Not only more, but most importantly, it differs per driver.
Which heavily depends on the impedance curve, but also on how well the Cms(x) and BL(x) are behaving.
Both will have also an effect on IMD.
Copy/cross-post from another topic, but it's relevant here as well;
One note about Sd modulation.
I found it a little far fetched to be perfectly honest.
In the worst case scenario the difference would be the entire size of the surround.
So lets assume that the surround is 10mm wide for a 6 inch woofer. (which is very substantial)
So if we take a 6 inch woofer with an Sd of 133 cm²
10mm extra on this will give us a Sd of 154 cm²
Which results in a difference of 20*log(154/133) = 1.27dB
This is really worst case, because even a very poor surround would not be that extreme.
In practice this means a very thick and wide surround, which is a extremely bad choice for a mid-woofer to begin with.
Again for situation with no or very limited excursion, this whole Sd story is non-existing.
Probably even similar argument can be made for speakers that are just only used for lower frequencies.
Another very important point, we don't have ANY objective data and measurements that backup this theory to begin with.
If they want to make a convincing point, please just show us the same driver with just different surrounds.
Very easy and simple, and will clear up all clouds.
And I am more than happy to change my skepticism in less than a heart-beat.
Because at this point the whole Sd modulation story, is nothing more than just a little theoretical thought.
Which is fine, but it doesn't hold any conclusions, nor any scientific value.
One note about Sd modulation.
I found it a little far fetched to be perfectly honest.
In the worst case scenario the difference would be the entire size of the surround.
So lets assume that the surround is 10mm wide for a 6 inch woofer. (which is very substantial)
So if we take a 6 inch woofer with an Sd of 133 cm²
10mm extra on this will give us a Sd of 154 cm²
Which results in a difference of 20*log(154/133) = 1.27dB
This is really worst case, because even a very poor surround would not be that extreme.
In practice this means a very thick and wide surround, which is a extremely bad choice for a mid-woofer to begin with.
Again for situation with no or very limited excursion, this whole Sd story is non-existing.
Probably even similar argument can be made for speakers that are just only used for lower frequencies.
Another very important point, we don't have ANY objective data and measurements that backup this theory to begin with.
If they want to make a convincing point, please just show us the same driver with just different surrounds.
Very easy and simple, and will clear up all clouds.
And I am more than happy to change my skepticism in less than a heart-beat.
Because at this point the whole Sd modulation story, is nothing more than just a little theoretical thought.
Which is fine, but it doesn't hold any conclusions, nor any scientific value.
You are stepping in it now buddy.
What you are speaking of now is exactly what Purify did. They completed their changes to the driver motor and used a rolled half round surround and still they were not happy with the distortion performance.
Next step was the development of the Purifi surround that reduced distortion by 20dB's.
Continue this uninformed skepticism, you will not make any friends.
Thanks DT
Take a second look at my chart b_force bigger drivers are outperforming the purifi in some IMD test like the 10" Peerless XXLS at 60 and 144hz (albeit in fractions of percent).
If you don't want to look at my data look at hobby hifi tests. Let say we are doing a 40 and 96 hz IMD test with the Purifi 6.5W04 and seas l26roy
Here's the distortion profile of the Purifi
and here is the Seas L26roy
96hz is not looking good for the Seas, but that's why I test at a variety of different frequencies. To be fair a 20hz and 48hz IMD test might tell a different story.
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Not exactly. Perhaps we don't have blind test data to demonstrate that Sd modulation, by itself and all else being equal, is an audible phenomenon... although it is possible that Purifi has exactly this kind of data, I don't know.
However, there is a lot of anecdotal evidence that the Purifi drivers are a step forward in audio quality compared to similarly sized drivers. When anecdotal evidence becomes replicated in many situations, it becomes statistically significant.
The good folks at Purifi have offered an explanation on why their drivers sound better. Their explanation is theoretically sound, from a physics and mathematical standpoint. The harmonic distortion and IM distortion performance is at least 10 dB better than their competitors.
So when you say " at this point the whole Sd modulation story, is nothing more than just a little theoretical thought ", that might be a valid criticism if there was no evidence that the drivers offered any benefit. But they do offer a measurable distortion benefit, and there are dozens (hundreds?) of applications where the finished project is judged to be quite excellent.
j.
We were talking IMD, this is just plain THD.
At what frequency is Hobby Hifi comparing again?
