# Help me understand 4th order tuning



## Jroo (May 24, 2006)

I have a few questions regarding tuning of 4th order designs. How does 4th order tuning differ from traditional ported boxes? I see some designs that are tuned low and it sounds like they want low end monsters? Am I correct that a low tuned 4th orders wont play much over the tuning? The 4th orders that I see with high tuning in the 60's and 70's seem to be the ones designed to be more musical? In a 4th with a high tuning does it play from the tuning down or will it start falling off like a ported box after that tuning point? So if I tuned a ported box to 60, it would peak there but then start falling off below the peak right? In theory a ported box like that would have great issues trying to produce anything in the 30's maybe in the 40's. This seems to differ with a 4th order?


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## The Baron Groog (Mar 15, 2010)

The Subwoofer DIY Page - 4th Order Bandpass Systems


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## sonikaccord (Jun 15, 2008)

4th order is a box type that includes ported boxes. All ported boxes are 4th order but not all 4th orders are ported boxes.
Car Audio Subwoofer Enclosures Fourth Order/Bass Reflex and Bandpass
the "4th order" part is the slope of the roll off after tuning or 24db/oct. Sealed boxes are "2nd order" and roll off half as fast, 12db/oct.


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## Jroo (May 24, 2006)

Thanks for the links above, but they really dont answer my question. I understand what a 4th order is. I am asking how raising or lowering the tuning of a 4th order affects the sound. I have two designs for a set of woofers done by two differnt people. Both designs are very close in terms of size of the sealed,ported sections, and port size. The thing that differs was the tuning of the port. One design has a tuning in the upper 60's and the other has the tuning in the lower 50's. Why would I use the higher tuning vs the lower tuning? How does that tuning affect the sound of a 4th order?


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## Oscar (Jun 20, 2010)

Winisd


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## Fricasseekid (Apr 19, 2011)

Start here:http://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html


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## SSSnake (Mar 8, 2007)

> I am asking how raising or lowering the tuning of a 4th order affects the sound.


In a 4th order bandpass you will start to see roll off above the tuned freq. So really the answer for what you want the port tuned to is dependent upon your expected low pass crossover freq. For example a box tuned in the low 50s with a 20 to 100 Hz passband is a VERY bad idea. The same box (tuned in the low 50s) would likely do OK lowpassed at 63hz. I typically want my subs to player higher than this so I would want the box tuned higher.

One suggestion if you are going to go with a 4th order tuned pretty darn low, you might want to look at 6th order bandpass. One thing to remember with any proted design is that you will get group delay (freq dependent delay in this case resutling from system/port tuning). This can make integration with the midbass more difficult. However, with 6th order most of the passband can be delayed and then you can use TA to integrate it more effectively with the front stage.


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## Fricasseekid (Apr 19, 2011)

SSSnake said:


> In a 4th order bandpass you will start to see roll off above the tuned freq. So really the answer for what you want the port tuned to is dependent upon your expected low pass crossover freq. For example a box tuned in the low 50s with a 20 to 100 Hz passband is a VERY bad idea. The same box (tuned in the low 50s) would likely do OK lowpassed at 63hz. I typically want my subs to player higher than this so I would want the box tuned higher.
> 
> One suggestion if you are going to go with a 4th order tuned pretty darn low, you might want to look at 6th order bandpass. One thing to remember with any proted design is that you will get group delay (freq dependent delay in this case resutling from system/port tuning). This can make integration with the midbass more difficult. However, with 6th order most of the passband can be delayed and then you can use TA to integrate it more effectively with the front stage.


Where is your information coming from?

If you have a box tuned to 50 hz (or 30 hz for that matter) it should have no problem playing to 100 hz and beyond. In fact it should play just fine all the way up to the first port resonance, which (if the box is designed well) should be way above the desired passband. But in thatsame box tuned to 50 hz, it's that 20 hz that you wont hit. Ported boxes generally lose an ability to control the cone below tuning frequency. 

