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Cabinets : Front Ported vs. Sealed vs. Rear Ported

#82
Probably. I don't recall seeing any reference work that gets things that wrong, but not everything that makes it into print is peer reviewed. It's not even that it's outdated, like reading a physics textbook from the 1950s that tells you the smallest subatomic particles are the neutron, proton and electron. It's just plain wrong.
 
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#83
Having used both front and rear ported cabs I cans say that the front ported cabs operates better than the rear ported. I owned and played through an Ampeg rear ported 410. Ported at the bottom not behind the drivers. The porting ran the entire lower part of the cab.

It had to be close to a wall to get any low end out of it.

I’m short I hated it.

Getting a GLC and SBX GK cabs solved all my problems. No more rear ports for me.

Oh and by the way. Sealed cabs suck to.
 
#84
Perry Wright said:
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Wow. I don't have the energy to respond to all of the problems I see with this, but being the crackpot that I am, I'll take a crack at one of them: The claim that rear porting causes interference [with the front wave] that results in non-linear response which varies with the distance from the wall. My response will be somewhat simplified for the sake of time.

First, let's look at a sealed box on a stage. When a sealed box cab has a wall behind it, some of the omnidirectional low-end output bounces off the wall and thus arrives at the bass player's ears after a corresponding time delay. At some frequency this time delay relative to the front wave will be equal to one-half wavelength and partial cancellation will occur. The result can be that some notes seem to "disappear" on stage, though they are usually still present out in the house.

With a front-ported cabinet, the output from the woofer and port both have the same path length for that wall bounce, and so they can have partial-cancellation dips at the same frequency. (In practice this may be mitigated by the fairly rapid phase rotation of the port's output.)

With a rear-ported cabinet, the port's wall-reflection path is significantly shorter than the woofer's wall-reflection path, thus the two do not have their wall-bounce dips at the same frequency. So if anything, they are more likely to fill in one another's wall-bounce dip. Therefore you are less likely to have notes that "disappear" on stage with a rear-ported cabinet. (This benefit is confined to the region where the port has significant output so rear porting is not "guaranteed" to be an improvement in this area in every situation, but it is clearly not a detriment.)

I have a few minutes left before smoking my mid-morning crack, so let's look at one more claim: "A sealed box will be safer for the speaker as well." While it is true that a ported box is unloaded below the tuning frequency, its excursion is significantly less at and little above the tuning frequency (where the cab's output is significantly louder than a sealed box). It is quite possible to tune a box with the intention of maximizing the excursion headroom and ending up with a worthwhile improvement over a sealed box. Finally, I have read papers where the cooling effects of various ports were examined, as voice coil overheating is the most likely failure mode for a woofer. A sealed box was included in the tests as a baseline. The various ports reached various equilibrium temperatures under the test conditions, but the sealed box never did: Its voice coil temperature rose faster and never stabilized but always continued to rise over time, until the signal was discontinued. So in conclusion a vented box can be safer than a sealed box from an excursion standpoint if that's the designer's intention, and it is always safer from a thermal management standpoint.

joel406 said:
Having used both front and rear ported cabs I cans say that the front ported cabs operates better than the rear ported. I owned and played through an Ampeg rear ported 410. Ported at the bottom not behind the drivers. The porting ran the entire lower part of the cab.

It had to be close to a wall to get any low end out of it.

I’m short I hated it.

Getting a GLC and SBX GK cabs solved all my problems. No more rear ports for me.
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I don't dispute your cab preferences, but I don't think the differences between those cabs was caused by the port location. That is an easy-to-see difference that has relatively minor audible consequence (not enough to make the difference between "hate" and "problem solved"), while the actual audibly significant differences were probably caused by woofer parameters and frequency responses and things that are not easy to see.
 
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#87
DukeLeJeune said:
The claim that rear porting causes interference [with the front wave] that results in non-linear response which varies with the distance from the wall.
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The flaw in that claim has a very basic explanation. For the front and rear waves to destructively combine they'd have to meet at or near 180 degrees apart, which is one half wavelength. Port output is pretty much confined to below 100Hz, where a wavelength is 11.2 feet, so there isn't going to be any cancellation because the wavelengths are too long. This brings up an earlier point, about having the port located where mids can exit the port. In that case the wavelengths are short enough that the waves from the port reflecting off the wall could combine destructively with the front wave. It's not a major concern, but it's something to consider.
 
