8th April 2003 - Direct Radiator Loudspeaker Systems with high Bl

On Tuesday 8th April, we were honoured to have John Vanderkooy from the University of Waterloo in Canada give a presentation to the Section. John began the lecture with a description of various modern magnetic materials, comparing BH curves. For example, by using Neodymium Iron Boron a BH product of 10 times a normal magnet can be achieved. A loudspeaker built with such a magnet would have a high Bl. The rest of the lecture explored various characteristics of high Bl loudspeakers.

The acoustic output of a high Bl loudspeaker is not flat, so EQ is needed. The equations for a sealed box system were studied. The mass of air near the cone was plotted against frequency. The air load must be included; normally this is done at frequencies less than 1kHz.

With high Bl, the impedance of the driver goes from 60 ohm to >200ohms. Also, the load is reactive over much more of the audio band. The corner frequency of roll-off is 25 times higher than a normal loudspeaker. In audio frequency band, the driver is velocity damped. The box damping now becomes almost insignificant. Self damping of the drive unit becomes very high. This is caused by the high back EMF.

Matlab was used to simulate the driver with normal Bl and 5x Bl. Pop music was used as signal material. The amplifier needs more voltage swing than with a normal driver, but the current is much reduced. The power dissipation is class B devices in the amplifier is reduced. The average power in the loudspeaker is reduced by 10 times. Speakers of up to 40% efficiency can be built compared to the usual 1 or 2%.

High Bl loudspeakers are best driven by class D amplifiers. This is because the loudspeaker returns power to the amplifier due to the high inductance of the coil/magnet assembly. Class D amplifiers use an inductor in the output section as part of the output filter and are able to both source and sink current.

John made a loudspeaker with an electromagnetic magnet. Unfortunately, the pole faces saturated, so this did not result in a high Bl drive unit.

Further simulations were done which reduced the size of the box, using a high Bl drive unit. The results showed that you can get deep bass from a sub-woofer with a small box with a high Bl drive unit. EQ is needed to give a flat response, but the EQ needed is quite easy to implement.

Next a drive unit with just double normal Bl was studied. Now the corner frequency changes only by a factor of 4 and much milder EQ is needed. Adamson Acoustics make a 9inch high Bl driver with a huge magnet. The frequency response lot shows 50Hz corner frequency, and good bass response with only a 9 litre box.

Next vented systems were studied. The cone's acceleration is given by a combination of the box characteristics and the vent details. Vented box loudspeakers have extended bass, but it rolls off faster than a sealed box. When Bl in increased by 5times, the port resonance becomes almost undamped, generating a peak in the response at the resonance. This peak then requires a sharp notch filter EQ. This is not practical. Therefore high Bl drivers need a sealed box.

How do you correct for High Bl frequency response without using electrical EQ? You could increase cone mass, but this is not very useful because efficiency goes way down. Using electrical EQ is best.

A driver with an adjustable Bl was made by Philips. The Bl could be adjusted by adding external steel pole pieces. Commercial high Bl drivers are made by JBL. An example was shown with a Bl of 32.

The main points of the lecture are that high Bl drivers are controlled by back EMF. Efficiency is very high. Small sealed boxes can yield good bass, but electrical EQ is needed. Strong boxes and driver construction is needed. A small amplifier can generate loud, high quality, audio with a properly engineered high Bl drive unit.

An lively question and answer session followed the lecture, including John Watkinson demonstrating a CD player with a loudspeaker system using many of the concepts described in the lecture.

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