Below its resonant frequency, a driver becomes less efficient. Ergo, it performs best above its resonant frequency which for a woofer should thus sit well below 20Hz. Unfortunately it usually sits somewhere between 20-60Hz right in the active bandwidth. Compounding the issue would be a sealed cabinet whose trapped air acts as a spring to drive the resonant frequency up higher – not what we want. Plus, we'd lose efficiency below that now raised figure for less bass. What to do? Again, one determinant of the resonant frequency is cone weight. Why not just make the cone heavier to drive down its resonant frequency? This too would lower the driver's efficiency. How about enlarging the cabinet to reduce the internal air pressure? Enter the monkey coffin. It's a working solution but less popular in the home. To maintain reasonable size, we get to ported cabinets which release some of the internal air pressure. When properly tuned with the right air volume inside the port tube, the air inside the cabinet will be in phase with the air outside the cabinet. The downside of this approach are port effects in the room; port noise; and time distortion because to work, the port must resonate/ring.

Enter feedback and Bruno Putzeys' famous slogan "one can never have enough feedback". Mister nCore designed an entire amplification league around the proper (linear across the bandwidth) application of copious negative feedback. With LF response in our sights, feedback may also be applied as motional feedback à la Velodyne's accelerometer-controlled subwoofers. Mind you, there's no leading 'e' with motional feedback. That comes later. For now, wouldn't it be nifty if we could measure cone movement in real time to compensate for response loss below the resonant frequency? How to harvest the data for the cone movement? Why not a microphone? Well, it might pick up other sounds to corrupt the measurement.

Another option is the introduction of a second coil to the driver to measure the voltage with its induction when the coil traverses the magnetic field. The challenge now becomes how to isolate the additional coil from the primary one. A better solution is fitting a small piezo-electric sensor to the cone, preferably in its centre. Cone movement will induce tiny voltages in the piezo to represent driver acceleration, making its data detection not susceptible to magnetic or acoustic interference.

This basic idea materialized in a 1973 Philips patent where a piezo-electric accelerometer in the heart of a loudspeaker cone created the input data for a correction mechanism at the loudspeaker's output. The first loudspeaker to be marketed with motional feedback or MFB was the 22RH532. Thanks to a FET introduced at the same time, a circuit fast and clever enough to cope with all the quirks MFB introduced was possible. Philips built the 22RH532 as an active 3-way with a 20-watt amp for the midrange and tweeter and a 40-watt amp for the woofer. The MFB accelerometer kicked in below 200Hz. We personally listened for many many hours to MFB loudspeakers way back in 1973/1974. Imagine a literally smoke-filled room without furniture except for some mattresses, psychedelic wall paintings and Pink Floyd's The Dark Side of the Moon rising up from the Philips MFB speakers. Delve deeper and imagine the track "Money" in that hazy room ping-ponging the stereo effect of coins and cash register. Ka'tchinnnng…

Back to present-day reality. At Grimm Audio, they already had guys with a Philips background, with a strong affection for music and a solid grasp on electronic traditions who wanted to improve the low-frequency response of their subwoofer. What more logical step was there than to learn from the past and modernize it for the future? Where the Philips MFB had been an analog circuit, today's digital tech suggested that the concept could be seriously improved upon.

By 2014, Rob Munnig Schmidt joined the Grimm Audio team. After studying mechanical engineering at the Delft University of Technology, Rob spent many years at Philips' R&D department. While there, he combined mechanics with his growing experience in analogue electronics, a combination which paved the way for a career as one of the first Dutch mechatronic engineers.

Rob was into audio from a young age. His first experiences with real high-fidelity sound happened with church organ recordings which he attended with his first and most important audio teacher, the recording engineer Luc Ludolph. This background knowledge paved the way to designing very accurate motion-control systems for the first wafer steppers at Philips Research around 1980, with Lorentz actuators and custom designed D-class amplifiers. These steppers were later used by Philips offspring and chip machine manufacturer ASML. And yes, it was Bruno Putzeys who at Philips perfected class D. Rob was appointed professor in mechatronics design at his alma mater in Delft in 2006. While still at Philips, Rob frequently visited their audio lab. At the time the lab was developing the MFB system which of course was in line with Rob's interests and expertise albeit as yet in another field. In his spare time he then built his own speakers.
Grimm in Munich. From left to right: Guido Tent, Rob Munnig Schmidt, Eelco Grimm, Hans van Bommel.
Rob's MFB speaker