Load Box vs Real Cab Dynamics?

[James Freeman]

I have only questions;
I've always wondered if there is a difference in dynamics between a real cab and a load box?
What part of the curve is affected the most when the speakers move a lot at higher volumes?
I know that the cabs' low frequency resonance is mostly affected by the cones' free air resonance and cab air volume combined, but what happens to the resonant frequency when the cone is playing several low frequency overtones like palm muting?

@FractalAudio
Have you experience with measuring cab response at various volumes?
Does speaker drive have direct affect on impedance curve in real amps?

 

[Shask]

I know there is a difference, but it is hard to explain, and it probably depends a lot on the particular amp and loadbox you are using. Playing my Recto with a Fractal LB-2 for example, produces more highs/presence than my cabs. The lower mids can come out different also. Many times I also reamp this signal through a solid state poweramp into my cabs, and the poweramp I use.

I have also noticed when taking ToneX captures that I get different feeling captures when I use my loadbox vs. connecting a cab and only using it as a line out.

 

[Deadpan]

Great topic. I look forward to the discussion. I have many loads and cabinets. Have tested unscientificlly. Would love to see some data.

 

[bikescene]

I recently bought a few 4x12s for some more first hand experience. I've been using a Suhr Reactive Load for several years now and am familiar with its sound. I notice a bit of difference in the low end in playback and in my own NAM captures when comparing my Greenback 4x12 cabs to the Suhr Reactive Load.

I don't know how it plays into the dynamics mentioned in the OP. I can't tell if the difference I hear is purely EQ.

I did take some impedance measurements. Not sure how helpful these will be. The volumes of the sweeps by the Dayton DATS are lower than I would use to play through the cabinets.

Suhr Reactive Load (Blue) vs Marshall 1960TV w/ G12M25 (Green) vs EVH 5150III w/ G12-EVH (Orange)

Though these 4x12 cabinets are 16 ohm, I felt compelled to compare because the SRL is supposed to emulate the response of a Greenback-loaded 4x12. Here you can see the difference in resonant peak, which is pretty close but not exact to my real-life references.

Here are comparisons to other 8 ohm cabinets that I have on hand. I haven't done any listening comparing the SRL to these as loads, but am planning on, since the curves are pretty close.
Suhr Reactive Load (Green) vs Mesa Traditional 4x12 w/ 8 ohm V30s (Blue)

Suhr Reactive Load (Green) vs Jet City 2x12 w/ 16ohm V30 and 16ohm G12H75 (Blue)

 

[0XI0]

this high contrast harmonic complexity between 40Hz and 180Hz has all my attention, I had a thread in FAS forum wish list requesting additional user editable SIC nodes/harmonics to do exactly as shown, I think thread is deleted

 

[Orvillain]

I can barely read this without my brain turning to mush:

https://pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/24_Klippel/AES_ALMA_and_Other_Papers/2006_LS_Non-lins_Causes_Parameters_Symptoms.pdf

Dynamical Measurement of Loudspeaker Suspension Parts, Wolfgang Klippel:

https://www.klippel.de/fileadmin/klippel/Files/Know_How/Literature/Papers/Dynamical%20Measurement%20of%20Loudspeaker%20Suspension_Klippel.pdf

But it seems entirely reasonable that real systems of connected power amplifiers and speakers, end up with various interactions, feedback loops, and outcomes that loadboxes don't.

Now....

In a loose sense, ChatGPT claims:

a real speaker is reactive and non-linear, while most load boxes are reactive but static (linear).

Which I found a puzzling statement, so I asked for more info:

A real speaker:

• Has a cone that moves air
• Generates back-EMF (the coil feeds energy back into the amp)
• Exhibits dynamic impedance changes depending on excursion, temperature, and frequency
• Shows compression at high SPL
• Has true acoustic resonances that shift with volume

=> The amp “feels” the speaker differently at 60 dB vs 110 dB.
Even the good reactive loads (Fryette, Suhr, Captor X, Fractal, etc.) simulate:

• the static impedance curve
• maybe some capacitive/inductive behavior

But they cannot:

• generate back-EMF in the same way a moving speaker does
• exhibit power-dependent impedance changes
• compress dynamically
• shift resonances with excursion

So the amp sees a perfectly predictable load, unlike a real speaker.

Result:
Real cab = more interaction, more sag, more “push back.”
Load box = more consistent, less dynamic, slightly stiffer feel.

What part of the curve changes the most at high volumes?​

When you push a speaker hard:

A. The low-frequency resonance (Fs) shifts upward​

As the cone excursion increases, the mechanical suspension stiffens (nonlinear compliance).
This raises the resonant frequency of the driver by several Hz — sometimes more at extreme levels.

B. The Q of the resonance drops​

The resonance peak becomes:

• broader
• lower amplitude

This is basically mechanical compression.

C. High-frequency behavior distorts​

Voice coil heating increases resistance → HF slope changes → top end softens.

Most affected area:
Low-frequency resonance (Fs and the big impedance peak around 70–120 Hz depending on the speaker).

What happens when you palm-mute (complex low-frequency overtones)?​

Palm muting is rich in low-frequency transients (30–200 Hz), meaning:

• The speaker gets hit with multiple LF partials simultaneously
• Cone excursion spikes hard
• Mechanical compliance stiffens momentarily
• Resonant frequency (Fs) jumps upward in real time

This is called “dynamic resonance shift.”
It also produces:

• temporary impedance rise
• more back-EMF
• different current draw from the amp
• momentary compression

Basically, palm muting makes the speaker behave like a stiffened spring for a few milliseconds.

