Purifi-based monitor - options

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IslandPink
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#46 Re: Purifi-based monitor - options

Post by IslandPink »

Nice presentation of what the Purifi driver does that others don't :
https://audioxpress.com/article/voice-c ... ed-solving
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#47 Re: Purifi-based monitor - options

Post by IslandPink »

Some measurements here - from approx 1m away :
First the full combo, then the Purifi MidBass only.
Not quite what I expected actually, although the 3 to 10kHz area is a bit raised. What I'm not seeing is an obvious lump around the 3 to 4kHz area where the Purifi ought to be sticking up in the mix. Odd.
Purifi_Monitor_2way_i_29JulPM.jpg
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Purifi_Alone_i_29JulPM.jpg
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#48 Re: Purifi-based monitor - options

Post by vinylnvalves »

That quite a respectable FR, yes maybe 2db to hot at the higher end. That’s not a bad thing for a more mature listener, who has lost some sensitivity to higher frequencies.

What’s the phase and delay look like as nothing on this plot which would suggest a harshness, at 3k . Chris may be able to add more insight. What’s causing the 100-200hz suck out - floor interaction?
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#49 Re: Purifi-based monitor - options

Post by Scottmoose »

I'm not Chris, but until he is able to provide feedback worth the reading, a couple of notes:

The rolloff is quite consistent; the Purifi unit doesn't have a great deal of excess output or major issues with undamped cone / suspension breakup in the 1KHz - 4KHz region; diffraction effects notwithstanding, just a gradual c. 3dB natural rise to about 3.5KHz. While it doesn't have the flattest of impedance curves (it can't with a coil of that size) it's very linear under more complex analysis and because the driver naturally rolls off above about 3.5KHz a relatively simple filter will tend to produce a more consistent acoustical slope than you'd be able to achieve with some others drivers, even without a correcting RC Zobel.

A nominal 1st order (6dB/octave) rolloff though can (can) expose you to some other characteristics. Withouth having full measurements I can't really comment on, for e.g., the phase interaction & on / off axis system responses, both of which can cause some subjective 'hardness' as consistency drops away. However, one point I would note is that the Purifi has a small peak in HD3 at about 1.7KHz and a similar peak in HD5 at about 2.4KHz -resonant amplified HD from its minor stopband cone modes. Taking nothing away from the excellent Purifi unit (which is about as good as it curently gets for a 7in midbass) without sufficient suppression those brief spikes may (may) cause some subjective colouration under certain conditions.
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#50 Re: Purifi-based monitor - options

Post by vinylnvalves »

Scott, I cited Chris for his measurements skills, being able to spot when measurements aren’t as they seem. Your perspective is always enlightening and educational.
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#51 Re: Purifi-based monitor - options

Post by IslandPink »

Quick comments, I'm out for the rest of the day.
Steve, i will correct the treble output, it sounds a bit hot to my ears, and this is going to be a monitor for Jon's studio so we want it flat.
The big lower-mid suckout is annoying and seems to be very common in this room with anything I test.
I hope to do some tests in Jon's studio, later. There's nothing I can do about it and it shouldn't be there ref the driver or the cabinet so I assume it's the room. I did have a big seat cushion on the floor for most of the tests here, though that was mainly helping the response from 300 to 1.5kHz.

I did have a look at the distortion on the last couple of runs , but didn't save it. I couldn't seen anything noticeable in the 1 to 5kHz region, it was all well down towards the noise floor.
The amounts & freq you quoted there Scott, are those from other measurements ( eg. HiFi Compass ?) because I can't see those exact peaks on the Purifi data sheet. They show some HD3 at 1.2 , 2.5 and 6kHz, but at 60 to 70dB down when the driver is playing at 94dB/1m.

ps. I need a better amp to drive this, I only have a 7wpc chip amp , I'm hoping to get a loan of Jon's new amp soon. So, there could be some harshness from the amp.

What's odd to me is that the woofer looks like it's got a low pass around 1.2kHz, it's already rolled quite a bit by 2kHz, whereas I calculated a Zobel plus a 2kHz LP on this first-stab. Given that the driver response goes up by maybe 4dB to 3.5kHz, I would have expected the response only to be down a couple of dB at say 4k, but it's a lot more than that.
However, I'll take it !

I'll do the tweeter adjust , and do some more measurements with distortion and a couple of off-axis scans, later in the week.
Last edited by IslandPink on Sat Jul 30, 2022 12:54 pm, edited 1 time in total.
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#52 Re: Purifi-based monitor - options

Post by IslandPink »

ps. I'm not sure the roll off above is 15kHz is quite that strong. I'm just using the standard calibration file for the ECM 8000 mic, and I saw a graph for the production spread on them, and they do flap around a lot in that region, so it may be that the calibration is off for this example.
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#53 Re: Purifi-based monitor - options

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IslandPink wrote: Sat Jul 30, 2022 12:48 pm ps. I need a better amp to drive this, I only have a 7wpc chip amp , I'm hoping to get a loan of Jon's new amp soon. So, there could be some harshness from the amp.
It just occurs to me that I should dig out the Pass F4 amp. I think there's a fair bit more gain I haven't used, from the Behringer interface box, so it might work OK. That will give me 25 wpc.
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#54 Re: Purifi-based monitor - options

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IslandPink wrote: Fri Jul 29, 2022 10:56 pm Some measurements here - from approx 1m away :
First the full combo, then the Purifi MidBass only.
Not quite what I expected actually, although the 3 to 10kHz area is a bit raised. What I'm not seeing is an obvious lump around the 3 to 4kHz area where the Purifi ought to be sticking up in the mix. Odd.

