Here's the discussion where I got my info and I've read other references that BLH's sound better using a driver with an Fs higher than the horn's LF cutoff.
http://www.decware.com/cgi-bin/yabb/YaBB.cgi?board=Imperial&action=display&num=1100918413&start=0
http://www.decware.com/cgi-bin/yabb/YaBB.cgi?board=Imperial&action=display&num=1100918413&start=0
Yes, I have read that tread, however, reference to some article isnt sufficient proof (to me, at least) that that was Jensens intentions. As far as having a driver with an Fs higher than cutoff, the P15-LL fits the bill. The cutoff frequency is 30Hz, so, with a Fs of 37, it is above cutoff. I am just questioning the need for a Fs above 40Hz. The G610 may have had an Fs above 40, and Jensen did indeed recommend that driver for home builders, but I think that was because using a coaxial (or triaxial in this case) was easier or maybe less intimidating for the typical home constructor, not because it was the optimum driver for the Imperial.
I'm too new to all of this to question the word of someone who has been building Imperial cabs for almost 50 years. Plus it doesn't sound like something someone would PFTA (pull from thin air). I think I'll just take his word for it.
I thought the Imperial was a 40hz horn in free air. Is that incorrect?
I thought the Imperial was a 40hz horn in free air. Is that incorrect?
Yes, but that's into 1/4 space (in wall and mouth at the floor) . I think it's a 40hz hypex horn contour and thus the need for a driver with an Fs of 40hz or more. The Selenium 15 has an Fs in the 50's. That may be the reason for response peak at 80-90hz that showed up using HornResp modelling. Although you can't truly model the full Decware version with HornResp because the chamber is larger than 99 litres giving it even deeper bass response.
John in Costa Rica,
It's pretty easy to access the output leads of the XO, so you can always start by running an 8 ohm in series and a 16 after that in parallel if you really don't like the high end. Or even cheat a little and run a 4 ohm in series. I'm just planning to EQ mine. I find the mid a bit more than I like. The high end is just fine with my old ears. And yes I have a 15" sub for the low end, I'm thinking IB for the drivers. (ceiling)
Cal
Edit: Spelling
It's pretty easy to access the output leads of the XO, so you can always start by running an 8 ohm in series and a 16 after that in parallel if you really don't like the high end. Or even cheat a little and run a 4 ohm in series. I'm just planning to EQ mine. I find the mid a bit more than I like. The high end is just fine with my old ears. And yes I have a 15" sub for the low end, I'm thinking IB for the drivers. (ceiling)
Cal
Edit: Spelling
Cal,
It's not that the tweeters are too loud. Instead it is a harshness that mucks up the detail. I'll probably just end up mounting a pyle bullet tweeter on the thick metal horn mouth which protrudes in the front. After tweaking they sound much better than the existing tweeters and have enough efficiency to keep up with the woofer. I'll probably get better time alignment too.
It's not that the tweeters are too loud. Instead it is a harshness that mucks up the detail. I'll probably just end up mounting a pyle bullet tweeter on the thick metal horn mouth which protrudes in the front. After tweaking they sound much better than the existing tweeters and have enough efficiency to keep up with the woofer. I'll probably get better time alignment too.
johninCR said:
OK, I misunderstood.
John, let's face it, it's nice cheap PA reinforcement coax and I'm betting you're just standing too close to them if it sounds harsh
and I'm not so sure 'detail' was on the mind of the engineers, I think it was more like: 
You can always EQ the nasties and add a bullet, although I'm not sure it's needed. I use the same tweeter in the 511 cowbell and it doesn't seem to need more. Remember, they're not for critical listening, just for
time. Cheers,
Cal
Hello my name is John and I'm a tweakaholic.
I agree totally that they are a great budget driver for a party cab. I've already got one of the dust caps off, so I need to do something anyway. I figure why not spend the $30 to upgrade the HF without sacrificing efficiency or power handling unless there's a way to spend less using some basic tweaks. eg the joint of the metal horn cone with the interior tube is not a very smooth one but it seems to be in there permanently. Maybe some thick rubberized paint could help in there. Also, opening the back of the tweeter exposes a slotted metal piece between the diaphram and the pathway to the front. A number of the slots have irregular forms resulting from an imperfect cast. Is trying to fix these imperfections in the build a waste of time or likely to be fruitful?
I'll use my pair as the guinea pig if anyone has some ideas.
I agree totally that they are a great budget driver for a party cab. I've already got one of the dust caps off, so I need to do something anyway. I figure why not spend the $30 to upgrade the HF without sacrificing efficiency or power handling unless there's a way to spend less using some basic tweaks. eg the joint of the metal horn cone with the interior tube is not a very smooth one but it seems to be in there permanently. Maybe some thick rubberized paint could help in there. Also, opening the back of the tweeter exposes a slotted metal piece between the diaphram and the pathway to the front. A number of the slots have irregular forms resulting from an imperfect cast. Is trying to fix these imperfections in the build a waste of time or likely to be fruitful?
