July 2022

The Trouble With Loudspeaker Drivers...
...and radio frequency interference (RFI) pickup by cables.
Editorial By Noel Keywood

  The trouble with loudspeaker drive units is they send sound backwards as well as forwards. Long ago they realised that If a cone moved forward, compressing the air ahead of it, this air would race around to the back, cancelling sound – inconvenient. Some bright spark realised this could be prevented by putting the drive unit in a box – which is where we are today. Trouble is that trapped sound causes other problems.

This has concerned KEF for some time – I recall they once had magic charcoal to counteract the problem – but all has gone quiet on that front. Now they have come up with Metamaterial. It's only for tweeters but certainly has quite an affect, You can read more about it on p11 where we review their new Reference 1 Meta stand-mount loudspeaker.

The subject of radio frequency (RF) pickup by loudspeaker leads is a fascinating one that gets raised on p34 where we look at a pair of filter blocks designed to remove it. As Martin Pipe says, loudspeaker leads can act as aerials and this they used to do in areas surrounding the Crystal Palace TV transmitter in South London. TV sound would break through into the hi-fi via the 'record player'. Strong TV signals were travelling from loudspeaker output to phono input, overloading it and getting rectified. Cure was to solder a small capacitor across the first transistor's base-emitter junction – simple and quick; I did it often.

These days we are surrounded by transmitters in our home so it is possible such problems still exist, even though there are now stringent rules to minimise spurious radiation. But this does not impact Wi-Fi or Bluetooth radiated power, nor the regular 3W bursts from a 'phone to handshake with the local 'phone mast'; we've all become walking transmitters, for better or for worse.

Helping resist this in today's turntables is balanced connection through screened leads, bringing me to Pro-Ject's latest X8 turntable on p71, ideal for a modern world full of RF.

Hi-fi has its strange sides and we look at them in this issue. I hope you enjoy.

— Noel Keywood
Editor, HiFi World

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