I get a box Q of 0.83 (assuming my WT3 is accurate). It may be because I refoamed the drivers and they aren't in spec anymore. The boxes don't have any leaks, so it can't be that.
It is more likely that your measurements are on a different part of the T/S curves and that both are valid.
dave
Zaph|Audio
Hi,
According to the above the Usher 8137A would be a good choice.
It models at about 90dB into half space so your looking at a 3way
with around mid eighties sensitivity after baffle step is considered.
Vas is 45L, Fs is 35Hz, Qts is 0.48. That models to a 38L sealed
box with a Qbox of 0.71, Fbox and -3dB are both 50Hz. For use
with a 80Hz AV amplifier roll-off you can reduce the box to 20L.
But its really about a proper 3 way design and I'd tend to go
for 2 x 6.5" rather than an 8". A 28L sealed version of this
would work very well : Zaph|Audio - ZDT3.5
rgds, sreten.
Hi,
According to the above the Usher 8137A would be a good choice.
It models at about 90dB into half space so your looking at a 3way
with around mid eighties sensitivity after baffle step is considered.
Vas is 45L, Fs is 35Hz, Qts is 0.48. That models to a 38L sealed
box with a Qbox of 0.71, Fbox and -3dB are both 50Hz. For use
with a 80Hz AV amplifier roll-off you can reduce the box to 20L.
But its really about a proper 3 way design and I'd tend to go
for 2 x 6.5" rather than an 8". A 28L sealed version of this
would work very well : Zaph|Audio - ZDT3.5
rgds, sreten.
If you're crossing this driver over to other drivers that will be handling the bottom end, I fail to see how effective box alignment will matter much. Pick a driver that can operate in the band you wish to run it in with low distortion and easy to deal with response characteristics (flat as possible through the range you are using). Give it a nice solid structure and damped chamber and call it a day. If you're high passing this driver down very low (below 200hz), pay close attention to resonance/impedance peaks and model out the effects of the box on these peaks and pay close attention to their effects on any attempt at a passive x-over. Go active for an x-over this low to avoid some major headaches and ridiculous passive impedance correction component costs.
Hi AndrewT,
The reasoning is based upon following.
Looking at the picture attached, one can see how different values of Qtc
affect the bass response around Fb. If a diyer would like to build a sealed
box with good bass performance ( good transient response and enough
sound pressure at Fb ) one should go for a graph of Qtc= 1,0 or similar.
Since sealed box increases the Qts of a driver to a value 1,41 times
greater than Qts, then it's only logical to divide the desired Qtc by 1,41
and get the wanted Qts which is at 0,7 or so.
Second important thing to be carefull about when choosing a driver is
Fs of the driver which we want to be as low as possible because we
want to be able to hear the bass and be satisfied with it. The lower the
Fs, the greater the Vas. Installing the driver in a big box is good for us
because we don't want the Fs to increase to much once installed.
Third important thing is if this box is going to be played really loud,
there should be appropriate peak to peak excursion value.
Practical example of a "perfect" driver to be installed into a sealed box:
Dynaudio 24W75
Rdc= 5,5 ohm
Fs = 33 Hz
Qts = 0,835
Vas = 88 liters
Once installed in a Vb= Vas, Qts rises to Qtc =1,17
After proper damping, the standing waves are nonexistent, and the
inner volume seen by the driver is increased which lowers the Fb and
Qtc to a lower value which is good.
If one would measure the impedance of this driver in a box of Vb= Vas,
measured values would be:
Fb= 37 Hz,
Qtc = 0,9-1,0
F3 = Fb
This is normal procedure for making a passive sealed box. Of course
there are other good examples of proper implementation of drivers with
lower Qts values, but the results are less optimal with a falling tendency
regarding very low Qts values.
Drivers with low Qts could also be installed in a box with Qtc = 1,0 but
this would be a small box compared to a Vas value and the Fb would be
to high for good bass response unless boosted with some kind of equalizer.
Hope this all is explained clearly enough not to sound like a myth
Attachments
Absolutely untrue.
The wanted Qtc of a selealed box can be anywhere from .45 to .8 - any of these values will do.
The driver's fs will determine where the rolloff will begin. A shallow rolloff of a lower Q is not undesirable, and either way none of this would matter with EQ circuits, as EQ changes Q.
What part of "I will be rolling in subs below 80hz" did you miss?
Low fs doesn't matter if you can get your corner down to around 75-80hz. Which plenty of 8" drivers, with varying VAS, varying sensitivity, and varying qts will do.
If you just try to push a driver to go as low as it can, you're just compromising total system performance with different forms of compression.
..and where is the logic behind this?
which is.. crappy.
maybe in 1965...That "falling tendancy" implies a sufficiently damped resonance. Just because it doesn't look as flat doesn't mean it's not desirable. I would never shoot for a box Q over 0.8. Once thermal compression becomes a factor with higher Q boxes they become really high Q and sound really punchy which is not accurate. I try to get Qtc =< 0.6 even if I lose a few Hz of extension. This generally ensures very accurate sounding bass in the driver's passband.
Add in that lower Q boxes require less amplifier boost to EQ flat on a whole, and it's clear why Low Q is desirable, not super high peaky Q.
A nice 8" driver for sealed is the Alpine 823D which is very nice for 40hz - 300+hz however sensitivity is low so you really want to run them active with a high powered amp amp like the Crown XLS 2500 (775w @ 4 ohm in stereo) to get the absolute best out of them.
Some suggestions for an 8, cost no object, go to the Seas Excel W22, and the Scanspeak 22W/4851T00.
However I realize that cost probably is normally an object, so a few rather nice 8s to look at:
CSS Trio 8 (i'm not sure if this is out of production or not though... might wanna ask Bob)
Dayton RS 225
Hi-Vi M8N
SB Acoustics SB23NRXS45-8
I don't think you could go wrong with any of the above. They'll each work great in 40L sealed as long as you cross them over to subs around 80hz you can feed them their full rated power easily.
Respectively, you could probably even get away with the RS225 in 21L, the M8N in 29L, and the SB23 in 31L, without much issue.
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Hi Lojzek,
Thanks for your detailed response. Let me make a few points that you should consider.
A system Q of 1 or 1.1 is a little higher than typically considered ideal. Most would consider a Q of .7 as maximally flat and with a little better transient response. I don't think that is an absolute and a Q of 1 will have its advantages.
Now, once a driver is picked, it can be placed in any conceivable box size and its resonance will vary accordingly. The stiffness of the box and the stiffness of the woofer will combine to determine the system resonance. Q of the system will track in proportion to system resonance. If resonance doubles then Q doubles. So the notion that Q goes up by 1.4 isn't right, it depends on how much resonance is raised by the enclosure.
If your driver Q is near to your target system Q then you must put the driver in a relatively large cabinet so as not to raise resonance too much. If your driver Q is fairly low you can use a smaller cabinet to raise Q and resonance (always together) to your target.
In the end, stiffnes from the cabinet and stiffness from the driver combine and one is equivalent (quality wise) to the other.
David S.
Thanks for your detailed response. Let me make a few points that you should consider.
A system Q of 1 or 1.1 is a little higher than typically considered ideal. Most would consider a Q of .7 as maximally flat and with a little better transient response. I don't think that is an absolute and a Q of 1 will have its advantages.
Now, once a driver is picked, it can be placed in any conceivable box size and its resonance will vary accordingly. The stiffness of the box and the stiffness of the woofer will combine to determine the system resonance. Q of the system will track in proportion to system resonance. If resonance doubles then Q doubles. So the notion that Q goes up by 1.4 isn't right, it depends on how much resonance is raised by the enclosure.
If your driver Q is near to your target system Q then you must put the driver in a relatively large cabinet so as not to raise resonance too much. If your driver Q is fairly low you can use a smaller cabinet to raise Q and resonance (always together) to your target.
In the end, stiffnes from the cabinet and stiffness from the driver combine and one is equivalent (quality wise) to the other.
David S.
Your argument is riddled with errors.
I pick out just one, the Qtc.
A high Qtc ruins the "naturalness" of the bass response.
You start with Qts=0.835 and end up with Qtc=1.17 which somehow changes back down to Q~0.9 to 1.0.
Rock & Speaker have peer reviewed your statements for us.
I would rather believe much of what Rock & Speaker state than any of your argument.
Your original claim is in my view flawed and carries no value as far as guiding a builder to creating a better bass or wideband speaker.
I do not see a need to go further.
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simple question that turned out to be not so simple.
The arithmetic just shows the fallacy of some of the suggestions.
Hi guys,
thanks for all your responses which are, I believe, meant to be informative
and benevolent, altough this may not seem at a first glance. It appears that
we all know the theory and things behind it, but we are not so liberal when
it comes to a decision which route to take. There is plenty to reflect on and
I'm not excluding myself from improving on the same level.
Regards
thanks for all your responses which are, I believe, meant to be informative
and benevolent, altough this may not seem at a first glance. It appears that
we all know the theory and things behind it, but we are not so liberal when
it comes to a decision which route to take. There is plenty to reflect on and
I'm not excluding myself from improving on the same level.
Regards
But that is exactly the type of generalization that gets people into trouble.
The point was that any desired sealed box system Q is achievable with some size box as long as you start with a woofer Q lower than the target Q. There is no ideal Q for an "acoustic suspension woofer". All woofer Q's can work if they are lower than the system target.
It is typical, but not mandatory, to aim for "high Alpha" designs, meaning putting a high Vas woofer into a significantly smaller Vb. This allows a smaller final system size than starting with a lower Vas woofer, (thus needing a larger box). If you follow such a rule you will be raising the woofer Q considerably and will want a low Q, low resonance woofer as a starting point.
David S.
Burning Daylight!
OK, it's time to pick a woofer.
We know 40 L sealed and stuffed box give or take. We know flat to 80 Hz. Need to know how high we need to play. (Important!) We know active crossover, assuming both low and high pass. We know a preference for non-paper cone if possible.
We assume a .65 - .8 QTC to be a good box performance target.
So, how much money do you have? And what speaker brands do you have available to you in Portugal?
OK, it's time to pick a woofer.
We know 40 L sealed and stuffed box give or take. We know flat to 80 Hz. Need to know how high we need to play. (Important!) We know active crossover, assuming both low and high pass. We know a preference for non-paper cone if possible.
We assume a .65 - .8 QTC to be a good box performance target.
So, how much money do you have? And what speaker brands do you have available to you in Portugal?
Hi,
The question is so open ended it cannot be answered.
It depends very much on the total design of the 3-way.
Basically its sensitiviity requirements, and that depends
on the amount of BSC incorporated. From that you can
then balance the size of the box with its bass extension.
Further requirements are the planned integration slopes
with the sub and if the drivers roll-off is part of that.
If not you'll be just listing drivers till the cows come home.
rgds, sreten.
40L is far too big for a sealed 8" not required to do bass.
The question is so open ended it cannot be answered.
It depends very much on the total design of the 3-way.
Basically its sensitiviity requirements, and that depends
on the amount of BSC incorporated. From that you can
then balance the size of the box with its bass extension.
Further requirements are the planned integration slopes
with the sub and if the drivers roll-off is part of that.
If not you'll be just listing drivers till the cows come home.
rgds, sreten.
40L is far too big for a sealed 8" not required to do bass.
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