4th order banpass build Log

[galacticmonkey 0]

Ill take the advice from the loudest Super Street competitor in history, over some guy on the internet.

Just a personal preference.

[sundownz 197]

Everyone keep it civil

There are different ways to go about being loud - eg: my truck is around 5:1 - but other people do it other ways and are very loud as well.

[galacticmonkey 0]

The input I gave is more for a loud streetbeater.

When it comes to a fartbox bandpass, I dont have any ideas.

[sundownz 197]

The input I gave is more for a loud streetbeater.

When it comes to a fartbox bandpass, I dont have any ideas.

The smaller front chamber will equal less of a peak around tuning.

[Megalomaniac 0]

A little background info for you, LB...

When we say "4th order" or "6th order", this refers to the rate at which the response rolls off at its upper and lower "corner" frequencies. In other words, above a certain frequency (say, 60 Hz), response will rolloff (the driver/enclosure combo will be quieter). The same can be said for below a certain frequency (say, 30 Hz).

So what are these rates of rolloff? Pretty simple: 1 order = 6 dB/octave rolloff. Extrapolating for a moment, you will see that a 4th order enclosure will have a rate of rolloff of 24 dB/octave, while a 6th order enclosure will have a rate of rolloff of 36 dB/octave.

To understand that part, you need to know that an octave above a frequency is double that original frequency; an octave below is half that original frequency. Also, note that this is just referring to a type of mechanical filter that behaves in the same way as, say, your low-pass filter on your amplifier.

To bring this all together, suppose a driver/enclosure combination that produces 100 dB from 30 Hz - 70 Hz. We'll say that the 30 Hz is the lower corner frequency (the frequency that the high-pass filter begins) and the upper corner frequency (the frequency that the low-pass filter begins) is 70 Hz. Suppose this enclosure is of the 4th order (24 dB/octave) variety. Looking at the low end of the spectrum, we will see that response is 100 dB at 30 Hz, 76 dB at 15 Hz, 52 dB at 7.5 Hz, etc (although it should be noted that at frequencies this low, your subwoofer will typically lack the throw to get much meaningful response). On the top end, we will see that response is 100 dB at 70 Hz, 76 dB at 140 Hz, 52 dB at 280 Hz, etc.

Now to apply this to specific types of enclosures. One of the simplest enclosures (the sealed variety) usually demonstrates a rolloff of 12 dB/octave (aka 2nd order). Bass Reflex enclosures (usually called "ported") exhibit a 24 dB/octave rolloff (4th order). However, neither of these two types will rolloff on the top end (aka they do not make for a mechanical low-pass filter).

Now bandpass is different. By definition, bandpass means you will see rolloff on the top end and on the bottom end (both low-pass and high-pass filters, respectively). So a 4th order bandpass exhibits the same rolloff as a ported enclosure, but this occurs below AND above the bandwidth it is used in. A 6th order bandpass also has rolloff below and above the bandwidth it is used in, although the rate of rolloff is obviously quicker (36 dB/octave). Why is this a good thing? Well, it isn't for many people as limiting the top end may be the opposite of what they want. For SPL competitors, however, this means that we gain more output in the bandwidth between these filters. In fact, the faster the rate of rolloff on the top and bottom end, and the smaller the bandwidth is, the more output at a central frequency you will see.

Hopefully that makes sense and provides at least a cursory understanding of what these enclosures are about. The problem with bandpass enclosures is that they are typically quite large (especially those of the 6th order or 8th order variety as the ports consume a fair bit of a volume themselves).

If you need to make a small step into understanding them further, you can simulate response with the spreadsheet on Brian Steele's website. If you want to visualize what some of these enclosures can look like, check out The12Volt.com.

As mentioned above, many drivers require enclosures built specifically for them using the Thiele/Small parameters supplied. The higher the order, the more sensitive the enclosure is to miscalculation and poor building. Read for a little while and ask questions if any of this is unclear.

I am a little confused. So a bandpass enclosure(gearing more at 4th orders on up) basically just acts as a Mechanical Filter?

Isnt it just meant to play that bandwidth solid so its not like a roller coaster peak and valleys in that bandwidth range....like the octaves you mention would mean there is no set 1 note tuning you could essentially play different notes as long as it falls in that octave.

[DevilDriver 28]

All enclosures are just filters.

[F3RR3T 1]

Couple major things that make no sense. You forgot the second chamber and a 4th order won't buy you what you are looking for as it will be quieter than a ported enclosure.

HMM so are 4th orders so prevalent in spl vehicles if its "quieter"?

Edited May 25, 2008 by F3RR3T