Back when Rick proposed the project, I was hot for the possibility of using a sealed box for smaller size, easy integration with a sub (no high pass filter required) and no port pipe resonances colouring up the midrange.
As it turns out, the 6.5" Purifi's Thiele Small parameters require a very small sealed box size.
Classic acoustic suspension design wisdom is that the air in a sealed box acts as a linear (and linearizing) spring, dominating over the driver's suspension compliance (which has its own distortion causing non linearity).
In reality, the thermodynamics of air show that an air spring isn't really linear and if air is compressed too much, it causes harmonic distortion on speaker's output that can easily exceed the distortion caused by the speaker's suspension.
Linkwitz
here estimated the % second harmonic distortion caused by air in sealed box as =0.014*SD (in cm^2)*driver displacement(in mm)/box volume (in L).
I simulated a sealed box for the Purifi, and then used Linkwitz's formula to estimate the
2nd harmonic distortion from the air trapped in the box itself, at different listening levels. Here are the results below, compared to the
total harmonic distortion of the Purifi vented prototype measured at ASR.
The sealed distortion is quite a bit higher than a vented and wastes the Purifi's low distortion potential. I also expect the mids might distort when played simultaneously with loud bass because the air itself is distorting (ASR tests don't measure this).
The 6.5" Purifi is all about playing loud with low THD, but it struggles to pull that off in a sealed box because of its unusual TS params.