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Introducing Directiva - An ASR open source platform speaker project

Follow up to post#18

UPDATE: The simulated drivers are the 6.5'' Purifi woofer and the SB Acoustics SB26ADC tweeter.
Cabinet is width x height x depth = 216mm x 406mm x 260mm

The order of the graphs is always:
1. sketch of the loudspeaker
2. frequency responses of the tweeter without crossover.
3. frequency responses of the tweeter normalized to 15°

If you arrange the tweeter on the baffle so that the top edge is only half as far from the center of the tweeter as the side edges, you get a slight improvement - version v2
1613426582277.png 1613426602973.png 1613426622225.png


The next step could be to bevel the side edges. So we grab the virtual saw and make a few bevels.
Version v4 with 40mm/30° bevel.
1613426701509.png 1613426724173.png 1613426748549.png
This significantly improves our design.


If you immediately hurt yourself with the saw, you can also grab the sandpaper and start sanding curves (for hours and hours...).
Version v5 shows 18mm rounding of side edges.
1613426880151.png 1613426897848.png 1613426915590.png
This optimization is also a significant improvement over Version v1 and v2.


Now we'll get cocky and add a rounding to the top edge as well. We don't even need to think about that, do we?
Version v6 shows 18mm rounding of side edges and top edge.
1613426948146.png 1613426976200.png 1613426995492.png
The result regarding the directivity is mixed, in the range 4-5kHz there is an improvement, but in the range around 3kHz there is worse directivity.

Now it's up to the marketing department to decide what to do next ;)
 
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The embedded images are very small and for some reason I can't click on them for viewing without getting this error.
1613425696576.png

I can't wait to see where this project goes!
 
TimW said:
The embedded images are very small and for some reason I can't click on them for viewing without getting this error.
Click to expand...
Yeah f.. the diagrams are copied from a private discussion with Rick. Will probably have to re-paste the diagrams - stay tuned.
 
Sorry for the mess!
Now all pictures and diagrams should be visible in post#21.
 
...so (once he saw the bevels) the marketing (engineer) aka me realized he needed to better define what was meant by a "simple" cabinet. My previous modular concept used a minimum baffle approach, so I asked @ctrl about it.

He politely pointed out that "The minimal baffle also has disadvantages. The tweeter dispersion is very wide, which can complicate the transition to the mid-bass driver." I took that as a big NO and tossed both ideas and moved on. He continued to model using the stock Denovo cabinet and its rounded edges and later shared this "The somewhat unfavorable radiation behavior of the tweeter has to be mitigated. The directivity of the tweeter related to the axial frequency response is not good in the range 2-4kHz"' It looked like this...

1613489520001.png

In case you are not familiar with these images, they should be more even and he was referring to the dark red bulge on the right. After a bit of clarification, the remedy amounted to upping the crossover frequency or use a waveguide. Not wanting to push the woofer towards its high end roll-off, had originally targeted a crossover frequency around 2kHz, so was not ready to compromise there. A separate waveguide violated my notion of simple, so despite its nice distortion profile, the SB tweeter got benched.

SB Acoustics had introduced 2 new tweeters with integrated waveguides. One was a Satori was pricey enough to blow the budget and the other was a relative unknown. When @ctrl proposed the Seas DXT, I knew it had a positive reputation, so it became our replacement tweeter. The Seas DXT was also the only tweeter that the entire team had mentioned as potential candidate on separate occasions. In case, you are not familiar with the DXT, it has a small waveguide and a deep chamber. It looks like this...

1613489125994.png

At about $70 each US, they are more expensive than the SB26ADC ($50) tweeter.
 

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Rick Sykora said:
The Purifi woofer is part of this implementation but not exclusive. This is what I intended as part of the flexibilty aspect. The idea is that others can use the initial work and apply the lessons learned. This is why it is an open source approach.

Hope this helps!
Click to expand...

I totally get it, and I think it is a great idea. I will happily read along and learn. I just feel that the Purifi woofer is a rough choice. It values bass extension that is not needed, it requires a passive radiator, and it is expensive. It seems to be such an idiosyncratic choice that I don't know how one would go to another woofer. But, as I said, I guess I will learn all of this in time. Best of luck and thanks to everyone for putting in their valuable expertise and time.
 
Rick Sykora said:
My previous modular concept used a minimum baffle approach, so I asked @ctrl about it.
Click to expand...
You know that such a concept does not work, but you still want to know exactly why not.

Although I do not know how the "minimal baffle" concept should have looked like, my motto here is "10mm plank with tweeter ":
1613433088086.png

Rick Sykora said:
He politely pointed out that "The tweeter dispersion is very wide, which can complicate the transition to the mid-bass driver."
Click to expand...
Well, I probably should have replaced "wide" with "narrow" with regard to a minimal baffle - Mister Omniscient :facepalm:
Because of the strong interference, the frequency response is very similar to a failed open-baffle concept.

