Driver Rms-value unknown, what to do? - diyAudio
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Old 4th January 2007, 02:24 PM   #1
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Default Driver Rms-value unknown, what to do?

Hi,

I'm currently modelling a few different drivers for a sealed music sub, one of the drivers I'm looking at does not have Rms specified, can this be estimated using other params?

Secondly; Bl is specified with the unit Tm, for other drivers, and in the spread sheet, the unit N/A is used, do I need som kind of conversion here?

I'm using the Linkwitz closed-box spread sheet and both Rms and Bl are required parameters in the spread sheet.

The driver missing Rms has the following other specs. Seems like a nice driver, though not very efficient, so I would really like to be able to model this one correctly ...

Znom 4 ohm
Re 3.44 ohm
Le@1kHz 1.2 mH
fs 21 Hz
Qms 3.53
Qes 0.52
Qts 0.46
Mms 149.3 g
Cms 384.8 mm/N
Sd 333.0 cm2
BL 11.3 Tm
Vas 60.6 ltrs
Xmax 11.6 mm peak
VC Ř 50 mm
Sensitivity
2.83V / 1m 83.3 dB
Nom. Power DIN 300 W
Magnet weight 2.1 kg

Regards,
M.A.
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Old 4th January 2007, 03:14 PM   #2
bjorno is offline bjorno  Sweden
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Hi mac-attack,

Quote:
I'm currently modelling a few different drivers for a sealed music sub, one of the drivers I'm looking at does not have Rms specified, can this be estimated using other params?
Yes.

Quote:
Secondly; Bl is specified with the unit Tm, for other drivers, and in the spread sheet, the unit N/A is used, do I need som kind of conversion here?
No.

Rms= 1/(w x Cms x Qms) ; w = 2 x PI x fs

b
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Old 4th January 2007, 05:24 PM   #3
Ron E is offline Ron E  United States
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Rms=2*pi*Fs*Mms/Qms

T*m = N/A
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Old 5th January 2007, 01:53 AM   #4
bjorno is offline bjorno  Sweden
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Hi Mac-attack,

Use Ron E's formulae regarding Rms instead of the one I submitted which is incorrect.

See my picture for a corrected formulae and an important observation of your supplied driver data.

B
Attached Images
File Type: gif rms.gif (6.5 KB, 84 views)
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Old 5th January 2007, 08:38 AM   #5
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Default Can the Cms value really be correct?

Thanks guys for your help, now I can continue my driver modelling. Nice to see a fellow Swede come to my aid.

Cms 384.8 um, well, I figured as much, millimeters didn't seem reasonable. I got confused for a while there though, reading the XLS 10" specs it said 0.52 mm, which in that case really is millimeters = 520 um. Why can't they all stick to basic units like meters and use scientific notation ...

Following some calculations Linkwitz did on the Thor sub, I find that it takes 6.7 amps to drive it to maximum excursion below the resonance frequency in the box (40 liters), I get ~4.4 amps for the XLS 10". So this driver is more stiff then the XLS 10" (lower Cms), takes a bit more power to drive, but the power reqs are still reasonable I quess.

What else can you guys derive from looking at the specs of this driver compared to e.g. the XLS 10"? For one thing the inductance (Le) is lower, does this to some extent imply lower distorsion? I want a nice and tight low distorsion driver for music apps ...

Regards,
M.A.
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Old 7th January 2007, 04:17 PM   #6
bjorno is offline bjorno  Sweden
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Hi mac-attack,

Quote:
What else can you guys derive from looking at the specs of this driver compared to e.g. the XLS 10"?
Look at the submitted .gif that makes an attempt to compare but without knowing the brand of other driver its impossible for me to comment which one I would prefer.

Quote:
For one thing the inductance (Le) is lower, does this to some extent imply lower distorsion?
No.

b

1(1)
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File Type: gif xversusxls10in.gif (22.2 KB, 64 views)
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Old 7th January 2007, 07:46 PM   #7
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Oh, sorry, the driver I'm talking about is the VIFA MA26WR09-04 http://www.madisound.com/pdf/vifa/ma26wr09-04.pdf.

I intend to build a sealed sub for music mostly, with a Linkwitz transform circuit, sub volume probably not more than 40 liters.

M.A.
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Old 7th January 2007, 09:51 PM   #8
bjorno is offline bjorno  Sweden
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Hi mac-attack

I'm familiar with that driver. Have you search this forum for MA26WR09-04? I found two where I posted to:

’Sealed box size for Vifa M26wr-0908 .?’ and ‘Sub technical spec.’

and there are more..

Do you really need to LT. This driver has a – 3dB cut-off at 32 Hz in a closed box of about 45 L.

Here is some plots I’ve made I think I never posted comparing SS 26W 4867T00 with the Vifa driver.
See the submitted pictures.

b

1(3)
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File Type: gif ssversusvifa1.gif (70.9 KB, 50 views)
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Old 7th January 2007, 09:52 PM   #9
bjorno is offline bjorno  Sweden
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2(3)
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File Type: gif ssversusvifa2.gif (33.1 KB, 45 views)
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Old 7th January 2007, 09:54 PM   #10
bjorno is offline bjorno  Sweden
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3(3)
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File Type: gif ssinvifabox3.gif (49.3 KB, 45 views)
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