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Introducing Directiva - An ASR open source platform speaker project

MrPeabody

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In a wide-baffle design like the LS1, is there a point at which the baffle is wide enough that the shape of the edges ceases to matter?
It goes without saying that in theory the baffle edges could be far enough from the driver for the sound pressure to have dissipated to the point where the edge diffraction will be too weak to matter. The question is just how wide the baffle would need to be in order for this to be an effective way to suppress the ripple associated with baffle edge diffraction. I don't really have a good sense of how wide the baffle would need to be in order for this to be a highly effective way to suppress diffraction ripple. But if I were to take a guess, my guess would be that with a baffle maybe twice wider than most conventional baffles this effect would be strong enough to be moderately useful, but probably not strong enough to be highly effective at suppressing diffraction ripple.

At the low extreme of a typical tweeter's range, the wavelength is about 5x greater than the diameter of the tweeter diaphragm. The main lobe at this frequency range (for a tweeter) naturally wants to extend well beyond 180 degrees (edge to edge) except that it is blocked by the baffle (assuming there is no wave guide, which changes everything). Since the baffle interferes with the main lobe, sound pressure at the edge of the baffle is appreciable and edge diffraction will be appreciable unless something is done about it. But things are very different at high treble frequencies, because at high treble frequencies, a typical tweeter is so highly directional that the sound pressure at the baffle edge (for a typical rectangular baffle) is too weak to produce significant diffraction ripple.

Now the irony. If edge rounding is modest, leaving the enclosure looking like a box with modestly rounded edges, the effect at suppressing edge diffraction will be limited to high treble frequencies where it isn't a concern anyway. To understand why modest edge rounding doesn't work at the lower treble frequencies where it would be helpful if it actually worked, it is helpful to think about the extent to which the rounding produces a spread in the distance from the tweeter to the edge. In the case of a sharp edge, there is just one distance. When rounding is applied, the edge distance covers a spread. The short end of the spread is the distance from the tweeter to where the rounding begins, and the long end of the spread is the distance from the tweeter to the point where the rounding ends, at the flat side of the enclosure. In order for the edge to not appear very sharp to the wavelength encountering the edge, the spread in the edge distance needs to be at least 1/5 as great as the wavelength, approximately. If the spread in the edge distance is not at least this great, the edge will look very sharp to the wavelength that encounters the edge. Said differently, if the wavelength is five times greater or more (approximately) than the spread in the edge distance, the edge will look very sharp to the wavelength that encounters the edge. Ideally, the spread in the edge distance should cover a full wavelength.

The spread in the edge distance is the same as 1/4 of the circular circumference corresponding to the radius of the roundoff. If you do the simple substitution and simplification, you find that the wavelength where the roundoff just begins to be a little bit effective is where the wavelength is 8x greater than the radius of the roundoff. For example, if the radius of the roundoff is 1/2", it will start to be a little bit effective at wavelength equal to 4", which corresponds to about 3.4 kHz. It becomes increasingly effective as frequency moves higher, and at lower frequency, the spread in the edge distance isn't great enough to be effective. This frequency is not a whole lot lower than the frequency where a typical tweeter becomes too highly directional to illuminate the edge of the baffle. The gist of it is that edge rounding is of no particular benefit unless the rounding is done to the extent that the enclosure takes on the appearance of a cylinder or a cone. Cylinders and cones are very effective at suppressing diffraction ripple. A bevel is very effective if the bevel clips the edges to the extent where the bevels are as prominent as the front of the baffle.

The most effective way to suppress baffle edge diffraction, for a speaker enclosure that looks more like a rectangular box than a cylinder or cone, is to use a modest waveguide. The modest waveguide has a strong affect on the tweeter directivity at the low end of the tweeter's range, preventing the tweeter from illuminating the baffle edges. At the high end of the tweeter's range the directivity is naturally such that the waveguide has little if any effect at further increase in directivity. A compact waveguide such as the DXT can be combined with strong beveling, and this combination should be highly effective at suppressing edge diffraction. Another benefit of this compact waveguide is that it permits the tweeter to be located closer to the midrange (or mid-woofer), which is beneficial to the polar response in the vertical plane.
 

