I was watching an old video from Erin's Audio Corner and noticed on the compression testing at high levels the port tuning frequency actually gets louder than the baseline. In addition the tuning frequency moves upwards. This caught be off-guard and I thought it was super interesting! Just look at 102dB, well above the 76dB baseline at 85Hz!
![Philharmonic%20BMR%20Monitor%20v2_Compression[1].png](https://www.diyaudio.com/community/attachments/philharmonic-20bmr-20monitor-20v2_compression-1-png.1186902/ "Philharmonic%20BMR%20Monitor%20v2_Compression[1].png")
I'd expect the port to increase efficiency and therefore reduce compression, but did not expect it to put out more than the amplifier gain!
I wondered if this is a known or studied phenomenon?
My guesstimate would be it's to do with air velocity being high enough to break down the boundary layer friction at the port walls, much like the way a golf ball can travel further when the boundary layer air is separated by adding dimples.
Encouraging this behaviour (B&W!) to gain higher efficiency seems like a good thing, but I'm not sure it is - this says nothing for how pure the sound is nor how linear the gain is at different levels. Still, if this additional efficiency could be had at all operational levels it might be very beneficial in PA systems.
I'd expect the port to increase efficiency and therefore reduce compression, but did not expect it to put out more than the amplifier gain!
I wondered if this is a known or studied phenomenon?
My guesstimate would be it's to do with air velocity being high enough to break down the boundary layer friction at the port walls, much like the way a golf ball can travel further when the boundary layer air is separated by adding dimples.
Encouraging this behaviour (B&W!) to gain higher efficiency seems like a good thing, but I'm not sure it is - this says nothing for how pure the sound is nor how linear the gain is at different levels. Still, if this additional efficiency could be had at all operational levels it might be very beneficial in PA systems.
Except you see similar behavior on sealed boxes at times also (links below).
And Erin's text states:
"Dynamic Range (Instantaneous Compression Test)
The below graphic indicates just how much SPL is lost (compression) or gained (enhancement; usually due to distortion)"
Distortion seems the likely explanation, instead of a desirable port effect at high levels.
https://www.erinsaudiocorner.com/loudspeakers/revel_w553l/
https://www.erinsaudiocorner.com/loudspeakers/kii_three/
And Erin's text states:
"Dynamic Range (Instantaneous Compression Test)
The below graphic indicates just how much SPL is lost (compression) or gained (enhancement; usually due to distortion)"
Distortion seems the likely explanation, instead of a desirable port effect at high levels.
https://www.erinsaudiocorner.com/loudspeakers/revel_w553l/
https://www.erinsaudiocorner.com/loudspeakers/kii_three/
Don’t forget a significant confounding factor. With most drivers the T/S will be quite differen tat those 2 levels and it will be out-of-tune for at least one of them.
dave
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Yeah, while the phenomenon is interesting it looks like it wouldn't sound very good, for small extra peak whole octave above (and below) is missing in this particular example. One note bass?
Takeaway is that a system pushed beyond its operating area looks like this. Good sounding system would not have that phenomenon happening with SPL levels one intends to listen.
Takeaway is that a system pushed beyond its operating area looks like this. Good sounding system would not have that phenomenon happening with SPL levels one intends to listen.
Wow thanks for all the replies!
Mattstat is right, it seems to happen on closed systems and even active ones, I had never paid enough attention.
Kii Three:
![Kii%20THREE_Compression[1].png](https://www.diyaudio.com/community/attachments/kii-20three_compression-1-png.1187119/ "Kii%20THREE_Compression[1].png")
In this case, I think Planet10 must be right about the parameters of the driver changing at high levels. Presumably the impedance drops at those frequency response peaks when driven hard??
AllenB, I think what you see with those crossover parts is not an increase in efficiency like I was guessing happened in the port, but rather a reduction in impedance and is not level dependant. Using a voltage source amplifier the power draw will increase as the impedance decreases, so not really getting anything 'extra'.
