• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Introducing Directiva - An ASR open source platform speaker project

tktran303

Addicted to Fun and Learning
Forum Donor
Joined
Mar 27, 2019
Messages
683
Likes
1,179
Great Metamaterial. Demonstrated by Lars Risbo/Purifi. Keeping it old school!
 

witwald

Senior Member
Forum Donor
Joined
Jul 23, 2019
Messages
478
Likes
502
Do you know what conditions HiFiCompass took those measurements? Notably, what baffle size?
HiFiCompass provided the following details about their test conditions (here, click "Measurement setup description"):

Test baffle size: 1650x850 mm

Measuring equipment:
  • measuring microphone Brüel & Kjær 4133
  • microphone preamplifier Brüel & Kjær 2639
  • preamplifier power supply Brüel & Kjær 2804
  • sound card EMU-0202USB
  • DSP Xilica XM2040
  • low THD power amplifier
 
OP
Rick Sykora

Rick Sykora

Major Contributor
Forum Donor
Joined
Jan 14, 2020
Messages
3,513
Likes
7,014
Location
Stow, Ohio USA
HiFiCompass provided the following details about their test conditions (here, click "Measurement setup description"):

Test baffle size: 1650x850 mm

Measuring equipment:
  • measuring microphone Brüel & Kjær 4133
  • microphone preamplifier Brüel & Kjær 2639
  • preamplifier power supply Brüel & Kjær 2804
  • sound card EMU-0202USB
  • DSP Xilica XM2040
  • low THD power amplifier

Thanks! Did not see the Measurement setup description on my iPad originally.

A few months too late to help the SB26, but given the large baffle, suspect it would not have changed the outcome.
 
OP
Rick Sykora

Rick Sykora

Major Contributor
Forum Donor
Joined
Jan 14, 2020
Messages
3,513
Likes
7,014
Location
Stow, Ohio USA
If this derails the thread PM me and I'll edit, but I thought some people might be interested in what you can achieve if you have a lot of room for ports, say in the bottom 2/3rds of floorstander, you can suppress about 20 db of port noise using resonance traps - as has been demonstrated in the DIYaudio thread.

Here are my rudimentary results from one nights tinkering. First pic was using a 60mm port for a baseline (didn't record length, you could work it out, looks tuned a bit too low). The second pic and all plumbing/port tinkering thereafter was done with 75mm ID piping. The FR's are not directly comparable, but you can compare the resonant peaks of a simple straight port vs. the spaghetti port I ended up with. The traps are stuffed quite generously with pillow stuffing.

Purifi long stroke woofer 6.5" in a 15L test box (as per directiva). Cabinet walls lined with foam. Port installed to base of box as if it were leaving the woofer chamber and running through the leg of a floorstander or stand i.e. there is no port volume within the woofer cabinet.


Do you know how this compares to a larger volume box (say 30L) with a short port?
 

McFly

Addicted to Fun and Learning
Joined
Mar 12, 2019
Messages
903
Likes
1,861
Location
NZ
Hmm, no I thought 30L the woofer would end up being too peaky at Fb, the upper limit of this woofer being like 22L imo. 15L seemed like the sweet spot for tuning in a bass reflex cab. Would still be interesting to see the resonance effect, but I don't have any cabinets near 30L at the moment.
 

MrPeabody

Addicted to Fun and Learning
Joined
Dec 19, 2020
Messages
657
Likes
942
Location
USA
I think that McFly may have thrown in the towel prematurely.

As best as I can judge from looking at Rick’s pictures of the baffle, the center of the woofer is about 8.3” from the top edge, and about 7.8” from the bottom edge. (Please pardon my use of archaic units of measure.) For the strongest, 1st peak in the edge diffraction, the distance from the center of the driver to the baffle edge is 1/2 wavelength, such that the resulting 180 degrees of phase change, combined with the 180 degrees you get with a “soft” reflection at the edge, yields 360 degrees of phase offset vs. the wave direct from the driver. The two wavelengths (top and bottom edges) are 16.6” and 15.6”. The corresponding frequencies (using v = 1125 ft/s which means that 1 foot corresponds to 1.125 kHz) will be 12/16.6 x 1125 kHz = 813 Hz, and 12/15.6 x 1125 Hz = 865 Hz. These two frequencies are so close together that we will anticipate a fairly broad peak at about 840 Hz, allowing for the inaccuracy due to the crude way I obtained the two distances. The 1st dip will occur one octave higher, at about 1.7 kHz, and the 2nd peak will be at about 2.5 kHz.