The resonant frequency changes dynamically with what you're playing.​

Load boxes do not replicate this behavior.

Emphasis all from ChatGPT.

Honestly, it kinda rings true for me. But I am no expert and this is a domain where I've just not built up any knowledge. One of the other clever clogs like @FractalAudio or @jay mitchell undoubtedly know more than me. So maybe ChatGPT is talking bollocks. But it does ring true, as I say.

I asked it to summarize:

I mean ..... again, that all rings true from my experience. I like my loadbox, but I'd rather mic a cab if I have the opportunity to do so.

 

[HotRats]

Emphasis all from ChatGPT.

meaning most of the gathered info's will come from Reddit. Geeeeez

 

[Orvillain]

meaning most of the gathered info's will come from Reddit. Geeeeez

Not at all. It pulls content from all sorts of places, particularly academic sources if you ask it academic questions, and if you ask it to back up what it says, it will. It will also fact check itself if you ask it to. Which I did.

On this issue, it is very insistent.

 

[EOengineer]

I’ve done comparisons capturing my 2204 and Mark III direct through a Suhr RL and 1960A. There is 100% a difference in how the amp sees and responds to each of them.

 

[MirrorProfiles]

I’ve shared this several times before but it’s one of my favourite tests I’ve done.

Same IR throughout, but I’m changing the load in each example between different types of cab and speaker and reactive load boxes. There’s obviously variables beyond what this video shows but it’s still very interesting how different cabs affect the sound of the amp, and also where reactive loads fit into the picture (where they get close, where they leave something to be desired).

 

[laxu]

Every loadbox is in its own way a compromise. I've always found that I can set my Fryette PS-100 pretty close to how my real cabs behave and that's good enough for me, considering I get something in return.

 

[Deadpan]

Every loadbox is in its own way a compromise. I've always found that I can set my Fryette PS-100 pretty close to how my real cabs behave and that's good enough for me, considering I get something in return.

Do you use the PS100 without a cabinet connected?

 

[Orvillain]

I’ve shared this several times before but it’s one of my favourite tests I’ve done.

Same IR throughout, but I’m changing the load in each example between different types of cab and speaker and reactive load boxes. There’s obviously variables beyond what this video shows but it’s still very interesting how different cabs affect the sound of the amp, and also where reactive loads fit into the picture (where they get close, where they leave something to be desired).

Yeah I do remember this. I'd forgotten how smothered and muffled the Suhr really does sound in comparison to a good cab.

That natty old Marshall cab with the K100's sounds siiiiccckk.

 

[MirrorProfiles]

Yeah I do remember this. I'd forgotten how smothered and muffled the Suhr really does sound in comparison to a good cab.

It’s interesting because I often read people saying they find the Suhr too bright - compared to Fryette, React IR, and especially Driftwood it can be but compared to a real cab it’s a bit closed off. I think it maybe exaggerates the 2-5kHz area a bit which is maybe where it sticks out to real cabs a bit.

Totally forgot I did this test too - several amps and load boxes but only comparing to a couple of cabs (otherwise the permutations just goes mad).

 

[laxu]

Do you use the PS100 without a cabinet connected?

Typically with a cab connected. The loadbox still needs to be set to the right switch combination.

 

[dk_ace]

View attachment 55352

this high contrast harmonic complexity between 40Hz and 180Hz has all my attention, I had a thread in FAS forum wish list requesting additional user editable SIC nodes/harmonics to do exactly as shown, I think thread is deleted

 

[jay mitchell]

I have only questions;
I've always wondered if there is a difference in dynamics between a real cab and a load box?

There always will be. The proper question is to what extent these differences are audibly significant. IOW, can they be heard and, if so, how big are the sonic differences?

Before getting into nonlinearities, it's important to note that the match between the linear behaviors of a load box and any specific cab will almost always cause audible differences. Unless those differences are eliminated, any nonlinear mismatch will likely be buried by them. IOW, the small-signal impedance vs. frequency of the loadbox must be an accurate match of the cab you're comparing it to. Unless you've accurately measured your own cab's impedance and designed a loadbox to match it, you're not gonna get there, and the remaining discussion is moot.

What part of the curve is affected the most when the speakers move a lot at higher volumes?

To answer that, you first need to know at what frequencies "speakers move a lot at higher volumes." The answer is low frequencies only. The peak displacement of a 12" speaker driven to incipient failure at ca. 1kHz is measured in thousandths of an inch.

I know that the cabs' low frequency resonance is mostly affected by the cones' free air resonance and cab air volume combined, but what happens to the resonant frequency when the cone is playing several low frequency overtones like palm muting?

First, two parameters determine resonant frequency: moving mass and spring constant. Moving mass includes some amount of air that stays attached to the cone and moves with it. Spring constant includes the speaker's suspension parts - spider and surround - and the additive contribution of the air in the box in the case of closed-back enclosures. Moving mass won't change with cone displacement. The spring constant will ultimately reach a linear limit, beyond which it will increase abruptly, assuming the speaker can survive being pushed that far.

Have you experience with measuring cab response at various volumes?

I'm not Cliff, but I have that experience. Here's a post of mine that is relevant: Axe-FX III Dyna-Cabs.