modeling enclosure size

[jaygeorge1979]

i have already used winisd to model some enclosures for subwoofers...i did not actually build these boxes, i was just looking at how box size/port size affected the response curve....it only really affects the low end of the sound spectrum....is this because the enclosure is used primarily to provide some bass? do different sized boxes do anything to the highs, since it doesnt affect it on the modeling software?

ALSO: how does modeling boxes for subs differ from modeling boxes for bookshelf/tower speaker enclosures? enclosures with two, three drivers in them? do i still look at how the low freq is extended? would i keep the low freq down since these would be used in addition to a sub? am i on the right track?

FINALLY: sorry i have so many questions: does having a steep rolloff on the winisd subwoofer modeling enclosure act kind of as a high pass crossover would for sub? keeping the really REALLY low freq out of the speaker, so it doesnt blow?

[richie00boy]

Due to the size of box usually used and the length/diamater of vents, the tuning only affects the bass region.

Modelling subs is no different to modelling any other speaker IMO with the exception that for a box producing full range sound you would be wise to consider 'baffle step correction', but many don't.

Ditto for multiple drivers, it just doubles or triples etc the box volume.

A steep roll-off serves nothing more than rapidly disappearing acoustic output. Beware with vented boxes that driving them below the tuning frequency unloads the cone so damage can occur, so best practice is to include a high-pass filter to protect them.

[jaygeorge1979]

since vented boxes can have such a steep rolloff, how is such loading even possible? what does loading mean exactly? i was under the impression that modeling the box kind of served to represent a high pass crossover in that it DOES have steep rolloff, keeping nasty SUPER low freq away?

[richie00boy]

No the curve you are referring to is simply the acoustic output. So if you are still feeding a signal in the cone will move but you don't get audible output. Look at the excursion graph for the real story of what happens.

[jaygeorge1979]

ahh i see....good point

do all subwoofers need high pass crossovers set to a very low freq in order to protect the subs? i know they need low pass crossovers to keep high freq out of the subs, but do all subs need high pass xovers also?

[richie00boy]

Any vented speaker should, in theory and to prevent worst case. A lot get away without though, due to particular application and diminishing low frequency content with most music types, and paying proper attention to design and tuning.

[jaygeorge1979]

does the high pass crossover point for all subs usually fall around the same place? where would this be? maybe, a high pass crossover at 15 Hz? how do u know what value to use?

[richie00boy]

The filter cut-off point will depend on the tuning frequency, obviously a higher tuning frequency requires a higher cut-off frequency.