First of all impedance of the two drivers: Nice! Both having a Fs of around 44-45 Hz. The coating of surround works fine and I hadn't expected to hit the same Fs for the drivers. Remarkably there's only a minor increase in impedance towards higher frequencies. This is usually only seen from symmetric drive (SD) speakers where the pole piece is covered by thin copper. As can be seen from the TS data sheet below, the induction at 10 kHz is low.

It appears that the Philips claims are not all that wrong and what's depicted above can take any shape depending on actual placement in your living room. If I put the speaker close to the wall, I would probably get even more response in the bass and lower midrange area. My workshop has a severe dip at 60-70 Hz, so this will account for lack of response in this area. However, it doesn't relieve the 9710 of having a severe peak at 3 kHz.

Above is seen the frequency response of the 9710 driver alone with no crossover attached.
I've recently had a mail exchange with a fellow running two 9710s in series on an open baffle and I'm still wondering how he can live with this. The open baffle may sound very different from what I hear from my boxed 9710s, but it won't remove the peaky treble. The frequency response of the 9710 not only looks awful on paper, it sounds awful too - to my ears. 10 dB peak at 3.5 kHz! Exactly where the ear is most sensitive! I honestly don't get it. As said previously, it's not an ear shredding kind of peak, but this is by any standard low-fi. I'll call it amplitude distortion as it surely changes the way instruments and voices sound.
I did survive this in the late Sixties having the speakers at the corners and on the floor of my room, where on-axis listening was impossible unless I was lying on the floor. And my B&O amp had tone controls... I don't recall if I used to lower the treble. However, the 9710 didn't last long and in came Peerless 3-25 kits, fitted with 12" bass drivers, 5" midranges and cone tweeters. Much better. Well, my hifi story some other time. Had I been able to make a crossover as seen below, my hifi career might have looked very much different. But we didn't have personal computers, CLIO and LspCAD back then.

Frequency response of individual drivers. The Peerless tweeters are connected in parallel and the FR is measured right between the tweeters at 1 metre distance. Placing a pair of tweeter on a horizontal axis has its problems as we will see later.

Impedance of drivers. Note the remarkably flat impedance of the two Peerless tweeters in parallel. Blue and yellow is phase. Vent tuning is 43-44 Hz.

Here's the final system impedance (crossover V4) from LspCAD. 6.3 ohms minimum where it hurts and a minimum of 4.5 ohms at 3 kHz. This is a dream for every low wattage valve amp.

Crossover version 4. Producing a linear response from the 9710 proved much more difficult than anticipated and quite some tweaking had to be tried before rendering what is seen above.
Every large 8" driver - and the 9710 is a large driver - will start to sound honky when it has to handle the upper midrange/lower treble. BUT, honkyness is not only a matter of dispersion, but also on level and not having an elvated level at 750-1500 Hz is of paramount importance. My V2 crossover ran for some time and I thought there wasn't much to do about it, but bypassing the first 1 mH coil with a 47 ohms resistor and bypassing the second coil with (2R7 + 1.5 uF) helped smooth this troublesome area and surprisingly the forwardness and slight honkyness was much reduced. Maybe we really can have an 8" running everything up 2.5 kHz... Well, it's never going to be like what a 4" can do, but possibly something we can live with.
The tweeters took a little equalisation too, hence the 0.1 mH paralleled by 2R7 at the entry.
Phase tracking may not be the best I seen, but I value amplitude higher than phase tracking here. The 9710 is lively cousin, and if you don't treat it right it'll shred your ears.

So how did the modelling fit with reality? Very good indeed as depicted above. Now the overall response is basically flat and vocals have improved compared to previous crossover versions. Red = FR, blue = minimum phase.
A guy visiting me brought along an MingDa MC34-A integrated amp (EL34 pp) and we ran the 9710s from this amp and don't say hifi can't be simplistic: An amplifier like this and a decent pair of 92 dB/2.8V speakers. Try Chris Minh Doky: "Listen Up", the intro track, and you'll jump in the seat when the drummer hits the snare drum hard. And try an old Garrard 401 turntable + the Denon 103 cartridge and you have a system that we could have listened to some 40 years ago. Amazing.
System sensitivity is 92 dB/2.8V based on average midrange response.