MrPeabody

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Thank you for all the invaluable information in this thread, in addition to your work on this awesome project! A more vague question, if you don't mind, since I know very little about diffraction: For a speaker with a dedicated midrange, is there a general trend of more or less diffraction of the tweeter's output between a recessed dome midrange (like a Morel MD55), a protruding dome midrange (like the Dayton RS52AN), and a small cone midrange (like the SS 10F/4424G)? I'm guessing that the specific tweeter and the spacing etc... play enough of a role that this can't be generalized, but I really don't know.
In general diffraction will occur at frequencies where the baffle edge is adequately well illuminated by the tweeter. As such, the more directional the tweeter, the less the diffraction effect, or the lower frequency range where diffraction effect will be significant. In general a cone will be more directional than a dome where the diaphragm diameters are the same. But you are asking about the midrange, and for the small midrange drivers you identified, about 2", the crossover point will likely be high enough such that baffle edge diffraction will be a concern for the midrange. The lowest frequency where baffle edge diffraction occurs is where the wavelength is twice the longer distance, from the driver center to the edge. So, for example, if the baffle is 12" wide and the driver in question is 4" from one edge and 8" from the other, the longer distance will be 8", [EDIT: the corresponding full wavelength will be 16", which corresponds to about 850 Hz], which would certainly fall within the range of most any midrange driver. The most effective way to suppress the diffraction ripple, assuming that the enclosure isn't cylindrical or conic or highly beveled, will be to use a waveguide, the same as with a tweeter. And if you look at some of the better 3-way speakers made by the likes of Genelec, you will see examples of waveguides used with midrange drivers. If you don't want to use a waveguide on the midrange, it will help for the midrange to be more directional than most, which is to say, for it to be a cone or else recessed slightly, but I'm not sure how much help this will be. Strong beveling is indicated, where the bevels are as prominent as the face of the baffle.
 
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ctrl

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So as I mentioned earlier, this is preliminary as am still reviewing the damping, etc...
Assuming that the measurement took place on the undamped enclosure, it seems that the leakage caused by the drivers is quite high.
If this is taken into account, then at least the simulation of the impedance agrees quite well with your measurement.
1616040979817.png
With the simulated frequency response you should always keep in mind that this applies to the half-space. The possible SPL is therefore 5-6dB lower and the baffle-step influences the frequency response.
 
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Rick Sykora

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Thread Starter #324
Assuming that the measurement took place on the undamped enclosure, it seems that the leakage caused by the drivers is quite high.
If this is taken into account, then at least the simulation of the impedance agrees quite well with your measurement.
View attachment 118845
With the simulated frequency response you should always keep in mind that this applies to the half-space. The possible SPL is therefore 5-6dB lower and the baffle-step influences the frequency response.
That Ql is not all that bad. Just need to verify if it just PR characteristic or have some leakage or both. Thanks!
 
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Rick Sykora

Rick Sykora

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Thread Starter #325
Input 1 is left and input 2 is right.
Yes, thanks. Should have mentioned in my post!

The bigger deal was that it was the opposite of my older minidsp wiring. The routing setup was a bit more vexing and will update post with it (it is on another computer).:cool:
 
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abdo123

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Yes, thanks. Should have mentioned in my post!

The bigger deal was that it was the opposite of my older minidsp wiring. The routing setup was a bit more vexing and will update post with it (it is on another computer).:cool:
I’m very excited about this project to be honest!

I think your biggest contender is the JBL Stage A130. Do you think you will surpass its distortion with the Purifi woofer?
 
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Rick Sykora

Rick Sykora

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Thread Starter #327
So quick update for today...

I ran tests to evaluate the cabinet for leaks and found none. Listening with a stethoscope yielded no obvious issues. Then I taped the woofer around the recess to see if it would affect the impedance and this changed inconsequentially.

