Dave, are you sure?
The various simulation programs seem to disagree with you - system Q in a finite sealed box must always be greater than the Qts of the driver you start with.
To the OP, a high-Q driver will be more efficient around Fs when used in an OB. Since OBs are inherently lossy, having a driver where the high Qts gives it a rising response down towards Fs is very useful - it can level out some of the dipole losses, and give you a reasonably flat low-frequency response down to Fs.
Keep in mind that more output = more excursion. A high-Q driver will use lots more excursion (and therefore have more output) than a low-Q driver at Fs.
Chris
Chris said it. A high Qts driver will have more bass - relative to the rest of its response.
That helps offset the open baffle bass losses and makes the design and crossover/EQ easier.
A low Qts drive may actually have more bass output because it's more efficient, but it will have much more mid and high output as well, especially relative to the bass. If you want to use that on OB, you'll have to trade away a whole lot of top end response the flatten the frequency curve.
That helps offset the open baffle bass losses and makes the design and crossover/EQ easier.
A low Qts drive may actually have more bass output because it's more efficient, but it will have much more mid and high output as well, especially relative to the bass. If you want to use that on OB, you'll have to trade away a whole lot of top end response the flatten the frequency curve.
As an approximation when using a driver OB it will have the specified free air resonance and Qt. So if you use a driver with a Fr of 30hz and a Q of .25 the output will be .25 which is something like -12db at 30hz. With a Q of .7 you will have an output of -3db. So you can see that you will get more bass response from the higher Q driver. The transient response is also impacted, but for moderate Q values it's not usually an issue. There are other factors, both acoustic and electrical that can effect the Bass output of an OB but as a starting point the driver Q has a big impact.
Yup, what bfpca said.
Let me make it a little simpler.
Something called Mass corner.
A driver will roll off at its Fs / qts on an infinite baffle (basically a wall)
So, it is better to have a fs and qts at say .7 and 100hz than a .3 qts, otherwise you lose bass.
Some deep woofs for open baffle can have qts 1.4 even up to 2, but some say it is better to have .4 or .5 because the woofer is too flabby sounding (ringing).
Open baffle will roll the bass anyway, 1/2 wavelength front speaker frame to back speaker frame it nulls, the 1/4 wavelength peaks, then it rolls off.
I had a 12'" on a phy open baffle (24" deep wings).
Not much bass.........
Let me make it a little simpler.
Something called Mass corner.
A driver will roll off at its Fs / qts on an infinite baffle (basically a wall)
So, it is better to have a fs and qts at say .7 and 100hz than a .3 qts, otherwise you lose bass.
Some deep woofs for open baffle can have qts 1.4 even up to 2, but some say it is better to have .4 or .5 because the woofer is too flabby sounding (ringing).
Open baffle will roll the bass anyway, 1/2 wavelength front speaker frame to back speaker frame it nulls, the 1/4 wavelength peaks, then it rolls off.
I had a 12'" on a phy open baffle (24" deep wings).
Not much bass.........
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Thanks SO much for all the knowledge!!!
For me, the key to understanding it is the "around resonance" idea..
Other than general knowledge for an upcomming OB project, the specific reason I wanted to know is that I have a pair of 12" woofers just begging to be used in said project..
Only thing that gave pause, is a relatively low Q...
They are a pair of very good woofers (that probably nobody has ever heard of) that didn't work out as intended in a project of many years ago...
They're nice, and not cheap, and sit in their boxes unused for a long time.... so I'd like to use them.
The specifics, that I was able to gather, are these:
FSn = 32
QTS = .373
VAS = 5.750
SPL = 93
To be used with a Dynavox T-215-RTF-64 - 8 Ω, and a Fountek ribbon.
Woofer "handing off" to the Dynavox around 200-300hz.
I'm not expecting deep bass.... don't even want it, really....
Sensitive neighbors and a small apartment with strong structure-bourne transmission..
I have a pair of great subs and a crossover that makes them easy to blend... But they won't get used here anyway.
SO... wondering if I can get away with those woofers.... just to have enough.... I guess mid-bass really..
That "ooomph" that the Dynavox would lack if run OB with no filter....
30hz fs.. say it's badly rolled an octave above that.... But OK down to 60hz.. more than enough for me..
Down to ~125hz with bad rolloff... meh... still could live with it i guess... provided it's not a real sharp rolloff between 125 and 60ish...
OB the Dynavox just doesn't have anything below 200hz..
I can live without the bottom 2 octaves.... just so long as the woofer fills in between that and the Dynavox's range,at least decently..
The woofers would have pretty decent transient snap too, for which the Dynavox are legendary, I'm told...
Wondering what this knowledgable group thinks?
While at it, ANY other suggestions about said project, would be most welcome!
Thanks again,
Greg
For me, the key to understanding it is the "around resonance" idea..
Other than general knowledge for an upcomming OB project, the specific reason I wanted to know is that I have a pair of 12" woofers just begging to be used in said project..
Only thing that gave pause, is a relatively low Q...
