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Cone excursion question

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0krizia View Drop Down
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    Posted: 08 April 2021 at 8:08pm
When I use winISD to simulate designs, the graph do not reflect reality. If I for example add a 6 db boost (Q=4) at 6 hz below tuning, the cone excursion below tuning spikes way over maximum cone movement above tuning, however when I test this in the real world, my designs usually bottom out above tuning first. Anybody knows how this can be the case? 






Edited by 0krizia - 08 April 2021 at 8:19pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 08 April 2021 at 9:38pm
Don't forget that your amplifier will have a subsonic filter incorporated in its design. Is that what's causing the effect you have seen?

If you want to test a speaker at frequencies below 20Hz of so at high power, you really need an industrial amplifier (as used for driving vibrator tables etc) with a frequency response down to DC.
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0krizia View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 0krizia Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2021 at 6:20am
Maybe I should have specified tuning better...

I usually tune my boxes around 33-43 hz. 
My designs are made with extra large ports (above 50% of cone surface area) this is because I want to take advantage of the low cone excursion at port tuning by boosting at tuning to force more output. 

With a boost at tuning, im thinking maybe the pressure is already high through the port, this protect the cone from high excursion at frequencies below tuning during music? 
With a tuning at say 42 hz and With such large ports, im thinking maybe a bass beat around 55 hz with almost no activity below 45 hz will then result in next to no pressure inside the large port(s), this might make the pressure inside the enclosure around 55 hz so low the cone moves like it is in free air. 

Does this make any sense or is my logic faulty?  

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Peter Jan View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Peter Jan Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2021 at 12:55pm
Originally posted by 0krizia 0krizia wrote:

I usually tune my boxes around 33-43 hz. 
My designs are made with extra large ports (above 50% of cone surface area) this is because I want to take advantage of the low cone excursion at port tuning by boosting at tuning to force more output.

Boost at tuning frequency does indeed take advantage of the low cone movement at tuning.
One thing has not much to do with the other, the SPL advantage comes from boost at tuning, not from (extra) large ports. You do need to make the ports big enough to avoid chuffing/breathing that could be the result of too much power, compared to port size or I should say port diameter mainly. You do need to make the ports large enough to keep speed of air going back and forth in the port under a certain value... WITH the added boost, because I see people forgetting about that part while simulating.  Add 6dB at 42 Hz (your port tuning) also means 4 times the power at 42 Hz and thus 4 times more air pushed back and forth in the port.
And something also not so much thought about... when boosting at port resonance, you will feed the speaker voicecoil that much more power (with a capable amplifier) at and around that frequency. The speaker will suffer no ill effect of boosting at port tuning on a mechanical level (keeping excursion at bay), but can easily run over it's electrical limits (burned voicecoil).

Originally posted by 0krizia 0krizia wrote:

With a boost at tuning, im thinking maybe the pressure is already high through the port, this protect the cone from high excursion at frequencies below tuning during music?

The cone will only move less (even about standstill) AT tuning only . A BR port is a very-small- bandpass resonantor, resonating in counterphase with the rear movement of the speaker, thus adding to what comes out of the front of the speaker as a result.
Below and above tuning it will have (gradually and fast drooping off) less of an effect.
Below tuning it will go out of resonance and start behaving as nothing more than a giant airleak. The frequency of air flowing in and out has become so slow, it just ooses in and out with relative ease.
Above tuning, it will also go out of resonance, but the port can't follow the faster moving air going in and out with rising frequency, air comes to an almost standstill and acts as an airplug. The speaker is going to "see" about the same conditions as if the port isn't there / closed off, but still working within the conditions of that "closed-off" enclosure and the amount of cone excursion that result from that particular frequency and the power at that frequency.

Originally posted by 0krizia 0krizia wrote:

With a tuning at say 42 hz and with such large ports, im thinking maybe a bass beat around 55 hz with almost no activity below 45 hz will then result in next to no pressure inside the large port(s), this might make the pressure inside the enclosure around 55 hz so low the cone moves like it is in free air.

"Moving in free air" is going on a bit below the 42 Hz tuning, not above.
55Hz is about completely out of the reach of a 42Hz tuned port, so the cone will move accordingly to the enclosure it sits in and the amount of power fed into it.
Also when boosting at port tuning, keep in mind that the boost Q should be pretty high (=narrow bandwidth) or the boost will also have an effect on frequencies a good deal below and above, where the speaker has no (mechanical) benefit of the port anymore, but will move according to the frequency, the power and the condition it sits in.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2021 at 2:06pm
Originally posted by 0krizia 0krizia wrote:

When I use winISD to simulate designs, the graph do not reflect reality. If I for example add a 6 db boost (Q=4) at 6 hz below tuning, the cone excursion below tuning spikes way over maximum cone movement above tuning, however when I test this in the real world, my designs usually bottom out above tuning first. Anybody knows how this can be the case? 


It might just be down to how you are testing this.
If you're using sine waves at known voltages (measured after the amp as per madboffin) then yes, something's wrong with either the model or your build.

However, if you're using normal program material, then it could just be a side effect of the fact that most (mainstream at least) music styles have much less content down under 40-odd Hz compared to levels higher up, where often the kick drum might be the loudest thing in the bass range, perhaps in the 50-80Hz range, which would explain your findings.

HTH,
David.
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0krizia View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 0krizia Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2021 at 8:08pm
Originally posted by Peter Jan Peter Jan wrote:

 "Moving in free air" is going on a bit below the 42 Hz tuning, not above.


