kimbo said:
Yes, that and to see if the drivers moved in tandem at all times at all frequencies. There will be no point of little movement as you have created a sealed box by closing the vent, instead the driver movement will be much greater at resonance of the box. However, this would eliminate any vent issues if the drivers move unevenly with the vent closed off.
I've noticed the same effect with the basement blasters, but it's less than 2:1 at most for this design. However, at Fb, even the woofer closest to the ports (they are at the bottom of a 5' tall box) shows a dramatic decrease in its excursion. Perhaps the additional air mass loading between the woofer(s) farther from the port affects excursion. For the Basement Blasters, since the ports are at floor level and the upper woofer also serves as a lower midrange driver, this effect is a good thing for this design.
For a floorstanding speaker, having the port(s) near the floor excites any local room mode near the port frequency a bit more, as would having a port at the back for a speaker near a back wall
For a floorstanding speaker, having the port(s) near the floor excites any local room mode near the port frequency a bit more, as would having a port at the back for a speaker near a back wall
kimbo said:
Hi Kimbo,
What I meant was that the + wires goes to the + terminal of driver A and then to the + terminal of driver B.
The - wire goes to the - terminal of driver B and then to the - terminal of driver A.
Still parallel wiring.
Then what I said, has no bearing on what your system is doing.
Seems it's real...
Hi folks,
I have seen this phenomenon in enough double woofer boxes to know that it's real.
If I follow logical thinking (well mine anyway) the woofer closer to the port will be provided with less resistance (less suspension) because the distance from it to the port is less and the volume of immediate air is less.
The woofer further from the port has more air volume providing stiffer suspension.
So, does this mean that the tuning of the box is also affected by the woofer - port distance. My thinking says it must be because of the very different ways the two woofers behave.
In my box I moved the port to be in the middle of the two woofers and now both woofer behave identically. This makes me think that it is a physical / mechanical characteristic rather than an electrical one.
One test would be to build two identical long single woofer boxes with identical woofers and ports. The port in one would be located next to the woofer and the other would be at the opposite end of the box.
Maybe one day I'll .......
Cheers
Hi folks,
I have seen this phenomenon in enough double woofer boxes to know that it's real.
If I follow logical thinking (well mine anyway) the woofer closer to the port will be provided with less resistance (less suspension) because the distance from it to the port is less and the volume of immediate air is less.
The woofer further from the port has more air volume providing stiffer suspension.
So, does this mean that the tuning of the box is also affected by the woofer - port distance. My thinking says it must be because of the very different ways the two woofers behave.
In my box I moved the port to be in the middle of the two woofers and now both woofer behave identically. This makes me think that it is a physical / mechanical characteristic rather than an electrical one.
One test would be to build two identical long single woofer boxes with identical woofers and ports. The port in one would be located next to the woofer and the other would be at the opposite end of the box.
Maybe one day I'll .......
Cheers
Re: Seems it's real...
Greets!
It's an acoustic pressure wave thing. Golden or acoustic ratio cabs have a ~uniform particle density down in the BW around Fb, so there shouldn't be any obvious difference in driver loading or Fb regardless of the driver(s)/vent locations, though there will be a difference higher up where some 1/4WL resonances are occurring if the XO doesn't attenuate them. IOW the cab's air spring is just 'feeling' one huge driver. The vent(s) OTOH will have its own harmonics that won't be affected by the XO, so with a long vent it could audibly comb filter with the mains if it's energized strongly enough.
Once the cab begins morphing into a mass loaded transmission line (ML-TL) due to an increasing aspect ratio, the pressure waves no longer have uniform particle density (visualize oscillating underwater currents for lack of a better analogy). Now we have a driver at one WL and another at a shorter WL WRT the vent if it's below them so they will be tuned slightly different, with the one closest to the vent being tuned at a higher Q, ergo slightly higher Fb. This assumes the vent is near/at the bottom. As it's moved up, another set of pressure waves develop between it and the bottom to further affect tuning.