They are a pair of very good woofers (that probably nobody has ever heard of) that didn't work out as intended in a project of many years ago...
They're nice, and not cheap, and sit in their boxes unused for a long time.... so I'd like to use them.
The specifics, that I was able to gather, are these:
FSn = 32
QTS = .373
VAS = 5.750
SPL = 93
To be used with a Dynavox T-215-RTF-64 - 8 Ω, and a Fountek ribbon.
Woofer "handing off" to the Dynavox around 200-300hz.
I'm not expecting deep bass.... don't even want it, really....
Sensitive neighbors and a small apartment with strong structure-bourne transmission..
I have a pair of great subs and a crossover that makes them easy to blend... But they won't get used here anyway.
SO... wondering if I can get away with those woofers.... just to have enough.... I guess mid-bass really..
That "ooomph" that the Dynavox would lack if run OB with no filter....
30hz fs.. say it's badly rolled an octave above that.... But OK down to 60hz.. more than enough for me..
Down to ~125hz with bad rolloff... meh... still could live with it i guess... provided it's not a real sharp rolloff between 125 and 60ish...
OB the Dynavox just doesn't have anything below 200hz..
I can live without the bottom 2 octaves.... just so long as the woofer fills in between that and the Dynavox's range,at least decently..
The woofers would have pretty decent transient snap too, for which the Dynavox are legendary, I'm told...
Wondering what this knowledgable group thinks?
While at it, ANY other suggestions about said project, would be most welcome!
Thanks again,
Greg
You can safely use a speaker with a small qts in open baffle. In Russia, people successfully used modern pro speakers with qts 0.19- 0.25. And if you weight the baffle, it plays better than closed box
because it gives a natural bass without wooden decoration of closed/reflex box and is not suppressed by filler.
Only the speakers need to pick up those that themselves give the current midbass and bass then it will be all right and each to allocate the necessary frequency so as not to climb where it does not need.
because it gives a natural bass without wooden decoration of closed/reflex box and is not suppressed by filler.
Only the speakers need to pick up those that themselves give the current midbass and bass then it will be all right and each to allocate the necessary frequency so as not to climb where it does not need.
OB woofers
Woofer Q can be a serious issue if you are employing a passive format crossover, and all things equal, less of an issue if you are employing an active crossover.
It is a balancing act with passive.... generally, and again assuming all things equal, as the woofer QTS declines, efficiency goes up. The main thing to remember is the front to rear path length the baffle provides usually sets the systems potential low frequency cut off and net operating efficiency. You can start out with, say a high efficiency 18" woofer with an FS in the mid - low 30's and an Xmax of say 4-6 mm peak that can really provide 97-99 db/watt of raw output. To do that, the QTS is going to be in the .23-.28 area. As you equalize, or tilt the mid bass, upper bass and lower mid range response downward in an effort to end up flat on the baffle frequency response, you typically trade away 9-12 of the initial (raw) efficiency. This is typical with a path length of 475-600 mm. This set up would provide a flat response of around 50 HZ with 6 db per octave roll off down to woofer resonance. Below resonance, the roll off becomes very steep.
If we instead use a high QTS woofer, say .9 - 1.5 with similar specs to our sample above, the raw efficiency will drop to around 89-92 db/watt. But because of the higher QTS, our correction requirement is quite a bit lower, in the area of 4-6 db for the same basic efficiency and low frequency cut off.
When you are all done, both drivers, after correction - equalization end up with similar low end cut off and efficiency down into the mid 40's. S. Linkwitz discusses and mathematically documents this at his web site. Hope this info helps.
Woofer Q can be a serious issue if you are employing a passive format crossover, and all things equal, less of an issue if you are employing an active crossover.
It is a balancing act with passive.... generally, and again assuming all things equal, as the woofer QTS declines, efficiency goes up. The main thing to remember is the front to rear path length the baffle provides usually sets the systems potential low frequency cut off and net operating efficiency. You can start out with, say a high efficiency 18" woofer with an FS in the mid - low 30's and an Xmax of say 4-6 mm peak that can really provide 97-99 db/watt of raw output. To do that, the QTS is going to be in the .23-.28 area. As you equalize, or tilt the mid bass, upper bass and lower mid range response downward in an effort to end up flat on the baffle frequency response, you typically trade away 9-12 of the initial (raw) efficiency. This is typical with a path length of 475-600 mm. This set up would provide a flat response of around 50 HZ with 6 db per octave roll off down to woofer resonance. Below resonance, the roll off becomes very steep.
If we instead use a high QTS woofer, say .9 - 1.5 with similar specs to our sample above, the raw efficiency will drop to around 89-92 db/watt. But because of the higher QTS, our correction requirement is quite a bit lower, in the area of 4-6 db for the same basic efficiency and low frequency cut off.
When you are all done, both drivers, after correction - equalization end up with similar low end cut off and efficiency down into the mid 40's. S. Linkwitz discusses and mathematically documents this at his web site. Hope this info helps.
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