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PostPosted: November 19th, 2017, 6:17 pm 
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Chris,

Thx for running numbers. I have no experience in speaker design.

When the fs is 20Hz, why does bass box show f3 going up as the volume goes up? What is the relationship between cutoff frequency and resonance frequency?


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PostPosted: November 19th, 2017, 6:33 pm 
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Pelliott321 wrote:
so only the woofer can come out not the horn?
I see the possibility of six braces
four front to back and two across

If you give me accurate inside dimensions I can over cut the six pieces, and trim to fit at your house

Actually I am think 1 dowels might work.


That’s right, horn is glued with silicone, extremely difficult to remove.

Thanks for the offer, I will take you up on the offer after I fm take some measurements of my system. Last night I realized that my microphone cable is too short for near field measurements of the woofer.


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PostPosted: November 19th, 2017, 8:05 pm 
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Cogito wrote:
Last night I realized that my microphone cable is too short for near field measurements of the woofer.


Guitar center...

Roscoe

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PostPosted: November 19th, 2017, 8:18 pm 
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Freeing silicon is a pain but can be done with a lot of patience and good exacto-knife


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PostPosted: November 19th, 2017, 8:22 pm 
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Shashi:

The spreadsheet I gave you with your driver parameters already installed will allow you to determine any combination of Fb, Q, and box size.

Personally I would leave what you have alone and get a separate sub to handle below 40 Hz. The driver you have works well in that box but even if you had a bigger box you will still be limited by the Xmax of the driver. Remember, if you want to go from 40 Hz to 20 Hz at the same SPL you will need 4 times the excursion. Older drivers sacrificed Xmax for higher sensitivity. If you try to push that driver to 20 Hz you will probably wind up with much more distortion. New drivers have Xmax ratings in the range of 3 - 5 times your existing driver.


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PostPosted: November 19th, 2017, 8:31 pm 
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Cogito wrote:
Chris,

Thx for running numbers. I have no experience in speaker design.

When the fs is 20Hz, why does bass box show f3 going up as the volume goes up?

The lowest frequency a driver/box combination produces is a different number for every driver, but it appears from an experiment I just did with another driver, as you extend in both directions toward super big or super small, the response rolls off in both directions. It's kind of like an upside down bell curve where the optimum is in the middle where upside down hump is (lowest frequencies) and you get a rise in F3 as you move toward the extremities.

What is the relationship between cutoff frequency and resonance frequency?


Cutoff frequency is not an appropriate term. F3 is the point at which the low end rolloff reaches 3 db lower than the primary bandwidth the speaker covers. It's not a cutoff because the speaker rolls off. F3 is just a reference point that in most cases signals that the speaker is beginning to roll off, however there is quite a bit of audible material below 3db down. For example if your speaker is playing at 94 db, 3db down is 91 db, 6db down is 88db, 12db down is 79db, 18 db down is 73 db. This is all still audible material and doesn't actually cutoff until the speaker SPL goes below the ambient sound pressure level in the room (60-70db in a quiet room). The point is that, by the time you reach 18 db down a sealed speaker could easily be reaching a whole octave below f3 and still be audible.

Resonant frequency is a little more tricky, but in more lay terms its the frequency at which the box/driver combination produces a contribution to the overall sound of the system by it's natural resonance. The relationship is: if your box is really big (Qtc .5) resonant frequency is below f3. If your box is really small (Qtc 1.1) the resonance is above f3. In the middle (Qtc .707) resonance and f3 are the same frequency. The implications of this I'm not completely sure of yet, but I suspect that having a resonance above f3 is more problematic to deal with than if it's right on, or below f3. (look up Qtc. if you are not familiar with what it is. It's pretty easy to understand).

Good questions!

Chris


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PostPosted: November 19th, 2017, 11:38 pm 
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Tom,

I am confused because, the spread sheet you gave me hows the resonance frequency going down as the volume of the sealed box increases. I am equating the resonance frequency with bass response. Chris's modeling shows there is not particular correlation between volume an f3.

Which is more relevant for determining bass response, resonance frequency fs or f3?

BTW, I found references online that xmax 7mm in each direction or 14mm total.


