Isobaric vented sonotube sub

[tizman]

Hello. I am about to build a tubular subwoofer that uses a doubled up 14" sonotube as its cabinet. The specs of the 10" woofers that I will be using are as follow: Fs 28 hz, Xmax 5mm, Qts .46, Vas 3.8 cubic feet (107.6 litres). The design is simply the two drivers in a clamshell configuration in one end of the tube, and two four inch wide and 26.25" long ports in the other end. The net internal volume of the cabinet will be 5.3 cubic feet or 150 litres. Drivers and vents will be set into 3/4" mdf rings that will be bolted onto another set of mdf rings in each of for ease of removal of vents and woofers. The enclosure will be tuned to 21 hz. I have modeled this with several free spreadsheet programs, and my primary concern is the excursion vs frequency component of the result. I have attached a screen shot of this chart to illustrate my concerns. It seems that the drivers will always be operating beyond their rated excursions. The blue line is for an existing sub that uses the same woofers but in a tube of 2.64 cubic feet with a 3" wide vent that is 8" long and is tuned to 26.5 hz. The red line is for the proposed sub. The existing sub has been working well for years, but I was hoping that the new design would go fown to 20 hz with some volume. Should I be concerned with the excursion vs frequency graph? I could also use a pair of 12" woofers that I have that have the following specs Fs 23 hz, Xmax 7mm, Qts .32, Vas 4.9 cubic feet. Please note that the design will be isobaric and that in both cases the Vas will be half of the one shown for a single driver. Thank you for any advice or thoughts you may have on my build. I intend on posting photos of the final product.

 

[Ron E]

Should I be concerned with the excursion vs frequency graph?

The excursion shown in that graph is for some power level entered in the program, reduce that power level until you are at the excursion line and you will be at the predicted level for max excursion and you can calculate the SPL output there and decide for yourself if it will perform adequately. With decent quality drivers, the prediction is conservative because the spider gets stiffer with larger excursions and you might be able to put ~1.5-2x the power before reaching xmax.

If you want output, don't do isobaric. 2 subs, each about 100 liters, tuned to 25Hz or so will give you ~6dB more output.

 

[tizman]

Thanks Ron E.

I didn't understand that the chart is power dependant. The blue line is for the sub that I already have. This sub works well, and is powered by a 150 watt per chan. Kenwood power amp. I will be using the same amp, and I never need to turn it up to anywhere near full power for my HT viewing. Do you think that based upon my graph, which reflects an input of 150 watts into one channel (although it is actually 150 watts into two channels as I have an isobaric setup) that I will have problems given the greater excursion shown by the red line?

Thanks Again!