When I look at the 2nd graph in post #660, these peaks and dips are unmistakable in the red trace (Amir NFS), but not in the blue graph (Rick gated). The same in post #672, which I assume is the same graph as in #660. Thus, the implied hypothesis is that the difference in the two response curves is due to one of them (Amir NFS) fully revealing the effect of baffle edge diffraction for the top and bottom edges whereas this effect is masked in the other response curve (Rick gated). Possibly there is something about the rig that Rick is using that allows the on-axis measurement to pick up an off-axis reflection, possibly a ceiling or floor reflection arriving within the gating window. Or, the explanation for why the two measurements are different in this respect might be that the woofers were oriented differently in the speaker that Amir measured vs. the one that Rick measured. If perchance this happens to be the true explanation, it then seems likely that in the speaker that Rick measured, the woofer was oriented such that the proud regions of the surround were at the North, East, South, and West compass points whereas in the speaker that Amir measured, these regions of the surround were not at these compass points (shifted from these compass points by 60 degrees I expect). It will likely be useful to confirm whether the two speakers were in fact different in this respect. If it turns out that the woofer orientations were the same, then my guess would be that in Rick’s setup, the diffraction effect was present and that the reason it doesn’t show up is that he was picking up an off-axis reflection.

While it will be useful to confirm whether the two speakers were in fact different in this respect, I think that it is indicated (regardless) to perform reflection-free measurements of the vertically on-axis and off-axis responses (if it hasn't already been done). It should only be necessary to do this for one edge, either at vertical angles above the top of the speaker or else below the bottom edge. As we have previously observed and noted, there is a conspicuous signature of baffle edge diffraction, which is evident when you look simultaneously at the on-axis and off-axis response curves. This signature occurs due to the fact that the affect on the on-axis response and the affect on the off-axis response are mutually opposite. At a diffraction dip in the on-axis response, there will be a peak in the off-axis response, which may be observed in a typical style of plot as a sort of pinch in the group of curves that includes the 0-degree off-axis response (i.e., the on-axis response). At a frequency corresponding to a diffraction peak in the on-axis response, there will be a spreading effect (instead of a pinch). I think that this may in fact be evident in the first two plots in post #650. (This signature effect - the pinch and spread at wavelengths separated by full octaves - is more plainly evident to me when the off-axis responses have not been normalized to the on-axis response.)

For anyone who might not be following what I'm saying, I ask that you look at the second graph in post #660, the red trace in particular, and take note of the frequency locations of the major peaks and dips you see in the red trace, then go back and look at the first two plots in post #560, with those same frequencies in mind. If the frequencies for the peaks and dips don't match up exactly with the frequencies I identified, the reason is almost certainly attributable to error in my crude measurements of the baffle, which I took off the screen from Rick's pictures.

If it turns out that this diffraction effect is appreciable and that the reason that it does not show up in Rick’s measurement is that the woofers were oriented differently, then the evident conclusion is that with this speaker it is important that the woofer be oriented such that the proud regions of the surround are at the N, E, S, and W compass points, such that the surround serves to prevent the baffle edges from being strongly illuminated. This is the more fortunate possibility, because the solution is trivial. The less fortunate possibility is that the reason for the difference in the two measurements has nothing to do with the woofer orientation, and is due to off-axis reflections being picked up in Rick's measurements. In this case, it may be that the true on-axis response, measured free of reflections, will always look like the response that Amir obtained, i.e., regardless of the woofer’s orientation. In this less fortunate possibility, we may have gained knowledge as to the explanation for the difference in the two measurements, yet be left in want of an easy solution.

I hope that this is helpful and that I haven’t just created confusion.
 
OP
Rick Sykora

Rick Sykora

Major Contributor
Forum Donor
Joined
Jan 14, 2020
Messages
3,513
Likes
7,014
Location
Stow, Ohio USA
Hmm, no I thought 30L the woofer would end up being too peaky at Fb, the upper limit of this woofer being like 22L imo. 15L seemed like the sweet spot for tuning in a bass reflex cab. Would still be interesting to see the resonance effect, but I don't have any cabinets near 30L at the moment.

