The Subwoofer DIY Page v1.1
Dipole Bass Systems
20 October 2018
Dipole subwoofers are quite different to the other subwoofer systems described on this site because of the way they treat the the driver's output. Your typical subwoofer driver produces sound from both the front and the rear of the cone, and the output from the rear is out of phase with the output from the front, which results in very reduced response levels, unless the rear wave is treated in some fashion. The other subwoofer systems described on this site all employ some means of dealing with the driver's rear radiation to improve overall low frequency response, the result being a "monopole" bass system that theoretically has the same response characteristics in all directions. However, for dipole bass systems, the rear radiation is left untreated, and instead the overall response of the system is adjusted by varying the size of the baffle and the "Q" of the system to achieve the best overall response characteristics.
The following table demonstrates the relation between the baffle's effective diameter (i.e. the diameter of a circular baffle that has the same radius as the smallest dimension of the baffle), Fpeak, and Fequal:
From the table, it's plain to see that it's nearly impossible to push Fequal much lower than 80 Hz unless a fairly large baffle is used. The tradeoff here is efficiency; the smaller the baffle, the lower the final efficiency of the dipole system. OTOH, the larger the baffle, the higher the efficiency, but response at the upper end of the passband could get somewhat irregular as Fpeak is reduced.
Almost all dipole bass designs incorporate some means of boosting the response at low frequencies to compensate for the baffle loss. Typically one or more of the following methods are used:
Let's say we want to build a dipole bass system using four 12" drivers with the following specifications: Vas: 164 litres., Fs=30Hz, Qts=1.10, Qes=1.30, Qms=7.0, R=8 ohms, Xmax=8mm, Sd=0.0547m^2 .
To maximize efficiency, the drivers will be wired in parallel, giving an effective Re of 2 ohms. We also want to know what SPL levels we can expect if we drive the system with 100W of power.
We decide to use a baffle with an effective diameter of 1.35m.
From this, Fpeak and Fequal can be calculated:
Fpeak = c/(D)
Fequal = Fpeak/3
We can model the baffle loss by using a spreadsheet that I put together for the purpose, called dipole.xls. The spreadsheet's simulation is accurate enough for our use below Fpeak. Above Fpeak, the response of the system is greatly influenced by the shape of the baffle, so no attempt is made here to include it in the simulation.
Below is shown the estimated response curve for the given driver mounted in the stated baffle:
To compensate for the 6dB/oct rollof, we select to do two things: increase the Qts of the system to 1.75 to flatten the low end response, and use a line-level or active 18dB/oct LP filter at around 80Hz to reduce the high frequency response.
The effect of the increase in Qts can be seen in the graph below:
The graph below illustrates the effects of an 78Hz 18dB/oct filter on the response:
As can be seen from the graph above, the increase in Qts and the addition of the filter will produce an on-axis frequency response that extends from 28Hz to 90Hz, 0,-3dB.
To increase the Qts to the target value, we can use a series resistor Rs, and calculate its value as follows:
As we plan to drive the system with 100W of power, assuming 10:1 differences between average and peak levels, we can use a 10W or greater resistor for Rs.
As the total resistance, Rs+Re, will be 3.6 ohms, the amplifier will have to be capable of driving at least a 3.6 ohm load. There will also be an efficiency gain as we're using 4 drivers, and an efficiency loss because of the baffle loss and the filtering. These need to be taken into consideration when working out the final efficiency of the system.
Special thanks to the following for corrections, links and other assistance with this page: