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About Matching Your Cabinet's Resonant Frequency

PacoCasanovas

Fractal Fanatic
Free air resonance frequency is 85Hz according to the manufacturers website, it changes slightly upwards when build into a cabinet housing, especially if the cabinet has a closed back* - the value of the resonance hump is dependent by the amp type, by the NFB-line, by the power tubes type, by various parameter.....use your ears, your experience when trying to match the data in your axefx power amp simulation when you're using a flat responce, high damping solid state power amp with your "real" speaker cabinet.


* a speaker is a two way transducer - at his resonance frequency the energy he takes to move the cone is the same as high as the energy inducted at the voicecoil when moving the magnetic field. The suspension, the cone diameter and the air pressure on the cone of a speaker result in a natural vibration-damping, which define the free air resonance frequency - if you close the back of your cabinet, the air pressure gets higher when the cone is moving backwards - this changes the vibration damping and increases the resonance frequency.


Cheers
Paco
 
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mtmartin71

Experienced
I've got a 2x12 OS Vertical Port City cab now with a Scumback M75/H75 mix. Normally I'll go direct and use IRs but I will occasionally use the amp/cab set up and I'm going through an Atomic Mono Block. I was thinking low frequency of 100 to see how that sounds, but what about the other values? Any suggestions? I have no way to measure
 

PacoCasanovas

Fractal Fanatic
Are these cabs wired parallel or in series?

Beside that - the atomic mono block is a tube amp, which will "feel" your speaker impedance distraction (unless you would use it with a passive crossover filter network, as they did in their reactor series "tube powered" active atomic FR cabs), so you don't need the power amp simulation from your axefx.

Have fun!

Cheers
Paco

EDIT: The MB50 has dual speaker outputs, according to their manual it's 8 ohm seach, since there is no 4ohm version of a T75 I would connect each single speaker to a separate output jack to preserve the amp from possible damage caused of wrong speaker loads (the axefx can simulate these "effects" of a falsely or wanted under- or overload in it's poweramp simulation without any possibility of damage.....that's why tube amps are already overrated:mrgreen:lol)
 
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mtmartin71

Experienced
Are these cabs wired parallel or in series?

Beside that - the atomic mono block is a tube amp, which will "feel" your speaker impedance distraction (unless you would use it with a passive crossover filter network, as they did in their reactor series "tube powered" active atomic FR cabs), so you don't need the power amp simulation from your axefx.

Have fun!

Cheers
Paco

EDIT: The MB50 has dual speaker outputs, according to their manual it's 8 ohm seach, since there is no 4ohm version of a T75 I would connect each single speaker to a separate output jack to preserve the amp from possible damage caused of wrong speaker loads (the axefx can simulate these "effects" of a falsely or wanted under- or overload in it's poweramp simulation without any possibility of damage.....that's why tube amps are already overrated:mrgreen:lol)

Hey Paco...thanks for the feedback. My new PC 2x12 is wired in parallel at 8ohms. I'm able to connect directly to it with a match on the Mono Block. I think the Atomic Mono Blocks were designed to act like a solid state amp in the sense that you're supposed to leave the amp sims on because they stay very clean all the way through the range of volume. Maybe someone else can correct me. I like a little more pushed sound than a preamp sound anyway. When I tried it without, it was too thin but I probably wasn't using the right models either.

With the new FW (12.04), I think I'm just going to play with dialing by ear to what I like. It does sound great as is. Just more of a fine tuning will be needed to get the bass to where I like it.
 

PacoCasanovas

Fractal Fanatic
The mb50 should be used with power amp sims on. It is an FRFR tube amp.

The only big thing that makes the MB50 more flat responce in the Atomic FR was the passive crossover network, which dampend the influences from the speaker. By definition the MB50 is a FullRange-Amp (btw. most tube amps are full range, this depends on the frequency response of your output tranformer.....), but it's not "flat response" by design.

There would be various possibilities to get more of a FRFR-tube amp, by increasing the damping factor or compensate the influences/interaction from the speaker load. The best possibility is, if you choose the screen of your output tube as a 2nd grid - this could be done if your output transformer has a 50% tap on his primary windings between plate and center tap, which loop back the signal in opposite phase to the plate of the output tube. Therefore this "negative feedback loop" also compensates the infulences from the speaker influences, which results in an almost constant power output over the whole frequency spectrum. This special circuit needs a special output transformer and is called "Ultra-Linear Circuit". It's not in a MB50.....

Sorry ;)
 

yek

Legend!
I can find the low resonant "frequency" of my real cabinet by ear.

But what are the implications of the latest beta firmware (12.04) for the Low Resonance value, when using a real speaker?
Not knowing the DC resistance of the speakers I use, I can't use the formula in the OP.

Does increasing the Low Res value (not frequency) work out the same as with IRs in 12.04?
 

PacoCasanovas

Fractal Fanatic
But what are the implications of the latest beta firmware (12.04) for the Low Resonance value, when using a real speaker?
Not knowing the DC resistance of the speakers I use, I can't use the formula in the OP.

Dear Alex

Rule of thump:

4Ohm Impedance = 3.2-3.6Ohm DC resistance
8Ohm Impedance = 6.2 - 6.6Ohm DC resistance
16Ohm Impedance = 12.6-13.6 Ohm DC resistance

Easiest way to get exact results is to measure it with a (cheap) digital multimeter...

Does increasing the Low Res value (not frequency) work out the same as with IRs in 12.04?

It does, since there is no change on the impedance curve itself, the improvements on 12.04 were mostly related to the dynamic interaction of the plate impedance on higher MV setting, which has a influence how the amp will interact with the impedance curve (IMO).

Cheers
paco
 
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joegold

Fractal Fanatic
Excuse my lack of math skills but...