1. frequency responses of the tweeter without crossover deg0-90.
2. frequency responses of the tweeter normalized to on-axis FR
3. spectrogram +-180° normalized to on-axis FR (tweeter only)

1613433563410.png 1613433593555.png 1613433855288.png
 
Actually you may want to rethink the cabinet volume and tuning and not try to copy my design. The experienced designers here would know why.
 
Selah Audio said:
Actually you may want to rethink the cabinet volume and tuning and not try to copy my design. The experienced designers here would know why.
Click to expand...

Thanks, am still mulling over the volume. I know there is more than one right answer. UPS just rescheduled the Purifi PRs. I actually want to do both vented and passive radiator. Just a matter of whether I need a change in volume or can get away with one box.

Stay tuned as I forgot to share box internals in my PM.
 
Measurements of the SB26 for those wondering:
https://hificompass.com/en/speakers/measurements/sbacoustics/sb-acoustics-sb26adc-c000-4

Good linearity and power handling/distortion. >10kHz isn’t ideal directivity wise, but that’s fine.

What I don’t get about the 6.5” Purifi is that while Hi-Fi Compass shows it to beam around 1500Hz:
ptt6.5w04-01a_offaxis_normalized_5-30db.png


The SPK5 measured here was nearly flat between 1000Hz-2000Hz even at 90° (crossover is 3kHz):
index.php


So not sure what to make of that.

My issue with the Purifi is the low sensitivity. Sure “watts are cheap”, but the majority of people have <100W 8ohm.

Assuming the measurements we have here are more realistic, then the crossover can be pushed higher than 2kHz, allowing for a wider selection of tweeters.

Looking just at <$60 SB tweeters measured by HiFiCompass, here are alternatives I’ve found with wider directivity <10kHz:

Main negative is higher distortion in the low end, it depends how much is acceptable.

Expanding the search to some other brands:


That last one really took me by surprise. Again, it’s distortion isn’t great, but I can’t think of any tweeter <$200/each with better FR & wide dispersion:
ne19vts-04_315mm_2v83_0grad_0.png

ne19vts-04_offaxis_normalized_5-30db.png


-3dB @ 60° @ 7500Hz, and no crazy resonances/breakups! :eek:

By comparison, the SB26ADC:
sb26adc-c000-4_315mm_2v83_0grad_0.png

sb26adc-c000-4_offaxis_normalized_10-25db.png
 
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MZKM said:
Measurements of the SB26 for those wondering:
https://hificompass.com/en/speakers/measurements/sbacoustics/sb-acoustics-sb26adc-c000-4

Good linearity and power handling/distortion. >10kHz isn’t ideal directivity wise, but that’s fine.

What I don’t get about the 6.5” Purifi is that while Hi-Fi Compass shows it to beam around 1500Hz:
ptt6.5w04-01a_offaxis_normalized_5-30db.png


The SPK5 measured here was nearly flat between 1000Hz-2000Hz even at 90° (crossoveris higher than 2kHz):
index.php


So not sure what to make of that.

My issue with the Purifi is the low sensitivity. Sure “watts are cheap”, but the majority of people have <100W 8ohm.

Assuming the measurements we have here are more realistic, then the crossover can be pushed higher than 2kHz, allowing for a wider selection of tweeters.

Looking just at <$60 SB tweeters measured by HiFiCompass, here are alternatives I’ve found with wider directivity <10kHz:

Main negative is higher distortion in the low end, it depends how much is acceptable.

Expanding the search to some other brands:


That last one really took me by surprise. Again, it’s distortion isn’t great, but I can’t think of any tweeter <$200/each with better FR & wide dispersion.
Click to expand...

Do you know what conditions hificompass took those measurements? Notably, what baffle size?
 
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MZKM said:
Not sure. I just updated my comment again with some other options. The last one is a Vifa one for just $25/ea on PartsExpress, it’s worth testing out.
Click to expand...

Ok, thanks.

As for tweeters, the Seas works fine and likely will finish with it. May consider others once we get first one built and tested.
 
Back when Rick proposed the project, I was hot for the possibility of using a sealed box for smaller size, easy integration with a sub (no high pass filter required) and no port pipe resonances colouring up the midrange.

As it turns out, the 6.5" Purifi's Thiele Small parameters require a very small sealed box size.

Classic acoustic suspension design wisdom is that the air in a sealed box acts as a linear (and linearizing) spring, dominating over the driver's suspension compliance (which has its own distortion causing non linearity).

In reality, the thermodynamics of air show that an air spring isn't really linear and if air is compressed too much, it causes harmonic distortion on speaker's output that can easily exceed the distortion caused by the speaker's suspension.

