# Bandpass or Ported box for 10" Infinity Ref.?



## stevja1 (Jan 25, 2004)

Hey All -
I'm kind of new to the audio section of the forums. I normally hang out in the engine forums, but lately I've been trying to set up a "system" in my car and I needed advice.

It boils down to do I go bandpass box, or ported, (or should I just keep the sealed box that its coming in?)

Here's the issue: I want SQL, and not really SPL. I don't know what a really wavy sound level line sounds like vs. a flat one. Will a bandpass make my music sound really wierd, or will it sound better? I have a wide range of music, from punk, to some rap/pop, to alternative, to classical, and even techno, when I'm having a weird week. I mostly listen to alternative and pop.

I've got a 10" Infinity Reference 1030W on the way, and I've looked at some numbers for the boxes. I like deep notes and would like to tune either box low.

For the bandpass, I'd like to build a stage 6 box, tuned at 25Hz and 50Hz. (I'm not sure if these are the best frequencies, if you guys have any input, please let me know.)

For the ported, I'm looking at tuning it at around 30Hz.

I've built two sealed boxes, and one ported. I've never built a bandpass before. I imagine its basically the same as two ported boxes stuck together, (with maybe some plexiglass showing everything off.)

There you have it. I've searched, and found some interesting threads about bandpass vs. sealed, and how ported boxes work and the like, but nothing really fit what I was looking for. If you are aware of a thread that could help me out, please respond with a link.

Thanks in advance.


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## asleepz (Feb 24, 2004)

Bandpass makes up for crappy speakers to make them sound loud. Not good, but loud.
Ported lets you make use of a lower power amp, again making it much louder but not exactly sounding like it should.

Your best option is to find the perfect cb. footage for your speaker and keep with the sealed. If you have a speaker worth a crap, it'll be loud and sound great all in one good package.


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## Sentrixx (Apr 30, 2002)

sealed... its the best way to go. But then again, it all depends on what you want it to sound like. If you want it to be loud.. then ported.. If you want it to sound tight and clean the sealed. Bandpass are crap.. Plan and simple.


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## erik2282 (May 17, 2005)

id go with _properly_ built/tuned ported box...


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## stevja1 (Jan 25, 2004)

erik2282 said:


> id go with _properly_ built/tuned ported box...


Thanks for the input.
I'm suprised that so many of you have suggested a sealed box. I honestly didn't think anybody would go for that. I thought you would lean more toward at least a ported. 

To sum what you guys have said, this is the situation, as I understand it.
So the sealed box will sound tighter, but not as loud and not as low.

The ported box will sound a little better on the low end, but not as tight.

The bandpass will be really loud at the lowend, but will wont be tight at all, and will sound funny.

I'm a little intimidated by a bandpass, simply, because I've never built one before. I'm confident in building a good, tuned ported box, no problem. Same thing with a sealed box.


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## sr20dem0n (Mar 19, 2003)

Well "tightness" isn't really a function of the enclosure, it has to do with transient response, which is the same thing as frequency response (time-based vs frequency-based measurement of the same thing). The reason sealed boxes are said to have superior transient response is because of their frequency response. See, cars have a property that's known as cabin gain, starting at about 50-60hz the cabin starts to boost output at a rate of roughly 12dB/oct. That means that for every octave to go below ~55hz, output increases by roughly 12dB, just because of the nature of the acoustics in a car. That means that in order to have a flat in-car response, you need a sub that will be flat down to ~55hz and then start rolling off at roughly 12dB/oct, so that when you add in cabin gain you wind up with a pretty flat response down very very low. A sealed box has exactly this response, and if you match the size of the box to your car's transfer function, you can wind up with a sub setup that's bone flat from 100hz down to the teens, literally.

You can not and will not get this kind of response from a ported box, at least not without compromising every advantage that a ported box typically offers (added output around tuning). You have to build the box very small (about half of the recommended size) and tune it very low (about 12-15hz), which will give you the same result as a sealed box, except it's more complicated, harder to build, and much larger. In order to get the added efficiency or output you need to build it bigger and tune higher, which introduces a large peak in the frequency response around tuning. This big peak at such a low frequency kills the transient response of the sub. Some people don't care about this though...

Bandpass boxes can give you a great response, basically the response of a sealed box with a much higher output potential, BUT you have to build it for your specific sub and your specific car. You need to build a simple sealed box for the sub, measure the transfer function of your car, and then use a program like bassbox pro to predict the behavior of the bandpass box in your specific car, then tune it accordingly. It's very complicated, but you can wind up with a nice result. If you don't want to take the time to do this then you'll most likely end up with something that has a response worse than ported, it just wouldn't be worth it.


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## captain_shrapnel (Sep 10, 2004)

While this may not apply to vehicle acoustics, it might be worth mentioning. Ported boxes have very good high frequency extention over any other kind. While this doesn't sound like a useful thing, if you are lacking in the mid bass department you can cross over a properly designed ported box higher to compensate. Granted, you will lose the "directionless" aspect to a low crossed sub, but if you need more midbass out of the sub a ported box will sound much more musical vs. a sealed. This also means that they will tolerate a shallow high pass crossover slope better.