Vertical dispersion. Very good indeed.
Red is at 1 metre distance at a height between 9710 and tweeters. The other graphs at +/- 10 and 20 cm from this position corresponding to +/- 30 and 60 cm at 3 metres listening distance.

Horizontal dispersion. Very bad indeed.
No surprise, placing two tweeter on a horizontal axis will produce this picture. Red = on axis, yellow = 10°, purple = 20° and green = 30°.
But it's not as bad as you may think. This is what a single, tiny microphone capsule picks up at a specific position, not similar to what our two ears perceive at different locations. But obviously it's not the perfect solution .

Crossover for 9710 and Fountek NeoCD3 ribbon tweeter.
This is a pretty straight forward 4th order filter and if you think 4th order filters destroys transient attack, try this!
And please don't confuse severe peaks in frequency response with good transient performance. This is very often the case.

Removing the whizzer cone has the effect seen above. Red = un-modified and blue = whizzer cone removed and rubber "phase plug" inserted. The whizzer cone is mostly active above 6-7 kHz and the response is reasonably linear up to 16 kHz. Not bad for an 8" driver. It also means the bump above 2-2.5 kHz is smoother and the response declines nicely above 7 kHz.

Coating of the 9710 cone is a serious matter. What type of coating should be used? If it doesn't work there's no way back. I don't have a whole range of coating materials - and not a whole range of 9710 drivers as well - so to cut a long story short, I chose to use the DAMAR coating. The 9710 cone is rather soft and the DAMAR will most likely make the cone more rigid. This may have several consequences: One is that the cone will operate more like a piston and the other is that rigid cones tend to break up - somewhere. Hopefully not in the area where it's supposed to operate. Thirdly the coating material will add weight to the cone and reduce efficiency.
Actually the DAMAR coating worked rather well on this cone, where I in other cases have seen serious cone break-ups around 2.5-4.5 kHz, just where the point of crossover is supposed to be. Seen below is the response from the 9710 before (red) and 7 days after one DAMAR application (blue).

Red = un-coated 9710 without whizzer cone. The frequency response is here merged with the nearfield response at 350 Hz.
What can be seen is an approx. 1-1.5 dB loss in sensitivity due to the coating but also that the steep rise in response above 2 kHz has moved some 4-500 Hz up. Good.

Green = un-treated cone. Red is 5 min. after application of DAMAR. Some serious loss in sensitivity, but most of the "DAMAR" is solvent that will evaporate over time.
Green is after 30 minutes. The response after 7 days is seen above.

Without doing further modifications to the crossover, here's the response of the 9710 + NeoCD3 before and after coating of the 9710.
Red = after coating. Blue = without coating + whizzer cone.

Summary:
1. removing the whizzer cone removes the response above 7 kHz, but leaves the area where I want to use the driver mostly un-affected.
2. Applying DAMAR coating reduces the sensitivity by some 1-1½ dB and moves the bump above 2 kHz up some 4-500 Hz.

So what about the sound?
The sound changes very much.
Getting rid of the whizzer cone immediately makes a more clean sound. The 9710 used as a "common" midbass driver really do not need a whizzer cone but I have a feeling that some will miss the more colourful sound of the original.
Adding a single thin layer of DAMAR coating makes yet another powerful impact on sound. Now this set-up really starts sounding like hifi - so take great care! - but it doesn't relieve the 9710 of having a sound much dictated by the very deep and conical cone.
The DAMAR coating greatly enhances the level of transparency and it is obvious that the very soft paper cones smear a lot of detail.
The 9710 still has "something" that the Monacor SP200X is short of. The sense of speed and sparkle. Monacor.
The transient attack of the 9710 is very hard to find elsewhere, but it comes at a price.

It has been great fun reviewing the 9710s here. I've enjoyed it a lot getting my old Sixties' drivers back and finding out what it was I was listening to back then.
The 9710s are not the end of story and I'm sure we can find better drivers today built from the same concept of a very lightweight 8" paper cone and maybe I should take a look at those Supravox drivers and find out what they can do.