So what to put under a midrange dome....M74T in my case? I am planning a wide baffle, 52cm or 20.5", with large radii (perhaps 4"), picture the Grimm LS1 with a little more depth. This gives a baffle step frequency of ~250Hz.
My first thought was, well, you haven't spared expense so far, so go for it, Purifi 8" or 10" and call it good. Then I was struck by their poor sensitivity v the M74T (Re 5.6R, 95.5 @ 2.83V/1m) and T34B (Re 3.3R, 97.5 @ 2.83V/1m). Certainly not a huge deal if crossed-over electronically, but still...and not to mention the expense (especially when I considered a second PTT8 crossed at the baffle-step so as not to drive down sensitivity even further (3.5 way), that is just simply too much $$$ when I am building 3- L,C, and Right).
I also thought a lot about the crossover frequency of 500Hz and C-C spacing the wavelength. The currently recommended C-C distances seems to be within 1/4λ or 1.2λ. Whereas typically the former is almost never achievable and the latter is used, in this case 1/4 wavelength spacing is possible and 1.2λ is 82.3cm or 32.4"........
Staying within 1/4λ sounded very appealing in terms of making it easier on a relative rookie to do the XO, particularly if the driver was nice and flat well beyond 500Hz. So I did some math.
The M74 chassis diameter is 12.1cm. So a woofer chassis (max) of 22.2cm is within 1/4λ C-C at 500Hz. That opens the possibility of 6 1/2" up to 8" drivers.
Still thinking about sensitivity, I began searching and modeling tons of drivers, hi-fi and pro. This is the power region, I want an effortless sensation of power, but also drivers that start and stop fast by CSD, are well-behaved above XO, don't have ripples in the impedance response in the intended frequency range, etc. I learned a lot, including:
I also realized that if 2 of the midbass drivers' C-C was < 1/4λ at 500 Hz I could possibly put them side-by-side under the mid-dome, increasing sensitivity, SPL, etc.
After all of that, here is a list I have come up with. If anyone has gotten this far and has any thoughts, I would love to hear them. I am ready to purchase and put this decision behind me. I am thinking of these drivers covering 80-120Hz up to 500Hz (the Satoris would extend deeper). One more driver will go below. And I have 5 JBL sealed 10" studio subs distributed in the room.
Thank you,
Bill
My first thought was, well, you haven't spared expense so far, so go for it, Purifi 8" or 10" and call it good. Then I was struck by their poor sensitivity v the M74T (Re 5.6R, 95.5 @ 2.83V/1m) and T34B (Re 3.3R, 97.5 @ 2.83V/1m). Certainly not a huge deal if crossed-over electronically, but still...and not to mention the expense (especially when I considered a second PTT8 crossed at the baffle-step so as not to drive down sensitivity even further (3.5 way), that is just simply too much $$$ when I am building 3- L,C, and Right).
I also thought a lot about the crossover frequency of 500Hz and C-C spacing the wavelength. The currently recommended C-C distances seems to be within 1/4λ or 1.2λ. Whereas typically the former is almost never achievable and the latter is used, in this case 1/4 wavelength spacing is possible and 1.2λ is 82.3cm or 32.4"........
Staying within 1/4λ sounded very appealing in terms of making it easier on a relative rookie to do the XO, particularly if the driver was nice and flat well beyond 500Hz. So I did some math.
The M74 chassis diameter is 12.1cm. So a woofer chassis (max) of 22.2cm is within 1/4λ C-C at 500Hz. That opens the possibility of 6 1/2" up to 8" drivers.
Still thinking about sensitivity, I began searching and modeling tons of drivers, hi-fi and pro. This is the power region, I want an effortless sensation of power, but also drivers that start and stop fast by CSD, are well-behaved above XO, don't have ripples in the impedance response in the intended frequency range, etc. I learned a lot, including:
- graphic demonstrations of Hofman's iron law- if I want sensitivity in the midbass I won't get deep extension and am looking at a 4-way(!)
- low Qts pro drivers roll off very high in sealed enclosures...
- finding good measurements of pro drivers isn't as easy as going to hificompass.com...
- B&C seems to have good correlation with their datasheets, 18Sound is somewhat hit or miss, and I don't have much confidence in Faital Pro.
I also realized that if 2 of the midbass drivers' C-C was < 1/4λ at 500 Hz I could possibly put them side-by-side under the mid-dome, increasing sensitivity, SPL, etc.
After all of that, here is a list I have come up with. If anyone has gotten this far and has any thoughts, I would love to hear them. I am ready to purchase and put this decision behind me. I am thinking of these drivers covering 80-120Hz up to 500Hz (the Satoris would extend deeper). One more driver will go below. And I have 5 JBL sealed 10" studio subs distributed in the room.
Thank you,
Bill
You could add one of the SB Audience Nero 10 or 12mwn700d drivers to the mix, even tho they don't go that low. All of the mwn700d series (including the 15") appear to have the same motor. High efficiency & power handling, fairly light paper cone, Fs 40~55 Hz, 7mm Xmax, robust motor & frame. Modest cost for what you get. The 12 is a very nice, neutral, dynamic driver.
I used well braced 2cf 44 Hz ported boxes lined with 1.5~2" thick foam. Near the wall & mounted high in a ~5000cf room, they go deeper than they should even near 110 dB spl. Live voices speaking into ordinary stage mics are produced with startling clarity. Extremely dynamic. Probably more so than any home HiFi woofer I have used.
Do you think that it could partly be that the speaker system designers were really talented and did a great job with the overall design, that allowed the great mid-drivers to really shine? I say this partly because it seems that really expensive speakers don't always live up to their inflated price tags, and some more modest ones seem to get things right. It seems like careful overall design is a big factor. Not trying to be contrary, just hope to provide some perspective.
They are a 10" sealed design, right? Do you know the woofer/mid cross point?
I have had a pair of those around for a while. One mid is dead. I might replace both mids with HIVI DMB-A. Since I have already suffered the learning curve with regard to the DMB-A, maybe the new crossover will not be too hard. The woofers need surrounds, and I would probably use new tweeters. The walnut cabinets are nice, but need new finish. It would be good to protect the DMB-A dome somehow; a typical round wave-guide for it is a no-go according to my experiments---it will only accentuate the problematic 4300hz null.
Maybe another, better behaved mid, could be made to mate with the existing Phillips wave-guide?