Maybe "moving in free air" was an exaggeration.
The reason I was thinking it happens above tuning is because of the large ports. 

The way I understand the physics of ports is that air compression inside port(s) have a significant impact on cone excursion around port tuning since it determine the air pressure the driver is working in. A 1" port in a dual 18" sub will make the enclosure work more or less like a sealed enclosure, a 10" port in a single 4" sub will make the driver work more or less in free air right above tuning, and in free air at and below tuning.

When ports have almost the same surface area as the driver, and when the driver play right above tuning, alot of the pressure escape through the port, if the port(s) are too large, there is next to no compression inside the port so the driver works in an environment with next to no pressure. 








Edited by 0krizia - 09 April 2021 at 8:55pm
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0krizia View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 0krizia Quote  Post ReplyReply Direct Link To This Post Posted: 09 April 2021 at 8:37pm
Originally posted by DMorison DMorison wrote:

Originally posted by 0krizia 0krizia wrote:

When I use winISD to simulate designs, the graph do not reflect reality. If I for example add a 6 db boost (Q=4) at 6 hz below tuning, the cone excursion below tuning spikes way over maximum cone movement above tuning, however when I test this in the real world, my designs usually bottom out above tuning first. Anybody knows how this can be the case? 


It might just be down to how you are testing this.
If you're using sine waves at known voltages (measured after the amp as per madboffin) then yes, something's wrong with either the model or your build.

However, if you're using normal program material, then it could just be a side effect of the fact that most (mainstream at least) music styles have much less content down under 40-odd Hz compared to levels higher up, where often the kick drum might be the loudest thing in the bass range, perhaps in the 50-80Hz range, which would explain your findings.

HTH,
David.

I didnt think of this, im sure this is a part of it.
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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 10 April 2021 at 10:08pm
Originally posted by 0krizia 0krizia wrote:

When I use winISD to simulate designs, the graph do not reflect reality. If I for example add a 6 db boost (Q=4) at 6 hz below tuning, the cone excursion below tuning spikes way over maximum cone movement above tuning, however when I test this in the real world, my designs usually bottom out above tuning first. Anybody knows how this can be the case? 







I stopped using WinISD nearly 20 years ago because the port tuning was never correct once measured under real-world testing. These were the days when the Pro version was not around.

Nevertheless, you would need to know the impedance curve of the driver in the box (I have yet to find a simulator that can get that correct once you literally measure the driver in the box) at the given frequencies. You may be feeding the driver more power than expected.

Also, pending on the size in addition to, the thickness of the materials used for the ports, you will lose room in the internal chamber of the cabinet, leaving the driver with less room than you might expect.

There are lots of variabilities in which, can be taken into the equation. However, I would suggest venturing to another software and, make a comparison of the two. By doing this, it will confirm if WinISD is indeed the problem or, something else.

Best Regards,
 

 
Elliot Thompson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Meat Substitute Quote  Post ReplyReply Direct Link To This Post Posted: 11 April 2021 at 10:09pm
Did you measure the impedance curve of the box you built? As the above poster has said, every nominal tuning of a box I've ever had from a design software has been wrong.

If you haven't measured then you should try. Fairly easy to do with some resistors and a multimeter if that's all you have. Once you have the impedance curve, post it up. From that folk here should be able to give you better advice based on the data.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 0krizia Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2021 at 2:10pm
Originally posted by Elliot Thompson Elliot Thompson wrote:



I stopped using WinISD nearly 20 years ago because the port tuning was never correct once measured under real-world testing. These were the days when the Pro version was not around.

Nevertheless, you would need to know the impedance curve of the driver in the box (I have yet to find a simulator that can get that correct once you literally measure the driver in the box) at the given frequencies. You may be feeding the driver more power than expected.

Also, pending on the size in addition to, the thickness of the materials used for the ports, you will lose room in the internal chamber of the cabinet, leaving the driver with less room than you might expect.

There are lots of variabilities in which, can be taken into the equation. However, I would suggest venturing to another software and, make a comparison of the two. By doing this, it will confirm if WinISD is indeed the problem or, something else.

Best Regards,
 

Today I downloaded an frequency app where I can play one frequency at the time, it turns out the “new” Winisd is faulty aswell! 

I gave the sub mentioned in this post about 150w and played one frequency test tone at the time while observing the cone movement close up, the cone was almost standing still at 37-38 hz! That is 4-5hz below what winisd simulation show! The internal volume has everything from wood thickness to handles, ports and driver volume included so the net volum is correct. Looks like im done with winisd, any suggestion for a better more accurate software? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JulianDA Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2021 at 2:22pm
Did you think about the "elongation" of the Port by placing it on the ground? This can quickly result in a tuning that is a few Hz lower than predicted...I would also advise to only use winISD as a tool to get a rough estimation for portlength. Then build a prototype, measure the impedance and correct the portlength so it reaches your desired tuning. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 0krizia Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2021 at 6:35pm
Originally posted by JulianDA JulianDA wrote:

Did you think about the "elongation" of the Port by placing it on the ground? This can quickly result in a tuning that is a few Hz lower than predicted...I would also advise to only use winISD as a tool to get a rough estimation for portlength. Then build a prototype, measure the impedance and correct the portlength so it reaches your desired tuning. 

I dont have a place to test the sub(s) unless im renting them out, but I placed one in my car, went into the forest where im not bothering anybody and used some test tones through my car amplifier while the back was door open so i could look at the cone from the outside of the car. Ill do another test with the sub outside the car tomorrow or so and see if it makes any difference, thanks for your comment
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