Best known as the working principle behind Velodyne’s patented High-Gain Servo System-equipped subwoofers, is motional feedback truly the future when it comes to hi-fi dynamic loudspeaker design?
By: Ringo Bones
Seasoned audiophiles, more often than not, had their first hand encounters with motional feedback technology via adverts – and hopefully purchases – of Velodyne’s world-famous servo-controlled subwoofers. These famed subwoofers are famed for their very low levels of harmonic distortion and coloration – when compared to their competitors’ offerings – that doesn’t use Velodyne-style High-Gain Servo System technology. But before we proceed any further, here’s a primer on what is motional feedback.
Motional feedback uses a second voice coil on the drive unit – typically a large woofer in stand-alone subwoofer systems – that provides a signal in which a tiny millionths of a second later is fed back into the amplifier, correcting distortion. The result – if correctly implemented – is amazingly powerful articulate bass / low frequencies devoid of coloration and distortion.
In those famous Velodyne subwoofer adverts of the 1990s, the company points to their use of a low mass – 2.5-gram accelerometer – that form the brains of their High-Gain Servo System. Velodyne mounts this amazing device directly on the voice coils of their subwoofer drivers, and measures the actual movement of the driver. The information is then sent back to a circuit, which makes corrections for any deviations from the pure output signal 3,500 times a second. Resulting in a virtually distortion-free bass even when the subwoofer is set at its highest frequency setting of 120-Hz.
My encounter with Velodyne’s FSR-18 subwoofer – one of the very best subs that we (me and my audio-buddies) can still afford when we still have Clinton administration era prosperity money during the 1990s. It can easily be described as a technological tour-de-force, imagine very loud and very clear bass subtle enough to portray the acoustic structure of the venue where the music being reproduced was originally recorded. Although early samples of the Velodyne FSR-18 subs were notorious for having loose wiring that came loose and slapped against the moving cone producing a tapping sound – evident when What’s Left by Lunachicks – from their Pretty Ugly album - was played during that particular listening session. Thus resulting in another flight to Hong Kong to our very friendly hi-fi dealer that gladly replaced our faulty subs.
In reality, motional feedback-type loudspeakers are a pain to design and make to work properly – tweaking one should only be a job for very gifted electrical / electronic / mechanical engineers – i.e. probably those skilled enough to make a humanoid robot that can run a hundred-meter dash in less than 10 seconds. It is not just Velodyne who had faced such problems developing and perfecting their very own motional feedback-equipped loudspeakers. Celestion and once-upon-a-time-loudspeaker-manufacturer Philips also experienced first-hand challenges one will likely encounter when flirting with motional feedback technology.
Graham Bank of Celestion once stated in a hi-fi magazine interview that a lack of positional reference in their prototype motional feedback loudspeaker design resulted in an enormous crack from the cone as it attempted to leave the chassis on musical peaks. Each time it did this, the connecting braids carrying the signal from frame to the voice coil broke, even though they were long enough to cope with the movement. Both Graham Bank at Celestion and Paul Mills at Tannoy have worked on motional feedback-type hi-fi loudspeakers during much of the 1970s and were somewhat convinced that that there were some deep seated difficulties in its application.
Even a notable demonstration of motional feedback technology by Philips – back when they were still making hi-fi loudspeakers – at their headquarters in Eindhoven, Holland, that motional feedback gave the sort of fast, tight and even bass most hi-fi enthusiasts dream about. Fantastic bass quality - provided that the user kept the volume down, otherwise, the driver instantly destroyed itself. The technicians at Philips also noted in their motional feedback loudspeaker experiments conducted during the 1970s is that much of the correction by the accelerometer and the servo system was being applied to signals above 100-Hz.
So is motional feedback the future of high-end hi-fi loudspeaker design? Well, remember how Velodyne’s FSR-18 subwoofer compares the deviation from the pure output signals and the one sensed by its low-mass accelerometer at 3,500 times per second? This is just with an audio bandwidth that rolls-off at 120-Hz – although it works beautifully, imagine this concept applied to the full Redbook spec CD audio bandwidth of 20,000-Hz. Corrections would be applied at 600,000 times a second – that’s just with a 20,000-Hz bandwidth. Imagine the difficulty with DVD-audio or Super Audio CD / SACD that reaches out to 100,000-Hz – a motional feedback hi-fi loudspeaker with a 100-KHz bandwidth would be doing servo corrections at 30-million times per second. Full audio bandwidth motional feedback hi-fi loudspeakers may be the future if its quirks are ironed out, but for now – and for simplicity’s sake – they’re only practical for subwoofers that only play notes as high as 120-Hz.