Linkwitz here estimated the % second harmonic distortion caused by air in sealed box as =0.014*SD (in cm^2)*driver displacement(in mm)/box volume (in L).

I simulated a sealed box for the Purifi, and then used Linkwitz's formula to estimate the 2nd harmonic distortion from the air trapped in the box itself, at different listening levels. Here are the results below, compared to the total harmonic distortion of the Purifi vented prototype measured at ASR.

index.php


The sealed distortion is quite a bit higher than a vented and wastes the Purifi's low distortion potential. I also expect the mids might distort when played simultaneously with loud bass because the air itself is distorting (ASR tests don't measure this).

The 6.5" Purifi is all about playing loud with low THD, but it struggles to pull that off in a sealed box because of its unusual TS params.
 
Back when Rick asked for some suggestions for the tweeter, I offered the DXT as a possibility, but my #1 recommendation was SB26ADC-000-4 for its better distortion performance (posted earlier, my bad).

Rick had initially suggested the SB26STWGC-4. but the top end is rough and would need extra components to EQ (mandatory)

index.php


The SB26STWGC-4 is an SB26STC (well regarded) with a waveguide. The small waveguide it uses just doesn't have enough gain (5 dB) to keep up with the distortion performance of other, better choices:
  • Revel uses a variant of the SB26ADC-000-4, which shows great distortion performance here and here. Great price. Troels makes it work OK (but not great) off axis here. The SB26ADC can be improved with a waveguide such as Augerpro`s free design here. The CAD file shows that the 4in waveguide would fit, and maybe even the 5ìn (see here and here).
  • An oldie but still goodie is the SB26STAC, MarkK tests and here
  • DA25TX00-08 is an option I want to explore some day based on this
  • Seas DXT is a good option for its built in waveguide but it has much more distortion 1-2k than the SB26ADC. I was also surprised to read this diva complaint
I then looked into ring radiators, which would need a waveguide to keep up with the Purifi's power handling capabilities
  • DX25TG09-04 and Monocor WG-300 waveguide, tests results here show very nice directivity and great distortion results. But the waveguide is big and the SB26 has better performance
index.php

  • XT25TG-30 and Monocor WG-300 waveguide, tests results here also show very nice directivity but the distortion is much higher, the DX25 combo wins in the battle of the ring radiators
index.php

The SB26ADC-000-4 with a waveguide is the front runner for me.

CTRLs diffraction models then simulated options. For a much more old school approach to this simulation but one that includes the tweeter's native frequency response as well, see my old tutorial here hosted at Charlie's page. There are better free tools around now, but the design concepts will be the same.
 
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MZKM said:
Measurements of the SB26 for those wondering:
https://hificompass.com/en/speakers/measurements/sbacoustics/sb-acoustics-sb26adc-c000-4

Good linearity and power handling/distortion. >10kHz isn’t ideal directivity wise, but that’s fine.

What I don’t get about the 6.5” Purifi is that while Hi-Fi Compass shows it to beam around 1500Hz:
ptt6.5w04-01a_offaxis_normalized_5-30db.png


The SPK5 measured here was nearly flat between 1000Hz-2000Hz even at 90° (crossoveris higher than 2kHz):
index.php


So not sure what to make of that.

My issue with the Purifi is the low sensitivity. Sure “watts are cheap”, but the majority of people have <100W 8ohm.

Assuming the measurements we have here are more realistic, then the crossover can be pushed higher than 2kHz, allowing for a wider selection of tweeters.

Looking just at <$60 SB tweeters measured by HiFiCompass, here are alternatives I’ve found with wider directivity <10kHz:

Main negative is higher distortion in the low end, it depends how much is acceptable.

Expanding the search to some other brands:


That last one really took me by surprise. Again, it’s distortion isn’t great, but I can’t think of any tweeter <$200/each with better FR & wide dispersion:
ne19vts-04_315mm_2v83_0grad_0.png

ne19vts-04_offaxis_normalized_5-30db.png


-3dB @ 60° @ 7500Hz, and no crazy resonances/breakups! :eek:

By comparison, the SB26ADC:
sb26adc-c000-4_315mm_2v83_0grad_0.png

sb26adc-c000-4_offaxis_normalized_10-25db.png
Click to expand...

Have used the Vifa and its bigger brother for the same reasons you found. However, their distortion profiles are not as good as the SB26ADC and have more options to deal with directivity than have ways to get lower distortion. So as @DDF illustrates above, it is the reason we had it is an option. Since I own all of them, we can try some variations once this first one is done.

IME developing products for over 3 decades, there is rarely one best answer and usually more than one good one. Between the good ones, there are usually tradeoffs (performance, reliability, quality, cost, longevity, etc.). As such, you take your best shot and usually do not get to revisit. Here we can (as long as the budget allows). :)
 
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