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#88
Rick James said:
This brings up an earlier point, about having the port located where mids can exit the port. In that case the wavelengths are short enough that the waves from the port reflecting off the wall could combine destructively with the front wave. It's not a major concern, but it's something to consider.
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I think that the mids coming out of a rear-facing port in effect combine with the front wave in semi-random phase, rather than resulting in audibly significant peaks and dips. By way of imperfect analogy, an in-room frequency response includes lots of midrange reflections that look like "hash" or "grass" on an unsmoothed curve, and thus appear disastrous to the eye, but are subjectively fairly benign. The net effect of these reflections is to increase the in-room SPL a bit, as the peaks and dips are far too close together to be of individual significance. I would expect midrange energy exiting a rear-firing port to be fairly weak to begin with, and therefore to have even less net effect.
 
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#89
DukeLeJeune said:
I think that the mids coming out of a rear-facing port in effect combine with the front wave in semi-random phase, rather than resulting in audibly significant peaks and dips. By way of imperfect analogy, an in-room frequency response includes lots of midrange reflections that look like "hash" or "grass" on an unsmoothed curve, and thus appear disastrous to the eye, but are subjectively fairly benign. The net effect of these reflections is to increase the in-room SPL a bit, as the peaks and dips are far too close together to be of individual significance. I would expect midrange energy exiting a rear-firing port to be fairly weak to begin with, and therefore to have even less net effect.
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I agree, there are enough contributing "boundaries" as well as the reflected mid level being significantly below the primary level that in practice it's probably not much of an issue.
 
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#90
I only brought it up because I've seen owners of Bag Ends that have the port directly behind the woofer comment on how they sound different when there's a wall behind them compared to when there isn't. Of course it might be attributed to a combination of factors, including Allison Effect and the Baffle Step, two more terms that I don't think we'd find in whatever source those pages came from.
 
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#91
The rear boundary can impact the response of the reflected sound, both the distance and the absorption/reflection coefficients. Think concrete versus a lightly framed wall covered with a stage drape. The effects will be different at low and high frequencies.
 
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#93
Rick James said:
I only brought it up because I've seen owners of Bag Ends that have the port directly behind the woofer comment on how they sound different when there's a wall behind them compared to when there isn't.
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Thanks for relaying that... I would have thought so, but don't recall actually hearing it in so many words.

Do you recall if they tend to prefer having the wall reflection versus not having it? Or is it just "different"?
 
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#94
I've really learned a lot from this thread. Thank you all. I understood the basics of using the T/S parameters but was pretty ignorant many of the issues discussed above.

So when you commercial designers design cabinets, are you frustrated that your well engineered work will be compromised because you don't have control over the room acoustics? It kind seems like you are designing a Formula One car that is then going to be driven on dirt roads. Do you make design changes (other than the addition of the clever port plugs) in anticipation that cabinets will often be used in venues with bad acoustics?

And this is priceless...:roflmao:
DukeLeJeune said:
I have a few minutes left before smoking my mid-morning crack, so let's look at one more claim:
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#96
SLivinghouse said:
I've really learned a lot from this thread. Thank you all. I understood the basics of using the T/S parameters but was pretty ignorant many of the issues discussed above.

So when you commercial designers design cabinets, are you frustrated that your well engineered work will be compromised because you don't have control over the room acoustics? It kind seems like you are designing a Formula One car that is then going to be driven on dirt roads. Do you make design changes (other than the addition of the clever port plugs) in anticipation that cabinets will often be used in venues with bad acoustics?
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Typically a good designer will design their product understanding that the acoustics are out of their control. Most will avoid creating issues that might make the results worse but won't worry about correcting something that's not correctable.

shadven said:
My experience say sealed for better mids = better mix. Ported first rehearsals and bedroom.
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The mid response is almost completely independent of the cabinet being ported or sealed.

It's very much a voicing choice by the designer, and many designers of sealed cabinets have made the choice of a mid forward voicing. Also, with the low frequency rolloff typical of a sealed cabinet, there may be the appearance of a more mid forward voicing.
 
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#100
gwangi said:
Okay here's my stupid question, so at the end of the day if you are going to have just one cabinet to use what would be the best all-round cabinet to cover everything: ported-rear, ported-front, sealed?
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I think that would depend on your criteria. Is size and weight a consideration? If I'm not mistaken, one could design sealed cabs to have similar extension to ported cabs, but ported, in theory, should be able to do "more" with less in many ways.
 

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