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ATC and bass roll-off - 3rd order?

dfuller

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Referencing this review of the SCM25A in Resolution: https://www.resolutionmag.com/wp-content/uploads/2016/03/ATC-SCM25A-Pro.pdf
Resolution Review said:
Low-frequency extension is very good with a 3rd-order roll-off with -10dB at around 30Hz.

So... How are they doing this? I was under the impression that ported systems always had a 4th order roll-off just as a matter of how bass reflex works. Maybe I'm misunderstanding, because I don't get it.
 

Tom Danley

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What they don't say is if the ultimate slope is -18 (which like you suggest isn't how a 4th order box works).
On the other hand, one can have an alignment with a lower than .7 Q response shape and initially the slope can be less than -24.
 
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dfuller

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What they don't say is if the ultimate slope is -18 (which like you suggest isn't how a 4th order box works).
On the other hand, one can have an alignment with a lower than .7 Q response shape and initially the slope can be less than -24.
Oh, the Q value makes sense. Somehow that didn't occur to me.

And it appears you're right. https://www.soundonsound.com/reviews/atc-scm25a
Sound On Sound said:
Like all of the larger ATC monitor designs, the SCM25A employs a ported cabinet to optimise the loading of the bass driver. However, ported cabinets inherently have a resonant frequency, which is determined by the cabinet and port dimensions. [...] However, ATC don't use the port to extend the bass as such, but rather tune it to load the bass driver correctly to control its motion, with the primary aim of reducing distortion [...] In technical terms, the difference is that the port resonance in ATC designs is well damped, with a relatively low Q of around 0.5, giving a fairly gentle and natural-sounding bass roll-off with very good extension. In contrast, most ported speakers are designed with more aggressive port tuning, Q values of 0.7 or higher being quite typical.

That would explain their oddly good low-end response on a waterfall too. A lower Q filter (and, essentially this is a HPF) should ring less than a higher Q one all other things being equal...
 
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