Measured LF Resonant Frequency Of A Marshall 4x12

Stratoblaster

Stratoblaster

Fractal Fanatic
Jay Mitchell said:
I've attempted more than once in the past to get this point across, but I gave up some time ago. Even within the Axe-Fx - i.e., using cab sims - in order to get an approximation of the interaction between tube amp and speaker, you've got to set the resonant frequency to a value that's in the ballpark (+/- 10% is more than close enough) of that of the original speaker. If you don't know what that value is, a little investigation is in order. There's no way to reasonably automate this in the Axe-Fx, because speaker and cab sim would have to exchange data in addition to realtime signal values.
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Jay's post inspired my curiosity so I measured the LF resonant peak of my circa 1982 Marshall 4x12 260 Watt cabinet with G12-65 speakers. Some people were interested in the LF peak on another thread, so I thought I'd measure my cab and post it.

The resonant peak of my cab is 115Hz.
 
Stratoblaster said:
Jay Mitchell said:
I've attempted more than once in the past to get this point across, but I gave up some time ago. Even within the Axe-Fx - i.e., using cab sims - in order to get an approximation of the interaction between tube amp and speaker, you've got to set the resonant frequency to a value that's in the ballpark (+/- 10% is more than close enough) of that of the original speaker. If you don't know what that value is, a little investigation is in order. There's no way to reasonably automate this in the Axe-Fx, because speaker and cab sim would have to exchange data in addition to realtime signal values.
Click to expand...

Jay's post inspired my curiosity so I measured the LF resonant peak of my circa 1982 Marshall 4x12 260 Watt cabinet with G12-65 speakers. Some people were interested in the LF peak on another thread, so I thought I'd measure my cab and post it.

The resonant peak of my cab is 115Hz.
Click to expand...

Procedure please.
How did you measure/determine this?
 
You need a signal generator, an oscilloscope, and a frequency counter makes things easier.

You put a ~240 ohm resistor in series with the output of the signal generator and the cabinet under test. Put the scope and the counter on the speaker input. The resistor is there to help keep the load constant on the sig-gen.

Apply a signal (I used 10V-peak sine wave) and sweep the signal generator starting from ~50Hz and work your way up until you see the waves peak on the scope. Go back and forth a bit until you're sure you've found the peak voltage, and measure with the freq. counter.

http://hyperphysics.phy-astr.gsu.edu/hb ... erres.html
 
Stratoblaster said:
You need a signal generator, an oscilloscope, and a frequency counter makes things easier.

You put a ~240 ohm resistor in series with the output of the signal generator and the cabinet under test. Put the scope and the counter on the speaker input. The resistor is there to help keep the load constant on the sig-gen.

Apply a signal (I used 10V-peak sine wave) and sweep the signal generator starting from ~50Hz and work your way up until you see the waves peak on the scope. Go back and forth a bit until you're sure you've found the peak voltage, and measure with the freq. counter.

http://hyperphysics.phy-astr.gsu.edu/hb ... erres.html
Click to expand...

OK. Thanks.
But that's too much for me.

I don't suppose there's any way to do this by ear, eh?
I was visualizing being able to play low notes on the low E string and just getting a feeling for the notes (and their vibrational frequency) that seem to make the cab resonate.
Is something like doable and or workable?

EDIT:
Actually... It'd be better to do this (assuming it's doable in the first place) with a bass as opposed to a guitar.
The low E on a guitar is only 88.407hz and many speaker cabs (like my EVM-12L in an open back cab) have resonant peaks well below that.
 
Jay said we've got a "close enough for horseshoes" scenario here, +/-10% I think he said ....

O.K., if the resonant frequency of the G12-65 speaker is 85Hz ..., and your cabinet is 115Hz ..., that's like 35% higher.

Any chance there's a percentage relationship between the speaker and the cabinet ( I think Jay already said no ) ?

And/or ..., any chance that the approximate resonant frequency is the same for any cabinet that has the same approximate internal volume ( probably not, but I had to ask ) ?

Finally, would the resonant frequency of a cabinet change if you replaced all the speakers ( given that the internal volume wouldn't change ) ?
 
There's a difference between the free-space resonance (Fs) of a given speaker and the LF resonance of the loaded enclosure.
 
xrist04 said:
There's a difference between the free-space resonance (Fs) of a given speaker and the LF resonance of the loaded enclosure.
Click to expand...

That much I DO know, unfortunately not much else ... :!:
 
Ahh, sorry RD, I guess I misread your post. :oops: Don't think there is any easily-calculable relationship between Fs and Fb, unless you bring in other data...
 
There's calculator available for this online (dont' remember where) but you also need other data to do the calculation like xrist04 said.
 
I use the "Woofer Tester II". Available from Parts Express and fairly inexpensive.
 
so is it safe to say if we started finding the resonant frequency of say a single Marshall 1960A cab loaded with T75s then everyone using that exact cab could enter that resonant frequency and be on the money? This would be great if we could start building a list of cab configurations and their resonant frequencies.
 
Jay Mitchell said:
Here's a way to make this simple: set speaker resonant frequency somewhere between 100 and 120 Hz, and fuhgeddaboutit. That figure is within 10% of the resonant frequency every guitar cab I've measured, and anyway it will only have an audible effect if you're using an amp sim with little or no negative feedback (e.g., Class A or Mr Z).
Click to expand...

While it would be interesting to see the numbers for various cabinets, I think Jay's thoughts on this matter are the best practical guidelines here.

That param is ~120Hz for any amp sim that I use a 4x12 cab with (I just start from the default and work my way up until I get what I'm looking for). Depending on how hard you run the Master and have the Damp tweaked, the overall effect of this setting varies. Different IR's also interact with all of this in interesting ways.
 
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