It's a bit better to look at VoiceCoil Testbench measurements, because in that case we also get the BL(x), Kms(x) and Le(x) graphs.
The problem is, it's rather difficult to compare certain speakers from a general point of view.
Reason being that it all depends how they were optimized.
The lack of a demodulation ring can already give a night and day difference for example.
Which creates a big problem for verifying certain specific claims (of certain "inventions").
Take this speaker for example;
https://audioxpress.com/article/test-bench-wavecor-wf275bd01-10-75-home-audio-woofer
Far more optimized for mid-frequency, which really shows in de distortion plot.
The same goes for this one;
https://audioxpress.com/article/test-bench-the-10nw76-pro-sound-high-spl-10-driver-from-b-c-speakers
Important note, this one was tested at 104dB!!!! (standard hifi speakers like Purifi are done at 94dB)
So if we extrapolate the distortion to 94dB, which is 10dB lower, in midrange this 10 inch will probably eat Purifi for breakfast. While having a dynamic range or maxSPL that a Purifi speaker can't even reach.
An much higher sensitivity also really helps with lower the need for amplifier power.
Also meaning less heat in the driver. The lower end is mostly 2nd order, which is far more preferable compared to the higher levels of 3rd order distortion for Purifi.
https://audioxpress.com/article/tes...udio-6-5-midbass-transducer-from-purifi-audio
Point is that it's rather hard to just pick a few cherries and say that a certain claim is true or untrue.
It's even harder to find out if certain selling points of a design do what they claim.
Again, in case of Purifi, the simplest way is to just slap a standard but high quality surround on a Purifi speaker.
(hopefully also keeping the Mms and Cms equal)
You can explain theoretical ideas till the cows come home, still don't make them objective.
With no actual data it's nothing more than a story.
I can find anecdotal evidence for many things, that also don't make them true.
Anyway, again, I am here for neutral, open minded and objective discussions about use cases or scientific data.
I still very strongly get a feeling that some people here will defend and mix up personal taste with that.
There is nothing wrong with personal taste and subjective impressions.
But they need to be called and stay that way.
If it's such an audible phenomenon, a blind test would be absolute peace of cake to proof.
So what are people preventing from doing one?
Also if it's THAT audible, it would also be very easy to measure.
Again, show me objective data with a regular high-end surround vs a special surround and I am happy to change my mind.
Still doesn't change the use case argument btw, because for that amount of money I can come up with quite some other ideas.
Why the snarky response? Kind of sad to discuss that way.
Especially if you clearly don't understand what peoples background are.
Because if it comes down to ACTUAL speaker driver development and production, as well as measurements, I can guarantee I have a bit more experience and knowledge here.
The fact that people have to explain how to even do proper tests and experiments already speaks words obviously.
Please stick to the subject with proper explanation and/or references instead of getting personal, mean and snarky.
That way you make yourself instantly unbelievable.
Makes me even wonder if I am dealing with objective people, or just fanboys who like to defend their little babies no matter what. In that case, I am out because that results in a rather lame and boring discussion.
I don't think you followed my point, if a larger surround can only deliver an extra output of less than 1.5dB.
Again, absolute worst case scenario, so again in practice this WILL be less.
How is it possible that the distortion reduces by 20dB?
Next, I would like to see the objective data about that 20dB reduction.
If people claim certain numbers, they should provide information.
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b_force, before you say a driver eats another for breakfast pay attention to the details.
The voice coil test bench measures midband at 104 db for pro drivers and 94db for home audio, but pay attention to the bass rolloff. If you subract 20 db from Vance's response and harmonic graph you get a 1m spl. So reading from the graphs on voice coil test bench articles and subtracting 20db i get this:
Purifi is at 82db at 50hz with .95% THD and 90db at 100 hz with .25% THD at 100hz, 90 db at 200 hz with 0.6% THD
The B&C is 68db at 50hz with 1.8% THD and 78db at 100hz with .7% THD at 100hz, 84db at 200hz with 1.0% THD
The B&C is looking good at 1khz and all things considered for a PA application I would choose the B&C, it has more power handling and a waterproof cone, but for home hifi I would choose the Purifi.
