Sealed subs are 12db
Ported can vary but 4th order is 24db
What is an open baffle woofer roll off?
I'm looking to use a 7" thats FS 35hz w/Qts of .45 so it starts to roll off at 77hz but how steep? It s going to be used with multiple subs but Im trying to model the midbass. Is my math off?
Any help appreciated.
Ported can vary but 4th order is 24db
What is an open baffle woofer roll off?
I'm looking to use a 7" thats FS 35hz w/Qts of .45 so it starts to roll off at 77hz but how steep? It s going to be used with multiple subs but Im trying to model the midbass. Is my math off?
Any help appreciated.
It depends on the qts of the driver. A very high q driver can roll off at 24 db below fs. A very low q driver will roll off at 12 db total below fs.
Your average driver with a q between .5 and .7 will roll off around 18 db total below fs.
I'm guessing your driver will be around 16 db?
Your average driver with a q between .5 and .7 will roll off around 18 db total below fs.
I'm guessing your driver will be around 16 db?
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Thank to both responses.
Why would a higher Qts roll off faster? Thats interesting.
Any input into how to gauge power handling free air?
Mmmm, sort of. The fundamental roll off rates are, as stated before, 6dB/octave above the driver resonance and 18db/octave below the driver resonance. This assumes that the OB rolloff (the 6dB/oct one) starts above the driver resonant frequency... which is typically the case. The frequency at which the 6dB/oct OB rolloff starts is determined by the effective baffle size, e.g. the pathlength from the front to the back of the driver radiating surface. Below the driver resonance, the additional 12dB/oct contributed by the driver causes an increase in steepness to an asymptotic 18dB/oct rolloff.
So then, what about driver Q? The driver response is "superimposed" on, or added to, the above fundamental OB rolloffs. You are essentially operating the driver in free air, and the vast majority of woofers have Qts < 0.5, with some exceptions. A response with such a low Q value has a drooping response, which is further diminishing output where you need it. This is why high Q drivers (as high as Q=2) are actually a good match to open baffle loading - the natural low end response near the free air resonance is actually flat or even bumped up a couple of dB (which acts to compensate for the 6dB rolloff a bit).
I think that we are saying more or less the same thing, but it's a little clearer (IMHO) if you break it up into the baffle response (6dB/oct transitioning to 18dB/oct) and the driver response resulting from its free air Q.
-Charlie
It has to do with the definition of "Q". Andrew is correct, a driver with a Q of .707 will roll off at -12dB/oct. For a Qts above that, the initial slope will be steeper, below it will be more gradual. The ultimate slope will always be -12dB, but that occurs below meaningful output (and only in theory, as you'll always have room gain). And Charlie's right as to why higher Qts drivers are preferred for OBs. The catch is, a high Qts pretty much guarantees a high Fc.
As to power handling, you have to investigate how the manufacturer measures it. If they measure in free air, you're all set. If they measure in a box that restricts cone motion, it can be a bit trickier. Myself, I'd take being measured in a box as a strong indication the driver was not suitable for OB use (though it some cases it may simply be the company's "standard practice").
Ack! Sorry, I'm afraid I explained nothing, except to those who already know the answer. 
Here's a link to a simple definition of "Q":
Passive Crossovers: The Filter Q
(Wikipedia used to be good for such things, but the internet show-offs have elevated every technical page to a doctoral level discussion.)
And yes, that's a page about filters, but what happens to the low end of a driver is basically a mechanical filter that follows the same rules as electrical ones do. This is the basis of all Thiele's work (Small was left with the job of sorting out the differences between the electrical and mechanical).
It's a "second order" filter (also called "two pole") and that accounts for the -12dB slope. Every order means -6dB. The cancellation of an open baffle creates a first order filter, and so adding the two together, we get the -18dB of a third order filter.
The speaker you mentioned, with a Qts of .45, exhibits the gradual slope of the green line in the graph on that page. To that you need to add the roll-off determined by your baffle width, and add in a factor for room gain. Then you can calculate a lowpass filter with the inverse roll-off for your subwoofers. Simple, no?
This is why measurement tools are so popular here. It's much easier to just build the thing, measure it in your room, and tweak the subs until it's flat.
Here's a link to a simple definition of "Q":
Passive Crossovers: The Filter Q
(Wikipedia used to be good for such things, but the internet show-offs have elevated every technical page to a doctoral level discussion.
)And yes, that's a page about filters, but what happens to the low end of a driver is basically a mechanical filter that follows the same rules as electrical ones do. This is the basis of all Thiele's work (Small was left with the job of sorting out the differences between the electrical and mechanical).
It's a "second order" filter (also called "two pole") and that accounts for the -12dB slope. Every order means -6dB. The cancellation of an open baffle creates a first order filter, and so adding the two together, we get the -18dB of a third order filter.
The speaker you mentioned, with a Qts of .45, exhibits the gradual slope of the green line in the graph on that page. To that you need to add the roll-off determined by your baffle width, and add in a factor for room gain. Then you can calculate a lowpass filter with the inverse roll-off for your subwoofers. Simple, no?
This is why measurement tools are so popular here. It's much easier to just build the thing, measure it in your room, and tweak the subs until it's flat.
Ack! Sorry, I'm afraid I explained nothing, except to those who already know the answer.
Here's a link to a simple definition of "Q":
Passive Crossovers: The Filter Q
(Wikipedia used to be good for such things, but the internet show-offs have elevated every technical page to a doctoral level discussion.
