If a port is placed on the base of a speaker will there be more bass output than on say the front or rear
You'd think.
In theory it makes no difference where you put it on the box, except where you put it in regards to room modes. Many of us have anecdotal evidence that putting it on the front/back/base is different. You'd have to ask whether it was the same in every way? Did the nearby floor affect the tuning, was there some higher frequency content coming out of the port, is the cabinet modal in the port band etc..
In theory it makes no difference where you put it on the box, except where you put it in regards to room modes. Many of us have anecdotal evidence that putting it on the front/back/base is different. You'd have to ask whether it was the same in every way? Did the nearby floor affect the tuning, was there some higher frequency content coming out of the port, is the cabinet modal in the port band etc..
If (and is a big if), the box acts as a bass reflex, then the port placement is irrelevant. In other words, the port output is the same independently on its position.
Point is that in most tall towers the box should be modeled as a MLTL, and in this case the port position matters.
Ralf
Point is that in most tall towers the box should be modeled as a MLTL, and in this case the port position matters.
Ralf
Both of the above. Assuming a pure Helmholtz vented (i.e. uniform internal air particle density & no eigenmodes), and the only variable is the vent location then as far as the box is concerned it makes no difference. If the box has eigenmodes present & / or is actively using them as a functional part of the alignment (i.e. it's an MLTL), then the vent's internal location does indeed affect the results. In both cases, proximity to boundary conditions / room modes will affect matters to some extent. A duct in close proximity to the floor may (may) pick up more boundary loading than some alternatives, but that doesn't automatically result in 'more bass output' as you can over-damp it -for e.g. having limited spacing from a down-firing vent and a plinth, having a down-firing vent with 2in of acoustic fiberglass directly below it 😉 etc.
The design I am looking at has a rear vent. But for waf I have to place the speakers 2 inches from the back wall and I am wondering if a bottom vent would be better again 2 inches from the floor.
A reflex box will always act as a transmission line, usually at a much higher frequency than the tuning frequency. See here, a bass reflex box that acts as a transmission line above few hundred Hz: https://sound-au.com/articles/boxstuff.htm
Adding damping material can damp out the transmission line effect and is why it is especially recommended to heavily stuff the ends of long/tall boxes with damping material. If you sim the box as a transmission line however, you may find that a calculated placement of the port can reduce your reliance on damping material to remove problematic transmission line effects.
If you add dual ports and place them at different locations on the box, the transmission line peaks can be dealt with even better than a single port.
Generally, a good rule of thumb is to place the port(s) not far from the bass driver, rather than down the other end of a long box. If you place the port on the side of the box is that close (2inches you say) to the wall/floor then it will act like the port is longer than it physically is, because of the restriction around the port mouth. Therefore if you to have your port exit on the bottom of the box, or on the rear of the box and then place it up against the wall, anticipate the tuning to be lower than expected and shorten the port's length to compensate. You will not get "more" bass because you can't defy physics, but the tuning will be lower than if for example you put the same length port exiting on the front or top of the box.
Adding damping material can damp out the transmission line effect and is why it is especially recommended to heavily stuff the ends of long/tall boxes with damping material. If you sim the box as a transmission line however, you may find that a calculated placement of the port can reduce your reliance on damping material to remove problematic transmission line effects.
If you add dual ports and place them at different locations on the box, the transmission line peaks can be dealt with even better than a single port.
Generally, a good rule of thumb is to place the port(s) not far from the bass driver, rather than down the other end of a long box. If you place the port on the side of the box is that close (2inches you say) to the wall/floor then it will act like the port is longer than it physically is, because of the restriction around the port mouth. Therefore if you to have your port exit on the bottom of the box, or on the rear of the box and then place it up against the wall, anticipate the tuning to be lower than expected and shorten the port's length to compensate. You will not get "more" bass because you can't defy physics, but the tuning will be lower than if for example you put the same length port exiting on the front or top of the box.
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By the way: If the design you are looking at has a rear vent chances are high it has been designed for a placement away from back wall. If you place it near the back wall you get reinforced and possibly bloated bass response.
More specifically, in a normal sized living room and at normal distances (e.g. comparing against the wall to say 2 feet out) the difference will be that when you place it against the wall that are a whole lot of nulls in the upper midrange and treble (due to reflections off the back wall) that make the bass seem more prominent. For all intents and purposes the bass will be the same if the speakers are placed 2feet or 0feet from the back wall, since the wavelength at bass frequencies is much much longer than 2 feet.
