A little different usage for REW analysis - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 5 Old 08-01-14, 01:26 PM Thread Starter
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A little different usage for REW analysis

I am an acoustic musician who joined primarily to gain access to REW software for specific testing purposes a little off the beaten path for home theatre discussions. I suspect there are many very savvy people here at this software and your input in my testing process is appreciated.

My testing goal is to see the specific full range acoustic output (fundamentals and partials) of the instruments I play, to allow me to design a system that reproduces them most accurately in amplified format.

Unfortunately, using microphones in a loud environment is not practical, so I am forced to use pickup systems (piezo) for the instruments and EQ them to sound as close as possible to natural. All of the transient attack, and input impedance matching issues with piezo's aside, the goal is to accurately measure the acoustic instrument output and make the amplified sound as close as possible to the natural sound of the instrument.

I have already seen graphic representation on several of the instruments on the display of a Behringer Ultracurve Pro, which indicated significant partials below the lowest fundamental note on my instrument. I am prepping this test in the studio now to avoid room influence on the measurements.

I ordered a USB calibrated mic (UMIK-1) and will start testing when it arrives. After instrument testing, I will test the piezo output in a similar way using very flat response studio monitors with no EQ coloring. The Piezo preamp will have an input impedance of 10 Megohm, so the only other hard issue to control will be the transient attack. Normally this is done with compression, but I want to do the tests both with and without compression to assure there is no big difference in the spectral output.

I haven't spent more than a few minutes withe the REW software after installing it, so I am hoping there is a possibility to store and print the tested curves for comparison.
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post #2 of 5 Old 08-01-14, 04:08 PM
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Re: A little different usage for REW analysis

Mandochamp wrote: View Post
My testing goal is to see the specific full range acoustic output (fundamentals and partials) of the instruments I play, to allow me to design a system that reproduces them most accurately in amplified format.
I’m not sure how useful REW will be for an exercise like this, as an aid to EQing an instrument through a sound system for “natural” sound (if I properly understand your post). Each note the instrument generates is a rich blend of fundamentals and harmonics. For any particular note, a measurement system will only capture (register) the loudest harmonic or fundamental, and that is really no reflection on what the note actually sounds like from one instrument to another. In other words, a “G” note on two guitars would probably register the same visual graph, even though you can audibly hear that they both sound different.

And exactly how can a measurement platform plot the response of a musical instrument? A loudspeaker system can generate a full-range pink noise that can be measured. This is impossible with a sound source that generates only a single “frequency” (note) at a time. A response graph could only be generated manually, by taking SPL measurements one note at a time and plotting it on something like graph paper. And again, each measurement would only reflect the loudest harmonic or fundamental. I suppose you could take that plot and use it to “accurately” equalize the sound system or guitar amp in use, but there’s not much chance it’s going to result in a “natural” sound. PA systems and guitar amps are notoriously colored (especially the latter) and as such add their their own sonic signature to the mix.

In short, I don’t know that this exercise will ultimately be any more useful or accurate than simply using your ear, but I’m certainly interested to see what you find, so please keep us updated!

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post #3 of 5 Old 08-03-14, 04:59 AM
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Re: A little different usage for REW analysis

You can use REW's RTA to view the responses from the mic and the piezo pickup, RTA responses can be saved but it can be tricky to capture the response to a transient event like playing a note and also tricky to play the note in exactly the same way each time you capture it. An alternative would be to capture the actual waveforms in each case and then select the sections of interest from each and look at their FFT to see the frequency response - Audacity can do that, but I expect most audio workstation software can do the same.
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post #4 of 5 Old 08-03-14, 11:24 AM Thread Starter
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Re: A little different usage for REW analysis

Thanks Wayne,

Yes, you are entirely correct about timbre; I won't be able to see this. And yes, each note is different, and probably must be sampled individually as well as the combination of notes changing the way they interact in the instrument to see slight differences. I have fairly good experience recreating very similar note attacks from using touch sensitive midi inputs for composition work with stringed instruments. It would be really nice to be able to see graphic curves of fundamentals and partials for each note, like the equivalent of a peterson strobe tuner with graphic file output, but if there is such thing made commercially, I bet I can't afford it!

All in all, this exercise wasn't intended to replace my ear, only to give me additional data in visual form to check against my ear. Basically all I am doing is looking at the overall frequency response curve of the instruments acoustic sound (all notes, all chords in all positions), to give me a baseline for comparison with the piezo output curve for the same instrument. The idea being that being able to graphically compare the frequency response of the pickup and match it by graphic and parametric EQ to the original curve would get me as close as possible to the overall EQ curve. Of course, just as you have noted, that overall curve is not representative of timbre.

I suspect that amp coloration will be insignificant in effect compared to pickup coloration, as I regularly use amplified sound with a good large diaphragm microphone (sometimes small depending on environment) and find the sonic output very acceptable at low volume levels in good acoustic environments. So the pickup is my focus at this point.

I may indeed find that this is an exercise in futility. But in the process of doing all of this experimentation, looking at the data and using my ear at the same time, I may stumble upon the best acceptable alternative 'tone' for the amplified instrument to 'simulate' a natural tone. I have already experimented with preamps, including those that add in varying amounts of recorded acoustic samples (Fishman Aura Spectrum). I have tried dozens of microphone/piezo pickup, preamp EQ rig combinations, ext., and obviously I haven't found something I find acceptably comparable to the mic'd sound. This is just one more step in the many things I have tried and I am hoping it yields another way to examine my sound and improve it. I can't fix the rooms I play in, and in many cases help the PA output of other people's systems, but everything I can do to make my end the best it can be (natural) will help.
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post #5 of 5 Old 08-03-14, 11:36 AM Thread Starter
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Re: A little different usage for REW analysis

Thanks John,

I think I ended up putting part of my response to you in reply to Wayne. I do have considerable experience playing very consistently (individual notes, chords, and attack format/intensity).

I am in process of capturing high quality audio samples at 24 bit/96KHz, of the calibrated microphone output, a good quality (AKG414) condenser mic and the piezo uncolored direct simultaneously. My intent is to do full range separate note samples and a sampling or many chords at different positions on the neck.

I have used RTAs for tweaking speaker arrays with pink noise and compensating for rooms, but have no experience with FFT at this point. Thats part of the reason I downloaded this software.

Thanks to both you and Wayne for the responses, Bruce
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