Big thumper - 6th order bandpass sub with Dayton Reference RSS315HO

Previously I introduced this design as a custom sub offered as an upgrade to a smaller bandpass sub they initially intended.  

The driver: Dayton Audio Reference RSS315HO 

RSS315HO is a driver that works in many designs. It's a driver intended for car audio use, mainly due to its parameters which suit a small enclosure. Often the HO versions of the Dayton Reference series work well in horn designs but in this case, it also works well in a bandpass enclosure. What I like about the Dayton Reference series is that they are well optimised for sound quality, avoiding the temptation to go extreme in xmax and instead focusing on aspects that give us high quality bass when the design is right. 

Why bandpass? 

The short answer is "because the client requested a custom bandpass sub." But why would someone choose bandpass over all the other options. In the past, bandpass subs have been unfairly associated with low quality subs. Often either really cheap computer mini subs or one note noise makers. Many audiophiles after decades in this hobby have not actually experienced a serious bandpass sub. In reality, bandpass designs offer some strong advantages with serious potential. However, let's cover the downsides first.  

Why to avoid bandpass 

Bandpass subs are more difficult to design than sealed or ported designs. They are highly sensitive to misalignment. The final result can often be different to a computer model in ways that makes the final design perform poorly. Many programs are not sophisticated enough to accurately predict their real behaviour. So there is always a little more risk that the final result won't rise to the level of expectations. Further, we see higher group delay with bandpass designs, coupled with port resonances out of band that can seriously degrate the sound. Often typical speaker box lining won't effectively damp the resonances, leaving us with a sub that doesn't sound quite right. Hence, in a poorly excuted design we could see multiple compounding problems.

In a well executed design we can achieve some compelling advantages, including: 

• higher sensitivity where we are willing to reduce bandwidth
• reduced cone excursion for a given SPL level
• lower distortion related to the above advantages
• the front chamber further lowers distortion due to acoustic low pass filtering
• greater maximum output and dynamics
• the ability to drive a sub harder before distortion becomes objectionable

 The cabinet and bracing design

This cabinet has two main horizontal braces that also form part of the port walls. Shelf ports also form the internal walls of the ports, which wrap around the enclosure. Since all the output of a bandpass sub passes through the ports, their design is critical. Bandpass subs are typically more prone to audible port chuffing. Hence this design has very large ports with flares on both inlet and outlet of both ports. 

Build photos coming ...