‘Tis often said in loudspeaker circles that a good big ‘un always outperforms a good little ‘un and this is certainly true when it comes to bass units. Today’s loudspeaker industry, succumbing no doubt to modern ‘lifestyle’ prejudices, is concentrating more and more on reducing the size of speakers and, therefore, drive units. The result is that nowadays a 6 inch bass unit is considered ‘large’. If we look back to the ‘60s, however, an 8 inch bass unit was considered ‘small’ and, if you wanted real bass, there was no substitute for the 10 inch, 12 inch and 15 inch units that abounded in commercial speakers of the time.

To be fair technology has marched on and we are not suggesting that a speaker with multiple 6 inch drivers could not produce bass levels that are certainly satisfactory given a suitably powerful amplifier. And therein lies the rub – as the drive unit diameter goes down so does the efficiency.

To put this into perspective it is relatively easy to create a loudspeaker with 90dB sensitivity for 1 Watt using a 10 inch bass unit. Drop down to a 6 inch and you would be hard pushed to achieve 84dB. This difference of 6dB may not sound much but it means quadrupling the amplifier power to achieve the same sound pressure level (each 3dB step requires doubling the power).

Many modern designs get round this by reducing the impedance. For a 2.83v input (specified because 2.83v into 8 Ohms is 1 Watt) you can, theoretically, improve apparent efficiency by 3dB by specifying a 4 Ohm drive unit. However this draws twice the current of an 8 Ohm unit; not a problem if your amplifier has a good power supply and large output transistors, but it does place more of a strain on the amplifier overall.

And, of course, if you are currently residing with a (relatively) low powered valve amplifier this sort of current requirement is that last thing you need. There is also the factor of ‘strain’ itself. Those of us who do hark back to the ‘golden’ days of hi-fi will remember the sense of ease that efficient speakers applied to the reproduction of music.

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