Dimensions of tone 3: suitcase tone controls

[Rob A]

If you haven't already, please have a look at dimensions of tone in which I explain the background of what is going on here.

In part three, we'll try to measure the precise effect of the tone controls on a suitcase (sorry stage owners--it's what I have on hand).

To accomplish this, I recorded a test signal: a .wav file with 10 seconds of pink noise. Pink noise is widely used as a test signal since it has 1/f power distribution. You can translate this into a useful musical term--every octave contains the same amount of energy. I played the test file out of my audio interface and into the effect loop return of the suitcase preamp (and hence through the amp and modified speakers). I measured the response with a  Behringer ECM8000 reference microphone, which has a reasonably flat response curve (you can look up the specs if you like, but it is "flat enough" within 20Hz-20kHz). That signal was sampled back in my Edirol UA-25 at 48kHz and 24 bit sample size (as previous measurements). The graphs were produced with Audacity.

First let's examine the spectrum of the input signal. On a log-log scale, you'd expect 1/f to be a straight line.


Looking good. That's a 3dB/octave slope. Now let's have a peek at the output spectrum with the tone controls set flat:


I overlaid this and the others on the input curve for easy comparison. What can we observe?

• bass response falls off rapidly below 45Hz. Lowest note on a seventy-three is about 41 Hz.
  
• From say 100Hz to 3kHz, the response tracks 1/f pretty well overall.
  
• There are a couple very prominent notches in the spectrum at 359Hz (F4) and 1423Hz (F6)
  
• Above 5kHz things roll off quite a bit more steeply than 3dB/octave. Yours will roll off even faster since I modified mine.
  

So the suitcase bottom colors the sound substantially. We knew that already, but this gives us a way to visualize it. The bass is boosted somewhat even though the tone controls are set flat.

Next I pegged the bass tone control. I was a little surprised at the resulting response:


You get a 6dB hump centered at around 91 Hz that's a couple octaves wide. What surprised me was the apparent cut in treble that came along for the ride. You wouldn't expect that from a graphic equalizer--I never considered that the bass boost may also cut treble.

Next I pegged the treble.


This seemed to take effect in the octave between 3kHz and 6kHz entirely. The EQ may actually have a wider band, but the rolloff of the speakers is swamping any effect that would have. And look at that, a corresponding bass cut (look at 95Hz--we lost 6db compared to the flat curve). Very interesting.

And then I pegged both bass and treble:


Interestingly, but not surprisingly now that we've seen each control separately, this results in something a lot like the flat curve, but with the middle dropped out a bit. I don't view this as a practical setting, I just threw it in for interest.

I'm working on getting you a good sound file to listen to the coloration (music, not pink noise). Bear with me while I work out mic placement issues.

Let me know if this kind of article is useful.

[Rob A]

http://music.linear1.org/mp3/mic-vs-direct-test-3.mp3

This isn't a demo of the tone controls, it's a comparison of direct and miked suitcase recording. I recommend you download it and listen with a program that lets you pan.

Behringer ECM8000 into Edirol UA-25 on the right channel, direct in the left. Both channels normalized. 48kHz/24-bit.

1976 suitcase 73 with piezo tweeter mod, tone controls flat.



On musical material rather than a test signal you get a pretty decent alignment between suitcase and direct spectrums, better than I thought actually. And they sound reasonably alike too. The treble rolloff going through the suitcase is pretty distinct though, and remember yours will be far worse (unless you have the piezo mod)

[Ben Bove]

this is all very interesting stuff.

[BJT3]

Rob, thanks for all the work. I've been doing some experimenting of my own trying to match an EQ curve as close as possible to your results to see how it sounds. So far, I like the results. I can try and post some audio clips if anyone would be interested

[Dan Belcher]

Very interesting study.  Now, if only you could figure out a way for my '78 to sound as sweet as your '76.  

[dnarkosis]

Rob:

On your last graph:

On musical material rather than a test signal you get a pretty decent alignment between suitcase and direct spectrums, better than I thought actually. And they sound reasonably alike too.

What do you think is at work here? I would not have expected this spectral result at all -- or am I misreading the graph? I would have expected a much greater difference.

The treble rolloff going through the suitcase is pretty distinct though, and remember yours will be far worse (unless you have the piezo mod)

Also very interesting, but much as I would expect. How might this falloff be addressed even with the piezo mod already installed? What part of your envisioned setup (in the other thread) might address this falloff?

[Rob A]

I also paused when I saw the alignment of the direct and through-air curves on a musical program.

Pink noise is broadband--it has lots of energy outside the "normal"  Rhodes frequency range. I think the correct explanation is that the suitcase bottom is actually pretty well tailored to the specific range of frequencies produced by the Rhodes. I took note of the 45Hz rolloff in the lower end--I really doubt that 's an accident (conversely, I believe it to have been a design decision), although I do wonder what happens with an 88 suitcase.

I'm also wondering if mic placement was a factor. I did change the mic placement before I recorded the music program sample, and I wondered if I would see a similar curve alignment if I close mic'ed the suitcase on a pink noise test (tell you what--it's easy to repeat the pink noise, so I'll do it with close mic and report).

You can bung in four more (or eight more!) piezos if you want more treble--they don't load the amp appreciably.

The very motivation for the new suitcase bottom design is that instead of trying to retrofit "flatness" (or any other desirable quality) onto a 40-year-old system design, you accomplish the goal  most effectively with a new design conceived to fulfill certain goals. And I'm speaking here of the electronics. I'm not qualified to discuss engineering improvements to the mechanical parts. (I'm not especially well-qualified on the electronics either but I can talk a good game.)

There's one specific thing about biamping--it reduces intermodulation distortion. I haven't yet set out to quantify this effect in the legacy suitcase. Since I'm impersonating a scientist here, my hypothesis is that the grungy lower mid you get with a suitcase is due to intermodulation distortion, and if I can prove that (stay tuned for dimensions of tone 4) to myself, then I will probably go forward with a prototype of some kind.

I'm just getting started--it's so easy to cram a test signal into the suitcase and measure the output, I'll probably be carrying on like this for some time to come. I hope that the result is some insight and not just colorful pictures with scientific-sounding blather attached.

By the way, I appreciate questions, please keep them coming.