This is true of non-vehicle speaker situations, but I can only speculate on in-car effect.


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## sr20dem0n (Mar 19, 2003)

How so? Around an octave above tuning the port resistance becomes so high that a ported box behaves identically to an equivalently sized sealed box. The high frequency extension is limited by the inductance of the sub creating a 1st order lowpass filter, which applies whether the sub is sealed or ported.


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## stevja1 (Jan 25, 2004)

sr20dem0n said:


> How so? Around an octave above tuning the port resistance becomes so high that a ported box behaves identically to an equivalently sized sealed box. The high frequency extension is limited by the inductance of the sub creating a 1st order lowpass filter, which applies whether the sub is sealed or ported.


Wow... now you guys are going into stuff I don't quite understand. I'm not sure what port resistance, high frequency extension, or sub induction are. I'm also not really familiar with the orders of lowpass filters. I'll be googling all of this stuff.

My friend has some free software that calculates dB over frequency of a box given some properties of the sub, and the box. Here is an image I had him make for me.










Purple = stage 6 (6th order??) bandpass box
Red & Orange = Stage 4 (4th order??) bandpass box and something else
Light Blue = Ported Box tuned at (I believe 25 or 30 hz...)
Green = Ported box at different frequency
Yellow = Sealed Box

First of all, I'm not very confident in the accuracy of this program, as it was free. This will give me an idea of how many dB/octave I get. I imagine the order has to do with that somehow. Thanks for your help and all the info. I love learning all of the technical stuff about things.


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## stevja1 (Jan 25, 2004)

I've also found a neat site for learning about this stuff. I thought I'd post it if anybody is interested.

The 12 Volt


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## captain_shrapnel (Sep 10, 2004)

sr20dem0n said:


> How so? Around an octave above tuning the port resistance becomes so high that a ported box behaves identically to an equivalently sized sealed box. The high frequency extension is limited by the inductance of the sub creating a 1st order lowpass filter, which applies whether the sub is sealed or ported.


To quote from The Loudspeaker Design Cookbook 5th ed, in the section describing benefits of a vented system:

"Lower cone excursion near the box resonance frequency, resulting in relatively higher power handling and lower modulation distortion. This attribute makes vented enclosures rather attractive for use in two way loud speakers." (p. 43)

While what you say is true, the limited excursion allows the sub to play more cleanly into the mid frequencies without modulation artifacts. Of course the sub's inductance comes into play, but usually at much higher frequencies than what I was talking about reproducing. I know many 15's can actually play into the 1.5khz region before inductance roll off, but I was referring to ~200 hz and lower.


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## sr20dem0n (Mar 19, 2003)

"order" is a way of measuring the rolloff of a filter. A highpass filter lets high frequencies pass and cuts off low frequencies, a lowpass filter lets low frequencies pass and cuts off high frequencies. But neither of these will just stop all frequencies that are beyond the cutoff frequency, they'll slowly attenuate them instead. So if you had a lowpass filter set at 100hz and you played a sine sweep (just a tone that starts at a low frequency and slowly increases in pitch), it would start off loud, say about the same until it got close to 100hz, and then it would start getting quieter. The higher the frequency, the quieter it would get, until it was basically silent at some very high frequency. How "fast" the filter attenuates these higher frequencies is called the slope of the filter, and it's rated in dB/oct (a 6dB/oct filter will attenuate the signal at a rate of 6 decibels for every octave that you go beyond the cutoff frequency). A 1st order filter has a slope of 6dB/oct, a 2nd order filter has a slope of 12dB/oct, 3rd order is 18dB/oct, etc.

Boxes have rolloffs too, as you can see in that WinISD plot you just posted. The blue, green, and yellow enclosures are all acting as highpass filters, the others are bandpass filters (they're a combination of both a lowpass and a highpass, hence the name bandpass, they only pass frequencies within a specified band, you can see this from the plots). Enclosures are rated in "order" as well, sealed boxes are 2nd order because they attenuate low frequencies at 12dB/oct, ported are 4th order because they attenuate low frequencies at 24dB/oct, 4th order bandpass boxes have a 12dB/oct highpass on the low end and a 12dB/oct lowpass on the top end (2nd order + 2nd order = 4th order), and 6th order bandpass boxes have a 24dB/oct highpass on the low end and a 12dB/oct lowpass on the top end. You can see all of these attenuation rates in that plot you just posted, just pick an enclosure, say the yellow one, you can see that at 10Hz it's at roughly 81.8dB, and one octave above (20Hz) it's at roughly 93.8dB, well 93.8-81.8=12dB, its output has fallen by 12dB in one octave, as I just mentioned would be the rolloff for any sealed enclosure.


Just trying to clarify some stuff for you, if you have any questions on the matter just ask


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## sr20dem0n (Mar 19, 2003)

captain_shrapnel said:


> To quote from The Loudspeaker Design Cookbook 5th ed, in the section describing benefits of a vented system:
> 
> "Lower cone excursion near the box resonance frequency, resulting in relatively higher power handling and lower modulation distortion. This attribute makes vented enclosures rather attractive for use in two way loud speakers." (p. 43)
> 
> While what you say is true, the limited excursion allows the sub to play more cleanly into the mid frequencies without modulation artifacts. Of course the sub's inductance comes into play, but usually at much higher frequencies than what I was talking about reproducing. I know many 15's can actually play into the 1.5khz region before inductance roll off, but I was referring to ~200 hz and lower.