In modern day high SPL apps this can/does destroy woofers so either they typically isolate the drivers in their own separate vented chamber or either put the vent(s) between the drivers or one above the top driver and one below the bottom driver, with each equidistant from an endplate. Or in some cases, one at two, or all four, diagonal corners.
So while the phenomenon has been well known since the beginning of vented cab design in the 1920s (and one reason they used aperiodic loading before the ducted port BR was invented in the early '30s AFAIK), it wasn't until MJK developed his PORTED WS that it could be simmed with any degree of accuracy outside a manufacturer's R&D group, so I recommend perusing his docs, download the MathCad demo if you don't have a licensed copy and play with his WS. It's not 100% accurate since he made some simplifications, but it shows the primary pressure wave action that dominates a typical tower (ML-TL or ML-TQWT) design.
For multiple drivers you will need to sim each driver/vent combo separately to see the difference. For simming a multiple driver design, just placing the drivers as close together as possible and using the center of the group as the center of the driver is close enough as long as the vent(s) is well away from the lower one, or better still, use a bi-polar layout.
GM
quasi said:
Greets!
It's an acoustic pressure wave thing. Golden or acoustic ratio cabs have a ~uniform particle density down in the BW around Fb, so there shouldn't be any obvious difference in driver loading or Fb regardless of the driver(s)/vent locations, though there will be a difference higher up where some 1/4WL resonances are occurring if the XO doesn't attenuate them. IOW the cab's air spring is just 'feeling' one huge driver. The vent(s) OTOH will have its own harmonics that won't be affected by the XO, so with a long vent it could audibly comb filter with the mains if it's energized strongly enough.
Once the cab begins morphing into a mass loaded transmission line (ML-TL) due to an increasing aspect ratio, the pressure waves no longer have uniform particle density (visualize oscillating underwater currents for lack of a better analogy). Now we have a driver at one WL and another at a shorter WL WRT the vent if it's below them so they will be tuned slightly different, with the one closest to the vent being tuned at a higher Q, ergo slightly higher Fb. This assumes the vent is near/at the bottom. As it's moved up, another set of pressure waves develop between it and the bottom to further affect tuning.
In modern day high SPL apps this can/does destroy woofers so either they typically isolate the drivers in their own separate vented chamber or either put the vent(s) between the drivers or one above the top driver and one below the bottom driver, with each equidistant from an endplate. Or in some cases, one at two, or all four, diagonal corners.
So while the phenomenon has been well known since the beginning of vented cab design in the 1920s (and one reason they used aperiodic loading before the ducted port BR was invented in the early '30s AFAIK), it wasn't until MJK developed his PORTED WS that it could be simmed with any degree of accuracy outside a manufacturer's R&D group, so I recommend perusing his docs, download the MathCad demo if you don't have a licensed copy and play with his WS. It's not 100% accurate since he made some simplifications, but it shows the primary pressure wave action that dominates a typical tower (ML-TL or ML-TQWT) design.
For multiple drivers you will need to sim each driver/vent combo separately to see the difference. For simming a multiple driver design, just placing the drivers as close together as possible and using the center of the group as the center of the driver is close enough as long as the vent(s) is well away from the lower one, or better still, use a bi-polar layout.
GM
This thread came to mind when planning the position of two ports in an enclosure for a single 10" driver. The box dims and use of scraps on edge inside to stiffen it up make placing driver and ports a bit of a dance. I may have to put one port on the front and one on the back or strip out some of the side bracing to give them room to be side by side.
. . . . . It'll work out, no probs, but it got me to remembering Quasi's posts here and wondering if placing the ports at different distances from the single driver would have a similar effect in reverse - ie. uneven loading on the ports.
I know there are lots of speakers out there with over/under ports beneath the driver but considering Quasi's observations it seems natural to ask the question. Does equidistant port placement deserve any consideration at all?
Thanks !
. . . . . It'll work out, no probs, but it got me to remembering Quasi's posts here and wondering if placing the ports at different distances from the single driver would have a similar effect in reverse - ie. uneven loading on the ports.
I know there are lots of speakers out there with over/under ports beneath the driver but considering Quasi's observations it seems natural to ask the question. Does equidistant port placement deserve any consideration at all?
Thanks !