I'll use my pair as the guinea pig if anyone has some ideas.
johninCR said:
John,
If cylinder heads can be ported with an air die grinder for more 'air' why not coaxial speakers?!
It is probably non-ferrous pot metal you are speaking of but nevertheless I'd be careful not to release metal particles which Murphy guarantees will end up in the coil gap. With my luck the magnet would cause the carbide cutter to impact the pole piece and shatter a tooth which would be magnetic and end up guess where?!
Those Seleniums just got a whole lot more valuable in case people haven't clued in yet. They are not available as of recently.
The slotted piece is easily separated from the unit, so I guess I'll take those out and bring them to a machine shop. The horn itself is a different matter. I'm thinking that some type of interior coating and or insert could damp out the harshness, but I don't know enough about the physics involved to make even an educated guess.
johninCR said:
Before you do, why don't you take a picture of your phase plugs (slotted metal piece) and post them so we can see the irregularities. Then I'll go downstairs and take mine apart to compare before you go grinding the things. I have four of the D205Ti.
rcavictim said:
Well, the replacement is available. Pretty much the same unit but the Fs is lowered to 41Hz from 50 and the new one has no XO.
Cal
> hey GM. Maybe I got it wrong in what he said but I am almost sure that was accurate. If what you are saying is true, and I believe it is, then it would make the project a lot easier because low QTS drivers are tougher to find. I never thought of the blh as a large br. That is very interesting and I can see what you are saying. I have never heard that description. What is your definition of highish QTS. number wise? Thanks for the input. Craig
=====
Greets!
Actually, I guess the more technically correct way to view it is as a BP with a room size rear 'compression' chamber.
I don't know of any clear breakpoint for hi Q other than the oft quoted 0.383, though I use 0.403, where Vb = Vas and Fb = Fs.
Noting that Fs and Vas are part of the equation (ergo there is no one 'right' driver, but the right combo for the desired BW/gain) and just plugging in the horn stats, the ideal would be where Vb = Vas and Fb = Fs, ergo since the horn resonates at ~35Hz and the filter chamber is ~5.166ft^3, the ~ideal driver would have a 35Hz Fs, 5.16ft^3 Vas, and 0.403Qts, but whatever Fs/Vas/Qts combo that yields a ~flat response in a~5.166ft^3/~25-45Hz Fb BR depending on the desired gain/BW should work fine regardless of driver size/qty.
====
> Over at the Decware forum, they like drivers with a Qt in the mid to high .3 range.
====
This is a little low based the horn's stats unless the Fs, Vas specs offset it, but not terribly so. One of the nice things about our lack of hearing acuity down low is that we are very tolerant of technically p*** poor alignments as long as they don't 'ring' so much that they are audible in the upper harmonics. Good thing too, or the room's affect would force us to limit in-door BW to ~100-200 Hz depending on the room, though their diminuitive size would have a high SAF.
====
> I have tried to find numbers on the P15-LL, but nobody seems to know the parameters of that driver, other than Jensens published Fs of 37Hz.
====
FWIW, my P15-LS measures 46Hz.
====
> I see what appears to be a lot of misinformation floating around on the net.
====
Oh really?!
====
>........I've read other references that BLH's sound better using a driver with an Fs higher than the horn's LF cutoff.
====
This would depend on how flat you wanted its response. Think about it, there's no way to reactance annull a BLH so just like any other vented design the ideal is to damp the driver's Fs, ergo it should = effective horn Fc (horn pathlength plus any boundary effect). The FR is then shaped by manipulating Fs/Vas/Qts, just like in a BR, so using a higher Fs driver will boost the horn's upper BW, which may/may not be desirable depending on the room's impact on the FR. Just don't confuse this with the advantage of using an Fs >Fc driver in a compression horn to help with reactance annulling where the rear chamber Vb may otherwise be too little to be physically practical.
Note that as a BLH is increased to its most efficient size, its acoustic mass will mass load the driver, shifting its Fs downward, first damping the lower impedance peak, then both, creating a new peak ~an octave below Fs. While theoretical BW/output is greatly increased, the driver's excursion will be highly non-linear and unless it has a huge BL/thermal power handling capacity it's been my experience that it will burn up if you try to use it this low at any usable power, and why I recommend not designing a BLH with an Fc <0.707*driver Fs unless its acoustically small for the desired flare rate, but as always YMMV.
====
>By the Jensen literature I've seen, they said it loads the driver down to 30Hz.