Think I may have misunderstood as the smallest floorstander cabinet I have is about 30L. You suggested just using the space to fit the port with the traps I gather?

If so, seems useful to compare anyway. With a 3 inch diameter port, you get an f3 of 45 Hz in a 30L cabinet. Here is a look if you modeled in VCAD...

1630680477778.png
 
OP
Rick Sykora

Rick Sykora

Major Contributor
Forum Donor
Joined
Jan 14, 2020
Messages
3,513
Likes
7,014
Location
Stow, Ohio USA
Rick, this kind of tuning rarely sound good. Slow gradual rolloff is what you should strive for. Acc to my sims with Purifi drivers, ~15l seems optimal.

While I get your reservation, it might be acceptable for a speaker that is well away from walls. Was not suggesting it was optimal, just what could be done in a larger box (so that did not have as nasty a port resonance). Always tradeoffs... :cool:
 
Last edited:

PKAudio

Member
Joined
Mar 25, 2021
Messages
48
Likes
76
It is not about placement in the room, though it could mitigate it. Ok try it, trials and errors....
Just wanted to express that any speaker designer would most most likely stay away from this tuning.
 
OP
Rick Sykora

Rick Sykora

Major Contributor
Forum Donor
Joined
Jan 14, 2020
Messages
3,513
Likes
7,014
Location
Stow, Ohio USA
It is not about placement in the room, though it could mitigate it. Ok try it, trials and errors....
Just wanted to express that any speaker designer would most most likely stay away from this tuning.

Have tried it, been doing this for 30 years. Based on that experience, am more open than some to trying borderline cases. The results I have had with Bassbox have been very good overall. I used VCAD as it happens to be on my office computer and Bassbox is not.

Btw, the alignment in Bassbox does not have the sort of hump that you seem so concerned about. Show me your sim and your assumptions, then we can talk specifics. Otherwise, if you want to purely judge results based on a sim, I need more info as some sims are better than others. Also, remember the initial discussion earlier here was about dealing with the ugly port resonance. Do you have some better way to model and remediate, if so, please share as well.

Thanks!
 

PKAudio

Member
Joined
Mar 25, 2021
Messages
48
Likes
76
I just recall several experiences where high port tuning lead to boomy and hollow sound, and this tuning was still very reasonable, no hump.

Ok, anyway, to be helpful, 20-25liters enclosure, 7cm port diameter, and either more stuffing inside cabinet, or you can try what troels did here in standmount version. I used this several times with good results (Ekta Grande upgrade was the latest experience), and even ~4cm long felt tube lowers port tuning significantly (it lowers port tuning and also output level and it is what seems to be desirable here). With this solution you could end up with port length 15-20cm.
http://www.troelsgravesen.dk/Illuminator-Monitor.htm
 
OP
Rick Sykora

Rick Sykora

Major Contributor
Forum Donor
Joined
Jan 14, 2020
Messages
3,513
Likes
7,014
Location
Stow, Ohio USA
How about no ports because they're stupid and make the phase messy and makes subwoofer integration a nuance?

Thanks for reminding me. This does seem familiar for some reason....

Anyway, you are in good company (as a number of prominent designers advocated as well, but not because ports do not have their place). Was not intending to revisit box tuning this one as has already been hashed many times over earlier in this thread. ;)
 
Last edited:

McFly

Addicted to Fun and Learning
Joined
Mar 12, 2019
Messages
903
Likes
1,861
Location
NZ
Think I may have misunderstood as the smallest floorstander cabinet I have is about 30L. You suggested just using the space to fit the port with the traps I gather?

If so, seems useful to compare anyway. With a 3 inch diameter port, you get an f3 of 45 Hz in a 30L cabinet. Here is a look if you modeled in VCAD...

View attachment 151250
What my intention was was to build a segregated floor stander cabinet, that is a cabinet that looks like a normal floor stander but is separated by an internal partition . 15L section at the top with the woofer and tweeter, and the port + traps occupies the space in the bottom. So it’s still a 15L speaker, just with a floor standing cabinet.

