I'm building a set of LCR speakrs for a hometheater, using a pair of Dayton Audio RS125-04 and an M&K Tweeter in each cabinet, the box design using Winspeakerz indicates a ported design works best. For such a speaker where is a good place to mount the port?
Front baffle, or rear? If I mount it on the front I will loose the symetrical layout I have, thus would prefer to have a rear firing vent or port, but am curious as to the pit falls
The center speaker will be on the top shelf of a big A/V cabinet and the L/R 's will be on the top of the whole cabinet, thus the center will basically be have another shelf above it, the L/R will be out in the open.
Thanks
Front baffle, or rear? If I mount it on the front I will loose the symetrical layout I have, thus would prefer to have a rear firing vent or port, but am curious as to the pit falls
The center speaker will be on the top shelf of a big A/V cabinet and the L/R 's will be on the top of the whole cabinet, thus the center will basically be have another shelf above it, the L/R will be out in the open.
Thanks
Ports can be split into 2 or 4 triangle shaped units (or smaller ducts) that can be placed around the cone, so can still be symetrical.
A rear loaded port tuning may be affected by placement close to the wall or other surfaces.
In the end, it does not make much difference, the frequencies reinforced by the port are low enough to be virtually omni.
As long as the cabinets are not too close to the wall the position is pretty arbitrary, except in the case of large shallow ports where reflected mid-high frequencies may be heard through the port.
A rear loaded port tuning may be affected by placement close to the wall or other surfaces.
In the end, it does not make much difference, the frequencies reinforced by the port are low enough to be virtually omni.
As long as the cabinets are not too close to the wall the position is pretty arbitrary, except in the case of large shallow ports where reflected mid-high frequencies may be heard through the port.
The location I'd think to be best (this is of particular interest since I'm designing the venting for some cabs right now) would be downfiring. This allows a lowpass filter effect for port resonance and other higher frequency components. Earl's approach would seem to have an advantage at frequencies where the integration requires some cross-cancellation, and reduce any localization effects (listener hearing the port separately from the woofer) at port resonances.
Earl, would you like to comment from a theoretical perspective? I've heard some speakers with the ports 'surrounding' the driver and they seemed to integrate well.
Earl, would you like to comment from a theoretical perspective? I've heard some speakers with the ports 'surrounding' the driver and they seemed to integrate well.
Leaving any subjective discussions aside, I have measured results that indicate that closer is better. Mind you it's very difficult to get good data at these low frequencies. I think that far too much importance is given to all of these effects. from what I can tell the details of any LF design are not even a close second to the rooms dominance. And since the room dominates the number of subs and location is far more important than any detail like where the port is.
It would be too difficult to dig up the data, I just remember that I started the Summa with a rear port but did not get the performance that I was looking for. I moved the ports to the front and it worked better. Don't recal much more than that, sorry. Theoretically it shouldn't make any difference - I never sorted that out, mostly because I don't use ports anymore.
I should really bow out of this discussion because I don't use ported enclosures in anything anymore. I do use them in the bandpass subs that I like, but that's not ported in the standard sense. As I said, when I found multiple subs all else fell by the wayside and I don't really care what the LF configuration is anymore as long as there are several subs supporting the mains.
I should really bow out of this discussion because I don't use ported enclosures in anything anymore. I do use them in the bandpass subs that I like, but that's not ported in the standard sense. As I said, when I found multiple subs all else fell by the wayside and I don't really care what the LF configuration is anymore as long as there are several subs supporting the mains.
badman said:
badman and B&W agree that raising the BASS cabinet on legs and putting a large diameter, large radius(inside and outside) port on the bottom of the cabinet has several advantages.
1) Takes full advantage of uniform floor gain. B&W adds a bottom floor board in case of carpeting, and includes a back board to block just the rear path.
2) Filters any midrange rear cone sounds from directly reaching the ear
3) allows longest straight port tube... ML TL favor this
Rear ports can sometimes rattle normal drywall construction and windows.
I only port deep bass alignments, never midbass, and also favor a downfiring port when possible. I put front bass ports near the floor for ground gain.
Re: C'mon doc
Rearward firing is best , but will make the speaker sensitive to wall proximity ...
Forward firing port is best located as close to woofer as possible ..
Close proximity makes for better coupling of port and woofer ....
forward firing will also be sensitive to speaker location and works best in near field application .....
DO not make the port more than 50% of effective cone area .........................
Downward firing works good in subwoofer application , i would not do so for a bass/mid driver .
2nd and 3rd harmonic distortion and time domain measurements tends to favor the port being of closer proximity when forward firing with bass/mid drivers ..
When forward firing in an subwoofer application this is not so critical due to the wavelengths involved ....
JBCon said:
Rearward firing is best , but will make the speaker sensitive to wall proximity ...
Forward firing port is best located as close to woofer as possible ..
