Saturday, 20 March 2010

What Constitutes a Good Drive Unit?

The Picture above is of the 5" drive unit we use in our Neutron Five loudspeakers. It is excellent and typical of the best available today.

Below are measurements pinched from a DIY forum to illustrate how they compare with an extremely expensive "high end" driver from another manufacture.

Plot one shows a smooth amplitude response to way beyond where a crossover would go and plot two shows  harmonic distortion.

Plot three shows a much more expensive "hi end" driver from another manufacture, which is remarkably similar in amplitude response and plot four shows that it is in harmonic distortion as well. The reality is that either drive unit will give excellent and similar results. However there are other characteristics to consider that will also affect performance.  For example; power handling and maximum linear travel, where ours still compares well. 

What these measurements show is that the performance variations between the between modern paper diaphragm drive units from reputable manufacturers are comparatively small,  though nothing like as small as those between DACs and other electronic components.

There is however a divergence of opinion of what constitutes the best diaphragm material and many prefer  hard or metal ones. 

The above illustration is of a good quality 5" metal diaphragmed driver, which has a smoother amplitude response to 4 kHz, will probably have slightly less harmonic distortion, but which breaks up catastrophically at higher frequencies. AVI doesn't like them and wouldn't use one, but others prefer them and use an extra filter in the crossover to reduce/remove the spike. We don't like this either because it is further distorting phase. 

In our opinion the paper or soft diaphragm is the best compromise because it acts as a shock absorber to suppress and iron out the tendency that cones have to break up above a certain frequency dictated by their size. I think 1.4 kHz for a 5".

These differences aside, all high modern drivers have relatively shallow cones to aid dispersion, large diameter spiders to minimise distortion, they are spaced off the front plate by about 10-12 mm with ventilation underneath to avoid resonances and they have longer coils than the front plate to give lots of linear travel. Ours has + or - 5 mm which is very good and double that before there is risk of mechanical damage. Some may remember the "clack" "clack" of the spider hitting the front plate of an old Kef B110 in LS3/5As!

This last illustration shows a large diameter and compliant spider and ventilation through to the voice coil underneath. This is typical of almost all modern, good quality drive units and has been for some years, although small incremental improvements take place all the time.


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