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PostPosted: November 20th, 2017, 9:12 am 
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Harbeth’s Alan Shaw discusses advantages of thin walled bass cabinets. Totally counter intuitive.

http://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/speaker-design/215-bbc-style-thin-wall-cabinets-why-so-special


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PostPosted: November 20th, 2017, 10:22 am 
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Cogito wrote:
Tom,

I am confused because, the spread sheet you gave me hows the resonance frequency going down as the volume of the sealed box increases. I am equating the resonance frequency with bass response. Chris's modeling shows there is not particular correlation between volume an f3.

Which is more relevant for determining bass response, resonance frequency fs or f3?

BTW, I found references online that xmax 7mm in each direction or 14mm total.


Shashi:

The resonant frequency does in fact go down to a point where it intersects with Fs or the resonance in free air. Resonant frequency only is an indicator of the degree of low extension, not the quantity of bass. Resonance is only the point where the totol mass of the moving parts and the total stiffness or inverse of compliance create a natural increase of movement of the cone because of the combination of those factors storing energy on each cycle. Think of a car with no shock absorbers (dampers) bouncing down the street after hitting a bump at a constant frequency.

I disagree with Chris on his assessment of changes of resonant frequency based on Q. Resonance is only a factor of mass and compliance. The volume of air inside the box is part of that compliance. Q is the behavior of the driver at resonance and around that point. It is easy to make the assumption that Q affects resonant frequency because both the resonant frequency and Q change as the box volume changes.

I agree with Chris that the term cutoff frequency refering to F3 is misleading. When you cut off a water valve no more water comes out of the hose. The F3 point as he says is where the output drops to -3dB with additional output below that. The slope of the decrease depends on the type of enclosure with sealed boxes having a more gradual decrease in output than ported boxes below the port resonance. Also ported boxes unload the driver possibly leading to damaging excess excursion. A better term to use is Fb which is the resonance of the driver/enclosure combo. That leaves the slopes below resonance and the position of F3 up to the box type.

The term Q has it roots in the early days of radio where it was very important to have the ability to build tank circuits with very steep slopes to allow greater selectivity. If the circuit had a very high peak and narrow bandwidth at resonance it was said to be a very high quality circuit. Thus the term "Q" relating to the quailty of the circuit in it's ability to have very narrow bandwidth and high amplitude at resonance. The Q had nothing to do with the frequency at which the circuit resonated, only the behavior at that frequency. Although high Q circuits are very good in radio tuning, they are bad in speaker/enclosure systems because of the deviation from flat response at resonance. While generally negative, this has been used to advantage in some cases. When early tube amplifiers had output transformers that were lacking in low frequency response, a bump at resonance would add some bass back that was being rolled off by poor transformer response. That came at the expense of transient response. Also, Carver used very high Q woofers in his Amazing design to help compensate for the dipole cancellation of the panel configuration. Everything is a compromise.


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PostPosted: November 20th, 2017, 10:43 am 
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Cogito wrote:
Tom,

I am confused because, the spread sheet you gave me hows the resonance frequency going down as the volume of the sealed box increases. I am equating the resonance frequency with bass response. Chris's modeling shows there is not particular correlation between volume an f3.

Which is more relevant for determining bass response, resonance frequency fs or f3?

BTW, I found references online that xmax 7mm in each direction or 14mm total.



I felt the need to jump in here. The simple answer to your question is both.

The lowest woofer resonance you can obtain is the free-air resonance. That's with the woofer sitting on a counter out in the open. Box loading raises the measurable system resonance. The bigger the box the lower the speaker resonance. Since any of the practical box sizes do not represent a true infinite baffle you will be limited by the box size. Linear excursion peak-to-peak is the other limiting factor. Woofers as a generic group are designed to "pump" air with long travel of the cone. Or -- you can use bigger woofers. Electrovoice made a 30-inch diameter woofer that could do 20-hz in the box. WOW!

F sub s would be the speaker resonance in the box and F sub 3 is the response that is 3-db lower than the over-all average from that frequency and higher. Or where the roll-off is considered to start and considered the low frequency response limit of the speaker.

I hope this helps.

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