How does a 4th order bandpass work?

[0kriz]

What about on the high end? is It feasible to limit the woofers bandpass frequency that much outside a vehicle? If you wanted to play down to 30 HZ in free air, you’d basically be limited to a passband of approx. 30-70 HZ. Wouldn’t you need a second set of woofers to cover the frequency range up to where mids take over? A 6th order seems like it would be better to cover a bit wider range in free air. Or a folded horn. Space is obviously not a concern as much in PA applications.

dual 15 woofers have no problem from 1000hz down to 50hz, a 2” compression driver deals with 1000hz +.
In PA, space is not a concern, we simply calculate how many boxes we need to achieve the spl we aim for. Convinient design for transportation and high efficiency is the main priority

 

[metalheadjoe 10+ year member]

I think OP wants a little more in-depth answer than the first few responses provided. I've also never understood the benefits of a 4th order.

Sealed boxes limit cone movement via unequal pressure inside and outside the box. There is no cancellation because the inside and outside pressures are separated. Makes sense.
Ported boxes add additional output over sealed boxes because they are tuned to a frequency where the air movement through the port aligns with the air movement from the cone. Makes sense.

Question is, what benefit does a 4th order have over a ported box? Seems like it combines the worst aspects of a sealed and ported box. Cone movement is limited more than in a sealed box by negative pressure in the sealed side and positive pressure in ported side, and you don't gain output like a ported box because the port is the only avenue for air to travel.

What am I missing?

 

[mrj858 10+ year member]

The “ported” chamber is more of a loading chamber….have you ever yelled in a tunnel or hallway and it seems louder? That’s how the ”ported” chamber of bandpass 4th order works….the “port tuning“is where the roll off point is….the sealed section dictates how it’ll sound I.e how low itll play etc….that’s why sub choices matters….low FS mid to higher Qtc subs with good excursio works better to get an optimal Fc and since it’s sealed the low frequency roll off isn’t as drastic as with a ported box and controls the cone better….that’s why theyre tuned higher but still play low….cabin gain helps a ton with usable bandwidth as well as manipulating the cabin with doors opened vs closed vs windows up or down etc….

 

[shredder2]

Mr858 nailed it, sub compatibility + tuning + install.
Match fs to tuning or close with a ratio to match environmental gains and its a winner.
Allow for power... That echo chamber needs a bit of area but understand the mechanics and how loading affects output.
Compatibility, tuning, ratio and install go into any 4th design.
Open air? Ok... Space makes bass.
Understand wavelengths at that point, throw out what you think any woofer needs space-wise and learn hornsrep then design around a pro audio woofer. A 4th may not be your go to at that point
35hz is low pro audio 30hz is insane. Without cabin gain I'd say best bet is something horn loaded as mentioned.

 

[0kriz]

[/QUOTE]
Mr858 nailed it, sub compatibility + tuning + install.
Match fs to tuning or close with a ratio to match environmental gains and its a winner.
Allow for power... That echo chamber needs a bit of area but understand the mechanics and how loading affects output.
Compatibility, tuning, ratio and install go into any 4th design.
Open air? Ok... Space makes bass.
Understand wavelengths at that point, throw out what you think any woofer needs space-wise and learn hornsrep then design around a pro audio woofer. A 4th may not be your go to at that point
35hz is low pro audio 30hz is insane. Without cabin gain I'd say best bet is something horn loaded as mentioned.
[/QUOTE]

For an amature weekend DJ, 35hz is low. For a concert or large event with music genres oriented around bass/sub frequencies, the rental company provide a soundsystem with the nessesary frequency response and sound pressure level, low 30hz is not unusual. Sound engineers tend to have more than enough “rig for the gig” on serious jobs. This however get outside the point, I was not sure if 4th order would work for pro audio, it was only a thought i was playing with, i know some have made it, but this thread was mainly because I was curious about an enclosure type i have never built before. I just could not make sense why this design would be effective compared to a bass reflex system.

 

[shredder2]

Sound engineers tend to have more than enough “rig for the gig” on serious jobs.

Ok then...