The voice coil test bench measures midband at 104 db for pro drivers and 94db for home audio, but pay attention to the bass rolloff. If you subract 20 db from Vance's response and harmonic graph you get a 1m spl. So reading from the graphs on voice coil test bench articles and subtracting 20db i get this:
Purifi is at 82db at 50hz with .95% THD and 90db at 100 hz with .25% THD at 100hz, 90 db at 200 hz with 0.6% THD
The B&C is 68db at 50hz with 1.8% THD and 78db at 100hz with .7% THD at 100hz, 84db at 200hz with 1.0% THD
| Driver | 1m SPL and THD at 50hz |
|
| ||||||
| Purifi PTT6.5X | 82db 0.95% | 90db 0.25% | 90db 0.6% | ||||||
| B&C 10NW76 | 68db 1.8% | 78db 0.7% | 84db 1.0% |
The B&C is looking good at 1khz and all things considered for a PA application I would choose the B&C, it has more power handling and a waterproof cone, but for home hifi I would choose the Purifi.
You have made sweeping statements.
It is your burden of truth to back them up.
This thread is about Purifi drivers.
Show us test results where the purifi surround does not make a difference.
See post number 76
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I suppose Purifi development version's data is not public. But those phases are well described by L. Risbo in several videos.
Here is a video of how the ragged surround looks like in motion
https://preview.redd.it/li8o74hbz0l91.gif?format=mp4&s=9cb7633145ce4f41a3acb980cd8e451fd886382b
Here is a video of how the ragged surround looks like in motion
https://preview.redd.it/li8o74hbz0l91.gif?format=mp4&s=9cb7633145ce4f41a3acb980cd8e451fd886382b
hi DT, I am not sure how to understand the question for me? We have normalised the IMD plots in the data sheet to SPL, ie adjusted the voltage to get a prescribed SPL. exactly like in shrub0’s excel sheet.
Going from infinite baffle to sealed box and the drive voltage for the bass tone needs to go up to keep the SPlL and we get more exposed to the force factor modulation (current dependent Bl modulation ) plus the cone and surround exposed to more back pressure
Hello lrisbo,
I installer the 8R 6-1/2 inch Purifi driver in a sealed 0.55 cubic foot enclosure, then ran an Audio Precision impedance sweep plus TS/P calculator and found the Total Q of the driver in the sealed enclosure to be Q=0.64. This is not bad for a sealed enclosure.
Then I set up The APx555 with the AP acoustic measurement software application, the output voltage was set to 2.83volts with the microphone at 1 meter. There was way too much room noise and room reflection. So I moved the microphone to within 4 inches of the driver cone (near field).
I used the same setup for the same reasons as above at 4volts for the Two Tone IMD voice test.
I am okay with the bass FR as is. If I need more bass I will turn on the sub-woofer. I want to avoid imputing more bass frequency current into the Mid-bass driver that plays the voice frequencies.
What I am asking you is, are you equalizing to raise the the bass SPL? That answer appears to be yes.
Thanks DT
I installer the 8R 6-1/2 inch Purifi driver in a sealed 0.55 cubic foot enclosure, then ran an Audio Precision impedance sweep plus TS/P calculator and found the Total Q of the driver in the sealed enclosure to be Q=0.64. This is not bad for a sealed enclosure.
Then I set up The APx555 with the AP acoustic measurement software application, the output voltage was set to 2.83volts with the microphone at 1 meter. There was way too much room noise and room reflection. So I moved the microphone to within 4 inches of the driver cone (near field).
I used the same setup for the same reasons as above at 4volts for the Two Tone IMD voice test.
I am okay with the bass FR as is. If I need more bass I will turn on the sub-woofer. I want to avoid imputing more bass frequency current into the Mid-bass driver that plays the voice frequencies.
What I am asking you is, are you equalizing to raise the the bass SPL? That answer appears to be yes.
Thanks DT
re back pressure... considering a normal position in a closed box... moving the cone 2mm inwards creates a pressure P, coming's back to rest position and now "pull" the cone 2mm outwards - isn't the pressure -P?
Is the problem the same for "front pressure"?
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Is the problem the same for "front pressure"?
//
reply copy pasted from other thread:
note that whilst 1.27dB may sound innocuous it represents a gain modulation of around 15%, similar to Bl(x) and Bl(i) modulation - both directly controlling the gain. In other words, 15% wide band IMD.
Sd modulation has been confirmed experimentally by changing only the surround and by a general measurement setup. Near field measurements close to a normal half roll is higher than close to the cone. Moreover, Sd modulation is confirmed by finite element simulation. It was first analysed in AES papers from 1994 and 1995. Technics invented a surround that reduces Sd modulation back in 1992.
it is pure Cu. We adjust the Cu/Alu ratio to meet our target T/S parameters.