Normal THD versus frequency plots are not meaningful because they don't represent what we hear. Numerous subjective studies into the perception of nonlinear distortions have shown that THD in loudspeakers is not a major concern - levels as high as 25% have been found to be inaudible. At Ai our main interest is in the power compression - the point where the system ceases to be an accurate transducer of sound. By industry consensus, this point occurs when the power compression exceeds 3 db, and which we define as the Max_SPL. We also specify the voltage needed to reach this SPL.

In a linear system, as the input power is raised from zero, the output power will track the input power. But a real loudspeaker doesn't do this, especially at higher power levels. As the input power is increased, the driver heats, there is nonlinear distortion, etc. and the output power increase will lag behind the input power increase. When this lag is 3 db then it is widely held in the industry that the system has "maxed out". More input power than this will almost certainly sound bad and will usually be accompanied by non-recoverable damage to the system. We call this the Max_SPL - a very important number.

Many companies calculate the Max_SPL from the sensitivity and the "thermal power" limit for the drivers. This number will virtually always be higher than the true Max_SPL measured by the system power compression. In fact it is unlikely that any system could actually be operated at the "thermal limit" for any period of time without permanent damage and or a serious sound quality degradation. In other words the system can't really produce this SPL number in practice - not a very useful specification in our opinion. The Ai Max_SPL number is realistic in that the system can actually be operated at this level without serious sound quality degradations for extended periods of time.