Can you guys provide some feedback here? This is a raw measurement of the Purifi PTT6.5X04-NFA-01 6.5" woofer in a 14 liter box with 2x 6.5" Purifi Passive Radiators. The box is solidly built with bracing and double walls. Does this look like a normal measurement? The low end is so much quieter it makes me wonder. This measurement was taken in my garage so there are clearly reflections but you get the idea of what's going on.
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Yes, tuned way too low; with a 0.26 Qts/low Vas this is compression driven prosound horn, BR specs driver, though even a max flat BR is tuned high, so normally a mid-bass driver for HIFI:
Vented net volume (Vb) (L) = 20*29*0.26'^3.3 = 6.8 L
(Ft^3 = (Vb)/~28.31685)
Vented box tuning (Fb) (Hz) = 0.42*33*0.26'^-0.96 = 50.5 Hz
F3 (Hz) = 33*0.28*0.26'^-1.4 = 60.9 Hz
(Qts'): (Qts) + any added series resistance (Rs): https://web.archive.org/web/20220707003028/http://www.mh-audio.nl/Calculators/newqts.html
Vented net volume (Vb) (L) = 20*29*0.26'^3.3 = 6.8 L
(Ft^3 = (Vb)/~28.31685)
Vented box tuning (Fb) (Hz) = 0.42*33*0.26'^-0.96 = 50.5 Hz
F3 (Hz) = 33*0.28*0.26'^-1.4 = 60.9 Hz
(Qts'): (Qts) + any added series resistance (Rs): https://web.archive.org/web/20220707003028/http://www.mh-audio.nl/Calculators/newqts.html
Is it possible to supply some details of the measurement set-up that was used? Were gated measurements used? If so, what was the length of the time window, and were any sound reflections included? What microphone was used and is it relatively flat down to 20Hz or so (with a calibration file if needed).
Your box build seems to follow the Purifi-suggested box dimensions to a considerable extent. I would expect the present enclosure to get close to Purifi's low-frequency cutoff specifications.
I'd suggest doing a near-field SPL measurement of the woofer to get an idea as to how it performs in the present enclosure. Follow this with near-field measurements of each of the two passive radiators. Taken together, those should give us a better overall idea as to what's going on to see if it looks normal. Try to keep the microphone-to-cone distance as identical as possible, with a long measurement window to get good low-frequency resolution.
My measurements are being taken at 1 meter on tweeter axis with the speaker about 1 meter above the ground. I'm using a Scarlet interface and an Earthworks M30 microphone with calibration file. The measurement was taken in my garage so there are some reflections but the overall picture is clear with it. I have taken near field measurements and they give the same picture. I see a 6db baffle step loss, which is normal but there seems to be another 3db loss of the bass region. Does this happen with the dual passive radiator alignment? This is what I'm trying to answer here.
Here's the modeled bass response. I've made some assumptions on PR alignment - that is no added mass, using Purifi 6.5" PR being PTT6.5PR-NF1-01.
Obviously a quasi-anechoic measurement would be best to remove all room modes / reflections to get a cleaner idea. I am unsure how parallel PRs are measured and summed in this approach.
There is roughly a 2dB drop from 300Hz to 40 Hz.
Edit - add any mass to these PRs, and you'll get a droopy bass response, eg. 20gms added (per PR I assume in VCad):
Obviously a quasi-anechoic measurement would be best to remove all room modes / reflections to get a cleaner idea. I am unsure how parallel PRs are measured and summed in this approach.
There is roughly a 2dB drop from 300Hz to 40 Hz.
Edit - add any mass to these PRs, and you'll get a droopy bass response, eg. 20gms added (per PR I assume in VCad):
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Seems reasonable to me; its upper mass corner is ~2*Fs/Qts = ~254 Hz, so the PRs are suspended off the end of a long, sagging lever so-to-speak that is only made worse by increasing net Vb > max flat. 
The type of passive radiator (PR) alignment that Purifi use for the PTT6.5X04-NFA-01 6.5" woofer has 2dB or so dip in the bass relative to the response in the passband. A VituixCAD simulation with this driver in a PR enclosure produces the response shown below.
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@Dave Bullet If not a hassle, could you run the Purifi 6.5-model in post #2546 with same cabinet but sealed? I am working on my newby WinISD modeling skills for a build with separate cabinets for (a) 10-inch Purifi X04 with passive radiators in a 60-70L cabinet below and (b) a sealed cabinet above with 6.5-inch Purifi M04. I have not modeled the smaller cabinet yet since was hoping Purifi would release the SPK16 design. I want to see if I can match your curves… as well as to pick out a crossover point. Though I assume the possible crossover between 10- and 6.5-inchers would be a pretty big range from 250 to 1000 Hz. I have active crossovers. So I could experiment without working in physical copper and caps.
Here's the closed 14L response. If you run these full range, then you will reach Xmax with only 15 watts. If however you high pass these at say 100Hz or more to your 10" drivers, you will not be excursion, but thermally limited (i.e. your 10" will reach Xmax long before the 6.5)
Thank you very much for the examples and advice. Will have more fun with this during an upcoming holiday.
it’s a raw ungated measurement with room modes and without baffle step compensation
the measurement is dominated by room modes so hard to see what the speaker does. you can put a mic in the box and get a very precise measurement of the bass response up to about 150Hz. The lack of baffle step correction is visible.
hi Witwald,
I assume this alignments is from the box tuning info from our web? we are in the process of updating the tables. The aim is to have the response flat to 0.5dB. I improved the optimisation.
cheers
Lars
Yes, I was just about to offer the opinion that @ZestClub 's project did not have enough (or did not have any) baffle step compensation...
This makes sense to me and that's why I think the measurement is normal. I have 6db loss due to baffle step and another 2-3db from the passive radiator alignment. Yes there are room modes in the measurement but you can see the drop. Cheers and thanks!
I know that this is only one measurement and obviously dominated by room modes but it shows about an 8db drop to the low end. I have also seen this with the same woofer in a ported box (though the drop was more like the normal 6db). It seems to me that I'm losing the normal 6db due to baffle step and another 2-3db from the dual passive radiators. Maybe my 14 liter enclosure is closer to 15 and is too big here. Otherwise, I think everything is fine. We know that there is a sensitivity sacrifice with these woofers when we take them down low. I was just wondering if this is normal. It seems that I'm losing an extra 2db of sensitivity below 100Hz with the passive radiators compared to a ported enclosure. I don't really care much about that since more power from an amplifier is easy to get. Plus, near field measurements and in room response shows that I do not need to compensate completely for all 8 or 9db of loss and my room gain will give me plenty of low end. I have been able to get an incredibly flat response and these woofers sound far better than anything I've ever used. I'm just new to the passive radiator alignment so I was wondering if it is in line with what other people are seeing with these woofers.
I don't have a ground plane measurement but this is the impedance of the final speaker with crossover.
