# Can anyone scientifically explain the 'Intensity' knob on an amplifier?

Discussion in 'Amps and Cabs [BG]' started by rbuist, Dec 3, 2012.

1. ### rbuist

Jul 29, 2012
Calling all sound engineers!

Hello all,
In my college physics course we are currently going over sound waves. I was excited to learn some of the science behind the loud noises I can make with my instruments.

Specifically, I wanted to know if anyone knew exactly what cranking up 'Intensity' knob does to one's tone- in scientific terms of course.

The definition of intensity as explained in lecture and via the exam formula sheet is Intensity = Power/Area or simply, I=P/A. From there, I can safely assume that the size of the speaker projecting the instrument's signal cannot increase (A is constant). Thus, when a musician turns up the intensity knob on their controls, the amount of watts sent to the speakers (the power P in watts) is increased, but this does not necessarily mean an increase in volume- it is just going from clean tone to a more distorted one right?

Though in the other respect, I could see the increased intensity increasing volume as
beta (in decibals)= 10dB x log(I/I[SUB]0[/SUB]).
Where I= new intensity and I[SUB]0[/SUB] equals the reference intensity.

Do I have the right idea or can some one tell me what's what with this stuff? My lecture professor is not a musician so he is curious about this as well. I searched all over the web and wasn't able to find an explanation on any forums, wikis, or guitar amp websites.

Thanks!
Ryan

Side note: I realize we do not see many 'Intensity' knobs on bass amps- the only ones I have seen were on old Fender guitar combos and reissues- any one have a bass head with such a control?

EDIT: Okay, I think I've got this all figured out.

The physical/scientific definition of intensity has nothing to do with the intensity knob on amplifiers- such knobs are only related to the on board-vibrato effect some amps sport.

The physical intensity I want to talk about actually relates to the amplifier's volume output. Loudness is given by
beta (in decibals)= 10dB x log (I[SUB]1[/SUB]/I[SUB]0[/SUB])

where I[SUB]0[/SUB] = 1.0 x 10[SUP]-12[/SUP] W/m[SUP]2[/SUP]

and I[SUB]1[/SUB] = the I value obtained by I=Power (in watts)/area (in m[SUP]2[/SUP]).

That very last formula can be simply stated by I=P/A.

<if any one really cares to see, I could show how much an increase in power would effect volume on a particular cab, I just don't have the time to think of an example myself at the moment>

Moving to the 'intensity' knobs on amplifiers- I can see it doing one of two things (sorry, I haven't had a chance to check out any of the links you guys have posted).
1) The knob can increase the presence of the effect; that is, lets say you have a clean tone (dry signal) and you wish to add vibrato (wet signal). Cranking this knob will increase the amount of vibrato present in your sound.
2) The knob will increase the rate of vibrations in your vibrato tone (See: surf music haha).

I guess all of this leads me to more questions about the way effects pedals can visually change a wave of sound. I'll do some looking on the internet but I also would like to do some recording myself- garage band (my only recording program) shows the waves on screen. I think it'd be a cool project to play the same tune with clean, distorted, vibrato and see the visual differences between each. I know my professor is pretty interested in it- I'll post my results if any one cares to take a look.

2. ### fdeckSupporting Member

Mar 20, 2004
Disclosures:
HPF Technology: Protecting the Pocket since 2007
The best way to understand any amp control from a technical perspective is to find out what it actually does. Schematics are available for many of the classic designs. I'm guessing it boosts a portion of the frequency spectrum associated with higher loudness. Or it adds some mild harmonic distortion.

3. ### nashvillebill

I believe you are actually getting confused with the Vibrato section of the Fender amps. The Intensity knob controls the "depth" of the vibrato; the Speed knob controls the timing of the vibrato.

4. ### Mr. FoxenCommercial User

Jul 24, 2009
Bristol, UK
Disclosures:
Amp tinkerer at Ampstack
The labels on amp knobs don't necessarily indicate their function. 'gain' being the most common mislabelled one.

6. ### crazyBassClown

Sep 25, 2007
sydney, australia
+1 to the above. Although the &ldquo;vibrato" is actually a tremolo.
Whatever the name most controls are simply some sort of eq or gain/volume.

