Klippel Distortion Analyser
In order for a driver to reproduce complex sounds accurately, the cone must move in exactly the same distance and manner in both directions of travel. If the cone moves 0.5 mm in the forward direction and only 0.4 mm in the backward direction in response to a given sine wave signal, the sound will be distorted. In order for the driver to move accurately in both directions, equal force must be applied to the cone, and the cone must be able to move as far and as freely in both directions. You’d think that would be easy, but it isn’t. Thankfully, Polk Audio owns a Klippel Distortion Analyser, a device that uses a laser and sophisticated software to do large scale analysis of drivers in motion. With this state of the art tool we can optimise the motor structure, voice coil alignment and suspension to yield the best possible performance. Figure 1 is a graph of the BL product of the SR6500 driver. BL Product is the product of the motor’s magnetic flux density times the length of voice coil in the gap. Put more simply it is a measure of the motive force applied to the driver cone. The important thing isn’t whether you have more or less BL product, but whether you have the right amount for the mass of the driver and whether the BL is applied uniformly. Ideally you would like to have the same amount of BL product applied to the cone at every distance of excursion in both directions of travel. The perfect driver would have a straight line graph plot. In the real world drivers have limitations. In these graphs the centre 0.0 vertical line represents the cone at the centre rest position. To the left of centre line is forward cone movement; to the right is backward cone movement. In this kind of graph, you’re looking for a symmetrical curve, centred on the 0.0 line and a broad, flat shape.
Another important Klippel analysis is that of stiffness or springiness of the suspension. Again, you’d like to have a symmetrical, broad, flat curve indicating that the tension strength of the suspension is consistent over a broad range of cone travel in both directions. Greater or lesser stiffness is immaterial, what is important is consistency.
Figure 2is the SR6500 showing a symmetrical and linear suspension stiffness over a broad excursion range.