave develop properly. Chief among them is flatter room power response than standard closed cabinets, which in my opinion mitigates against "boxy" sound by virtue of the increased reverberant field above the baffle step frequency.
Cheers,
Francois.
DSP_Geek said:
Here is the measured response of a Vifa P13W bipole vrs monopole
An externally hosted image should be here but it was not working when we last tested it.
Bipoles.
dave
Is that just the bass that's better Planet10, or the whole musical spectrum; timing, seamlessness, etc?
Sorry to be so pedantic!
I'll be trying some tommorow evening with the bottom 300Hz rolled off with an electronic 24dB/oct L-R X-over to a monacor sph-135 basss unit. It should be interesting to see (hear) how much that ruins things...
We may even get round to adding a tweeter, a bit more off axis treble response would be welcome IMO, but that will probably ruin the sound too, we'll see...
Sorry to be so pedantic!
I'll be trying some tommorow evening with the bottom 300Hz rolled off with an electronic 24dB/oct L-R X-over to a monacor sph-135 basss unit. It should be interesting to see (hear) how much that ruins things...
We may even get round to adding a tweeter, a bit more off axis treble response would be welcome IMO, but that will probably ruin the sound too, we'll see...
Cheers for that Dave.
Mark -knowing Dave, I'm 99.99% certain he means the whole frequency range. Bipoles usually are superior to monopoles; however, there are very few people, if any, who can design this sort of enclosure / system as well as Dave can. If he says they are special, then they will be very special indeed. I'm going to try a set of his Bipolar Bipoles with twin FR125s myself when funds and time permit. Which is a vote of confidence, if you like.
Mark -knowing Dave, I'm 99.99% certain he means the whole frequency range. Bipoles usually are superior to monopoles; however, there are very few people, if any, who can design this sort of enclosure / system as well as Dave can. If he says they are special, then they will be very special indeed. I'm going to try a set of his Bipolar Bipoles with twin FR125s myself when funds and time permit. Which is a vote of confidence, if you like.
i noticed that the bipoles you've built were in a sealed 25l cab...would it be possible(advisable) to build the same outer cabinet, but split it into two, and port both the top and bottom. keeping the same config in the top, but a eside firing extremis in the bottom tuned to 30Hz, say? I mocked up this...
Thanx,
Mantisory
Thanx,
Mantisory
