Lower frequencies with ported boxes.

[Arx]

I'm building a cabinet for a bass (guitar) amplifier using 4 8" speakers

When I plug the TS parameters into "GSpeakers" and click the optimize button it recommends a volume of 24.1 litres, and a port tuning of 37.3Hz

The bass it will be used with is a 5 string (30.87Hz on the B string), but the plot from GSpeakers says it will be down about 12dB at 30Hz.

If I mess with the numbers, say triple the box size to 75 litres, and drop the port tuning to 30Hz I get a bit of a dip (2-3dB) at around 60hz, then a peak back up to -1dB at around 38Hz, and am only -3dB at 30Hz. Not exactly flat, but it looks like a fair tradeoff for an extra half octave.

I'm assuming there's something else I'm not seeing on a simple frequency response graph.

Will I run into any big problems if I build it this way? I know that obviously at 30Hz I'll be limited by excursion, so I'm not expecting any miracles in that regard (though the port resonance should help a bit), but a less loud amp that will play much lower would be fine with me.

Any thoughts?

-Nick

[sreten]

Hi, driver Vas, Fs, Qts ? /sreten.

[richie00boy]

The dip around 60Hz indicates loss of power handling capacity (lack of excursion) in that area. I suspect you will have significant energy content in this area so you may want to see if you can reach your desired SPLs.

[luvdunhill]

look into "extended bass shelf"

http://www.diysubwoofers.org/prt/ported5.htm

[Arx]

Originally posted by sreten
Hi, driver Vas, Fs, Qts ? /sreten.

Vifa MG22WO0908
Size: 23cm
Voice coil diameter: 40mm
dB 2.8V: 89
W: 80
Z: 8
Re: 5.5
Le: 1.1
Ce: 36
Fs: 21
Vas: 154
Qms: 1.6
Qes: .27
Qts: .23
Xmax: +/-6.0
Cutout: 186mm

richie00boy: It looks more to me like that dip is just the roll-off starting to occur due to the box size. The port is slightly too low to even it out.
I don't think this program even takes power handling into account.



There's the plot that I'm talking about. The red is "optimized" at 24.1 litres 37.3Hz. The blue is 75 litres 30 Hz.

-Nick

[AudioFreak]

Group Delay probably suffers quite a bit also.

[freddi]

how much fundamental is in your low notes?

[Arx]

Originally posted by AudioFreak
Group Delay probably suffers quite a bit also.

Okay. I'll have to look into that one a little. There doesn't seem to be much good speaker design software for linux.

Originally posted by freddi
how much fundamental is in your low notes?

The more the better. There should be plenty of fundamental on th e input side. I just need to try and keep as much as I can on the output side.

-Nick

[sreten]

Hmmm..........

Shown are 4 drivers in a box.
12 cu.ft is ridiculous for a 4x8 and will sound awful.
Too big to lug around and will overload easily.

Around 3 cuft 80 litres for all 4 drivers with a port tuned to
around 33hz for power handling is the best bet for a 5-string.

Add a 3 to 4 cuft 15" 8 ohm for more juice if the amp handles 4 ohm.

/sreten.

[sreten]

Hmmm..........

Shown are 4 drivers in a box.
12 cu.ft is ridiculous for a 4x8 and will sound awful.

:0/sreten.