Toccata Grande, Part 1

By Lars Mytting
20 Jan 1998

How many Bel?

But: How can we use midranges with lower sensitivity than the woofers!? The two woofers have a total sensitivity of 98 dB, don't they?

Well, yes, but that is their average sensitivity. Remember that sensitivity figures for drivers are taken as an average, and the exact sensitivity may vary greatly with frequency. The CA25FEY drivers have a steeply rising response, and at 2000 Hz they have a sensitivity of 100 dB! But where we plan to use them -- up to 3-400 Hz, they are at a more normal level of ca. 92 dB pr. driver.

Also, remember that no crossover cuts each driver completely silent after the crossover frequency. If we make a low-order crossover that rolls the woofer of gently -- or if it has significantly higher sensitivity -- it will fill in the otherwise missing output of the midrange.

So -- what do we have? Two 10" and two 5". That gives a total Sd of 910cm2. Later measurements proved the sensitivity to be satisfying, circa 94 dB/1w (or 9,4 Bel -- decibel is one tenth of a Bel, named after Alexander Graham Bell, inventor of the telephone).

Let us compare this with the industry standard 6.5" speaker of 8.6 Bel sensitivity and Sd of 120 cm2. When 120 cm2 is to move 1.5 mm at a given frequency, 910cm2 needs to move just 0.2 mm, because the cone area is 7.6 times bigger [4]. A amp connected to the 6.5" must deliver 10 watt to achieve the reference SPL, but the 9.4 Bel speaker gives the same output with only 1.25 watt invested.

Thus, the Toccata works 7.6 timer easier than the single 6.5", and the amp connected will work 3 times easier.

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