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Cabinets : Front Ported vs. Sealed vs. Rear Ported

#21
James Hart said:
I was just coming back to edit...

I do know exactly what you mean... just needed to defend the little S15. Like any piece of gear "you need to know how to use it". Poorly executed or not... they are a great little cab if you know how to use it (as I know you know how to)... but yes, some rooms / stages make it hard to place them right.

Perhaps the tiny rear port(s) on the S12 (and my current D12) are the reason I prefer the 12" version so much better?
Click to expand...

+1 To me, it's more of a 'quirk' than a disaster with the little 115 cube.

Again, I'm getting over my head here, but based on all my discussions with Alex, Greenboy, and reading Bill F's post, it seems one of the ways you can mess up with rear porting is having a large port sitting literally right behind the driver. That's the only Bag End that seems to have this design. The others are smaller ports placed in more offset locations.

I'll let others who know more comment at that level, but my limited gigging experience with a couple of other Bag End cabs makes me think that the issue I mention is primarily with that single cab and not the others.
 
#24
I've had problems in the past with a rear ported SWR GoliathII and an Ampeg 115 combo being hugely inconsistent from room to room.

The 115 outside was terrible. I had to crank the bass to 4:00 o'clock, the problem with that was I was going direct out of the combo which gave FOH too much bass. I had to put a rack door behind the amp to give me some low end.

I understand scientifically lows are omni directional, but there's something about a rear ported cab that doesn't work for me in live situations.
 
#25
LowDown Hal, I would say that the differences between front and rear porting are inconsequential in a bass guitar application, and practicality is probably the overriding factor. Any differences you hear between front and rear ported cabinets are most likely due to other factors.

The difference in operation between sealed and ported cabs is significant, and the difference in results can be, although there is a fair amount of overlap: One can design a sealed box that comes pretty close to emulating the frequency response of a vented box, and vice versa. And frequency response is the dominant factor in what we hear at low frequencies.

Briefly, a vented box is tuned to have a Helmholtz resonance below the frequency where the woofer would normally roll off if the box were sealed. This is used to extend the bass deeper than it otherwise would have gone. The advantage of a vented box over a same-size sealed box is the vented box can either go deeper or be more efficient; or, for roughly equivalent performance, the vented box can be made smaller.

In general, a sealed box starts rolling off higher up, but more gradually, than a vented box. A vented box will usually have more bass output and higher excursion-limited power handling down to its tuning frequency, but below that bass output and power handling plummet. In general, sealed boxes are more likely to have good pitch defiintion and "tightness" in the bass region, while vented boxes are more likely to have a lot of bass weight - but this is not a hard and fast rule. Woofers whose parameters are suitable for a vented box are more likely to have good midrange articulation because their magnets are more powerful relative to the cone weight.

As a designer, I like the flexibility that vented boxes give me in shaping the low frequency response, but I used both types depending on the application.
 
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#26
the only difference between front and rear ports is how far you need to keep them away from a wall. You need some room around ports or you'll change the sound of the cab. Front ports usually have lots of room because you're not placing anything right of against there.
 
#28
From what I remember reading:

Sealed cabinets have more punch with loss of a small amount of volume and resonance. The air in the cabinet stays where it is, limiting the speaker's movement.

Ported cabinets allow air to flow freely, resulting in more volume and resonance with a "looser" sound because the speaker can vibrate more fully.
 
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#29
Bill, I'm curious if you can answer me this in regards to this subject. I will be replacing my 18 driver in my current cab in the near future. After plotting everything out in WinISD, It would appear that I may not have enough room on the front baffle for porting to keep port velocities at acceptable levels. Aside from the obvious phasing issues at higher freq's (which this cab won't see), are there problems with having vents in both the front and rear of an enclosure?
 
