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Old 3rd May 2010, 02:30 AM   #1
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Default Fb Hz lower than F3?????

So maybe its me with some antiquated formulas but I keep coming up with my F3 cutoff before my resonant frequency!
Whats up here?
Is my chosen driver unsuitable for vented enclosure? The EBP is spot on at 93.33.....
Here are the specs...could someone double check my math...perhaps my chosen formulas are flawed.
Fb 35.830... Hz
F3 41.188...Hz

__________________________________________________ _____Rick....
Fs 28 Hz
Vas 15.6 cu Ft.
Qts .29
_
SPL 99Db
Qms 8.28
Qes 0.3
41-2.4K Hz
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File Type: jpg Port calc.JPG (27.0 KB, 248 views)

Last edited by Richard Ellis; 3rd May 2010 at 02:32 AM. Reason: The numbers
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Old 3rd May 2010, 03:01 AM   #2
Loren42 is offline Loren42  United States
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I don't know the make and model of your driver.

However, you can align or misalign any cabinet to yield all kinds of alignments.

Perhaps if you published the driver make and model and give me some information on what you are trying to achieve I could try modeling what you are doing.
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Old 3rd May 2010, 03:39 AM   #3
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Default Specifics

Thanks Loren..

The make is the Eminence Sigma Pro-18A2...an 18" driver from the "pro" application side of things.
The intent is to create a highly efficient system(100+ Db) to use a low powered single-ended tube amp.
I'm tending to shy away from the intensive woodworking side of horned enclosures & trying to stick with large driver, ported, Horn Mids & HF drivers.
My limited skills with wood and relative lack of appropriate tools make for straightforward port versions being the way to go.
This driver at 99Db seems to be the highest I have found with an appropriate low Fs for this project.

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Old 3rd May 2010, 04:01 AM   #4
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Hi Richard,

It is not at all unusual for the F3 to be higher then the FB in a bass reflex enclosure. I just threw the published params for my 10" woofers into akabak and brought up the alignment list (see attachment).

This is a vifa M26WR-09-08 which is recommended for reflex enclosures.

Note that all the alignments shown have F3 higher than FB.

Tony.
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File Type: png alignment compare.png (11.2 KB, 231 views)
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Old 3rd May 2010, 11:45 AM   #5
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Richard Ellis View Post
Thanks Loren..

The make is the Eminence Sigma Pro-18A2...an 18" driver from the "pro" application side of things.
The intent is to create a highly efficient system(100+ Db) to use a low powered single-ended tube amp.
I'm tending to shy away from the intensive woodworking side of horned enclosures & trying to stick with large driver, ported, Horn Mids & HF drivers.
My limited skills with wood and relative lack of appropriate tools make for straightforward port versions being the way to go.
This driver at 99Db seems to be the highest I have found with an appropriate low Fs for this project.

________________________________________________Ri ck.........
Well, here is the comparison of your selected driver compared to a JBL 2245. I did this because when I saw the response curve for the Eminence I was very disheartened. I would not choose that driver and I will explain why shortly.

Click the image to open in full size.

The problem with the Eminence is that the cone breakup at 1kHz and beyond is very nasty. While the SPL is high, it is measured at 1 kHz, a frequency that you would never want to produce with this driver anyway. As the frequency goes down it drops at least 3 dB, so the actual region where the driver makes sound has an SPL closer to 95 - 96 dB.

I ran the cabinet volume way up to 18 cubic feet to get as much bottom end as possible.

Secondly, all that breakup will require a lot of crossover work to suppress. This will further rob valuable amp power with inductors.

I compared the Eminence with the pro JBL 2245H. You can view the raw driver data HERE.

I modled the JBL in a 16 cubic foot cabinet. The F3 is about the same as the Fb. If you drop the volume down to 10 cubic feet F3 goes up to 34 Hz.

Obviously, there is more to this story than simply the woofer, so a lot depends on what the rest of the drivers are that you intend to use and the system architecture.

If it were me I would not bother trying to squeeze a super high SPL out of a loudspeaker at the expense of performance of that loudspeaker. My system is only 93 dB/W/m, but I have 60 WPC.

My personal opinion with SET amps is that they are more a novelty amp and not something I would use for serious listening. A class A or AB push pull would yield more power and better result, which gives you a wider array of choices for drivers.

A second tact to increase efficiency is to use two woofers in parallel. This gives you a 3 dB boost in efficiency, but you are required to double the cabinet volume. If you have a lot living space (and money) you can get away with this, but you cut the impedance in half and the amp still needs to handle the lower impedance (i.e., 4 Ohms).

Lastly, you can further improve the performance of the system by bi-amping and using an active crossover instead of the passive crossover. The bass (500 Hz and lower) is where 50% of your amp's power is required. So, building a second stereo SET would give you two things. First, you would gain 3 dB in system power. Second, you would eliminate the low end crossover for the woofer, which will make the woofer operate more efficiently without the huge inductors.

Using an 18" woofer implies that you are at least considering a 3-Way system. You can get away with a passive crossover for the mid and tweeter and then use an active solution for the woofer to mid. The results would be superior to a passive 3-way crossover and nearly as good as a fully active system.

