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Crashpc 
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Joined: 26 February 2008 Location: Czech Republic Status: Offline Points: 465 |
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 Posted: 23 March 2019 at 8:08pm |
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Uh, oh! Corell is right, but there is no reason to decrease Bl. There is no easier way than EQing things out, as the driver will like it. At no point, it would bottom out sooner than "lower Bl driver" when all else being equal. Quite the opposite. 21" in compact box already suffers from low cone excursion and poor cooling. Putting some Juice in greatest cone excursion range is about good thing, to get the coil ends cooled down. In compact box, there is not much of a dip to speak about. Rather steady sensitivity decrease. That is easy to handle. Weaker driver already has more cone excursion to begin with, because more power goes in with usually lower impedance. Higher BL driver gets less power, due to the higher impedance. Only where strong motor driver struggles is amplifier capabilities. One needs greater voltage swing to feed the driver with sufficient power. I would greatly recommend RCF LF21N551 if money allows, then 21SW152 for torture, then 21DS115 for digging deep with sensible program content.
Edited by Crashpc - 23 March 2019 at 8:12pm |
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smitske96
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Joined: 16 February 2016 Location: The Netherlands Status: Offline Points: 1092 |
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Posted: 24 March 2019 at 11:08pm |
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@Crashpc
Do you have any measurements of your current box?
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Group2
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Joined: 14 January 2019 Location: Denmark Status: Offline Points: 10 |
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Posted: 25 March 2019 at 6:02am |
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interesting, following.
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Crashpc
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Posted: 25 March 2019 at 7:46am |
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Smitske: Not the one I would like to share, but it is coming soon. Slight peak at BR tuning frequency is there, but within 3db range, which would help to make the characteristic flatter after HPF subsonic application.
It is more complex than that though. With efficient driver, one should even think about very different approach. I'm now exploring possibilities to drastically downsize the bassreflex port volume without any side effects for my setup, as the port might be used less (for PA, cut HPF at, or above port tuning frequency). I have yet to compare efficiency compared to closed box, and the port might actually only remain in the box as a cooling tunel changing air in the box for fresh one. I'll keep you updated.
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Nikon and Canon people should not be married to each other. Why did you let this happen?
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Crashpc
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Posted: 25 March 2019 at 8:13pm |
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Okay. After some sims, it really looks to me like this approach with cutting at the (low) port tuning frequency might actually work well for PA. In the meantime, I jumped on the other project of my adequately small 10"+1" tops. Hopefully I don't stagnate, finish tuning these (3d printed custom horn for driver can be pain in the arse to develop) in oncoming two days, and get back to finish squeezing that 21" box into absolutely final version at dimensions 58*58*73cm(75 with legs). Measurements will be made and published.
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Nikon and Canon people should not be married to each other. Why did you let this happen?
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smitske96
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Posted: 26 March 2019 at 2:17pm |
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Okay current options:
21" reflex with the 21DS115 which would need some EQ at 50/60 Hz.. 18" reflex with 18SW100, RCF LF18N405 or the 18TLW3000. Current amp is the itech 8000 which can do 200 V max (k20 or so in the future). The 21DS stays well within excursion limits when feeding it 2000 Watt. 18TLW3000 goes past xmax around 50 Hz, and both 405 and SW100 stay within their limits. I have to add I uses Xvar for the SW100, don't know how far the other drivers can be puched within some limits. I'm now leaning to 4 18" BR with the option to add two in the future.. So many options and dilemma's
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toastyghost
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Posted: 26 March 2019 at 2:33pm |
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Those drivers might not exceed excursion at that power level but you can be damn sure they’ll get hot if you give them that long term. The ‘real power’ for the 21SW152 from the Klippel data provided by B&C shows approx 900w max with 4dB+ power compression in free air.
So make sure you have peak and RMS limiting if you need to eke out maximum performance as well as some semblance of longevity from the box. |
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smitske96
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Posted: 26 March 2019 at 3:53pm |
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I had some spare time so did some quick simulations in WinISD.
21DS115 180L 32Hz:   LF18N405 180L 33Hz:   18TLW3000 180L 32Hz:   18SW100 180L 35Hz:   18SW115 180L 32Hz:   Max SPL when fed 2000Watt:  Blue: 18SW100 Yellow: 18TLW3000 Red: LF18N405 Green: 21DS115 Purple: 18SW115 Edited by smitske96 - 26 March 2019 at 4:07pm |
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Crashpc
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Posted: 26 March 2019 at 9:35pm |
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Not that I would encourage anybody to torture their speakers, but in real life, it might be less demanding than klippel test. If we consider different program content, better cutoff between 30 and 95Hz, real power VS apparent power, those beasts would withstand 4kw peak and 1kW RMS from the amp without much issue. BUT,one still can burn the speaker, if it is pushed steadily as one note bass line situated on box bassreflex tuning frequency, where apparent power is also real power, and where the voice coil doesn't move s lot. Therefore poor cooling.
I cannot agree with these sims. Very inaccurate. Hornresp might do better job regarding SPLmax.
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Nikon and Canon people should not be married to each other. Why did you let this happen?
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toastyghost
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Posted: 26 March 2019 at 9:41pm |
Which is why I tend to do the maths and then drive the box hard with 6dB crest factor stressful music content to see if there’s any smell, clear thermal compression or change in impedance before settling on limiter values. |
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MarjanM
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Posted: 27 March 2019 at 12:59am |
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As i have said, the highest BL drivers are the worst.
180lit box is really too small to justify using a strong 21 inch in it. I am not shure why you are focused on such small box. Something of 230-250 pure internal volume would make significant difference. As someone have said earlier, there is no replacement for displacement.
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Marjan Milosevic
MM-Acoustics www.mm-acoustics.com https://www.facebook.com/pages/MM-Acoustics/608901282527713 |
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Crashpc
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Posted: 27 March 2019 at 8:06am |
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toastyghost:
Yes, that is one way to do that. And if it burns down for some reason, and it
isn´t caused by operator, then one can conclude
that he didn´t want such poor driver anyway. :-) MarjanM: No. You cannot
fully trust the sims here. Higher BL drivers have less desirable curve in
voltage sensitivity graph. That is the only issue, rather related to the
amplifier voltage “feeding” capabilities. Look at it this way. The driver has
most displacement (cone area * cone excursion) capabilities. It has most motor
force (Newtons pushed per Watt). It has very nice force to
cone mass and cone area ratio, usually better than its peers. At no point there
is a reason it should behave worse, than weaker smaller drivers. When we establish this, we
need to do corrective action for the outcome we see in the simulator… If a simulator tells you
this wrong piece of data, then it is otherwise – something is wrong with the
simulation. For the most part, it is the voltage driving of the driver. The simulator does not show
“2000W” of power to the speaker in the least. It cannot. It picks some arbitrary
point, by which it decides it is 2000W, but it is not. It cannot be - for
impedance and phase reasons. It seems that nobody cares about these, yet these
are vital part of the driver evaluation. You might spot, that those
18”s do have significantly more cone excursion in the same sim, than 21”. Cone
excursion * cone area will make SPL. The allegedly weaker driver moves more air
than strong 21” in the simulation. Doesn´t this look weird to you yet? The 21DS115 still works superb down to 125l (tested), and will outdo most 18”. It will not run far though, but if one is after most power output density in smallest box, this is the way...
Edited by Crashpc - 27 March 2019 at 8:15am |
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Crashpc wrote: