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BMS 12S330 - PLANS Help
BMS 12S330 - PLANS Help
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Old 21st October 2019, 11:27 PM   #1
Tonefarm is offline Tonefarm
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Default BMS 12S330 - PLANS Help

Hello Sub builders...

I have settled on a new mini project consisting of a BMS 12S300 in a very low tuned enclosure... BMS Specifies a 60L container for the given response with 110 mm / 517 mm long (presumably) circular port..
3DVIEW.jpg
I would like a slot designed port and given my confused state i can't seem to find anyone coming up with definitive calculations for getting the correct port length etc.. maybe i am missing something. I have a rumour in my head the WinISD doesn't calculate specifically slotted ports.. maybe i'm being corrupted by something somewhere else on DIY.

BMS Recommends to use a port length almost half the size of mine and a circular cross section of 9500mm2, again thats almost half as much as my 21840mm2 (6X36X102cm) slot port.
PORTLENGTH.png
Given the simulated response i just don't think i am getting the SPL i should be getting considering this type of driver... it has a super low FS. I have already tried to get more SPL out of the enclosure by increasing the volume by 8 litres...
RESPONSE.png
I have attached some pictures of the simulated response with WinISD and some pictures of the enclosure... my instinct tells me that the port length is two small in width and too long in its entire length to the back.

The parameters as follows....

BOX = 67.8 L
Tuning Freq = 29Hz
VENTS = 6cm X 36.4cm
Calculated Vent Length = 102cm
1st port resonance 167hz (A bit undesirably low... LPF will be set around this point)
VELOCITYat500W.png
After putting together the box in CAD and calculating the volume displacement with the amp modules am i accounting for the rear volume independent of the port volume? i have tried to balance this with the suggested port length 102cm X width and size to somehow find the balance with a slotted port that will fit in unison with the suggested internal 67.8L.
VOLUME.png
Are there any subwoofer building experts out there that could at least help me approve what i have done as a valid design? I feel that i am overthinking things when in reality it could quite possibly be a bit more straight forward..

I really would appreciate any help where you can,

Lots of Kind Regards,

Dec
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Old 22nd October 2019, 07:44 AM   #2
GM is offline GM  United States
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Maybe this will help: https://www.diyaudio.com/forums/atta...ort-length-jpg

GM
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Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
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Old 22nd October 2019, 09:33 AM   #3
David Morison is offline David Morison  Scotland
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Quote:
Originally Posted by Tonefarm View Post
Hello Sub builders...

Given the simulated response i just don't think i am getting the SPL i should be getting considering this type of driver... it has a super low FS. I have already tried to get more SPL out of the enclosure by increasing the volume by 8 litres...
Unfortunately, the relationship between Fs and efficiency is a cubic one, so even a modest reduction in Fs results in a big loss in efficiency. Raising Vb will get you more output around the tuning frequency but doesn't change the underlying efficiency of the driver itself. Also, the lower you tune a vented box the less power it takes to reach Xmax in the main passband, again limiting max SPL in a low tuned box.

Quote:
After putting together the box in CAD and calculating the volume displacement with the amp modules am i accounting for the rear volume independent of the port volume?
Yup, Vb in WinISD is net of all inclusions, so it's the volume left over after accounting for the port, backside of the driver, bracing, amp modules (if built into the speaker box) etc etc.

HTH,
David.
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Old 22nd October 2019, 10:48 AM   #4
papasteack is offline papasteack  France
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Sim in 2pi with hornresp at 120w (31v) to limit excursion to +/- 8mm :
capture 1 response. +/- 45cm depth simed, hear you see box resonnance effect that will need some damping.
Capture 2 is port velocity at this power. It's seems me a bit high, but ok...
Capture 3 port output to show it first resonnance spike, at +/- 170 hz
Capture 4 is the effect of adding a simple stub of 8cm diameter to put at half lengh of the port, that is heavily stuffed, of 45cm lengh, to null the ugly 170 hz port resonnance and, the distortion it would involve of H2 at 85hz and H3 at 56hz.
Capture 5 show the stub sim shematic.
Attached Images
File Type: png Capture.PNG (24.7 KB, 172 views)
File Type: png port velocity.PNG (20.1 KB, 171 views)
File Type: png port.PNG (28.4 KB, 174 views)
File Type: png SH2 stub.PNG (70.3 KB, 177 views)
File Type: png stub shematic.PNG (8.8 KB, 175 views)

Last edited by papasteack; 22nd October 2019 at 10:58 AM.
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Old 22nd October 2019, 11:34 AM   #5
Tonefarm is offline Tonefarm
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Quote:
Originally Posted by GM View Post
Maybe this will help: https://www.diyaudio.com/forums/atta...ort-length-jpg

Thanks
GM

I have seen this already but struggled to decode the message.. i can see the end corrections etc.

Given the volume of my enclosure without the port for now is that sufficient, before i even get to the port. i have had to design it larger to account for fitting the slot port.