And yes, that's a page about filters, but what happens to the low end of a driver is basically a mechanical filter that follows the same rules as electrical ones do. This is the basis of all Thiele's work (Small was left with the job of sorting out the differences between the electrical and mechanical).
It's a "second order" filter (also called "two pole") and that accounts for the -12dB slope. Every order means -6dB. The cancellation of an open baffle creates a first order filter, and so adding the two together, we get the -18dB of a third order filter.
The speaker you mentioned, with a Qts of .45, exhibits the gradual slope of the green line in the graph on that page. To that you need to add the roll-off determined by your baffle width, and add in a factor for room gain. Then you can calculate a lowpass filter with the inverse roll-off for your subwoofers. Simple, no?
This is why measurement tools are so popular here. It's much easier to just build the thing, measure it in your room, and tweak the subs until it's flat.
I can measure after but I'm looking to start with a trapezoid front panel of 15" wide at the base tapering to 9"at the top and 42"tall for an ear listening height of 36. (Think apogee)
It will be an 80-100hz 4th order elect roll off. Im not looking for huge SPL, just satisfying audio with great depth illusion. Its using a Neo 3PDRW dipole for the top end and RS180 for the midbass with a 2K 4th order cross, all active.
Its a 21 foot"x 21 x 9.5 room. I own all these drivers so I'm sort of testing here. Its not a final draft.
Any suggetions for a better midbass for use with the Neo3PDRW? I have a pair of focal 6V4311 with an FS of 52 and Qts of .57 if I recall. Those cones can cross really high.
It is for midrange and not for bass.
Its a 2 way trapezoid needing no real output under 80hz. I have subs and I'm happy with them.
The size has been altered to be 12" wide at the top and 15" wide at the base very similar to an apogee stage sitting on a rectangular base.
This is the Qts-induced roll-off (around Fs) of the driver in infinite baffle:
The-x axis is divided into multiples of the drivers Fs: 2.0 is double Fs, 0.50 is half Fs etc. You can easily see how a low Qts driver rolls off at higher frequencies than a driver with Qts=1. The 6dB dipole roll-off has to be added to this.
The-x axis is divided into multiples of the drivers Fs: 2.0 is double Fs, 0.50 is half Fs etc. You can easily see how a low Qts driver rolls off at higher frequencies than a driver with Qts=1. The 6dB dipole roll-off has to be added to this.
lol, because you chose to answer the question you wanted instead of the one I asked in the first post.....
I asked what was the rate of roll-off of a driver in an open baffle?
I didnt ask about baffle width or what was the best sub configuration.
Since the first post also stated it was a 7" and the FS/QTS, it stated the exact application was midbass and specifically stated that my subs were separate from this application. Its all good tho.
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Hi.,
The ultimate roll-off is 18dB/octave.
However excursion follows the driver infinite baffle response, (12dB
/octave) and the extra 6dB of baffle loss per octave chucks away driver
excursion, it is not just a simple manipulation of response, it is a loss.
The response can be EQ'd to anything you like, the real issue is SPL
capability at 80Hz with narrow baffles, it will be very poor compared
to boxes, baffle roll-off is a SPL loss, not the response of the driver.
rgds, sreten.
The ultimate roll-off is 18dB/octave.
However excursion follows the driver infinite baffle response, (12dB
/octave) and the extra 6dB of baffle loss per octave chucks away driver
excursion, it is not just a simple manipulation of response, it is a loss.
The response can be EQ'd to anything you like, the real issue is SPL
capability at 80Hz with narrow baffles, it will be very poor compared
to boxes, baffle roll-off is a SPL loss, not the response of the driver.
rgds, sreten.
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Yes, I'm aware but its a test build to see how my set-up works with speaker location and the dipole effect. I have space but its a home theater so first to see if dipoles work in the locations I have then a RD50 with 6-8"drivers (dayton subs with 9mm excursion) crossed at 500 or so with the same 80hz low end would suffice. If it doesnt cut it, it will be a hybrid system.
OB
Hi there SpM: Just ran a SPL check on an experimental OB (18" curved to 16.25") Uncalibrated generator, DVM and SPL meters: 2.83v:
130hz-70db, 140hz-65.8db, 150-58db, 160hz-71.4db, 170hz-75.3db, 180hz-76db,
190hz-78db, 200hz-78.8,300hz-84db, 400hz-8.3db, 500hz-85.2, 1k-84.4db, 1.5k-97.5db
2k-91db, 3k-97.2db, 4k-97.3db, 5k-97.4, 6k-97.5db, 7k-82.2db, 8k-87.5db, 9k-87.5db,
9k-87.5db, 9.3k85.1, 10k-62.8db. Since I intend to cross over to the bass unit around 150hz, I did not record the fall off below 130hz. ...regards, Michael
Hi there SpM: Just ran a SPL check on an experimental OB (18" curved to 16.25") Uncalibrated generator, DVM and SPL meters: 2.83v:
130hz-70db, 140hz-65.8db, 150-58db, 160hz-71.4db, 170hz-75.3db, 180hz-76db,
190hz-78db, 200hz-78.8,300hz-84db, 400hz-8.3db, 500hz-85.2, 1k-84.4db, 1.5k-97.5db
2k-91db, 3k-97.2db, 4k-97.3db, 5k-97.4, 6k-97.5db, 7k-82.2db, 8k-87.5db, 9k-87.5db,
9k-87.5db, 9.3k85.1, 10k-62.8db. Since I intend to cross over to the bass unit around 150hz, I did not record the fall off below 130hz. ...regards, Michael
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