Anyway, back to our regular programming -
 

MrPeabody

Addicted to Fun and Learning
Joined
Dec 19, 2020
Messages
657
Likes
942
Location
USA
It seemed as though the question, of why Rick and Amir obtained different primary response curves for what was thought to be the same speaker, was an important question that needed to be answered. I studied this question and arrived at what I think is the answer, essentially, but I can't tell whether anyone has read what I wrote. So I'll try once more before I give up and shut up. The answer, I believe, is that with the speaker that @amirm measured on NFS, diffraction associated with the top and bottom edges of the baffle influenced the on-axis measurement, and this did not occur in the measurement that @Rick Sykora obtained.

But why did this occur with Amir's measurement and not with Rick's? One strong possibility is what was suggested previously by McFly, which is to say, the woofers are oriented differently in the two speakers, such that the top and bottom edges of the baffle were in the shadow of the woofer surround with the speaker that Rick measured, but not with the speaker that Amir measured. Another strong possibility is that this diffraction effect is present in the speaker that Rick measured but that it didn't show up in the measurement, the presumptive reason being that his on-axis measurement was picking up a reflection of the vertically off-axis response, likely from the floor or ceiling. Since these are the only two strong possibilities that come to mind, I suggest that they be treated as two alternative hypotheses.

Regardless of which of these two alternative hypotheses is correct, I have very little doubt that this is the correct, underlying explanation for why the two measurements are different. The reason I am confident of this is because if you calculate the theoretical frequencies for the three primary response anomalies (strong peak followed an octave higher by a strong dip then another strong peak another octave higher) that should occur due to this diffraction, associated with the top and bottom edges of the baffle, peaks and dips at these specific frequencies are readily apparent in Amir's measurement but not in Rick's measurement, and this difference, the presence of these peaks and dips in Amir's measurement but not in Rick's, accounts for the major differences between the two measurements.

For the record, I would not have written this post, following up the previous one, if anyone had acknowledged the prior post. My reason for doing this is that based on what I read in a series of posts, the discrepancy between the two measurements was deemed to be a significant concern for the progress and ultimate success of the project. In this particular respect I may have misjudged and erred, but I honestly have no idea.



As best as I can judge from looking at Rick’s pictures of the baffle, the center of the woofer is about 8.3” from the top edge, and about 7.8” from the bottom edge. (Please pardon my use of archaic units of measure.) For the strongest, 1st peak in the edge diffraction, the distance from the center of the driver to the baffle edge is 1/2 wavelength, such that the resulting 180 degrees of phase change, combined with the 180 degrees you get with a “soft” reflection at the edge, yields 360 degrees of phase offset vs. the wave direct from the driver. The two wavelengths (top and bottom edges) are 16.6” and 15.6”. The corresponding frequencies (using v = 1125 ft/s which means that 1 foot corresponds to 1.125 kHz) will be 12/16.6 x 1125 kHz = 813 Hz, and 12/15.6 x 1125 Hz = 865 Hz. These two frequencies are so close together that we will anticipate a fairly broad peak at about 840 Hz, allowing for the inaccuracy due to the crude way I obtained the two distances. The 1st dip will occur one octave higher, at about 1.7 kHz, and the 2nd peak will be at about 2.5 kHz.

When I look at the 2nd graph in post #660, these peaks and dips are unmistakable in the red trace (Amir NFS), but not in the blue graph (Rick gated). The same in post #672, which I assume is the same graph as in #660. Thus, the implied hypothesis is that the difference in the two response curves is due to one of them (Amir NFS) fully revealing the effect of baffle edge diffraction for the top and bottom edges whereas this effect is masked in the other response curve (Rick gated). Possibly there is something about the rig that Rick is using that allows the on-axis measurement to pick up an off-axis reflection, possibly a ceiling or floor reflection arriving within the gating window. Or, the explanation for why the two measurements are different in this respect might be that the woofers were oriented differently in the speaker that Amir measured vs. the one that Rick measured. If perchance this happens to be the true explanation, it then seems likely that in the speaker that Rick measured, the woofer was oriented such that the proud regions of the surround were at the North, East, South, and West compass points whereas in the speaker that Amir measured, these regions of the surround were not at these compass points (shifted from these compass points by 60 degrees I expect). It will likely be useful to confirm whether the two speakers were in fact different in this respect. If it turns out that the woofer orientations were the same, then my guess would be that in Rick’s setup, the diffraction effect was present and that the reason it doesn’t show up is that he was picking up an off-axis reflection.