Every sound source close to the boundary will be boosted anywhere from 3dB to 6dB. Just tune the port/bass-reflex frequency little bit lower (with longer port).
While often true, this is not always the case, which is why I would advise checking. For instance, I regularly use rear vents in many smaller designs, but they employ a well-damped alignment and are perfectly happy in reasonable proximity to a front wall. The positioning is a result of ensuring any flow-noise is minimised while avoiding excessive duct length (and the 1/2 wave modes resulting).
I feel here is the biggest problem with ports, not the helmholz resonance or coupling with the room but the midrange that can come out as well. Careful positioning of the inlet inside the box reduces the leakage. Worst case there is some midrange peak coming out as loud as the helmholz resonance. A pipe resonance or internal mode(s) leaking out. Such mid leakage could be less obstrusive if the port is on the back, or anywhere else than towards listening spot. Better to tweak the inlet position though, to reduce the leakage altogether.
If the box is small and helmholz tuning very low, like 30hz which is ~10 meter long wavelenght, I suspect few tens of centimeters difference in the port location won't affect room modes at all. Or is there some evidence it does? If tuning was 100Hz and box big, then it is more likely as the port position moved significantly in comparison to wavelength.
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Flow noise and 1/2 wave modes from the duct can definitely be an issue, especially since most modern 'hi-fi' midbass et al drivers are targeting the smallest possible Vb requirements. Cue inevitable balancing act, since to keep the duct length down (I set 6in as the maximum I'm willing to use) & avoid the 1/2 wave modes, the CSA is often compromised. In those cases, I generally think compromising on the latter & accepting a slightly higher air velocity through the duct is preferable as you can usually render it ~inaudible with a rear vent position, which isn't always the case with pipe harmonics.
Ah yeah didn't even think the chuffing 😀 Yeah definitely less audible from back of the box.
I see the box modes come out quite often, ment these with modes on last post. Latest one here https://www.diyaudio.com/community/...708-ish-dimensioned-2-way.377634/post-7011499, even if it eventually wasn't too audible it can mess up measurements and cause headache for the designer. At least few previous I remember seeing on this forum was box front/back distance 1/2wl resonance coming through.
I see the box modes come out quite often, ment these with modes on last post. Latest one here https://www.diyaudio.com/community/...708-ish-dimensioned-2-way.377634/post-7011499, even if it eventually wasn't too audible it can mess up measurements and cause headache for the designer. At least few previous I remember seeing on this forum was box front/back distance 1/2wl resonance coming through.
Thank you for all your replies and advice, although I must admit I am a little confused.
As I said the design I am looking at has a rear port and due to waf speakers must be no more than 2 inches maybe 3 inches from the wall behind.
I dont think a port on the side will be tolerated either so must have bootom or rear port.
The cabinets are approx 38 litre floor standers and the ports tune the 2 bass/mid units to approx. 30hz.
So bearing all that in mind what would be my best choice base or rear port, thanks for your help.
As I said the design I am looking at has a rear port and due to waf speakers must be no more than 2 inches maybe 3 inches from the wall behind.
I dont think a port on the side will be tolerated either so must have bootom or rear port.
The cabinets are approx 38 litre floor standers and the ports tune the 2 bass/mid units to approx. 30hz.
So bearing all that in mind what would be my best choice base or rear port, thanks for your help.
If you can choose some other design/kit, made for close wall positioning, I'd change to that one. Altering ready made kits in any way, including changing port location might bring "issues" like discussed in the thread. The issues can be mitigated but this is some extra work for you both learning, communicating and building. It is fun process though, at least for many 😀 Not sure how you feel about it so changing to a speaker that is designed for the application is better bet for both sound quality and ease for you to build.
If you are set for the project and absolutely want to build it then I'd suggest to build it like in the plan, close the ports with stuffing and they will probably work fine close wall and the port location doesn't matter, keep it as in the plan. There are many ways to think about the issue, just keep it simple 🙂
If you are set for the project and absolutely want to build it then I'd suggest to build it like in the plan, close the ports with stuffing and they will probably work fine close wall and the port location doesn't matter, keep it as in the plan. There are many ways to think about the issue, just keep it simple 🙂
It will be same for both positions, if port opening distance from the wall (if mounted on the back) is the same as the distance from the floor (if mounted at bottom).
correcting and refining my statement above here is a simulation of back wall distance 40" and 2" done with jeff bagby's excel-tool.
@colmo: keep in mind that this may not apply to your speaker, it's just an example for an idealized speaker, assuming arbitrary values!