#31
Regarding placing rear-ported cabinets close to a wall, you might get perhaps 1 dB more boundary reinforcement near the tuning frequency relative to a front-ported cab close to the wall. In order for the wall to interfere with the port tuning, the distance from port to wall would have to be 1 port diameter (or 1 port height, if a slot port) or less before it will affect the tuning frequency. And even then the effect would be to lower the tuning frequency, which would be the right direction to partially offset the effects of increased boundary reinforcement. As long as you don't push a rear-ported cab so close to the wall that you start to physically choke off the airflow, you'll be fine.

The increased "punch" of a good sealed-box system arises from the shape of its frequency response curve and how the ears interpret it. It is possible to design a vented box that is nearly indistinguishable from a sealed box, while retaining an edge in the box size/bass extension/efficiency department. Now most vented bass cabs are not designed with emulating a sealed box as their top priority, but Barefaced's "Vintage" cab has an adjustable tuning system that allows a very low tuning which is claimed to sound like a sealed box. I've modelled that woofer in a low-tuned vented box, and I believe the claim.
 
#32
Blues Cat said:
I understand scientifically lows are omni directional, but there's something about a rear ported cab that doesn't work for me in live situations.
Click to expand...


Don't tar all rear ported cabinets with the same brush. I have never had any low end issues on any sized stage with my Eden D410T cabinets.

Cheers,
Duff2
 
#33
I hadn't thought of this before, but when you're standing right in front of the bass cab the impact of the bass that you hear will be affected by whether or not the port is in the front. You see, in the nearfield SPL falls off by 6 dB for every doubling of distance (simplifying here a bit), so when the ports are on the rear of the box (roughly twice as far away) the port's relative contribution may well be 6 dB less from the perspective of the bass player. In the farfield (where the audience is), it makes no difference.
 
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#34
DukeLeJeune said:
I hadn't thought of this before, but when you're standing right in front of the bass cab the impact of the bass that you hear will be affected by whether or not the port is in the front. You see, in the nearfield SPL falls off by 6 dB for every doubling of distance (simplifying here a bit), so when the ports are on the rear of the box (roughly twice as far away) the port's relative contribution may well be 6 dB less from the perspective of the bass player. In the farfield (where the audience is), it makes no difference.
Click to expand...

Huh? I've always thought that the nearfield is where the SPL doesn't fall off by 6 dB per doubling of distance.
 
#35
fdeck, my understanding is that the nearfield is the zone where the direct sound from the source (in this case a bass cab) delivers more energy to the ears (or microphone) than the reverberant sound does. Depending on the directional characteristics of the source and reflective properties of the room, maximum near-field distance varies from perhaps five to twenty feet or so. At considerably greater distances the reverberant energy dominates, and SPL changes relatively little with distance. At some distance, the soundfield reaches a steady-state condition such that there is no change in SPL with distance. Up closer, in between the nearfield and farfield, there is also a transition zone (or midfield) within which the relative contribution of both direct and reverberant energy is comparable.

In an anechoic chamber (or outdoors), all we would have would be nearfield conditions because there would be no reverberant contribution. Well not really - we'd still get a reflection off the ground outdoors. But under true anechoic conditions, SPL falls off by 6 dB for each doubling of distance from a point source. Nearfield is closer to true anechoic than are any of the other in-room distance zones, as it's where the relative reverberant contribution is the least. My oversimplification in my post above was ignoring the reverberant field contribution, which reduces that theoretical 6 dB per doubling of distance even when we're in the nearfield.

While we're on the subject, under anechoic conditions SPL falls off by 3 dB per doubling of distance from a line source, and by 0 dB per doubling of distance from a planar source. At very close range (like a few inches), a large cone or array of cones might simulate a planar source and therefore the SPL would be constant over short distances. I have encountered this phenomenon with very large electrostatic panels, but it only holds up out to a foot or so.

I have seen different terminology conventions used to describe this, and it's possible that I'm not using the correct terminology here.

Since most of the listeners in a large room are at a distance where the reverberant energy is dominant, I think that the higher priority ought to be placed on the power response (summed total output) of a bass cab, rather than on the on-axis response. But, that's another topic for another day.

By the way, fdeck, your website is great. Your DIY project is very well thought-out and explained, and quite educational.
 
