Test Purifi Audio PTT6.5W04
Copyright 2020-22 © Troels Gravesen

I had a pair of drivers from Purifi Audio for testing, PTT6.5W04-01B, 6½ inch, 4 Ohms drivers. The name was later changed to PTT6.5X04-NFA-01.
They may look like any other 6" drivers except for - and very much so - the rubber surround. This rubber suspension has indeed a very odd shape and is meant to eliminate the all too common edge resonances, which I've been fighting for years. Read here.
These resonances in the surround can be a real pain, not only in the design of the crossover, but certainly also in terms of sound, as any rapid phase shift in upper-mid/lower treble is clearly audible. Not necessarily unpleasantly, but nevertheless distortion of the music we feed our drivers. And we can only know what it does once it's gone.

There are a number of well known people behind this driver - and so far, two other drivers, the same as here in 8 Ohms impedance and one 4" midrange driver. Also produced is a passive unit meant for the 6" drivers. I suggest taking some time reading all the info on their website. https://purifi-audio.com/
There's a thorough introduction to the people behind the company and a more thorough test of the same driver as will be presented here: https://hificompass.com/en/reviews/purifi-audio-ptt65w04-01a-midwoofer

But to learn what a new driver can do and not do we need to perform our own testing under familiar conditions. What tickled my curiosity was obviously the promise of a ruler flat response in upper-mid/lower-treble as I had drivers before with "funny" surrounds that overcame this problem.

Conclusion:

What we have here is a driver that indeed has a ruler flat response in the critical areas, which is revealed in my frequency response measurements and also - and not least - impedance measurement. Not a single wrinkle can be seen from the impedance profile. It may not be flat compared to drivers featuring fully Symmetric Drive, but nothing indicates any resonant troubles.

Despite its 4 Ohms impedance, this is not a particular sensitive driver managing only 88 dB/2.8V/1 meter, which translates into something like 85 dB for 1 watt. And I guess it wasn't mean to be, rather a 6" driver that could do some serious bass due to its Xmax of +/- 10 mm given its 24 mm voice coil length and rather narrow 4 mm magnet gap. Quite some Xmax for a 6" driver with only 133 cm^2 membrane area.
Such heavy excursion will impact midrange quality, but from a 2½-way from the 8 Ohms versions, we may be able to produce quite a soundstage nevertheless and a decent overall system sensitivity. From a single 4 Ohms driver on a narrow baffle given its baffle step loss, I reckon we can manage a system sensitivity of 85 dB with some room-gain helping it out in the low end. See simulation below.
I would like to see a similar 8 Ohms driver optimised for midrange with a shorter voice coil and higher sensitivity.

Going through the list of persons involved in this project, it is also clear this is not a niche project aimed at high-efficiency SET nerds, rather digital people with high-order filters and tonnes of digital watts available. And it's aimed at an audience asking for quality small speakers with decent bass extension, that is - the majority of the consumer market.

Purifi claim superb distortion properties at all levels, thus I've made distortion test from very low to quite high signal input. And I wasn't disappointed. Very much the same picture regardless of sound levels from ~79-94 dB/1 meter. I do not have an anechoic measuring chamber, so please disregard figures below 300 Hz. Also at very low levels we see high distortion above 7 kHz, which is an artefact. This driver is not meant to be used above 2-3 kHz.

Download Purifi data sheets here:

PTT6.5W04-Data-Sheet-1.01.pdf (the driver tested here) - and now PTT6.5X04-NFA-01
PTT6.5W08-01A-Data-Sheet-0.92.pdf (8 Ohm version of the above)
PTT4.0W04-01-Data-Sheet-0.91.pdf (4" midrange driver)

SPK5-Crossover-App-Note-1.22-1.pdf
(2-way construction with crossover details. Strangely enough no cabinet details included)

A friend said: My God, this is plain ugly!
To be honest, I don't give a damn how it looks as long as it doesn't distort my music!

Not only does this driver have an unusual rubber surround, but also a triangle of paper glued to the rim of the cone - on the back side.
All sails set to eliminate edge resonances.

This driver also has an unusual voice coil winding, apparently 3-4 layers and not evenly distributed over the voice coil bobbin.
Click image to view large.

The driver flush mounted on my 100 x 120 cm test baffle.

Simulation from the excellent ScanSpeak excel file.

SPL of the two drivers on my 100 x 120 cm test baffle.
Not much we can say about that, except exceptional linear midrange all the way up in the treble range.

Impedance from the two drivers, measured in free air. Fs around 32-33 Hz.

Dispersion at 0, 10, 20, 30 and 40 deg. off-axis.

Waterfall plot. Small glitz at 4 kHz as seen below,.

Distortion measured at a level equivalent to 79 dB/1 meter.

Distortion measured at a level equivalent to 82 dB/1 meter.

Distortion measured at a level equivalent to 85 dB/1 meter.

Distortion measured at a level equivalent to 88 dB/1 meter.

Distortion measured at a level equivalent to 91 dB/1 meter.

Distortion measured at a level equivalent to 94 dB/1 meter.

2023:

After having released the Purifi-6661 speaker construction I made a comparison on my 80x120 cm baffle of the PTT6.5X04-NFA-01 and the PTT6.5W08-NFA-01 and this is what it looks like.
The "X" is indeed a bit easier on crossovers than the "W".

Here the same from 200-10000Hz.
These drivers do have an exceptionally smooth and linear midrange.