Test tones

2003-09-07 21:05:17

I am purchasing a car deck in the near future with a 7 band EQ, and i'd like to tweak it for flat frequency response. I have a radioshack SPL meter, so I suppose I just need test tones, right?

Where can I find such a thing, and...


Any other tips?

Test tones

Reply #1 – 2003-09-08 03:37:16

Hey Jebus;  I think you're going about this the wrong way.  You should adjust the equalizer to what sounds best to you.  Don't use test equipment.  Picture this:  you get the sound close to "flat" using pink-noise generators and spectral analyzers, and then observe that you like how it sounds with the 40 hz slider set a bit higher.  You put it back where the "test equipment" said it should be and say "Darn, I realy wanted that little extra bass."

Sounds silly, right?  It is.  Just adjust the EQ til it sounds however you like it.  Its your car and your stereo.   

And just for the record, if you really want to go the "test-equipment" route, just for curiousity's sake, what you really need is a pink-noise generator and a spectrum analyzer.  The pink noise is fairly easy.  Try this free tone generator, which includes pink-noise.  Simply record however many minutes of noise you want, then burn it to a CD to play in your car. (Or cassette, or whatever you're using.)

The spectral analyzer will be a bit harder.  The reason you can't really use your SPL meter is, a properly balanced system wouldn't have the same sound-pressure-level at, say, 15khz that you'ld have at 40 hz.  You'll also need a very good microphone, which will have its own sonic charateristics.  You can see how this starts to get messy.

Like I said initially, just use your ears.  They're better than you think.   

Dex

Test tones

Reply #2 – 2003-09-09 22:02:49

In foobar2000 you can open the "open location" doalog box and enter "tone://x,y" where x is frequency and y is duration (in seconds). Then burn that to CD for your car.

Test tones

Reply #3 – 2003-09-10 07:40:12

This page could be useful: How to compensate for suboptimal frequency response of your speaker using EQ.

The idea is to generate a tone for each of the bands your eq has, and each reproduced tone should have the same perceived volume.

But for public performance, you could use your meter and tune to the Fletcher Munson curves:
Here is a bigger image.

As you can see, tones at 3500hz should be the easiest to hear, but to hear a 20hz tone you would need more than 70db (assuming your equipment can properly reproduce the frequency). So below the 0 phons curve nothing can be heard. This content below the 0 phons curve is usually discarded by lossy encoders.

Because these curves represent the average person, but not necesarilly your hearing, you may better want to use the perceived volume method.

Here is a little program you can use to generate noise, including the test tones: Stomper Hyperion.

Note that lossy encoders base their psychoaccoustics on these curves, so its actually a good idea to perform this calibration, and its also why is bad to apply eq carelessly to lossy encoded content.

Test tones

Reply #4 – 2003-09-10 08:03:49

Yep, what he said.

You need a flat microphone,  (actually the right $2.50 Panasonic mic capsule would do)
a fairly decent sound card,  a copy of RMAA...  and away you go.

You send the RMAA test tones to a file (bet ya didn't know about that feature) burn them on a disk,  and use your flat microphone and sound card,  and starrt looking at your responses.

Know that the position of the mic in the car changes things,  as well as being too close to one speaker,  woofer,  tweeter,  sub,  etc...  so you'ld want to try to put it around where your head would normally be.  Some times a few inches makes a difference,  especially in the higher frequencies.  So take several runs,  in different positions,  and get an average.

Speaker builders do this kind of thing to determine crossover points and such,  do a little research on that and you'll find articles on building your own mic..  (eric wallin's page as an example)

It's fun to fool around with this kind of stuff,  if you're into that..  but like he said..  you can also do a pretty good job by listening to the system with several kinds of music and adjusting by hand..  but you don't get those really cool spectral graphs!!