How does a 4th order bandpass work?

0kriz

CarAudio.com Newbie
Jun 3, 2022
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As the title says, how does it work?

A 4th order bandpass has one sealed chamber and one ported chamber with the driver installed in the wall between them, so far so good. But when the cone moves into the ported chamber, the pressure in the ported chamber increases, and the pressure in the sealed chamber decreases. Why don't they cancel each other out? My logic tells me this should be a very ineffective design with wasted potential, but this is obviously not the case.

2 sealed chambers cancel each other out, but add a port in one chamber and magic happens... or does it has something to do with phase response?
 
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SlugButter

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Nov 5, 2019
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The sealed chamber in a 4th order is controlling the movement of the cone more than any other function. You wouldn’t have frequency waves crossing over each other to cancel each others energy out. if You were to put a sealed box and a ported box in your car separate, it would cancel some of each other’s energy out because the different waves would interact with each other in the cabin of the vehicle.
 
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mat3833

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Mar 12, 2008
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4th orders are really simple when you think of them as 2 seperate enclosures. I'm going to WAY over simplify here, but this is a pretty solid base level example.

Speakers produce 2 pressure waves 180 degrees out of phase with each other. A sealed enclosure just traps one of those waves so it can't cancel out the other and boom, you got sound. The low-end of a sealed enclosure tapers off at about 6 decibels per octave.

A ported enclosure traps one of the waves, but also adds a secondary chamber that acts like an additional driver at a specific frequency and gives you a boost around that frequency. This enclosure type is making the normally unwanted "rear wave" useful and getting some output from it, usually a +3 decibel gain. The downside is that below that frequency, your speaker is essentially playing without an enclosure. The low end of a ported enclosure rolls off MUCH steeper, usually between 18 and 24 decibels per octave.

Now, a 4th order basically utilizes the benefits of both designs. You have the sealed section which let's you play really low and not exceed your speakers mechanical limits, and you have a port that gives you a boost in output. In this case, the "rear wave" is trapped in the sealed section, and the "front wave" is playing through a port. From the ported enclosure example, you know that below a ports tuning frequency it basically acts like a big hole and does nothing. But the closer you get to the ports tuning, the more output "boost" you get. Generally you can expect a +6 dB gain over a small window, usually an octave; for example from 30hz to 60hz. However, above that frequency, your output drops off just like in a ported enclosure.

Matt
 

Buck

Why so Sirius?
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A 4th Order BP is just a sealed box inside of a “booster” ported box. The extra ported box plays more evenly away from both sides of the ported section, which is why they’re tuned higher for the same frequency response as a normal ported box, because the sealed side resonates low enough and keeps the cone controlled and keeps the woofer from unloading.

A 4th order BP is primarily a sealed box on a ported box’s steroids. By far, the most important thing is the sealed side. If you don’t understand the Qtc and Fc/Fsc/Frc, and how that changes the nature of sealed boxes, then you’re probably not going to make the best 4th order BP, unless you get lucky enough to do the right sub in the right volume.
 
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Buck

Why so Sirius?
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The most important part to a 4th Order BP is choosing the right sub to play in sealed and making sure the sealed section will do what you want, before you even try to think about the ported section.

For people who want low lows, I wouldn’t go above a 45 hz or so Fsc. Over 50 Hz Fsc, I say do ported or series 6th, for any kind of “low” bass.
 

Buck

Why so Sirius?
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Like a series 6th BP is just a ported box inside of a ported box, fundamentally, but the ports and their needed port area and length are somewhat of an art form.

4th order BP: sealed box inside ported box, sealed is primary sound.

Series 6th BP: ported box inside of ported box, inner ported box is primary sound.
 

0kriz

CarAudio.com Newbie
Jun 3, 2022
14
4
  • Thread Starter Thread Starter
  • #8
4th orders are really simple when you think of them as 2 seperate enclosures. I'm going to WAY over simplify here, but this is a pretty solid base level example.

Speakers produce 2 pressure waves 180 degrees out of phase with each other. A sealed enclosure just traps one of those waves so it can't cancel out the other and boom, you got sound. The low-end of a sealed enclosure tapers off at about 6 decibels per octave.

A ported enclosure traps one of the waves, but also adds a secondary chamber that acts like an additional driver at a specific frequency and gives you a boost around that frequency. This enclosure type is making the normally unwanted "rear wave" useful and getting some output from it, usually a +3 decibel gain. The downside is that below that frequency, your speaker is essentially playing without an enclosure. The low end of a ported enclosure rolls off MUCH steeper, usually between 18 and 24 decibels per octave.

