Toccata Grande, Part 1

By Lars Mytting
20 Jan 1998

Common sense and sensitivity

Another natural goal is high sensitivity. Sensitivity corresponds to the sound level you get for the power that is fed into the speaker. Normally, this is measured by the sound level at one metre for 1 watt input [2]. The average 6.5" speaker we mentioned earlier usually has a sensitivity of 85-88 dB. (Very often, manufacturers cheat with the specifications, so that the real sensitivity may be as much as 4-5 dB less than claimed!)

When sensitivity is improved by 3 dB, the output needed from the amplifier is halved. If a 82 dB speaker needs 30 watt to reproduce a certain music signal, a 85 dB speaker will require only 15 watt. And if we could increase sensitivity to 91 dB; the amplifier would just have to deliver 3.75 watt for the same sound level.

The less demand you can put on the amplifier, the better. The amp will use less of its power reserves, making it easier to deliver all the needed output; another step towards our goal of good transient response.

The following example explores the benefits of high sensitivity and low excursion: Say that we want to achieve 100 dB SPL at 150 Hz, corresponding to a air displacement volume of 70 cm3. This is how two typical drivers will manage this: (Sd corresponds to cone area).

Driver 1: 5", Sd 80 cm2, sensitivity 85 dB/1w. The cone must travel 9 mm p-p, and the amp must deliver 32 watt.
Driver 2: 10", Sd 350 cm2, sensitivity 94 dB/1w. The cone must travel 2.1 mm p-p, and the amp must deliver 4 watt.
With driver 2, the amp will have to deliver just 1/8 of the output needed for driver 1, and the driver excursion is only 1/4.

Is it just to load up as many drivers as the baffle can hold then? Sorry, no. Performance of large drivers decrease in the higher frequencies, with poorer radiation and more cone breakup. Big cones require big cabinets. Using multiple small drivers may introduce problems with the radiation pattern. The best midranges are often 4" or 5" big. So ultimately, the final sound quality of a speaker is not only determined by whether the drivers are large or small.

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