Speaker enclosure "orders"

[DigitalJunkie]

Hey guys,hopefully a simple question for ya..
(and yes,I searched for a while,but didn't find what I'm looking for...maybe I missed it?)

I've seen the terms "4th order" "6th order" etc. in referance to subwoofer (bandpass) enclosures..but never understood what comprises the different "orders".

I've seen designs for 4th order and 6th order bandpass enclosures,and noted that the 6th order had a port on each side of the driver,as opposed to only one ported side (the other side's sealed) in a 4th order..

Is the difference just in the porting (# of ports)/tuning of the enclosure,or is there more to it? Maybe something to do with freq. rolloff like a crossover network's order aswell? (6,12,18,24dB etc.)

A friend asked me what the difference was,and I didn't really know what to tell him...So I thought I'd ask those "in the know"..
One of those things I've never really put much thought/investigation into..

[DigitalJunkie]

Hmm.. My friend found
http://www.faqs.org/faqs/car-audio/part3/

There's also a similar explaination on www.the12volt.com (great site for car audio/alarm/wiring,etc info,and handy ohms law calc's too [useful in those wee morning hours ])
http://www.the12volt.com/caraudio/boxes6.asp

[03blueSI]

As far as I know the order has to do wuith the natural rolloff of the enclosure.:

1st order 6db/octave
2nd order: 12db/octave
4th order: 24db/octave

if you add a 4th order crossover (linkwitz/riley) to a 2nd order enclosure you get a 6th order or 36db/octave rolloff. Hope this helps

[Ron E]

Order is the total number of energy storage components. The components are analogous to masses and springs. A mass stores energy in its velocity Em = 0.5*mass*velocity^2. A spring stores energy in displacement Es= 0.5*k*displacement^2, where k is the spring constant. A piston acting on a volume acts like a spring.

The driver is always 2 orders, because it is a piston and has a mass. The enclosure in the case of a sealed box just acts to stiffen the spring so it is still second order. An enclosure and port (the port acts like a piston) act as two energy storage components independent of the speaker, so a vented box is fourth order. A 4th order bandpass is just a vented box minus the contribution from one side of the driver. A 6th order bandpass has 2 orders from the driver, plus 4 from two ports and two volumes.

The sealed box has a 2nd order rolloff, the ported box has a 4th order rolloff. The 4th order bandpass has a second order rolloff below tuning and another above tuning. A 6th order bandpass box has a 4th order rolloff below the low tuning and a 2nd order rolloff above the high tuning.

[03blueSI]

That explains it better than I have ever seen it explained before, thanks

[DigitalJunkie]

That is indeed a great explaination! Thanks!
Pretty similar to the conclusion I came to after reading a bit..
Thanks for filling in the blanks!

[Svante]

Originally posted by Ron E
Order is the total number of energy storage components.

Nope. A closed box has cone mass, cone suspension compliance and a box compliance. Yet it is second order.

The order of a LS system is the order of the denominator polynomial in the transfer function.

[Ron E]

Originally posted by Svante


Nope. A closed box has cone mass, cone suspension compliance and a box compliance. Yet it is second order.

The order of a LS system is the order of the denominator polynomial in the transfer function.

Hi Svante,

Well..... the reason that the closed box does not add an order is because the compliance of the box volume is in parallel with the speaker compliance. All the closed box does is stiffen the loudspeaker spring, so to speak. For those who don't understand that, take a speaker and put it in a closed box of volume Vb. It will have a Qtc and Fc and an efficiency, etc. If you made a speaker with Fs=Fc, Qts=Qtc, and Vas2 = Vas*Vb/(Vas+Vb) and placed it on an infinite baffle, it would have the same response as the original speaker.

With that caveat, my discussion does a good job at explaining the reason for the orders without getting overly technical.

Maybe you should kick out some acoustic equivalent circuits and explain LaPlace Transform techniques while you are at it. Also explain that a ported box is only a 4th order system if you make certain simplifying assumptions, etc....