While it will be useful to confirm whether the two speakers were in fact different in this respect, I think that it is indicated (regardless) to perform reflection-free measurements of the vertically on-axis and off-axis responses (if it hasn't already been done). It should only be necessary to do this for one edge, either at vertical angles above the top of the speaker or else below the bottom edge. As we have previously observed and noted, there is a conspicuous signature of baffle edge diffraction, which is evident when you look simultaneously at the on-axis and off-axis response curves. This signature occurs due to the fact that the affect on the on-axis response and the affect on the off-axis response are mutually opposite. At a diffraction dip in the on-axis response, there will be a peak in the off-axis response, which may be observed in a typical style of plot as a sort of pinch in the group of curves that includes the 0-degree off-axis response (i.e., the on-axis response). At a frequency corresponding to a diffraction peak in the on-axis response, there will be a spreading effect (instead of a pinch). I think that this may in fact be evident in the first two plots in post #650. (This signature effect - the pinch and spread at wavelengths separated by full octaves - is more plainly evident to me when the off-axis responses have not been normalized to the on-axis response.)

For anyone who might not be following what I'm saying, I ask that you look at the second graph in post #660, the red trace in particular, and take note of the frequency locations of the major peaks and dips you see in the red trace, then go back and look at the first two plots in post #560, with those same frequencies in mind. If the frequencies for the peaks and dips don't match up exactly with the frequencies I identified, the reason is almost certainly attributable to error in my crude measurements of the baffle, which I took off the screen from Rick's pictures.

If it turns out that this diffraction effect is appreciable and that the reason that it does not show up in Rick’s measurement is that the woofers were oriented differently, then the evident conclusion is that with this speaker it is important that the woofer be oriented such that the proud regions of the surround are at the N, E, S, and W compass points, such that the surround serves to prevent the baffle edges from being strongly illuminated. This is the more fortunate possibility, because the solution is trivial. The less fortunate possibility is that the reason for the difference in the two measurements has nothing to do with the woofer orientation, and is due to off-axis reflections being picked up in Rick's measurements. In this case, it may be that the true on-axis response, measured free of reflections, will always look like the response that Amir obtained, i.e., regardless of the woofer’s orientation. In this less fortunate possibility, we may have gained knowledge as to the explanation for the difference in the two measurements, yet be left in want of an easy solution.

I hope that this is helpful and that I haven’t just created confusion.
 

ctrl

Major Contributor
Forum Donor
Joined
Jan 24, 2020
Messages
1,616
Likes
6,086
Location
.de, DE, DEU
One strong possibility is what was suggested previously by McFly, which is to say, the woofers are oriented differently in the two speakers, such that the top and bottom edges of the baffle were in the shadow of the woofer surround with the speaker that Rick measured, but not with the speaker that Amir measured.

Another strong possibility is that this diffraction effect is present in the speaker that Rick measured but that it didn't show up in the measurement, the presumptive reason being that his on-axis measurement was picking up a reflection of the vertically off-axis response, likely from the floor or ceiling.

As explained in post#667 and post#675, the protruding surrond of the woofer as a secondary sound source affects the axial frequency response, with the first minimum at 5.3kHz and the first maximum at 2.6kHz. So a direct influence can be excluded - we all agree on that.
This means that it is only a question of whether the protruding surround "shadows" - i.e. attenuates - the sound propagation to the vertical edges of the loudspeaker cabinet.
1630773002895.png
Source link

If this were the case, the secondary sound sources produced by the vertical cabinet edges would weaken.
This means that the first minimum affecting the axial frequency response around 1.7kHz, caused by the vertical edge diffraction, would be less pronounced - we all agree on that too.

In Amir's measurement, the protruding surround, was therefore not in the way - in the case described by @MrPeabody. The simulation did not consider the protruding surround either.
Therefore Amir's on-axis measurement should follow the frequency response of the simulation better than Rick's measurement does.