Simple rule, don't believe the name on the knob.

7. ### slyjoeSupporting Member

Jun 28, 2008
Valley of the Sun (AZ)
It isn't a scientific term in this context - it's a marketing term used by amp manufacturers.

Not sure how you get that from the "intensity" knob - from the definition, it looks like it is a power control. As you point out, A is a constant. If I goes up, so does P. So you can't tell (see point about marketing).

That is the equation for power, so ok. As pointed out, it is really only an increase in I, not P since you have scaled P by A. But don't confuse power with actual volume. The rule around here is 10dB power increase equals 3dB in volume (double volume). Sound is a pressure wave, not an electrical signal. In addition, speaker sensitivity and efficiency plays into this - its not linear over any and all power (or intensity).

Well...I'll leave that for you to decide.

8. ### nashvillebill

Exactly, tis really a tremolo.

Listen to CCR's "Born on the Bayou" for moderate-to-high intensity and moderate speed. Tommy James "Crimson and Clover" has the intensity turned up even more.

9. ### nashvillebill

Nope, he's just confused on the Fender tremolo/vibrato effect.

10. ### wneff

May 27, 2003
Woburn, MA
Congratulations on taking a physics class on sound. I taugth a class like that at Buff State College, very interesting, and hopefully helps you cut through some of the BS.

Intensity knobs usually adjust how strong an effect sounds (like said above many times)

...so....what am I contributing here....????? /me scratches his head

11. ### slyjoeSupporting Member

Jun 28, 2008
Valley of the Sun (AZ)
Well, for his test he probably should use the instructor's definition.

13. ### nashvillebill

Ahh, but now I understand--the professor was not referring to Intensity as meant by the Fender knob but rather as a general term for the sound pressure; the OP saw the term on the Fender amps and thought that knob meant the same thing.

Hence my confusion as well.

14. ### slyjoeSupporting Member

Jun 28, 2008
Valley of the Sun (AZ)
Yea bill - it's even more confusing since it is unclear whether the professor is using P for power or pressure. I would bet power.

15. ### will33

May 22, 2006
austin,tx
+1 to all the above.

+1 to you for getting into this.

The "intensity" almost certainly refers to the subjective, meaning "sounds more intense, man.", and almost certainly not to the scientific definition of "intensity".

Although, yes, when you turn up the intensity on a vibrato effect, or other type effect, it's affect becomes more "intense", that is, it has a bigger effect on the sound, it varies more wildly, or otherwise alters the sound more in comparison to the "clean, unaffected sound".

16. ### xk49wSupporting Member

Apr 13, 2008
It is a term which roughly translates to "yet another tone control."

17. ### vegedge

Jul 14, 2012
i would love an ongoing thread about the science behind amps and what different junk does!

18. ### nashvillebill

For amp "science" basics: look for a book called "Electric Guitar Amplifier Handbook" by Jack Darr, with a little searching you may find it downloadable free online.

19. ### B-stringSupporting Member

I'm going to take a different track here. Assuming the "intensity" is actually the "Volume" control (as in intensity of amplifier drive). When a speaker reaches it's maximum linear power input any additional power sent to it results in non-linear operation (distortion) and of course heat. Guitarists can rely on old Alnico speakers to lend a specific distortion character when power compression happens (and pay heavily for old Alnico Jensens).
Since this is a mechanical distortion and not electronic it is generally void of the sharp edge that electronic distortion can have. Input power above the start of this compression has a diminishing return on spl.

20. ### IPYF

Mar 31, 2011
The intensity knob is for helping you concentrate. If you're feeling a bit washed-out you can turn the intensity up so your amp knows how much to compensate for your bum-notes by replacing them with stock tones imprinted on the amplifier's microchip. It's great for when you're hung over, drunk or playing with people you just don't like.

I had an 'Enhance' knob on one of my amps which I think adjusted my skill level depending on who I was playing for. My garage band didn't want much, but the time I played for the King of Siam I needed everything it had to offer.

21. ### slyjoeSupporting Member

Jun 28, 2008
Valley of the Sun (AZ)