Regards

Dec


GM
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Old 22nd October 2019, 11:49 AM   #6
Tonefarm is offline Tonefarm
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Join Date: Jun 2019
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Quote:
Originally Posted by papasteack View Post
Sim in 2pi with hornresp at 120w (31v) to limit excursion to +/- 8mm :
capture 1 response. +/- 45cm depth simed, hear you see box resonnance effect that will need some damping.
Capture 2 is port velocity at this power. It's seems me a bit high, but ok...
Capture 3 port output to show it first resonnance spike, at +/- 170 hz
Capture 4 is the effect of adding a simple stub of 8cm diameter to put at half lengh of the port, that is heavily stuffed, of 45cm lengh, to null the ugly 170 hz port resonnance and, the distortion it would involve of H2 at 85hz and H3 at 56hz.
Capture 5 show the stub sim shematic.
I have to admit, papa... i am not the best expert in horn resp, so unfortunately i cant imagine how these graphs translate into a type/box/enclosure.

What can you tell me from my existing designs?

Thanks

Declan
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Old 22nd October 2019, 12:42 PM   #7
papasteack is offline papasteack  France
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It's your cab I simed, in hornresp. Just added a long stub at half length of your port to damp first port resonnance in last two pictures.
Low frequency Q would need Eq too and high pass to protect speaker from over excursion.
I would prefer bandpass design for such a speaker, but it add some complexity and tweaking to make it right. So exept the port resonnance problem, this is a good design for the conservative simed output (120w). It would need more bracing since it s for such low frequencies.

Last edited by papasteack; 22nd October 2019 at 12:49 PM.
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Old 22nd October 2019, 01:19 PM   #8
Tonefarm is offline Tonefarm
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Quote:
Originally Posted by papasteack View Post
It's your cab I simed, in hornresp. Just added a long stub at half length of your port to damp first port resonnance in last two pictures.
Low frequency Q would need Eq too and high pass to protect speaker from over excursion.
I would prefer bandpass design for such a speaker, but it add some complexity and tweaking to make it right. So exept the port resonnance problem, this is a good design for the conservative simed output (120w). It would need more bracing since it s for such low frequencies.
Thank you for taking the time to place in the parameters, it's helpful to see.. I need to finalise the designs today to get the wood sent off for CNC. Trying to settle on getting the most low end/output out of this driver.

Any other enclosure suggestions are welcome.

Thanks

Dec
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Old 22nd October 2019, 01:30 PM   #9
David Morison is offline David Morison  Scotland
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Join Date: Nov 2013
Location: Aberdeen, Scotland
Quote:
Originally Posted by Tonefarm View Post
I have seen this already but struggled to decode the message.. i can see the end corrections etc.

Given the volume of my enclosure without the port for now is that sufficient, before i even get to the port. i have had to design it larger to account for fitting the slot port.

Regards

Dec
OK, so what the equations GM provided are dealing with is a factor called End Correction (apologies if I'm covering stuff you already know - I just feel explanations usually work better with a bit of background).

So, a normal port, mounted on a baffle, with its backside in the middle of the cabinet, behaves reasonably predictably as far as modelling its length.

With anything other that that scenario, the different conditions at the back end of the port change how the air at the end of the port behaves a little. End Correction is a factor added into the equation to calculate the port length to account for this.

Unfortunately, WinISD only calculates that standard scenario properly.

The more that you have any of the walls of the port extending beyond the "nominal" port end, the longer the port will behave. A slot port is about the maximum example of this - 3 out of 4 port walls (ie the sides and base of the cab, typically) are a lot longer than the shelf itself.

So the best thing to do with the info given by GM is throw it into a spreadsheet and instead of using the 102cm predicted by WinISD, use the revised (and shorter) port length you will get from that.

Something like this should do it: Port End Correction.xlsx - Google Drive

You need to input the port dimensions, number and box info and it'll calculate the new length for you - don't overwrite the bolded cells or it won't work.

HTH,
David.
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Old 22nd October 2019, 01:36 PM   #10
Tonefarm is offline Tonefarm
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Join Date: Jun 2019
Location: U.A.E
Quote:
Originally Posted by papasteack View Post
Sim in 2pi with hornresp at 120w (31v) to limit excursion to +/- 8mm :
capture 1 response. +/- 45cm depth simed, hear you see box resonnance effect that will need some damping.
Capture 2 is port velocity at this power. It's seems me a bit high, but ok...
Capture 3 port output to show it first resonnance spike, at +/- 170 hz
Capture 4 is the effect of adding a simple stub of 8cm diameter to put at half lengh of the port, that is heavily stuffed, of 45cm lengh, to null the ugly 170 hz port resonnance and, the distortion it would involve of H2 at 85hz and H3 at 56hz.
Capture 5 show the stub sim shematic.
Screenshot 2019-10-22 at 16.34.07.jpg

I have placed the stub your talking of in the enclosure, is this what you were suggesting? (check attachment photo)

Cylindrical in nature and governs the width of the port, 8CM in diameter, presumably that acts as an acoustical notch filter... interesting to see how this may work.

Thanks

D
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