In green you can see the simulated on-axis frequency response, which deviates greatly overall because the Purifi driver has a paper cone and this breaks down into eigenmodes early and thus deviates from the ideal.

However, we are only looking at the course of the frequency response, i.e. where are the humps and dips.
1630775430070.png

We can see a flat dip in the frequency response around 1.5kHz in the simulation and a hump around 2kHz followed by a wide dip up to about 3.2kHz. Rick's measurement shows approximately such a curve progression, Amir's NFS measurement not at all - so exactly the opposite as expected.

Now let's look at the vertical frequency responses of Amir's NFS measurements and the simulation.
I smoothed Amir's measurements with 1/3oct, frequency responses are normalized to on-axis FR.

Vertical up
1630776611437.png

Vertical down
1630777263190.png

Vertically upwards, the frequency response curves of NFS and simulation fit together quite well.
Vertically downwards, however, it looks bad, especially at larger angles.


For those who have doubts about the accuracy of the simulation, here is the comparison of the horizontal NFS measurements with the simulation for the tweeter with waveguide (measurements with 1/3oct smoothing, frequency responses are normalized to on-axis FR):
1630777993998.png


What the hell does it all mean?
Well, that's the million dollar question. Rick's measurements seem to be okay, anyway.

Spontaneously I would have guessed, but I know that Amir doesn't make such a mistake, that the measurement platform was larger than the base area of the speaker. This would then explain why measurement and simulation fit better vertically upwards than vertically downwards.
 
Last edited:

McFly

Addicted to Fun and Learning
Joined
Mar 12, 2019
Messages
903
Likes
1,861
Location
NZ
I now believe differences are more likely to be in variations to the woofers due to production date. Purifi have tweaked the woofer over time making it slightly more sensitive. I compared my woofer in a similar baffle to ricks measurements and my woofers a different response again.
 

MrPeabody

Addicted to Fun and Learning
Joined
Dec 19, 2020
Messages
657
Likes
942
Location
USA
As explained in post#667 and post#675, the protruding surrond of the woofer as a secondary sound source affects the axial frequency response, with the first minimum at 5.3kHz and the first maximum at 2.6kHz. So a direct influence can be excluded - we all agree on that.
This means that it is only a question of whether the protruding surround "shadows" - i.e. attenuates - the sound propagation to the vertical edges of the loudspeaker cabinet.
View attachment 151420
Source link

If this were the case, the secondary sound sources produced by the vertical cabinet edges would weaken.
This means that the first minimum affecting the axial frequency response around 1.7kHz, caused by the vertical edge diffraction, would be less pronounced - we all agree on that too.

In Amir's measurement, the protruding surround, was therefore not in the way - in the case described by @MrPeabody. The simulation did not consider the protruding surround either.
Therefore Amir's on-axis measurement should follow the frequency response of the simulation better than Rick's measurement does.

In green you can see the simulated on-axis frequency response, which deviates greatly overall because the Purifi driver has a paper cone and this breaks down into eigenmodes early and thus deviates from the ideal.

However, we are only looking at the course of the frequency response, i.e. where are the humps and dips.
View attachment 151449
We can see a flat dip in the frequency response around 1.5kHz in the simulation and a hump around 2kHz followed by a wide dip up to about 3.2kHz. Rick's measurement shows approximately such a curve progression, Amir's NFS measurement not at all - so exactly the opposite as expected.

Now let's look at the vertical frequency responses of Amir's NFS measurements and the simulation.
I smoothed Amir's measurements with 1/3oct, frequency responses are normalized to on-axis FR.

Vertical up
View attachment 151452

Vertical down
View attachment 151462

Vertically upwards, the frequency response curves of NFS and simulation fit together quite well.
Vertically downwards, however, it looks bad, especially at larger angles.


For those who have doubts about the accuracy of the simulation, here is the comparison of the horizontal NFS measurements with the simulation for the tweeter with waveguide (measurements with 1/3oct smoothing, frequency responses are normalized to on-axis FR):
View attachment 151463


What the hell does it all mean?
Well, that's the million dollar question. Rick's measurements seem to be okay, anyway.

Spontaneously I would have guessed, but I know that Amir doesn't make such a mistake, that the measurement platform was larger than the base area of the speaker. This would then explain why measurement and simulation fit better vertically upwards than vertically downwards.

I appreciate that I am not being ignored, so I no longer feel creepily like Glenn Close in Fatal Attraction.

And I appreciate your expanded discussion of some of what you found with the simulation and in comparisons of the simulation with the measurements.

But I think you might be missing my point, which is with a very strong hypothesis for why Rick's measurement and Amir's measurement differ. Based on my reading of the previous 2 -3 pages of this thread, this discrepancy was deemed a strong concern. If no one cares about the discrepancy or with the explanation of it, then neither do I. But if this is something that matters, the exceedingly likely explanation is that Amir's measurements reveal diffraction associated with the top and bottom edges of the baffle, while Rick's measurements do not reveal this effect. This is not just a wild guess. As I have previously explained, diffraction from the horizontal baffle edges, i.e., the top and bottom edges, in theory creates on-axis response peaks and dips at specific frequencies; these predicted peaks and dips and readily apparent in Amir's measurement but are not apparent in Rick's measurement. And the major differences between the two measurements are at these frequencies. I'm not just saying that the speaker exhibits these anomalies. I would have no motivation for saying something like that. The point I'm trying to get across is that this is the explanation for why the two measurements are appreciably different: these peaks and dips are present in Amir's measurement but not in Rick's. I don't know how else to say. If it doesn't matter to you or to Rick, then it certainly does not matter to me, but what I see in the most recent 2 - 3 pages suggests to me that this does matter to both of you, and as such it seemed appropriate to me to point out that the major discrepancy between the two measurements are explained thusly, that one (Amir's) shows the effect of diffraction associated with the top and bottom edges of the baffle, while the other one (Rick's) does not. Of course this raises the follow-up question of why Rick's measurement does not show this diffraction effect. There would seem to be two possible explanations for this, one of which is that the woofer is oriented differently in the two speakers, such that in the speaker that Rick measured the horizontal edges (top and bottom) are not strongly illuminated (in the shadow of the protruding part of the surround) whereas in the speaker that Amir measured the horizontal edges are strongly illuminated.
 

MrPeabody

Addicted to Fun and Learning
Joined
Dec 19, 2020
Messages
657
Likes
942
Location
USA
I now believe differences are more likely to be in variations to the woofers due to production date. Purifi have tweaked the woofer over time making it slightly more sensitive. I compared my woofer in a similar baffle to ricks measurements and my woofers a different response again.

No harm in mentioning again that you changed your mind on this question. But I am not persuaded that you had a particularly good reason to change your mind. You showed the specs for three generations of the woofer, revealing that there has been an evolution in the woofer. This suggests the possibility that the differences in the two measurements - Amir's vs. Rick's - are explained by the use of different versions of the woofer. This possibility is suggested, but the suggestion of a possibility is not enough for formulation of a hypothesis, much less a conclusion.

The theoretical understanding of baffle edge diffraction, applied to this speaker specifically, predicts peaks and dips at specific frequencies. These specific peaks and dips are plainly evident in Amir's measurement but not in Rick's, and these particular differences, the diffraction peaks and dips that are plainly evident in Amir's measurement but not in Rick's, are the prominent differences between the two measurements. The only important question that isn't easily answered is why these peaks and dips that are predicted by theory and also plainly evident in Amir's measurement do not appear in Rick's measurement. Your prior idea, that it has to do with different orientation of the woofer and a resulting difference in how strongly the baffle edges are illuminated, is one of the two strong possibilities. The other possibility that begs not to be ignored is that Rick's on-axis measurement was affected by vertically off-axis reflections. I do not know the correct answer to why this diffraction effect is not evident in Rick's measurement, however I am inclined to think that it is almost certainly one or the other of these two distinct possibilities.

When sound, fundamental theory predicts a very specific thing and this very specific thing is plainly evident in the measurement taken on the NFS, it is very difficult to disavow the agreement between theory and measurement. The diffraction effect should be there, according to theory, and it is there, according to Amir's measurement. But why isn't it evident in Rick's measurement? This is the question that remains to be answered, and if Rick or someone else figures out the answer to this question, we will then have the answer to why the two measurements are not in closer agreement.
 
Top Bottom