Custom biquad filters open the door to a wide range of audio filtering applications, up to this point never available on low cost Digital Signal Processing platform. With the introduction of miniDSP Advanced plug-ins, building a Linkwitz transform filter is just an example of how flexible these pocket size DSP boards can be.
The linkwitz transform (also called LT) was initially invented by Siegfried Linkwitz and now widely used in many subwoofer configurations. The purpose of the LT filter can be summarized as extending the frequency response of a closed box in the low end of the spectrum. With precise equalization based on the driver & box Thiele & Small parameters, one can indeed remove the typical bump before the natural driver roll off but also increase the response on the low end.
Some very good reference sites will provide more detailed information about the basics of an LT. We could recommend as a first read the following:
- Linkwitz Lab: The reference for many filtering applications as it includes all the theory from the inventor Siegfried Linkwitz
- Elliot Sound Products: Another great reference when it comes to Audio theory. Rod Elliot's site remains a great source of information when it comes to DIY audio projects.
One of the beauty of miniDSP is how versatile these boards can be. Whoever built his own loudspeakers already experienced how simulation can sometimes be quite off compared to the real world. With trial & error, updating filtering or equalization of a box in real time is a very powerful feature of DSP products. The second positive aspect of the advanced plug-in is how easy it becomes for miniDSP community users to experiment custom biquad filters. The Linkwitz Transform project, powered by a valiant team of DIYers from Netherlands, is a living proof that creativity, combined with the right tool, can open new doors.
With constant improvements to the excel based biquad calculation spreadsheet, the following Forum thread is a great resource. All credits go to "Team Netherlands" for their great documentation and experimental work. Make sure to drop a thank you note if you find the spreadsheet useful for your application.
Download the biquad calculation spreadsheet
If you also want to experiment how Linkwitz transform will improve your system, you only need:
- A closed box along with its parameters
- A copy of the latest version of the Linkwitz Transform spreadsheet available from the forum link above
- 1 x miniDSP platform (kit/2x4/2x8/8x8)
- 1 x plug-in with the "Advanced Biquad programming feature" (please check table here for confirmation)
- Finally, a freeware measurement software with a microphone to easily measure the effect of pre/post filtering. Arta/REW/Smartlive... are just some examples of the many software package available for free or for sale online.
Got a great concept for a custom filter? Want to share ang get your idea featured on the miniDSP website? Simply get involved in the community!