Exploring Purifi Woofer Speaker Builds

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#1,061
bcodemz said:
The driver parameter may not have changed, but the box changed, and therefore the lumped parameters of the box + driver has changed. This is why the excursion is different, and why the response shape is different depending on the box volume. If you normalize for excursion, you'll see the SPL is exactly the same for a given amount of excursion and frequency no matter the box size.



This is definitely not true. You can easily see this through your own example. Take a 100L subwoofer box built with 1" MDF, which is very common. By your logic this is the same as a 4L cabinet with 1/25" walls, or 1mm thick walls. Clearly that's not the case.

Stiffness is the product of the material modulus of elasticity and the section moment of inertia. For a rectangular cross section, the moment of inertia is (wh^3)/12, where w is width and h is depth. The important part to note is that the relationship between stiffness and thickness is cubic, meaning if you double the thickness, the stiffness increases by a factor of 8. If you triple the thickness, the stiffness increases by a factor of 64.
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The excursion is only off by 0.2mm between the 4L and 7L.

Yeah and a 100L cabinet made from 1” MDF would sound super tubby. Unless it had very extensive bracing. So what you're saying is the benefit of making a smaller cabinet with the same material thickness is much greater than I was thinking.
 
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#1,062
Ok I did the 7L over with exactly 12mm excursion. Added about 0.2dB SPL.

Purifi 7l.JPG
 
#1,063
fluid said:
There is a difference in excursion between the 4L and 7L simulations that would favour the 4L in output terms.

If you look closely the 7L does not get to 12mm and at 30Hz the difference between the two is about 1mm extra excursion in the 4L box. 7L is 7mm as shown below, 4L is at 8mm.

View attachment 1005226
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Ok now I see what you're saying. Even after I redid the 7L. I wasn't looking at the 30hz excursion. I was looking at 10hz. Which isn't realistic because not much music has frequencies that low. So yes at 30hz the excursion is lower with the bigger box sizes. So I suppose these drivers are capable of much more SPL yet with actual music before bottoming out.
 
#1,065
Amplifier dude said:
That’s about 11.8mm and the 4l is 12mm. Would have made about a 0.2dB difference in spl if I added the tiny bit more power to hit 12mm.
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Odd that the difference at 30Hz was more like 1mm. The low frequency output of a driver in a sealed box is solely governed by Vd which is a property of the driver and not the box. You can trade box size and amplifier power but the max output is the same in both cases.

You seemed to be suggesting that Vd is not the limiting factor, which is why I posted.

In the sims above the group delay responses are different and so is the rolloff shape from the Q change, if those were equalized the level differences should go away too.
 
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#1,066
Amplifier dude said:
Ok now I see what you're saying. Even after I redid the 7L. I wasn't looking at the 30hz excursion. I was looking at 10hz. Which isn't realistic because not much music has frequencies that low. So yes at 30hz the excursion is lower with the bigger box sizes. So I suppose these drivers are capable of much more SPL yet with actual music before bottoming out.
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Sorry I didn't see this when I posted above.
 
#1,067
lrisbo said:
hi Paul and mainframe99,

Regarding the 'compression distortion': briskly correctly explained the problem above. The concern is that the excursion of the driver will modulate the box volume and thus cause a nonlinear air compliance. However, this nonlinear spring will simply add to the drivers own typically nonlinear suspension and the net effect will still typically be domniated by the drivers typically progressive suspension. Such Kms(x) disortion mostly causes harmonic disortion down towards fs and below (where it is not much audible) and very little IMD higher up (see our blog post Low frequency harmonic distortion is almost inaudible. So what’s the point of low distortion drivers? - PURIFI). Again, this mechanism comes from changing the box volume and has nothing to do woth the drriver design.

A related issue is that the driver surround deforms in a non linear way due to the high back pressure in a small selaed box - essentially a nonlinear modulation of the the drivers effective radiating area Sd. This causes a substantial modulaiton of the air spring stiffness (stiff ness of the piston is proportional to Sd squared).

For this back pressued induced Sd modulation, our Sd neuttral surround is much more robust to back pressure than a normal half roll so in that way the PTT6.5X is more suited for small sealed boxes than most other drivers.

In the end, everyone can measure the distortion coming out of a driver in a sealed box and that is what counts. Mark Bushinsky has published numerous such measurements in this forum including IMD and the PTT6.5X.

Cheers,


Lars
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My favourite part of that blog post is “armchair psychoacoustician”. Good thing these guys don’t exist on audio forums 🙂
 
#1,068
Amplifier dude said:
My favourite part of that blog post is “armchair psychoacoustician”. Good thing these guys don’t exist on audio forums 🙂
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I agree with your idea that purifi woofer is a great candidate for sealed active speaker (especially the new alu version). Your plan is an in wall design but what do you think of a middle size active speaker with a waveguide/tweeter + 6.5 purifi sealed (but without EQ, maybe f3=85) and a 8 inch sealed subwoofer (or two) in the back (for instance sb23mfcl in 10liter) ? With your dsp capabilities, it will allow some control directivity ?
 
#1,070
nanou64000 said:
I agree with your idea that purifi woofer is a great candidate for sealed active speaker (especially the new alu version). Your plan is an in wall design but what do you think of a middle size active speaker with a waveguide/tweeter + 6.5 purifi sealed (but without EQ, maybe f3=85) and a 8 inch sealed subwoofer (or two) in the back (for instance sb23mfcl in 10liter) ? With your dsp capabilities, it will allow some control directivity ?
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The cabinets I shared pictures of will have another layer over the baffle, with a waveguide, and special diffraction elimination material. However that’s just a speaker for smaller rooms. For my flagship in wall speaker I’m going to with 4x of the new aluminum Purifi 8” woofers in a small sealed cabinet. With the new 3” Beryllium Bliesma mids (which I have on the way BTW) and the T34B in a MTM configuration. For subs I’m using the 15 and 18” BMS woofers. Until Purifi comes out with something better that is. Next down from that will probably be dual Purifi 8’s and single 3” Bliesma mid with T34B tweeter.

For Atmos theatres the specs call for 105dB continuous from the front mains and centre channels at 2/3rds the way towards the back of the theatre, seated in the middle. So for theatre applications, the speaker choice will be contingent on the room size.

Also for the centre and front mains in an Atmos Theater, F6 @ 40hz is all that’s needed to meet spec. So will be able to pull that off with the 8’s with minimal boost.

https://professional.dolby.com/siteassets/cinema-products---documents/dolby-atmos-specifications.pdf
 
#1,072
There’s only 4 areas in high end audio that have shown strong year over year sales growth over the last 3 years. Listed below:

1: Headphones
2: Portable Bluetooth speakers
3: Active sound bars
4: In-wall speakers

So based on this reality, the only interests I have for commercial manufacturing would need to fall in one of those categories. I’m not much of a headphone guy. And that market is flooded with them anyways. So that only leaves the other 3 categories as viable. The 4” Purifi along with the upcoming smaller Purifi cone drivers will be interesting for categories 2&3.
 
#1,073
Amplifier dude said:
There’s only 4 areas in high end audio that have shown strong year over year sales growth over the last 3 years. Listed below:

1: Headphones
2: Portable Bluetooth speakers
3: Active sound bars
4: In-wall speakers

So based on this reality, the only interests I have for commercial manufacturing would need to fall in one of those categories. I’m not much of a headphone guy. And that market is flooded with them anyways. So that only leaves the other 3 categories as viable. The 4” Purifi along with the upcoming smaller Purifi cone drivers will be interesting for categories 2&3.
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Cat 2 and 3 usually cost the same as one or maybe two Purifi driver(s)... so how does that add up?
 
#1,076
High end audio in the traditional form factor is for the few. That’s why nobody’s buying it anymore. These form factors are the only thing that the many will accept these days.

There isn’t less lovers of great sounding music today than there was 40 years ago. They only moved with the times regarding electronics form factors. That’s why the flat screen TV made the CRT obsolete within 3 years of launch.
 
#1,077
the other thing is that has enabled such amazing sound in major developments in signal processing, class D amplifiers and battery technology, better understanding of psychoacoustics. Better sound has never been cheaper, smaller, and more affordable.

If you transported anyone from 1981 to 2021 to listen to the latest 1L battery powered wireless speaker in mono, stereo or multichannel they would be gob-smacked! What wizardry or magic is going on here!?

Convenience and quality can go up AND price down.

As for the "high end" ("I'm not technical, I just want a high standard of excellence to fit in with our decor") has been about custom installs (invisible speakers ) for at least 2 decades now.

The acceleration of the decline of movie theatres and increased adoption of streaming video at home means in-wall and in-ceiling is only on the way up...
 
#1,079
fluid said:
Odd that the difference at 30Hz was more like 1mm. The low frequency output of a driver in a sealed box is solely governed by Vd which is a property of the driver and not the box. You can trade box size and amplifier power but the max output is the same in both cases.

You seemed to be suggesting that Vd is not the limiting factor, which is why I posted.

In the sims above the group delay responses are different and so is the rolloff shape from the Q change, if those were equalized the level differences should go away too.
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Correct me if i am wrong, but i think that the long term max output would not be the same, because compensating lovered F3, caused by the smaller and therefore stiffer spring working in the smaller enclosure is very steeply raising the thermal load of the driver, which is also higher in smaller enclosure for obvious reasons, so the driver will eventually reach its point of thermal compression, where the output is dropping and the distortion jumping up much sooner than without the DSP correction and naturally lower F3 in bigger, naturally also more heat absorbing (dissipating) enclosure.
 
#1,080
The IEC268-5 18.2 power rating is based on a 28 minute test with pink noise. And the IEC268-5 18.2 power rating for the 6.5’s is 250w. In my simulations only 200w peak is applied to the woofer To reach that excursion level. And with music you don’t run at the peak continuously.

Also a key takeaway for me from this Purifi blog:

https://purifi-audio.com/2019/12/12/imd/
“The science was right: even gross bass distortion is remarkably difficult to hear. The fine print says yes, provided it stays in the bass.”

Which is why I run my woofers up to 120hz, and mid from 120hz. This keeps 100% of the distortion out of the audible range.
 
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