Purifi_Monitor_2way_i_29JulPM.jpg

Purifi_Alone_i_29JulPM.jpg
Mark,

I've noticed that the traces you've posted are averages. What measurements were averaged?

If any of them were off-axis, a steeply-declining off-axis response could pull the average down quite easily. You can weight the averages by repeating the measurement(s) you wish the average to be weighted towards.

ie, if you want the average to be mostly on-axis, but with some off-axis consideration, you might take two or three on-axis measurements and one off-axis, and average those. The resulting average would be closer to the on-axis response.

FWIW, >10kHz, mic orientation becomes relevant.

Chris
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#55 Re: Purifi-based monitor - options

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They were averages of about 6 or 7 axial or near-axis measurements. I just did small movements of the microphone and a couple of shifts of the loudspeaker, mainly sideways. Just tried to get a bit of a change in the room modes.
However , all these were within 5° of axis .

Thanks for the tip on Mic orientation, I'll look at it more carefully, but it's always a faff getting the mic to 3 or 4 feet off the ground with a standard microphone stand !
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#56 Re: Purifi-based monitor - options

Post by chris661 »

Further thought: the dips in the lower-mid/upper-bass area might be floor and/or ceiling reflections. I ended up adding another driver near the floor to fill in the dips - https://www.audio-talk.co.uk/phpBB3/vie ... php?t=8257

Which worked well. The speakers gained a solid upper-bass "slam" which was missing previously.

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#57 Re: Purifi-based monitor - options

Post by Scottmoose »

IslandPink wrote: Sat Jul 30, 2022 12:48 pmThe amounts & freq you quoted there Scott, are those from other measurements ( eg. HiFi Compass ?) because I can't see those exact peaks on the Purifi data sheet. They show some HD3 at 1.2 , 2.5 and 6kHz, but at 60 to 70dB down when the driver is playing at 94dB/1m.
See Yevgeniy's measurements under 2.83v; you'll spot them. What's interesting of course is that the Purifi drivers are one of the handful (they do exist) of MC drive units where some minor modes don't make themselves obvious (or even known) in some way on the impedance curve.

Minor note re the box drawing: that slot-duct is of a very high aspect ratio; you'll have some frictional / boundary losses creeping in so the box may end up with a lower Fb than expected.
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#58 Re: Purifi-based monitor - options

Post by IslandPink »

Thanks yes, had a look at his measurements now, see what you mean.
I need to do more listening once i've got the treble down another dB ( just about to do some soldering )

I was wondering myself about the slot port. Maybe that's an advantage ? Not sure how you'd simulate it, but main plan is just to listen. Will be able to compare the Mk.1 and Mk.2 cabinets side by side fairly soon.
chris661 wrote: Sun Jul 31, 2022 2:03 pm Further thought: the dips in the lower-mid/upper-bass area might be floor and/or ceiling reflections. I ended up adding another driver near the floor to fill in the dips - https://www.audio-talk.co.uk/phpBB3/vie ... php?t=8257
Thanks. That's another project and an additional £750 !
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#59 Re: Purifi-based monitor - options

Post by IslandPink »

Just adjusted the LPad resistors to 2.3 into 2.5 on the tweeter.
Seems closer, has bumped up the performance usefully.
The comments on the crossover are very informative.
I'm looking at that 'bottomless notch' on the woofer and wondering how it works - I will play around with it on LTSpice later.
I have a similar problem understanding that as I have with the 'series notch' that is often shown across a driver !
Anyway - could be easy to clip-lead something in there and see if I can kill this occasional 'snarl' that i can hear in the treble.

Edit : I just got the series notch etc. sussed, I was just doing something stupid on Spice.
Last edited by IslandPink on Mon Aug 01, 2022 8:33 pm, edited 1 time in total.
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#60 Re: Purifi-based monitor - options

Post by chris661 »

The bottomless notch, in that implementation, is a network which will give a near-dead-short at a particular frequency.

That gives maximum electrical damping at that frequency, and (via the woofer inductor forming a potential divider) puts a deep notch in the frequency response. Without the bottomless notch, the breakup peak is attenuated by about 40dB. When you consider that it was around 12dB up on the nominal response to start with, that means it's only 28dB down. I suspect that's on the borderline of audibility, but I haven't done much testing in that direction.

The notch pushes it down by another 20dB, to almost 50dB below nominal. I'm pretty sure that will be inaudible, but, again - not tested.


It's worth noting that the Seas driver I'm using has a whole series of resonances above 4.5kHz. It's just that 4.5kHz happened to be the strongest in terms of SPL, and was closest to the crossover point. The 3rd order slope takes care of the resonances further up the range.

Chris
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