If a tweeter is to fully respond to the dynamics of music, it must run as cool as possible, especially when driven hard. The problem is that a tweeter’s voice coil has to be small and light for it to respond to very high frequencies and so it heats up very quickly. The hotter the coil gets, the higher its electrical resistance becomes and the current through it, which determines for force exerted on the dome, ceases to be proportional to the voltage signal from the amplifier. Compression is the result, with the music losing its true life.
The use of neodymium-iron-boron magnets for their small size can make the problem worse because they make a less effective heat sink. That’s why the Nautilus™ tubes of 800 Series tweeters are metal. They help carry heat away from the voice coil. Forcing more magnetic energy into the voice coil gap would reduce the current required to reach any given output level, allowing the coil to run cooler. But how, given that the gap is small and the steel parts are already close to saturation? Normal motor systems use a single magnet (1), sandwiched between a top plate and a backplate/centre pole and the voice coil sits in the gap between the top plate and the centre pole. In order to focus the magnetic energy right where the coil sits, the top plate is little more than a millimetre thick and is easily saturated.
Increasing the size of the magnet results in very little extra magnetic energy in the gap. The trick is to use extra magnets that are polarised in the opposite direction to the field produced by the main magnet. The first is placed behind the backplate (2). This is the position often used in shielded magnet systems, where the extra magnet is used with a metal can that surrounds the whole motor system. In this case we do not need the can, but a further two magnets on top of the top plate (3) and on the tip of the centre pole (4) serve to direct the energy through the coil. This construction can add around 2dB to the sensitivity of the coil, which reduces the power requirement by around 40%. As a result, the tweeter runs cooler, compression is reduced and the music comes back to life.
Find out what Bowers & Wilkins customers and audio enthusiasts are talking about on our blogs, and read in-depth articles in the Sound Lab.
Be part of ‘One Album Per Hour’, with BeardymanJuly 10,2014