"if the speaker's resistance is 6 ohms and the impedance at resonance is 60 ohms then our impedance gain would be 20*log10(60/6) = 20 dB. Dividing by 2.4 gives a Low Res value of 8.3."

My Mac came with a fairly sophisticated calculator which includes a "log10" item/key.

How, exactly would I enter the above numbers into my calculator to get the same result, 20db?

Does "*" mean multiply by?
On my Mac, that's what it means.

My calculator doesn't do fractions, but 60/6 = 10.

As I recall, a number in front of some other numbers in parentheses means to multiply the result of the terms in the parentheses by the preceding number.
So 20*Log10(60/6) means: 20 times Log10 times (60/6), no?

But when I enter in 20 times Log10 times 10 I get a sum of 13.01, not 20.

So how do I use my calculator to enter in this formula?
 

yek

Legend!
60 divided by 6 = 10.

In CALC (scientific mode) enter 10, then press Log10. Result = 1.

* means multiply.

So 20 x 1 = 20.

20 divided by 2.4 = ...
 

yek

Legend!
Applied to my KTS70 in a 1x12:

LFR is 81 (approx.).
Divided by 6.2 (approx. DC resistance acc. to Paco)
= 13

Log10 of 13 = 1.12

20 x 1.12 = 22.3

22.3 divided by 2.4 = 9.3
 

Moltenmetalburn

Self-Admitted Software Thief
The only big thing that makes the MB50 more flat responce in the Atomic FR was the passive crossover network, which dampend the influences from the speaker. By definition the MB50 is a FullRange-Amp (btw. most tube amps are full range, this depends on the frequency response of your output tranformer.....), but it's not "flat response" by design.

There would be various possibilities to get more of a FRFR-tube amp, by increasing the damping factor or compensate the influences/interaction from the speaker load. The best possibility is, if you choose the screen of your output tube as a 2nd grid - this could be done if your output transformer has a 50% tap on his primary windings between plate and center tap, which loop back the signal in opposite phase to the plate of the output tube. Therefore this "negative feedback loop" also compensates the infulences from the speaker influences, which results in an almost constant power output over the whole frequency spectrum. This special circuit needs a special output transformer and is called "Ultra-Linear Circuit". It's not in a MB50.....

Sorry ;)



That is a bummer, at one point I thought about buying one but I would have been disappointed I truly thought they were FRFR.
 

joegold

Fractal Fanatic
60 divided by 6 = 10.

In CALC (scientific mode) enter 10, then press Log10. Result = 1.

* means multiply.

So 20 x 1 = 20.

20 divided by 2.4 = ...

OK thanks.
I think I get it, but to me the original formula looks ass backwards.

I would have written the formula something like this to better match the way it gets entered on the calculator:
(60/6)Log10 X 20

Obviously that's the wrong way to write it, but still...

Thanks again.
 

PacoCasanovas

Fractal Fanatic
20 times Log10 times (60/6), no?

But when I enter in 20 times Log10 times 10 I get a sum of 13.01, not 20.

So how do I use my calculator to enter in this formula?

it's 20 times the logarithm of "impedance at resonance to DC resistance ratio" - aka 20 x log(60/6) which means 20 x log(60/6=10) or 20 x log(10)

logarithm of 10 is 1, so as Alex said - 20 x 1 = 20 ;)
 

Jeff B

Inspired
So, call me stupid... but here's the information I found on the Celestion website for a single G12M Greenback (I'm running a 2x12 cab, 16 Ohms each of these speakers):


Nominal diameter...... 12", 305mm

Power rating...... 25Wrms

Nominal impedance...... Available 8Ω or 16Ω

Sensitivity...... 98dB

Chassis type...... Pressed steel

Voice coil diameter...... 1.75", 44.5mm

Voice coil material...... Round copper

Magnet type...... Ceramic

Magnet weight...... 35oz, 0.99kg

Frequency range...... 75-5000Hz

Resonance frequency, Fs...... 75Hz

DC resistance, Re...... Available 6.7Ω or 13.1Ω


So, what numbers here am I actually supposed to be looking at and how does it fit into the formula? I'm sorry if this is a stupid question, but you're talking to a guy here that flunked algebra (even though I aced geometry!!). Just a bit over my head here with all the jargon and such. Any help is appreciated, as I'm one that is struggling with getting my AFX to do the trick for me and trying all I can from the forum here.

Also, if I choose to run it to an FRFR cab... how do I set things then??

Thanks so much!!
 

PacoCasanovas

Fractal Fanatic
So, what numbers here am I actually supposed to be looking at and how does it fit into the formula? I'm sorry if this is a stupid question, but you're talking to a guy here that flunked algebra (even though I aced geometry!!). Just a bit over my head here with all the jargon and such. Any help is appreciated, as I'm one that is struggling with getting my AFX to do the trick for me and trying all I can from the forum here.

Also, if I choose to run it to an FRFR cab... how do I set things then??

Thanks so much!!

Okey....a 2x12" cab with 16Ohms each, means your speakers were wired in parallel. Means your total load is 8Ohms. Since then the DC resistance according to your infos from the Celestion website would be 6.55Ohms (why not 6.7? This is because you wired two of them parallel, which halved the total load, so it goes for the DC resistors....)

You only need the correct impedance at resoance frequency! Since there is no impedance curve, I would ask your local Celestion distributor if he knows the impedance at resonance frequency - otherwise we need to measure it (which is a bit difficult, but possible).

Cheers
Paco
 

JGR

Inspired
I measured the impedance curve of my Mesa Recto 2x12 loaded with 16 ohm series wired EV12L's a while back and the peak was right at 100 Hz. I'll see if I can dig up the curve and post it if anyone is interested.
 
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