====
I didn't try to calc it to the Hz, but it should be solid down to at least 35Hz/half space (mouth sitting on floor), so in-room could easily be good down into the mid 20s in a corner with a ~20-25Hz Fs driver.
====
> That may be the reason for response peak at 80-90hz that showed up using HornResp modelling.
====
Right. Again, think about how the driver's Fs affects the summed response of a fixed Fb (Fc) cab.
WRT spraying diaphragms with lacquer to raise Fs, this will only work on diaphragms which have their perimeter terminated into a rolled surround and assumes the spider is very high compliance. This lowers Vas, Qts and raises Fh as well as its break-up modes, so unless the horn is designed around the revised specs I don't recommend it.
For drivers with high compliance surrounds, the spider will need stiffening instead, which is a lot tougher to do uniformly once installed so not a tweak I would try on an expensive and/or rare driver, but as always YMMV.
====
> Boy, i am really confused now.
====
Hopefully somewhat less so now.
GM
=====
Greets!
Actually, I guess the more technically correct way to view it is as a BP with a room size rear 'compression' chamber.
I don't know of any clear breakpoint for hi Q other than the oft quoted 0.383, though I use 0.403, where Vb = Vas and Fb = Fs.
Noting that Fs and Vas are part of the equation (ergo there is no one 'right' driver, but the right combo for the desired BW/gain) and just plugging in the horn stats, the ideal would be where Vb = Vas and Fb = Fs, ergo since the horn resonates at ~35Hz and the filter chamber is ~5.166ft^3, the ~ideal driver would have a 35Hz Fs, 5.16ft^3 Vas, and 0.403Qts, but whatever Fs/Vas/Qts combo that yields a ~flat response in a~5.166ft^3/~25-45Hz Fb BR depending on the desired gain/BW should work fine regardless of driver size/qty.
====
> Over at the Decware forum, they like drivers with a Qt in the mid to high .3 range.
====
This is a little low based the horn's stats unless the Fs, Vas specs offset it, but not terribly so. One of the nice things about our lack of hearing acuity down low is that we are very tolerant of technically p*** poor alignments as long as they don't 'ring' so much that they are audible in the upper harmonics. Good thing too, or the room's affect would force us to limit in-door BW to ~100-200 Hz depending on the room, though their diminuitive size would have a high SAF.
====
> I have tried to find numbers on the P15-LL, but nobody seems to know the parameters of that driver, other than Jensens published Fs of 37Hz.
====
FWIW, my P15-LS measures 46Hz.
====
> I see what appears to be a lot of misinformation floating around on the net.
====
Oh really?!
====
>........I've read other references that BLH's sound better using a driver with an Fs higher than the horn's LF cutoff.
====
This would depend on how flat you wanted its response. Think about it, there's no way to reactance annull a BLH so just like any other vented design the ideal is to damp the driver's Fs, ergo it should = effective horn Fc (horn pathlength plus any boundary effect). The FR is then shaped by manipulating Fs/Vas/Qts, just like in a BR, so using a higher Fs driver will boost the horn's upper BW, which may/may not be desirable depending on the room's impact on the FR. Just don't confuse this with the advantage of using an Fs >Fc driver in a compression horn to help with reactance annulling where the rear chamber Vb may otherwise be too little to be physically practical.
Note that as a BLH is increased to its most efficient size, its acoustic mass will mass load the driver, shifting its Fs downward, first damping the lower impedance peak, then both, creating a new peak ~an octave below Fs. While theoretical BW/output is greatly increased, the driver's excursion will be highly non-linear and unless it has a huge BL/thermal power handling capacity it's been my experience that it will burn up if you try to use it this low at any usable power, and why I recommend not designing a BLH with an Fc <0.707*driver Fs unless its acoustically small for the desired flare rate, but as always YMMV.
====
>By the Jensen literature I've seen, they said it loads the driver down to 30Hz.
====
I didn't try to calc it to the Hz, but it should be solid down to at least 35Hz/half space (mouth sitting on floor), so in-room could easily be good down into the mid 20s in a corner with a ~20-25Hz Fs driver.
====
> That may be the reason for response peak at 80-90hz that showed up using HornResp modelling.
====
Right. Again, think about how the driver's Fs affects the summed response of a fixed Fb (Fc) cab.
WRT spraying diaphragms with lacquer to raise Fs, this will only work on diaphragms which have their perimeter terminated into a rolled surround and assumes the spider is very high compliance. This lowers Vas, Qts and raises Fh as well as its break-up modes, so unless the horn is designed around the revised specs I don't recommend it.
For drivers with high compliance surrounds, the spider will need stiffening instead, which is a lot tougher to do uniformly once installed so not a tweak I would try on an expensive and/or rare driver, but as always YMMV.
====
> Boy, i am really confused now.
====
Hopefully somewhat less so now.
GM
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