I just came across some product documentation that supports a concept I learned many years ago from Toby Guynn.
WIKI – midwoofer-tweeter-midwoofer (MTM)
The basic concept is that with a typical 2 way midwoofer-tweeter (MT) design, you have lobe tilting away from what you would expect to be the center axis of the loudspeaker. MTM
WIKI – Loudspeaker Time Alignment
Take a look at the following image. It comes from the spec sheet for an EV FIR-2082 loudspeaker. Most manufacturers give us useless information like “40hz to 20k” “90 degree dispersion”. This is basically useless information.
Questions:
What is the frequency response between 40hz and 20k on axis? The loudspeaker may reproduce frequencies down to 40hz and up to 20k but at such a low volume that it’s irrelevant. Some manufacturers will state another spec that is “usable frequency response”. Yes please. Please tell me useful information instead of fantasy marketing specs.
In case it’s not already very clear, no speaker is direction at all frequencies and even the best speakers money can buy do not radiate sound at every frequency equally in the same dispersion pattern. A general rule of thumb is that LF is omni directional (a sphere shaped dispersion) and HF is narrow. MF are somewhere in the middle depending on the crossover frequency, size of the various drivers, spacing of those drivers, size of the loudspeaker baffle, etc…
QSC provides a -6db spec for their passive speakers. Let’s take the ADS12 speaker as an example:
Here is what they provide for the specs of that box.
While I can’t verify that this is a marketing ploy, their own graphs state clearly that the frequency dispersion is NOT stable through out the frequency response of the loudspeaker.