Later I ran some nearfield measurements for the bass section. @ctrl has processed those and we will share soon. :cool:
 

sgoldwin

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Great, another cliffhanger.

this is becoming the Breaking Bad of DIY speaker builds, hard to watch in real time due to all the waiting for the next development; nearly overwhelming to binge watch with all the detail. Sub-plots too. And little comic diversions...
 
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Rick Sykora

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So, this is a look at the dual PR bass system without any eq. @ctrl took my raw REW nearfield measurements and doubled the passive radiator output and adjusted for membrane effects. One caveat is these measures are very sensitive with respect to microphone distance. I tried to keep to around 1 cm away, but keep in mind the PRs are mounted at 75 degree angle...

1616166030922.png
 
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tktran303

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@ctrl

Are you splicing or blending the near field and farfield woofer measurements.
What software are you using?

I’m watching with interest to see how the traditional measurements stack up against the Klippel acoustic field scanner.

And if the NFS gives a level of accuracy near the passband, to influence crossover redesign/refinement.

@Rick Sykora
Any chance to ground plane farfield measurements?

I wonder what’s causing the ripple around 600Hz and just over 1KHz, that’s seen in the impedance plot and amplitude response.
 

tktran303

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So quick update for today...

I ran tests to evaluate the cabinet for leaks and found none. Listening with a stethoscope yielded no obvious issues. Then I taped the woofer around the recess to see if it would affect the impedance and this changed inconsequentially.

Later I ran some nearfield measurements for the bass section. @ctrl has processed those and we will share soon. :cool:
I find that the modelled bass tuning is always slightly different to the actual tuning. The actual tuning seems to be a bit lower than simmed. I’m not yet sure why, and whether it needs a fudge factor, or correction factor somewhere.

Anyway, a few Hz here or there is not enough to be a material difference IMHO, particularly when everything below the Schroeder frequency is dominated by your room, anyway...

But it does make me think about port dimensions- that port that you thought was too long to fit into your box, well, maybe your port doesn’t have to be as long, as simulated...
 
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ctrl

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Are you splicing or blending the near field and farfield woofer measurements.
What software are you using?
Rick provided me with near field measurements of the woofer and a PR.
I then adjusted the sound pressure of the PR according to the ratio of the sound radiating surfaces to the woofer and added up the sound pressures of the woofer and the corrected PR.

After that, a rough baffle-step correction was performed.
I did not correct the phase frequency response of the two near field measurements.

For a better resolution of the low frequencies, I downsampled the resulting impulse response.

The software used was Arta. You can also use VituixCAD, there you would have even more possibilities for a most realistic preparation of the measurements.

I wonder what’s causing the ripple around 600Hz and just over 1KHz
The resonance around 600Hz, could be influenced by a standing wave in the cabinet. But could also be a resonance of the PR - Rick is currently experimenting with cabinet damping.

The resonance around 1200Hz is caused by the driver and can already be seen in the measurements published by Purifi (could be a surround resonance).
 
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Rick Sykora

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Thread Starter #334
@ctrl

Are you splicing or blending the near field and farfield woofer measurements.
What software are you using?

I’m watching with interest to see how the traditional measurements stack up against the Klippel acoustic field scanner.

And if the NFS gives a level of accuracy near the passband, to influence crossover redesign/refinement.

@Rick Sykora
Any chance to ground plane farfield measurements?

I wonder what’s causing the ripple around 600Hz and just over 1KHz, that’s seen in the impedance plot and amplitude response.
Weather is starting to warm, so may get to ground plane measures soon.

Rippling at those frequencies looks to be passive radiator related, but as @ctrl mentioned, am still trying out damping materials. Still sorting out some things, so not overly concerned just yet. Certainly feel free to continue to ask questions, but am not sending to Amir until team is comfortable with results. :cool:
 

Morpheus

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So, this is a look at the dual PR bass system without any eq. @ctrl took my raw REW nearfield measurements and doubled the passive radiator output and adjusted for membrane effects. One caveat is these measures are very sensitive with respect to microphone distance. I tried to keep to around 1 cm away, but keep in mind the PRs are mounted at 75 degree angle...

