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About Matching Your Cabinet's Resonant Frequency

FractalAudio

Administrator
Fractal Audio Systems
Moderator
This post is aimed at those who use a solid-state power amp into a conventional guitar cab.

As is described in the post "About Speaker LF Resonance" a guitar cabinet has an impedance resonance in the low frequencies. This typically falls in the range of 50 to 100 Hz.

A tube amp, being essentially a current source, will have a voltage output that follows the impedance curve. Speakers, being electromotive devices, respond to applied electromotive force (EMF) which we know as voltage.

A solid-state power amp is a voltage amplifier and, hence, will not be influenced by the impedance of the speaker.

When using a solid-state power amp into a conventional guitar cabinet the experience will be different if the simulated speaker in the Axe-Fx II is not adjusted to match the actual speaker. Whether or not this is important is up to the individual but I imagine a lot of the posts about "in-the-room using power amp and cab is not the same" are due to this. Unfortunately the Axe-Fx II cannot measure the speaker impedance characteristics as it is not directly connected to the speaker. No device can measure the speaker impedance without being directly connected to the speaker, despite what their marketing claims may infer (cough, ahem...), since impedance is, by definition, V/I and we cannot measure these unless connected to the speaker terminals.

The only truly accurate way to set the simulated speaker is to measure the speaker being used with an impedance measuring device. These can be had relatively inexpensively in the form of the Woofer Tester 3 (from Dayton Audio IIRC). You can also make your own using a small value resistor (0.1 ohms or so) in series with you power amp and measure the voltage across the resistor.

The next best method would be to estimate the impedance using published data from the speaker manufacturer. If the make and model are known the data may be available. Add approximately 10% to the published resonance frequency if the speaker is in a sealed box.

The worst method, and the subject of some contention, is finding the resonance by "feel". No power amp has perfect damping. If you put a sine wave (use the Synth block) into the speaker you may be able to observe or feel the resonant frequency. The cone will have increased excursion at this frequency. Of course you may just be feeling the room resonance. I have used this technique successfully on several speakers but it takes practice. The main drawback is that the magnitude of the resonance is unknown.

The Axe-Fx II's Low Res parameter is displayed in dimensionless units from 0 to 10. Each unit corresponds to 2.4 dB of impedance "gain". We define this as a gain since the our current source power amp will experience a voltage gain. This is relative to the DC resistance of the speaker. For example, 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. Since a tube amp isn't a perfect current source these values should be reduced slightly. The exact value of the Q isn't too important. About 2.0 is a good starting point. Adjust up or down to taste. If you are anal more information is in the aforementioned post about deriving the value of Q.

Once the simulated speaker is set correctly you may notice a difference in low-frequency behavior and pick attack.
 

joegold

Fractal Fanatic
The next best method would be to estimate the impedance using published data from the speaker manufacturer. If the make and model are known the data may be available. Add approximately 10% to the published resonance frequency if the speaker is in a sealed box.
Is there a simple percentage you'd trust (relatively speaking) for an unsealed box/open back cab?

E.g. EVM-12Ls are supposed to have a free air low res freq of 55hz.
Jay once told me to use 65hz for an open back cab with 12Ls in it on the Spkr page of the Amp Block.

I tried to do it by ear once and it came out to 69.3hz but I usually just use 65 hz anyway.
Seems to help, but I've never been sure.
 

yek

Moderator
Moderator
Is there a simple percentage you'd trust (relatively speaking) for an unsealed box/open back cab?

E.g. EVM-12Ls are supposed to have a free air low res freq of 55hz.
Jay once told me to use 65hz for an open back cab with 12Ls in it on the Spkr page of the Amp Block.

I tried to do it by ear once and it came out to 69.3hz but I usually just use 65 hz anyway.
Seems to help, but I've never been sure.
EV 12L spec sheet:

Thiele-Small Driver Parameters
fs (free-air resonance frequency): 55 Hz
Qes (electromagnetic Q at fs): 0.245
Qms (mechanical Q at fs): 4.37
Qts (total Q at fs: (Qes Qms) / (Qes + Qms): 0.232

I had a cab with 2 EV 12Ls. I measured the LFR by ear using the old trick*, resulting in 72Hz IIRC.

* "One way to find the SRF is to put a Filter block after the amp block. Set the type to Peaking, Q to 5 or so and Gain to 10 dB. Start with a Freq. of 50 Hz. Play some chugga-chugga and slowly adjust the Freq. until you hear and feel the cabinet resonate. Make a note of the frequency. Remove the filter block and set the amp block SRF to match."
 