Now, a 4th order basically utilizes the benefits of both designs. You have the sealed section which let's you play really low and not exceed your speakers mechanical limits, and you have a port that gives you a boost in output. In this case, the "rear wave" is trapped in the sealed section, and the "front wave" is playing through a port. From the ported enclosure example, you know that below a ports tuning frequency it basically acts like a big hole and does nothing. But the closer you get to the ports tuning, the more output "boost" you get. Generally you can expect a +6 dB gain over a small window, usually an octave; for example from 30hz to 60hz. However, above that frequency, your output drops off just like in a ported enclosure.

Matt
you wrote:
"A sealed enclosure just traps one of those waves so it can't cancel out the other and boom..."
_____________________________________________________________________________________________

It traps some of the waves, there are still some that escape right?
if not this statement becomes faulty

"You have the sealed section which lets you play really low"
______________________________________________________________________________________________

I still don't see how they don't cancel each other out just because one side has a port in it :S

The port gain starts to have a small effect from about an octave above tuning and provides some output a few hz below tuning.
but the higher the frequency relative to the port tuning, the more the enclosure act like a sealed chamber.
so if a 4th order is tuned to 40hz, at 70 hz, there are a small port gain, but mostly the ported chamber will act like a sealed enclosure, and should to some degree cancel the sealed chamber out?
 

0kriz

CarAudio.com Newbie
Jun 3, 2022
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  • Thread Starter Thread Starter
  • #9
Thanks for all replies! anybody is free to add their view on my reply above. I would really love to understand how a 4th order BP actually work...
 

Grimmjoww

CarAudio.com Well Known
Apr 18, 2021
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I'm a little confused that the above replies didn't answer your question, the guys that responded are very well versed in how enclosures work. Guess I will break it down Barney style and see how that goes. If you have any type of sealed container and you pressurize it with air you now have a pressurized container or "vessel." Since a woofer is capable of moving in opposite directions this almost acts like a piston in a 4th order. The woofer pressurizes the sealed chamber, the pressure in the sealed chamber has no way to be spent unless it 1 - moves the woofer back, or 2 - there is leak around the woofer cone or seal to let it escape into the ported chamber. Since the chamber is sealed it will also control excursion limits unless it's overpowered. So in my view what you get is a piston firing back and forth with added pressure from the sealed enclosure. This will let the woofer down travel with more force into the ported enclosure. Your sound actually comes from the port in the box.
 
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some dude

CarAudio.com Regular
Sep 7, 2020
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Inside the sealed portion no pressure waves escape. Therefore, there is no 2nd pressure wave source to cancel anything out.
Right?
 
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0kriz

CarAudio.com Newbie
Jun 3, 2022
14
4
  • Thread Starter Thread Starter
  • #12
Inside the sealed portion no pressure waves escape. Therefore, there is no 2nd pressure wave source to cancel anything out.
Right?
I would not say "no pressure wave escape". From what I understand, the reason why 4th order bandpass designs are less peaky when the sealed chamber is large, is because the output from the sealed chamber has a broader bandwidth and because the larger the sealed chamber is, the more spl it will provide (within reasonable size ofc).

if my logic is wrong, pleas correct me ;)
 

mat3833

CarAudio.com Elite
10+ year member
Mar 12, 2008
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182
Inside the sealed portion no pressure waves escape. Therefore, there is no 2nd pressure wave source to cancel anything out.
Right?
You have a pressure wave in front of the cone, and one behind the cone. Sealed enclosures trap the rear wave so you don't get cancelation, that is true. Depending on what the lowest frequency you want to play is, you don't actually even need an "enclosure". You can just use a baffle of a specific diameter, I don't recall the formulas to calculate this unfortunately, to prevent the pressure wave from canceling out in the band you want to play. That's called an "open baffle". Some midrange drivers use this type of design.

Matt
 

some dude

CarAudio.com Regular
Sep 7, 2020
151
69
I would not say "no pressure wave escape". From what I understand, the reason why 4th order bandpass designs are less peaky when the sealed chamber is large, is because the output from the sealed chamber has a broader bandwidth and because the larger the sealed chamber is, the more spl it will provide (within reasonable size ofc).

if my logic is wrong, pleas correct me ;)
No pressure wave escapes because it is sealed.
Sealed = no leaks. How can something escape if it has no way out?
 

0kriz

CarAudio.com Newbie
Jun 3, 2022
14
4
  • Thread Starter Thread Starter
  • #15
No pressure wave escapes because it is sealed.
Sealed = no leaks. How can something escape if it has no way out?
A thin layer of wood cannot block all pressure waves, even 10" of concrete cannot block low frequencies totally.
 
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