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| By: Scraph on Fri, 3 Oct 21:03:37 -0500
| | There is one common assumption and misconception about what places the limit on how loud a speaker can play. It is generally assumed that power (Wattage) is all that imposes this limit. The reality of the situation, however, while still very intuitive is slightly more complicated.
There are actually three factors which limit the "clean" SPL attainable from a speaker and their prevalence varies with frequency. At the lower frequencies, the exact definition of lower relative to the size of the speaker in question, the SPL is generally limited by the Xmax (peak excursion) of the speaker. As the wavelength at these lower frequencies is long the motor assembly (magnet and coil) does not have to accelerate the cone significantly (governed by F=ma). Instead, the dominate force is that required to displace the speaker against the spring-like suspension (governed by F=kx). Therefore, the SPL at low frequencies is limited by the maximum displacement the speaker cone can undergo.
Above these frequencies, the maximum SPL is generally limited by the power available from the amplifier. This is because the higher frequencies require much higher acceleration of the cone and therefore F=ma becomes dominant. At even higher frequencies, the power handling of the speaker voice coil limits the SPL as it becomes less than the increasing power requirement for some dB level. The third source of SPL limitation that sometimes occurs is around the impedence peak of the speaker. The voltage of the amplifier is limited and essentially constant so near at and around this impendence peak the constant voltage requires a drop in current. This reduces the power output of the amplifier and accordingly the SPL at this frequency.
The general conclusions that can be drawn are this... more amplifier capacity does not always mean a louder system. To produce louder low frequencies (35Hz +/- ~15Hz), either a subwoofer with more excursion or more subwoofers are required, as power is not the major requirement but cone displacement is. To produce more volume at high frequencies more amplifier power is required and the limitation is set by the power handling of the speaker. Once this voice coil limit is reached more speakers will be required to produce more volume above this frequency. So, low frequencies don't require the power, high frequencies do. But the definition of high power of course varies with the sensitivity of the driver.
To see a graph of this behavior familiarize yourself with some computer speaker design software (such as WinISD (copyright LinearTeam)).
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