Ah alright, yeah IMD will be reduced due to the lowered excursion around tuning, and you also won't have those pesky harmonics creaping into the signal either. Both of those are only problems if you're approaching the point where BL has dropped to 70-80% of its rest value though (aka - near Xmax), as long as you're staying linear neither of those will present a problem.


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## stevja1 (Jan 25, 2004)

sr20dem0n said:


> "order" is a way of measuring the rolloff of a filter. A highpass filter lets high frequencies pass and cuts off low frequencies, a lowpass filter lets low frequencies pass and cuts off high frequencies. But neither of these will just stop all frequencies that are beyond the cutoff frequency, they'll slowly attenuate them instead. So if you had a lowpass filter set at 100hz and you played a sine sweep (just a tone that starts at a low frequency and slowly increases in pitch), it would start off loud, say about the same until it got close to 100hz, and then it would start getting quieter. The higher the frequency, the quieter it would get, until it was basically silent at some very high frequency. How "fast" the filter attenuates these higher frequencies is called the slope of the filter, and it's rated in dB/oct (a 6dB/oct filter will attenuate the signal at a rate of 6 decibels for every octave that you go beyond the cutoff frequency). A 1st order filter has a slope of 6dB/oct, a 2nd order filter has a slope of 12dB/oct, 3rd order is 18dB/oct, etc.
> 
> Boxes have rolloffs too, as you can see in that WinISD plot you just posted. The blue, green, and yellow enclosures are all acting as highpass filters, the others are bandpass filters (they're a combination of both a lowpass and a highpass, hence the name bandpass, they only pass frequencies within a specified band, you can see this from the plots). Enclosures are rated in "order" as well, sealed boxes are 2nd order because they attenuate low frequencies at 12dB/oct, ported are 4th order because they attenuate low frequencies at 24dB/oct, 4th order bandpass boxes have a 12dB/oct highpass on the low end and a 12dB/oct lowpass on the top end (2nd order + 2nd order = 4th order), and 6th order bandpass boxes have a 24dB/oct highpass on the low end and a 12dB/oct lowpass on the top end. You can see all of these attenuation rates in that plot you just posted, just pick an enclosure, say the yellow one, you can see that at 10Hz it's at roughly 81.8dB, and one octave above (20Hz) it's at roughly 93.8dB, well 93.8-81.8=12dB, its output has fallen by 12dB in one octave, as I just mentioned would be the rolloff for any sealed enclosure.
> 
> ...


Thanks for the info. Very helpful.
Are any of you aware of a free program that graphs cone excursion for a given sub, box and amp?

Wouldn't the resonate frequencies of the car change depending on the materials that the car is made of, as well as the size?


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## sr20dem0n (Mar 19, 2003)

stevja1 said:


> Thanks for the info. Very helpful.
> Are any of you aware of a free program that graphs cone excursion for a given sub, box and amp?
> 
> Wouldn't the resonate frequencies of the car change depending on the materials that the car is made of, as well as the size?



WinISD alpha will do cone excursion, same program that was used to generate that plot you just posted

The resonant frequency and starting point of cabin gain doesn't really depend on the materials in the car (they'll all have their own individual resonant frequencies, but for the most part they don't contribute to the car's resonant frequency), but it does depend on the size of the car. The frequency were cabin gain will start is roughly equal to the speed of sound divided by 2 * the longest measurement in the car (like the distance between the foot well and the trunk lid).

So if our cars measure 9ft (I'm not sure what it is, just guessing, too lazy to go outside and measure), that would be 1137/(2*9)=63Hz. Most cars are about 50-60Hz, so my 9ft estimate might be a little small, but then again maybe not, our cars are pretty short. Regular cab trucks are more like 70-80hz, large SUVs are more like 50hz.


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## stevja1 (Jan 25, 2004)

I think this is what I'm going to do. The sub comes with a Qlogic sealed enclosure. I'm just going to listen to it and see how I like it. If it sounds nice and loud, and crisp, I think I might just leave it. I'm definately not going to try to build a bandpass box. ( :thumbup: That was close...)

I kindof want to build a ported box, just for the heck of it though and try it out and compare. I've got a lot more information about this than I had before, and will take this into account when I build the new box. Thanks for all of your help!


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## sr20dem0n (Mar 19, 2003)

Make sure you seal up the joints in that Q Logic box well, they're pretty leaky

Sounds like a plan, it's always best to try both and see which you prefer


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## stevja1 (Jan 25, 2004)

sr20dem0n said:


> Make sure you seal up the joints in that Q Logic box well, they're pretty leaky
> 
> Sounds like a plan, it's always best to try both and see which you prefer


I got it installed and it sounds great. I think I'm gonna stick with the sealed box. The only problem I have is after the install, my idle is really screwed up.  I've got another thread set up for that though.

Thanks for all the help!


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