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-   -   F3 and Fb significance (http://www.diyaudio.com/forums/subwoofers/215560-f3-fb-significance.html)

Shpilev 3rd July 2012 05:09 PM

F3 and Fb significance
 
Hey,

I'm in the middle of deigning an enclosure for a Audes 75w104-8. The subwoofer would be used in a room. The room itself is about 35 cubic meters and in the shape of a rectangular prism.

Here is what I've come up with so far :

http://img585.imageshack.us/img585/2...111111111s.jpg

The parameters for the driver are as follows:

10''
8 Ohms
RMS: 130 W
Max: 200 W
Range 30 - 2000 Hz
Sensativity (2,83 V/1 m) 92 dB
Fs 31 Hz
Mms 37,5 g
BL 12,37 Tm
Q ms 2,83
Q es 0,31
Q ts 0,28
V as 100,5 ltr

This driver is also available in the latest BassBox pro 6 database with all the parameters.

Now is the fact that my F3 is higher than the Fb a problem? Are my F3 and Fb too close to each other? Are they too high at the moment for good SQ? I'm looking more for SQ than SPL.

Any assistance would be great. Thanks.

OscarS 4th July 2012 12:54 AM

it's not a problem so long as you are happy with the predicted frequency response. OTOH, I personally would try modeling it with a tad more enclosure volume and slightly lower tuning to extend the low-end a bit more.

bjorno 4th July 2012 01:27 AM

1 Attachment(s)
Quote:

Originally Posted by Shpilev (Post 3080307)
..Now is the fact that my F3 is higher than the Fb a problem? Are my F3 and Fb too close to each other? Are they too high at the moment for good SQ? I'm looking more for SQ than SPL...

Hi Shpiley,

You've done a good simulation job, No worries, everything is in order ..Will post a MJK:sim. tomorrow to prove my statement.Have a look at this:

b:)

Shpilev 4th July 2012 07:15 AM

Quote:

Originally Posted by OscarS (Post 3080774)
I personally would try modeling it with a tad more enclosure volume and slightly lower tuning to extend the low-end a bit more.

Well... I wouldn't dare doing that. Although I agree that the tuning might want to be even a little lower - 55 liters is big enough as it is for this 10'' driver. Actually people usually recommend about 45 liters for this driver, however I did not achieve satisfactory response with an enclosure of that size. Hence I went up to 55, but that's probably as high as I'll go. If I remember correctly - a bigger enclosure than this actually tended to increase the F3 again.

Quote:

Originally Posted by bjorno (Post 3080795)
Hi Shpiley,

You've done a good simulation job, No worries, everything is in order ..Will post a MJK:sim. tomorrow to prove my statement.Have a look at this:

b:)

Haha... well thank you:D. I'm really trying to model the best box I can. It is my first project.

bjorno 4th July 2012 01:17 PM

2 Attachment(s)
Quote:

Originally Posted by Shpilev (Post 3080966)
..It is my first project..


As promised:

b:)

djk 4th July 2012 08:05 PM

36hz box design.

Driver Properties
Name: 75W104-8 (2003)
Type: Standard one-way driver
Company: Audes
No. of Drivers = 1
Fs = 31 Hz
Qms = 2.83
Vas = 100.5 liters
Cms = 0.689 mm/N
Mms = 37.5 g
Rms = 2.633 kg/s
Xmax = 4.5 mm
Xmech = 6.75 mm
P-Dia = 202 mm
Sd = 320.5 sq.cm
P-Vd = 0.144 liters
Qes = 0.31
Re = 6.4 ohms
Le = 0.933 mH
Z = 8 ohms
BL = 12.37 Tm
Pe = 130 watts
Qts = 0.28
no = 0.931 %
1-W SPL = 91.84 dB
2.83-V SPL = 92.81 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box w/ Active HP Filter
Shape: Prism, square (optimum)
Vb = 1.141 cu.ft
Fb = 33.21 Hz
QL = 6.983
F3 = 35.84 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 2.75 in
Lv = 10.48 in
-----------------------------------------
Active 12 dB/oct. HP Filter
Fx = 35.54 Hz
Qx = 1.93

djk 4th July 2012 08:09 PM

Alternate 31hz box design.

Driver Properties
Name: 75W104-8 (2003)
Type: Standard one-way driver
Company: Audes
No. of Drivers = 1
Fs = 31 Hz
Qms = 2.83
Vas = 100.5 liters
Cms = 0.689 mm/N
Mms = 37.5 g
Rms = 2.633 kg/s
Xmax = 4.5 mm
Xmech = 6.75 mm
P-Dia = 202 mm
Sd = 320.5 sq.cm
P-Vd = 0.144 liters
Qes = 0.31
Re = 6.4 ohms
Le = 0.933 mH
Z = 8 ohms
BL = 12.37 Tm
Pe = 130 watts
Qts = 0.28
no = 0.931 %
1-W SPL = 91.84 dB
2.83-V SPL = 92.81 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box w/ Active HP Filter
Shape: Prism, square (optimum)
Vb = 1.5 cu.ft
Fb = 31 Hz
QL = 6.983
F3 = 31.2 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 3 in
Lv = 10.74 in
-----------------------------------------
Active 12 dB/oct. HP Filter
Fx = 31 Hz
Qx = 2

Shpilev 4th July 2012 08:21 PM

Thanks for the help.

djk the first option - maybea 2.75 in vent might be a little small ? Could it start causing vent noise ?

I'll definitely consider the second box tough.

sreten 4th July 2012 08:36 PM

1 Attachment(s)
Hi,

For the bass end in general its not best to optimise F-3dB. F-6dB
is a lot better and sometimes even F-10dB for some drivers.

This allows room gain to work properly and low frequency vent
tunings help prevent overloading below the vent tuning frequency.

Ideally you would add a peaking high pass filter at around
30Hz for AV use, probably not needed in a music context.

The red alignment has near the same -3dB, better - 6dB
much better -10dB. It will sound deeper and tighter.

rgds, sreten.

I'd tune 55L to 30Hz, but reckon 40-45L 33Hz is about right.

djk 4th July 2012 10:20 PM

"maybea 2.75 in vent might be a little small ? Could it start causing vent noise ?"

The computer calculated the minimum vent size based on the Sd, x-max, and Fb of the design. On the 31hz design I forced a larger vent than what was the minimum required.

As suggested by sreten, you might be able to reduce the Q of the high-pass filter if you have a lot of room gain in the 30hz region.


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