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In Reply to: RE: Exactly what I did posted by Ugly on May 24, 2007 at 16:47:58
Hi.
Never use pure sinewaves of single frequencies to test indoor acoustics as the sine waves will be intermodulated by its own reflection from the interior surfaces to provide erroneous readings.
For accurate indoor measurement of acoustical frequency response, always use pink noise (scienficially, 1/f noise, which gets equal energy per octave - volume decreass logarithmically with frequency). Similar to the perception sensivity curve of our auditory system.
Research shows almost all musical melodies tend toward a pink noise spectrum. Pink noise is a standard test signal for audio engineering, commonly used in audio spectrum analysers & equalizers.
c-J
Follow Ups:
Huh? A sine wave run through my stereo will certainly create sound, I am guessing this would be true of the OP's system as well. Since that is true the SPL can be measured. If I happen to choose a frequency my system responds to and this frequency doesn't happen to cancel where I happen to be listening from as a result of room acoustics this test sound can tend to be quite loud.
Are you implying that I won't get a response from my SPL meter if I only use sine waves. Kinda funny if so.
Hi.
Of course, you esound coming from your speakers, reproduced music signals (a mixture of many pure sinewaves from your programme sources) or pure sine waves from an audio signal generator as you suggested.
But we are talking about acoutical measurement, whereby pure audio sine wave signals should be avoided.
I say it again: pure sine waves will generate standing waves inside your listening area, due to constructive interference of the reflected waves from the wall/ceiling superimposing on the incident waves. This is very undesirable even for normal listening , less alone SPL measurements.
That's why a mixture of waves should be used instead of pure audio sine waves. Pink noise is commonly used for such audio test applications.
Do you have a clue what I am talking about?
c-J
Of course
Don't forget: I never said don't use some other signal or that a sine wave is the only perfect choice. I only suggested a sine wave might be a better choice than some piece of music. It is still a true statement. The only reason I am even bothering challenge what you said at all is it's my belief that your statement is going overboard quite a bit. For a quick and dirty SPL test sine signals are perfectly adequate except in a small but statistically unlikely given sample set of random choices of arbitrarily chosen test signal frequencies. In other words you have to choose poorly but the good news is that is unlikely to happen in most circumstances. Consider also that in the case of some user with only a set of analog sources these fancier noise signals you speak of may be a lot more difficult to come by than sine waves.
"Do you have a clue what I am talking about?"
I understand that you are apparently assuming quite a bit about me and the OP and then using that stretch to try and bust my balls for something here.
Obviously the assumption you are making is that there will be either additive or destructive interference of the given test signal at all infinity test signal frequency possibilities at the measurment location and if some poor soul makes the grave error of choosing a sine wave as a test signal to measure SPL it will automatically yield completely bogus SPL measurement results. I just say that bogus results aren't very likely in most systems and this infinitely bad room you speak of is not even physically possible in the first place and therefore frequencies exist which would be considered good test signals. In my room and system especially after EQ you'd have trouble even finding a really bad test frequency signal to choose from since it has been a goal to trend my response toward flat. In other words just about any frequency you choose at my house would be as information rich,within a small margin of error of course, as any complex waveform to a fairly simple piece of measurement equipment like a SPL meter. For a quick and dirty measuement like those done via SPL meter this margin of error is going to be nearly completely negligible, again unless you're unlucky in your choice.
If for example one intelligently chose a sinewave frequency which is not a problem for this room/system and listening/measuring position and one merely cares about SPL's capabilities at a given volume knob position as would necissarily be the case if using a SPL meter since that is the output of that type of device and which by the way is what the OP was talking about, there is no more amplitude information available in pink noise, white noise or anything else over a pure sine wave.
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