Except for some models like the SCM19 or SCM20, most ATC speakers are ported, not sealed.
I am not an expert in that field, but ported speakers can be designed in a lot of ways, with woofer TS parameters, enclosure internal volume, port shape, cross section and length all being interconnected.
AFAIK, ATC ported speakers tend to have quite high internal volumes, and short ports. By increasing enclosure volume and/or increasing vent length, one can lower the port tuning frequency. Longer vents, depending on the application, have some drawbacks. Ports will generate harmonic resonances based on their length. The longer the vent, the lower the resonance frequency. In a subwoofer application, this may not be an issue since they do not reproduce audio much higher than 100hz. But ATC 3 way speakers have their lower crossover at 380hz. Too long of a vent may produce audible resonances below that frequency. The issue is of course even worse with ported 2 way speakers, since their crossover is much higher.
Ported enclosure design also affect the FR shape. Some box design can result in a peaking resonance at the port tuning frequency. More port output, at the cost of a "one note sub" type of sound, especially with room gain. Because of their design choices, ATC tend to have low port output, which result in leaner bass, which looks more like typical sealed box FR. This design choice is, in my opinion, probably one of the reasons why ATC speakers have long been popular with recording engineers. Just like the famous NS10, for some engineers it is better to have lean, but clean and non resonant bass, than lower extension at the cost of linearity, especially in the time domain.
Modern speaker design with equalized sealed enclosure equipped with large, high excursion woofers in combination with lots of class D power tend to nullify those ported speaker issues. But those design have only recently been possible, and are still rare, expensive and sometime plagued by questionable design choices (some Barefoot midwoofer/tweeter section IMHO).
I am not an expert in that field, but ported speakers can be designed in a lot of ways, with woofer TS parameters, enclosure internal volume, port shape, cross section and length all being interconnected.
AFAIK, ATC ported speakers tend to have quite high internal volumes, and short ports. By increasing enclosure volume and/or increasing vent length, one can lower the port tuning frequency. Longer vents, depending on the application, have some drawbacks. Ports will generate harmonic resonances based on their length. The longer the vent, the lower the resonance frequency. In a subwoofer application, this may not be an issue since they do not reproduce audio much higher than 100hz. But ATC 3 way speakers have their lower crossover at 380hz. Too long of a vent may produce audible resonances below that frequency. The issue is of course even worse with ported 2 way speakers, since their crossover is much higher.
Ported enclosure design also affect the FR shape. Some box design can result in a peaking resonance at the port tuning frequency. More port output, at the cost of a "one note sub" type of sound, especially with room gain. Because of their design choices, ATC tend to have low port output, which result in leaner bass, which looks more like typical sealed box FR. This design choice is, in my opinion, probably one of the reasons why ATC speakers have long been popular with recording engineers. Just like the famous NS10, for some engineers it is better to have lean, but clean and non resonant bass, than lower extension at the cost of linearity, especially in the time domain.
Modern speaker design with equalized sealed enclosure equipped with large, high excursion woofers in combination with lots of class D power tend to nullify those ported speaker issues. But those design have only recently been possible, and are still rare, expensive and sometime plagued by questionable design choices (some Barefoot midwoofer/tweeter section IMHO).
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