Close proximity makes for better coupling of port and woofer ....
forward firing will also be sensitive to speaker location and works best in near field application .....
DO not make the port more than 50% of effective cone area .........................
badman said:
Downward firing works good in subwoofer application , i would not do so for a bass/mid driver .
2nd and 3rd harmonic distortion and time domain measurements tends to favor the port being of closer proximity when forward firing with bass/mid drivers ..
When forward firing in an subwoofer application this is not so critical due to the wavelengths involved ....
Will keep both suggestions in mind.
I have done a bit of reading and as stated, the general consensus seems to be it doesn't matter, but my own preceived feeling (based on hours after hours of listening to various speaker designs over the years), is that it works better inline with the drivers, however what I don't know (or don't recall off hand), is there a "preceived difference" when using a small coned midrange, versus system susing larger 6" and above midwoofers.
My design with a pair of RS125's in a .27 cu ft box calls for two 1.5 inch ports of about 4 " each for a tuned frequency of 94 or so hertz. That is a 4.87 sq in. recommended port volume. I guess I could go with a larger port diamter, but I already have the prefab port tubs on hand and would prefer to use them.
What are the rules of having one port on the front baffle and the second on the back, as long as they are not on same ends of enclosure?
I have done a bit of reading and as stated, the general consensus seems to be it doesn't matter, but my own preceived feeling (based on hours after hours of listening to various speaker designs over the years), is that it works better inline with the drivers, however what I don't know (or don't recall off hand), is there a "preceived difference" when using a small coned midrange, versus system susing larger 6" and above midwoofers.
My design with a pair of RS125's in a .27 cu ft box calls for two 1.5 inch ports of about 4 " each for a tuned frequency of 94 or so hertz. That is a 4.87 sq in. recommended port volume. I guess I could go with a larger port diamter, but I already have the prefab port tubs on hand and would prefer to use them.
What are the rules of having one port on the front baffle and the second on the back, as long as they are not on same ends of enclosure?
JBCon said:
Regarding perception please keep in mind that the perceived bass will be far more dependnet on the room and setup specifics than the woofer design details.
In general ports all add together as acoustic masses. Just determine the total acoustic mass and divide it up into the various ports. They can be different lengths and areas, etc. and don;t even need to be the same acoustic mass. I would put all of the ports as close together as possible however and not at different locations like different ends of the enclosure. If they are not close together then they will not add as simply as I have described.
Re: Re: C'mon doc
I'm going to be venting somewhere between 30-40Hz. Can you explain more about the measurements you're quoting? As we all know, an undocumented measurement didn't happen.....
In any case, at 30-40Hz, there is plenty of wavelength to swamp most of these effects, I'd imagine.
a.wayne said:
I'm going to be venting somewhere between 30-40Hz. Can you explain more about the measurements you're quoting? As we all know, an undocumented measurement didn't happen.....
In any case, at 30-40Hz, there is plenty of wavelength to swamp most of these effects, I'd imagine.
Helmholtz resonance is unaffected by port location, but the influence of internal standing waves nodes is determined by the positions of the driver and port in relation to cabinet boundaries.
The take away is that for small cabinets or low frequency passbands (subs), port position matters little but as cabinet size grows and/or passband is wider, port size and position matter more. A long port can develop a pipe mode in the passband, for example. Another thing that can happen is standing waves line up inside a cabinet so the port falls in a pressure node. Those are transmission line behaviors, and they influence response even if the inteneded tuning mechanism is bass-reflex.
I usually suggest a few things be done when working on a new design, especially one that is large enough that internal standing waves occur in the bass to lower midrange. First, use Martin King's spreadsheets to model the system. This will show the effects of internal standing waves. Second, consider putting insulation spanning the cross-section in large cabinets. Spacing the insulation away from the cabinet panels makes it more effective at lower frequencies, down into the midrange. That usually isn't needed on small cabinets because the standing waves are high in frequency. But in a larger cabinet, the standing waves are lower and if you want to damp them, the insulation needs to be in the right place. As always, depend on measurements to verify anything you have modeled to be sure it is working as expected.
The take away is that for small cabinets or low frequency passbands (subs), port position matters little but as cabinet size grows and/or passband is wider, port size and position matter more. A long port can develop a pipe mode in the passband, for example. Another thing that can happen is standing waves line up inside a cabinet so the port falls in a pressure node. Those are transmission line behaviors, and they influence response even if the inteneded tuning mechanism is bass-reflex.
I usually suggest a few things be done when working on a new design, especially one that is large enough that internal standing waves occur in the bass to lower midrange. First, use Martin King's spreadsheets to model the system. This will show the effects of internal standing waves. Second, consider putting insulation spanning the cross-section in large cabinets. Spacing the insulation away from the cabinet panels makes it more effective at lower frequencies, down into the midrange. That usually isn't needed on small cabinets because the standing waves are high in frequency. But in a larger cabinet, the standing waves are lower and if you want to damp them, the insulation needs to be in the right place. As always, depend on measurements to verify anything you have modeled to be sure it is working as expected.