I would be interested in seeing what your plans are for the rest of the system and what drivers you are considering. My conclusion would be to either reconsider the Eminence driver, possibly substituting the JBL or reconsider the way you amplify your system.

Last edited by Loren42; 3rd May 2010 at 11:52 AM.
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Old 3rd May 2010, 02:51 PM   #6
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You might consider doing an IB thing, if you own your home... You'll get all the way to 28Hz. then.

But 40Hz. is quite reasonable!
Most systems although they say they are, are not really flat to 40Hz.
Some are, and some go lower of course.

Your box calc does not take into account "room lift" or room placement either.

Depending on what that does and where you put the boxes that will have a big effect. In the corners, you will get significant boost as you go lower.

Placed in a large triangular pyramid like box and shoved into the corners with the driver low to the floor will give you rather remarkable results, depending still on the actual room and floor materials... etc.

Using a SE amp, your Qt figure will likely tend to be a bit higher than the actual figure in practice...

Although as the previous poster suggested a biamp'd system has numerous merits. If ur going tube only, I'd suggest a P-P amp for the bottom end myself.

Also, the lower you go with the "mid" horn the better... check the L'Clerc'h horn thread(s) for one way to go there...

SE amps are not novelty amps - they are not all equally good, but matched properly to the right speakers they are capable of magic. Improperly designed, built and matched, they are not very good.

Definitely SE amps are not "best" in most cases for bass drivers.

Paralleled woofers are something to consider. You get 6dB if you count the halving of the impedance... but in a biamped system, who cares? Just set the levels properly!

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Old 3rd May 2010, 03:02 PM   #7
Loren42 is offline Loren42  United States
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Originally Posted by bear View Post
You might consider doing an IB thing, if you own your home... You'll get all the way to 28Hz. then.
_-_-bear
No way could you use the Eminence driver in an open baffle. F3 approaches 90 Hz and 28 Hz is 15 dB down.

However, what other IB driver do you know of that has a high SPL?
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Old 3rd May 2010, 03:11 PM   #8
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You could use it in o/b if the Qts is raised by some means such as Linkwitz transform.
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Old 3rd May 2010, 03:16 PM   #9
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In any ported system designed for maximum flatness the lowest cutoff frequency is achieved with a woofer Qts of .383 or very slightly less if a passive crossover is used. Q other than this cutoff will always be above box frequency.

I have no idea why one would use an 18 at 1kHz and the importance of the response up there. That cross should be like 200Hz into a suitable mid.

As for that JBL mentioned it squawks so bad we spray the entire cone surface with the cheap undercoat from Walmart automotive department and set them in the Sun to dry.

Please remember, small ports don't work so if the port is not at least 1/3 the area of the woofer it is pretty much a waste of time. Port will go into power compression long before the driver runs out of ump.

Large drivers do not at all mean low bass automatically. I have a 3 inch here that goes down to 40Hz but it does not play loud at all as it runs out of cone excursion. For low bass choose in a small cabinet choose a driver with a low resonance and smaller cone with Qts= .383. All things being equal like Fs, resonance, and Vas the smaller driver will have a proportionally smaller cabinet. The reality of fabrication of a large cabinet like 16 cubic feet is quite difficult and takes a lot of bracing. A quality cabinet that size will weigh like 250 pounds- 110 kilos. That makes smaller better in a lot of ways.
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Old 3rd May 2010, 03:34 PM   #10
Loren42 is offline Loren42  United States
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Originally Posted by sumaudioguy View Post
I have no idea why one would use an 18 at 1kHz and the importance of the response up there. That cross should be like 200Hz into a suitable mid.
The idea is that you still have to deal with the cone breakup! Even though you apply a crossover slope of 6 or even 12 dB per octave, the breakup is so obnoxious that it will still intrude into the midrange frequencies.

At 200 Hz and 12 dB/octave you are only 24 dB down at 800 Hz and 36 dB down at 1600 Hz. Looking at the graph you have a wide 6 dB spike, which will require additional notch filtering to get flat. The more components you throw at this driver (i.e., steeper slopes and notch filters) the more power is dissipated as heat and not music. This is very bad for a flea powered amp.

If you cross at 200 Hz you need a midrange with very low Fs, at least 50 Hz if not lower, and it still needs to have a high SPL. The alternative is a 4-way, which is a monster to try to design.

Crossing low implies large inductors, which are both expensive and power robbing. It is better to cross as high as possible. 400 to 500 Hz is realistic, but then that aggravates the problem with the cone breakup.

The bottom line is the Eminence 18 is not a good driver for the design goals.

I do agree with you about port area and it would be prudent to design your system for higher power applications in the future (even though the SET will never develop enough port velocity to be audible), but that is a side issue compared to all the other problems he is trying to juggle.

Personally, I would disagree with you about driver size and bass capacity (particularly for his application). In this instance I would recommend at least 12" to 15" for deep bass production, assuming a very efficient driver. A 3" driver simply can not effectively produce a good polar response, let alone move enough air to be heard with a SET.
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