 

[Buck 5,000+ posts]

I think OP wants a little more in-depth answer than the first few responses provided. I've also never understood the benefits of a 4th order.

Sealed boxes limit cone movement via unequal pressure inside and outside the box. There is no cancellation because the inside and outside pressures are separated. Makes sense.
Ported boxes add additional output over sealed boxes because they are tuned to a frequency where the air movement through the port aligns with the air movement from the cone. Makes sense.

Question is, what benefit does a 4th order have over a ported box? Seems like it combines the worst aspects of a sealed and ported box. Cone movement is limited more than in a sealed box by negative pressure in the sealed side and positive pressure in ported side, and you don't gain output like a ported box because the port is the only avenue for air to travel.

What am I missing?

I’m really not a huge fan of 4th order BP’s. The simple fact that the only way to tune the sealed is by the size means many people don’t put all the parts together to allow the sealed size to play well, vs a series 6th where you can force tuning in a very specific way in the rear.

4th orders allow sort of double pressurization at certain frequencies and don’t unload like a ported will below tuning frequency. To take good advantage of this, you have to have a sub that’ll play low in the sealed. The quality of a 4th BP and how it plays is almost all woofer design and nature.

Sealed boxes have peak tuning frequencies based on size and woofer. You can match that with the front ported section for more bass, in some cases.

 

[Buck 5,000+ posts]

A 4th order BP is literally just a sealed box that has a front or outer “booster” ported chamber. A 4th BP is simply just a sealed box with a booster ported chamber on the front to help extra pressurize a certain bandwidth that the sealed reproduces. A 4th BP is literally just a sealed box dropped inside of a ported box, where you tune the ported box to boost a certain bandwidth of the sealed with a certain peak of frequency with the tuning. In a big wall or something, you can have a Fsc of say 45 hz and tune your front port to 45 hz and that’ll be a banger, where in home audio you can have a Fsc of like 30-35 hz with a huge sealed portion and a tiny ported portion tuned up into the 60 hz range. You can do it however fancies you. You can even put a sealed box inside of a horn or t-line, and it still functions like a 4th order BP, just with different roll offs/ dB octaves per slope of fade from the peak. I’ve done sealed-t-line hybrids before and they work well for SQ.

Any sealed box has you relying heavily on a proper woofer with proper Fs and Vas and Q values to meet your required listening for your bass.

I rarely see people talk about Qtc and Fsc when talking about 4th order BP’s, which is honestly the most important two stats.

Any bandpass enclosure like we do in car audio audio has a primary chamber that the rest of the chambers are based on, and the sealed chamber is the primary chamber for 4th BP’s. You must absolutely have a perfect spec sealed chamber before you even make another step in designing the ported section. Ported sections on 4th BP’s are just boosters. Like a v8 drag racing motor is the sealed section and the ported section is just the nitrous you use to go faster, but the ported nitrous can’t be used at all without the motor. My point being a 4th order BP is an advanced sealed enclosure, fundamentally, and maybe people don’t understand what that really means or the dynamic that gives the enclosure.

Sealed has its limitations that it carries into 4th order BP’s, so you have to master sealed boxes to even begin to master 4th BP’s.

 

[Beatin' 10+ year member]

Somebody in this thread asked 'what is the point of 4th order?'

I run two 15's in a 4th order, over 7cu/ft net. The point is I feel that typical ported boxes are too bloated on the low end with the vehicles cabin gain and the upper end "attack" of transients are not strong/loud enough. So I'm willing to filter out some 75-80hz+ notes to get more skull cracking bass from 45-70hz and let cabin gain increase output under 40hz. Works well for me.

 

[KicktheSDbasshead]

I just saw where this thread was going and decided to chime in, reading all the comments with ensuing questions must have left me a little bit weary.

"Cool kids?"... sure Wilbur. That was a pic of one of the many 4ths I've designed. Without defending its pros and cons it was a lower powered 4 10" musical 4th. NOT a boomy spl enclosure...
Here's another couple pics of another lower powered musical 4th for you to critique since you felt the need...

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