View attachment 119019
Quick thought: drivers, and passive radiators, are engineered and measured for operation in upright position. If you lay them flat, cone sags slightly and uniformly but straight down the usual axis of operation, so you loose xmax in one way, but no new distortion arises if you don't push it.
OTOH, if you tilt them, the sagging is minute, but, depending on the mass distribution and suspention of the moving assembly, they will be positioned very slightly misaligned with the operation axis, wich could cause some rocking motion on the cones and cause new distortion/resonances not seen in usual driver testing, right? Could that be in play here, since other causes seem to be out? Has anyone tried measuring a loudspeaker on axis, but tilted, say 10 degrees to the floor compare that to when it is measured upright?
 

tktran303

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I think 10° is too subtle to cause a difference that is a measurable, or audible.

45° maybe

For many years I used passive radiators that were downfiring and there was never a measurable difference after many years of use, at least I was unable to be discern it separately from effects of loosening of suspension over time (driver “break-in”

The main problem with a large downfiring passive radiator is that the air force and/Newton’s third law causes the whole subwoofer enclosure to vibrate up/down and back/forth, and hence move around when placed on hard flooring. This is a much bigger and immediate problem than the theoretical cone sag causing distortion/resonances/drift over time.
 
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Rick Sykora

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Thread Starter #337
If you noticed, I pulled my z offset delay measures from my earlier posting. As part of the nice summed nearfields produced by @ctrl, he had noticed that they showed a phase reversal. I removed the minidsp and they went away. After some further troubleshooting, I reset the minidsp to OOB defaults and checked the impulse response on both the analog and USB inputs. The USB ones (I had recently started to use), are negative phase from the analog inputs. :eek:

Not sure if the source is the minidsp or my computer or something else, but will retake the delay measurements are repost. I am using the USB cable that was supplied with the minidsp. Searching the net a bit, did yield some older hits on other minidsp products. Checking with Amir too as I know he tested a while ago.
 

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If you noticed, I pulled my z offset delay measures from my earlier posting. As part of the nice summed nearfields produced by @ctrl, he had noticed that they showed a phase reversal. I removed the minidsp and they went away. After some further troubleshooting, I reset the minidsp to OOB defaults and checked the impulse response on both the analog and USB inputs. The USB ones (I had recently started to use), are negative phase from the analog inputs. :eek:

Not sure if the source is the minidsp or my computer or something else, but will retake the delay measurements are repost. I am using the USB cable that was supplied with the minidsp. Searching the net a bit, did yield some older hits on other minidsp products. Checking with Amir too as I know he tested a while ago.
I have noticed a bug with the MiniDSP that 'restore to factory settings' does not clear everything sometimes.

Make sure to click 'restore to factory settings' + 'this configuration only'

Sometimes the setting would appear at default in the GUI (after a restoration) but in reality the old settings is still there.

I experienced this with the gain settings, better revert all settings manually by hand (one by one) or use 'this configuration only' from now on.
 

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I can confirm that my 2X4HD inverts phase when using USB. TOSLINK and analog are OK. See below for impulse response confirming this. My board is marked R1.2 and dated Jun 06, 2016. I am using the 1.17 plugin with 1.12 firmware.

Michael

2X4HD Phase.png
 
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Rick Sykora

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For today, reran some more damping testing. In this case, was seeing how it might affect the passive radiator...

I started by using the bottom lined with Sonic Barrier as a baseline. Next I added about half a cabinet of Acousta-stuf and this resulted in almost no effect. Next, I replaced the Acousta-stuf with the cotton-based damping from my SPK5s. In this, case, pretty much packed the cabinet interior. This did not have much affect on the Passive radiator output either.

Conclusion is that more damping is not going to impact the PR performance significantly. ;)

This motivated me to remove the Sonic Barrier. That appears to be more revealing. Will share that with my measurements in the next post...
 

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