#36
DukeLeJeune said:
fdeck, my understanding is that the nearfield is the zone where the direct sound from the source (in this case a bass cab) delivers more energy to the ears (or microphone) than the reverberant sound does.
Click to expand...

Aha. I have to admit that I was thinking of a definition that has more familiarity in optics and antenna design.

By the way, fdeck, your website is great. Your DIY project is very well thought-out and explained, and quite educational.
Click to expand...

Thanks for the kind words! :)
 
#37
DukeLeJeune said:
Briefly, a vented box is tuned to have a Helmholtz resonance below the frequency where the woofer would normally roll off if the box were sealed. This is used to extend the bass deeper than it otherwise would have gone. The advantage of a vented box over a same-size sealed box is the vented box can either go deeper or be more efficient; or, for roughly equivalent performance, the vented box can be made smaller.

In general, a sealed box starts rolling off higher up, but more gradually, than a vented box. A vented box will usually have more bass output and higher excursion-limited power handling down to its tuning frequency, but below that bass output and power handling plummet. In general, sealed boxes are more likely to have good pitch defiintion and "tightness" in the bass region, while vented boxes are more likely to have a lot of bass weight - but this is not a hard and fast rule. Woofers whose parameters are suitable for a vented box are more likely to have good midrange articulation because their magnets are more powerful relative to the cone weight.
Click to expand...

ok, starting with this (somewhat dated quote from DukeLeJeune) I wanted to dig into the question of sealed and ported/vented cabs, and whether there is a reason to either combine them or avoid such combinations.

Specifically, I currently have an Ampeg PF500/PF115HE (sealed 1 x 15˝ ceramic Eminence driver +
1˝ HF compression driver, freq response: 50 Hz - 17 kHz
Sensitivity: 99.5 dB SPL @ 1w/1m ). It sounds fine, and as suggested, the sealed 115HE is pretty punchy to my ears.

Now, I have been thinking of adding another 1x15 for the (rare) occasion that I need more volume/coverage than the 115HE can provide. An obvious option is to go with another PF115HE to keep the cabs/specs the same. One sounds good, two would probably sound just as good, only "bigger".

OTOH, Ampeg offers a PF115LF (Shelf ported 1x15 no horn, freq response: 57 Hz – 3.7 kHz Sensitivity: 99.5 dB SPL @ 1w/1m).

Now I see that the response of the HE is 50Hz-17kHz, and the LF (oddly, given its name - LF) is 57 Hz-3.7kHz, but, the question I want to ask in general is, if you are going to run 2 cabs (with the same driver, in this case 1x15") why not take advantage of the characteristics of the two designs and run one ported cab tuned more towards LF, and a sealed cab with its softer roll-off, and potentially "punchier" characteristic ?

I know the world would not end if this happend (at least I hope not) but what are the arguments for/against this thinking ?
 
#38
pfschim said:
I know the world would not end if this happend (at least I hope not) but what are the arguments for/against this thinking ?
Click to expand...
The problem with it is that you cannot predict the result. If you're able to try the mixed combination, along side two 115HE and two 115LF, and have a preference for the mixed pair all well and good. But you're being able to do so would be the very rare exception.
 
#39
billfitzmaurice said:
The problem with it is that you cannot predict the result. If you're able to try the mixed combination, along side two 115HE and two 115LF, and have a preference for the mixed pair all well and good. But you're being able to do so would be the very rare exception.
Click to expand...

well, money is not the issue, it's more about all the logistical PITA that would have to happen to get 3 more cabs here (2 LF's and 1 HE), on top of the one 115HE I already have at my disposal, and then try them out in a reasonably realistic scenario like a gig, and not just a practice space.

but all that aside, I am interested to hear that the idea is not completely flawed from the start.
 
#40
I think everybody needs to read back through this whole thread...it's not that long. Between Bill's hard engineering, and Duke's more lengthy explanations and examples of the same, there is a ton to be learned....beyond even port placement.
 

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