Mattstat is right, it seems to happen on closed systems and even active ones, I had never paid enough attention.
Kii Three:
In this case, I think Planet10 must be right about the parameters of the driver changing at high levels. Presumably the impedance drops at those frequency response peaks when driven hard??
AllenB, I think what you see with those crossover parts is not an increase in efficiency like I was guessing happened in the port, but rather a reduction in impedance and is not level dependant. Using a voltage source amplifier the power draw will increase as the impedance decreases, so not really getting anything 'extra'.
Then again, what is happening at 2KHz here? Impedance of the crossover drops as the coil core saturates, or...?
Revel W553
Is there any measurements to predict how a low frequency driver works in normal to high power situations? If the classic T/S parameter calculations is not good for this, then what's the point of it after all? Or it's not accurate but still good enough?
Increasing distortion in that region is typical of many dome tweeters. Could be an artifact related to that. I couldn't find the crossover point listed anywhere though, so just a guess.
Klippel testing can do quite a bit on that front. They have a lot of application notes on their site, but they're kind of spread around. They also have some e-learning courses available.
https://www.klippel.de/
One of their presentations:
KLIPPEL Customer Seminar “Mastering Large Signal Testing in QC and R&D”, 2014, by Robert Werner
https://www.klippel.de/uploads/medi..._Signal_Testing_in_QC_and_R_D_by_R.Werner.pdf
Just picked the first one:
https://audioxpress.com/article/voi...tal-pro-s-10hx230-coaxial-10-pro-sound-driver
dave
Yes, I saw these graphs in audioxpress tests.
But is it possible for example to calculate a box alignment for flat response for a woofer like with the classic T/S parameters just for specific driving power?
Anyway, according to the audioxpress tests and graphs, the tendency is roughly looks like (to me) the Qts is increasing and the Fs is decreasing with more driving power. At least in the case of 2.83 V vs 40 V.
But is it possible for example to calculate a box alignment for flat response for a woofer like with the classic T/S parameters just for specific driving power?
Anyway, according to the audioxpress tests and graphs, the tendency is roughly looks like (to me) the Qts is increasing and the Fs is decreasing with more driving power. At least in the case of 2.83 V vs 40 V.
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If you have the specific measures for the power level you will be using.
Typically, for this very reason, i find typical factory specs (from reputable makers) are much more useful than the typical diy kit we use to measure.
T/S are vectors, and the numbers we see are collaped to scalars. The fatory kit measures at a more reaslistic level.
On of the concepts of the miniOnken alignment i use, adds some R to the vents to reduce dynamic changes in response due to different levels.
dave
Typically, for this very reason, i find typical factory specs (from reputable makers) are much more useful than the typical diy kit we use to measure.
T/S are vectors, and the numbers we see are collaped to scalars. The fatory kit measures at a more reaslistic level.
On of the concepts of the miniOnken alignment i use, adds some R to the vents to reduce dynamic changes in response due to different levels.
dave
That's true but I'm not convinced distortion would give such an increase, certainly not in a sweep rather than pink noise. My money would still be on the crossover parts. I guess I can look over more graphs from active vs. passive designs to get a better idea.
What's happening is thermal power compression due to rapid (exponential) heat rise with increasing power/time, which HR can do up to 270/518 deg C/F, so yes, it's basically just increasing Qts, making the box alignment increasingly under-damped.
FWIW, the pioneers' version of what I assume you want was to preload the cab in that while a max flat alignment (by their definition) was Vb = Vas/1.44, Fb = Fs and for prosound Fb = 1.56x Fs to ensure it wouldn't exceed some value, which nowadays seems to be the VC 'blows' open before Xmech, though while still relevant for many of today's drivers being little more than higher power handling versions of them, this may be moot for the latest super high power VCs/Xmax 'monsters' where it might be best to just add (build in?) thermistors to protect them.