Toccata Grande, Part 1
By Lars Mytting
20 Jan 1998
No compression -- no depression
Low compression is vital for good transient response. Put simply, compression starts at the point when the input and output of a driver no longer are linear. If the signal corresponds to a output of 99 dB and the speaker output is only 98 dB, compression has occurred. This may happen because the thermal, mechanical or electrical limits of the driver is exceeded.
A general rule is that compression increases greatly with cone excursion. Also, drivers with large voice coil diameters usually have less compression than standard drivers. With standard drivers, compression may occur at low levels, maybe just 3-4 watt. This is something that most speaker driver manufacturers does not like to talk about; but you can have a meaningful discussion of it with professional PA driver manufacturers like ATC and JBL. Also, the happy band of horn speaker enthusiasts is usually able to provide details on this topic. Drivers with low compression gives huge benefit for the dynamic capabilities of a speaker. It will sound more open and free, and the soundstage will be more relaxed at all sound levels -- but the difference will be most noticeable when playing loud.
Less excursion, please
The way to reduce compression with standard drivers is to reduce their amount of work. Their work corresponds to the sound level, which corresponds to the quantity of air being moved, which corresponds to the product of cone area and excursion. So if we want to reduce compression, we should increase the cone area.
Doubling the cone area will reduce excursion to the half [1]. Four-doubling it will reduce it to 1/4. A average 2-way speaker with one 6.5" driver usually has 120 cm2 cone area. If we are using four 6.5" drivers, or a single driver with a cone area of 480 cm2 (like a 12"), excursion will be 1/4 while achieving the same sound level.
It is especially advisable to have this in mind when choosing crossover frequency, because excursion will be four-doubled for each octave. A 25 mm tweeter must move 0.1 cm3 of air to give 90 dB at 2000 Hz, requiring a excursion of 0.2 mm p-p. For the same SPL at 1000 Hz it must move 0.4 cm3 of air, resulting in excursion of 0.8 mm p-p. For 100 dB the necessary excursion is 0.8 mm at 2000 Hz and 3.2 mm (total damage!) at 1000 Hz.
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