JBCon said:
Hi,
Some quick modelling in winISDbeta indicates a 6dB peak at 100Hz,
and of course no bass power handling or effective bass below 80Hz.
Red is one 1.5" diameter x 4" long port.
/sreten.Measurement from www.zaphaudio.com used.
Attachments
Re: Re: Re: C'mon doc
This is why it becomes 2 and 3 rd harmonic ....
If your bass driver is producing outputs in the 400- 800 hz range you will have issues with a forward firing port of sizable dimensions..
Nearfield will show this effect more than farfield ......
Energy in the cabinet is still audible , chuffing heard thru ports for eg. have shown to create noise that will cause muddying of the bass while not audible to the listener as port chuffing...
Really large ports forward firing will allow higher freq to be audible and will have phase issues ...
Step response and waterfall plots will show the interference....
Now while low frequency is omni, radiating output is not , so for large
speakers with large ports , rear or downward firing is best , radiating output is dispersed more evenly and will not present the listener with an " here I'am " Moment ..
badman said:
This is why it becomes 2 and 3 rd harmonic ....
If your bass driver is producing outputs in the 400- 800 hz range you will have issues with a forward firing port of sizable dimensions..
Nearfield will show this effect more than farfield ......
Energy in the cabinet is still audible , chuffing heard thru ports for eg. have shown to create noise that will cause muddying of the bass while not audible to the listener as port chuffing...
Really large ports forward firing will allow higher freq to be audible and will have phase issues ...
Step response and waterfall plots will show the interference....
Now while low frequency is omni, radiating output is not , so for large
speakers with large ports , rear or downward firing is best , radiating output is dispersed more evenly and will not present the listener with an " here I'am " Moment ..
As a general comment these claims seem to be somewhat unsupported.
"Energy in the cabinet is still audible , chuffing heard thru ports for eg. have shown to create noise that will cause muddying of the bass while not audible to the listener as port chuffing... "
Is there a reference to this as I am unaware of any valid studies to this effect.
"Really large ports forward firing will allow higher freq to be audible and will have phase issues ..."
This seems unlikely to me and what are the "phase issues"?
"Now while low frequency is omni, radiating output is not , so for large speakers with large ports , "
The port is still small compared to the sound radiated and would still be omni-directional. It would take a huge port for this to not be true.
"Energy in the cabinet is still audible , chuffing heard thru ports for eg. have shown to create noise that will cause muddying of the bass while not audible to the listener as port chuffing... "
Is there a reference to this as I am unaware of any valid studies to this effect.
"Really large ports forward firing will allow higher freq to be audible and will have phase issues ..."
This seems unlikely to me and what are the "phase issues"?
"Now while low frequency is omni, radiating output is not , so for large speakers with large ports , "
The port is still small compared to the sound radiated and would still be omni-directional. It would take a huge port for this to not be true.
Greiner, some years ago, showed that locating the port as close as possible to the woofer improved performance of the system. I looked through some of my AES reprints and copies but didn't find the specific article. I did find that Earl has a number of publications that have made it into the AES Loudspeaker Anthologies. Well done, Earl!!!
Regards,
Paul
Regards,
Paul
pblossom said:
I'd be interested in seeing how it does so. If it's a matter of port harmonics being cancelled by the in-phase output from the front of the driver, this would make sense, though it would reduce response linearity along with the distortion (some notches, with THD reduction proportional to notch size) unless I'm misthinking this.
But, are we really concerned with cancelling the low order harmonics at the cost of increased higher order harmonics? Down or rear firing will tend to perform better bandpass filtering of the port, where front firing right next to the woofer wll lead to more higher frequency components getting to the listener, both termination noise/edge diffraction at the opening and higher frequency port resonances (and midrange components coming through the port).
This is an interesting discussion, I can certainly see wanting to keep the outputs integrated from vent and speaker, if it's at some higher frequency (though I'm against venting anything above 50Hz or so, I find myself very sensitive to 80Hz vents)
gedlee said:
Unsupported in what way ?
Are you saying chuffing from the port is in no way audible or measurable ?
As to port noise and issue with phase
This was in relation to port size vs cabinet volume vs effective cone area ... too large a port will have those issues..
Localization of a subwoofers diminishes when port and woofer output are dispersed in such a manner where there summation takes place into the room as opposed to the front baffle....
In a small room for eg ... where high DB levels are requested 20 yrs ago we had done research into how sub frequencies energizes a specific room ( studios ) and found this to be true , hence the subwoofer design at the time was done for equal radiating outputs localized at the central height of the room ..Integration was improved 3 fold .
This effect is why in small - medium room downward firing ported woofers systems have better dispersion and performance ...over there front firing stablemate , who really shout " look at me " I'm over here
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