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miketheman

Experienced
Maybe the Axe-Fx III comes with a speaker resonance measurement tool, SRMT. ;)
Yeah, but don´t you think it should preferbly be named Speaker Measurement Acquisition Reference Tool?
If it were me and my creation, I would even consider adding an extra A and two S... but I´m out of words for the moment.

:lol

@Cliff, thanks for these notes you are publishing. Nothing better than consolidated info like this, where you get both "How" and "Why" reasoning regarding the subject!
 

Fabio KTG

Fractal Fanatic
Is this method applicable to FRFR systems? I'd quite like to implement this into my preset building with my RCF.
 

yek

Moderator
Moderator
PS: I'm in the process of making such a device! ;) (not an AxeFx III of corse) - comin' soon ;)
That'd be nice. I owned the Woofer Tester. But not knowing anything about electronics and being clumsy, I couldn't figure out how to measure a mounted speaker in a closed enclosure.
 

Nagi Mysore

Inspired
A lot of users on the forum have purchased Matrix NL12, CFRs and Atomic CLRs. May help if someone can suggest what q factor to adjust (BTW where is q?)...

I have a NL12...☺..any suggestions?
 

xpenno

Power User
A great write up as usual Cliff!

One thought that cropped up and that might help Poweramp/Cab guys is the idea of a global Low Freq Resonance Setting. This would allow you to override the setting for all amps depending on what guitar cab you are using at the time. This would be useful if heading out to a fly show or a rehearsal space where you probably wouldn't take your own cabs. Otherwise you may have to tweak quite a number of patches which could be time consuming. As usual just a thought.

Cheers

Spence
 

yek

Moderator
Moderator
A great write up as usual Cliff!

One thought that cropped up and that might help Poweramp/Cab guys is the idea of a global Low Freq Resonance Setting. This would allow you to override the setting for all amps depending on what guitar cab you are using at the time. This would be useful if heading out to a fly show or a rehearsal space where you probably wouldn't take your own cabs. Otherwise you may have to tweak quite a number of patches which could be time consuming. As usual just a thought.

Cheers

Spence
Excellent idea! Whenever I change cabs (I switch between 1x12 and 2x12), I have to adjust the LFR in each global amp slot that I use (and remember to that).
 

yek

Moderator
Moderator
@yek- Have you measured the LFR of the ASW KTS70?
Yes, in my 1x12: 81 Hz.
Note that this a "Wave" Port City cab, the specific ported design can have an impact.
I didn' find any published information by ASW about the resonant frequency.

I'm still waiting for my 2nd KTS-70, to be mounted in a PC Wave 2x12 vertical, with EV 12L. (I got one but it was defect) So measuring that 2x12's RF is on hold.

Back on topic: I'm curious how resonant frequency measurement turns out, applied to a multispeaker mono cabinet.
 

elchampion

Inspired
Yes, in my 1x12.
It's .... :drumroll .... 81 Hz.
Note that this a "Wave" Port City cab, the specific ported design can have an impact.
I didn' find any published information by ASW about the resonant frequency.

I'm still waiting for my 2nd KTS-70, to be mounted in a PC Wave 2x12 vertical, with EV 12L. (I got one but it was defect) So measuring that 2x12's RF is on hold.

Kind of curious how resonant frequency measurement turns out, applied to a multispeaker cabinet ....
Sweet! Yeah I couldn't find any info either. Mine is loaded into a closed 1x12 ASW cab but this will give me a starting point. My second cab is 1x12 PC Wave OS with a Jensen Jet Torpedo Neo and if I understood what the graph said it measured around 85 hz.
 

Strumzilla

Power User
* "One way to find the SRF is to put a Filter block after the amp block. Set the type to Peaking, Q to 5 or so and Gain to 10 dB. Start with a Freq. of 50 Hz. Play some chugga-chugga and slowly adjust the Freq. until you hear and feel the cabinet resonate. Make a note of the frequency. Remove the filter block and set the amp block SRF to match."
I'm going to give this a try with my Port City OS 2x12 (WGS ET65s) tonight. I haven't been in love with the tone I'm getting compared to FRFR so far, maybe this is a critical element.
 

Semih Yanyali

Power User
I am not %100 sure but for nl12 i remember robin from matrix saying me 85khz. And for frfr this doesnt matter or?

A lot of users on the forum have purchased Matrix NL12, CFRs and Atomic CLRs. May help if someone can suggest what q factor to adjust (BTW where is q?)...

I have a NL12...☺..any suggestions?
 
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