Also group delay isn't that big of a deal with subwoofers. When you deal with a sound wave that's 40ft or longer in length it takes alot of group delay to make it audible. 

To the OP: did you read the article I posted? In general a lower tuning will create a flater response and a higher tuning will boost output at the tuning frequency but this usually results in a response curve that has peaks and dips.


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## Chaos (Oct 27, 2005)

The advantage of the 4th order bandpass is that the sealed chamber will still dampen the sub regardless of the port tuning frequency, so even if it doesn't sound right, the speaker will most likely survive the experience.

A program like WinISD can describe how the sub will behave in different alignments within an anechoic environment, but it can't tell you how it will sound. All you can do is build it to hear for yourself.


As if all of this isn't interesting enough, just wait until you get to the next level: Quarter Wavelength Loudspeaker Design


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## The Baron Groog (Mar 15, 2010)

The tuning is as much car dependant as driver/box dependant-either box you outline could work well IF their roll off matches your car's cabin gain.

There are two main schools of thought on this-either tune very low and EQ out the peak or tune to be around car Fs and make use of the cabin gain to get a flat response.

XtremeRevolution has a thread running where he'll workout your cabin gain and advise/design enclosures for you


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## SSSnake (Mar 8, 2007)

> Where is your information coming from?
> 
> If you have a box tuned to 50 hz (or 30 hz for that matter) it should have no problem playing to 100 hz and beyond. In fact it should play just fine all the way up to the first port resonance, which (if the box is designed well) should be way above the desired passband. But in thatsame box tuned to 50 hz, it's that 20 hz that you wont hit. Ported boxes generally lose an ability to control the cone below tuning frequency.
> 
> ...


I think you are referring to response from a rear ported sub NOT A 4TH ORDER BANDPASS. My information comes from experience (actually designing building and using 4th order bandpass enclosures), modeling programs, and multiple authoritative written sources.



> The tuning is as much car dependant as driver/box dependant-either box you outline could work well IF their roll off matches your car's cabin gain.
> 
> There are two main schools of thought on this-either tune very low and EQ out the peak or tune to be around car Fs and make use of the cabin gain to get a flat response.
> 
> XtremeRevolution has a thread running where he'll workout your cabin gain and advise/design enclosures for you


This is true to an extent. What happens to group delay above the port tuning? It changes drastically. The audible result is very poor integration with the mid/midbasses dependant upon the freq content of the source material. I have also noticed lots of noises using this type of application. They could likely be addressed by port treatment (rounding, flares, etc). But with the group delay issue it was IMO never worth it...


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## SSSnake (Mar 8, 2007)

BTW - I am of the school of thought that you build it to sound flat anechoically and then EQ it to deal with the environmental influences (too tough to model in most cases). Again this is only my approach and I do concede that good results can be gained through other methods.


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## Fricasseekid (Apr 19, 2011)

SSSnake said:


> I think you are referring to response from a rear ported sub NOT A 4TH ORDER BANDPASS. My information comes from experience (actually designing building and using 4th order bandpass enclosures), modeling programs, and multiple authoritative written sources.


I suppose your right. I have a good understanding of fourth order enclosure but don't know alot about the bandpass enclosures which, I'm now assuming, act differently. 

I'm not sure why everyone is talking about fourth order bandpass enclosures though since the OP simply asked about fourth order tuning?


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## Fricasseekid (Apr 19, 2011)

SSSnake said:


> BTW - I am of the school of thought that you build it to sound flat anechoically and then EQ it to deal with the environmental influences (too tough to model in most cases). Again this is only my approach and I do concede that good results can be gained through other methods.


X2 
low frequency peaks + peaks EQed out = flat response with less excursion/distortion


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## SSSnake (Mar 8, 2007)

Typically rear ported is just called ported and 4th order refers to single reflex bandpass. I can't speak to the OP intent.


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