In modern guitar amp modelling software often convolution techniques are used to capture the sound of the speaker cabinet. Via impulse response files the sound of the cabinet is sampled and can easily be applied to any input. Although this is a very popular technique it is in my opinion not perfect for getting a great speaker cabinet sound.
An impulse response is like a sample that can be superimposed to a signal. It is a linear process which can’t add non-linear compression or saturation to a signal since it’s a linear technique. Using an impulse response the input signal won’t be able to drive the output in non-linear ways. This means that many of the irregularities of a real speaker are not present when using an impulse response. A speaker can breakup in a non-linear way and we’re not able to capture this in an impulse response.
An impulse response of a speaker cabinet captures the frequency spectrum of the speaker cabinet with a fixed amount of breakup plus the ambiance of the room in which the cabinet is placed.
A perfect cab filter
In 1993 I bought a Palmer PDI-03 Speaker Simulator. The speaker output of my Koch, Vox and Marshall tube amps can be attenuated into a line-level signals when using the PDI-03 plus its build-in filter that will simulate the sound of a speaker cabinet. The Palmer in fact is a loadbox + speaker simulating filter. Its sound is very direct, lacking any ambiance. The sound is also extremely dynamic since it lacks the irregularities and breakup a real speaker will add to a signal. But this can be a nice thing I found out! It’s a much cleaner signal, but one without the irregularities of the speaker and without colorisation of a cabinet.
A guitar amp speaker cabinet is a midrange system. It’s not like a HIFI speaker system but often uses only 10 or 12 inch speakers. It’s frequency curve can be simulated using an EQ to filter the sound. The Palmer showed me that this can create a fantastic sound which supreme dynamics.
But I also believe that the non-linearities are an important factor for getting a great guitar sound that reacts nicely on playing dynamics. An impulse response can capture the sound of a cabinet but it is not able to capture the breakup sound of the speaker. And this is a bit of an issue in my opinion.
I must say I am not a technical genius and probably never will be one (read: I am not a scientist and don’t wanna become one!). But I have trained my ears over the years so I am now more capable than ever before to listen in 3D and explain what I am hearing (frequency peaks, saturation, compression). And I am now able to design the sounds I am hearing in my head for many years. Sounds which are an improved version of the equipment I have been using for many years, like some of the tube amps I mentioned above. Because I always felt something was not right. Some amps have a great overdriven tone using the bridge pickup but start to sound muddy when using the neck pickup. That’s why I came up with the Rockmen ReFill, using nothing but the stock devices of Propellerhead Reason. I was able to create my own custom AMP which worked great on all my guitars using all kinds of settings. No impulse responses or Rack Extensions are needed.
Also published on Medium.