I actually found some posts from John that may have cleared up my confusion. It sounds that using the RTA function versus the Spectrum function is most effective with Pink PN not requiring any averages.
Always go with what John says over anyone else's advice. He is The Man!
Periodic PN gives the steady result that needs no averaging. It is peak limited (the crest factor (peak to average ratio) is limited),
and... it REPEATS!:grin2:
Just to clarify
a little, using RTA function will give a horizontal measurement plot with pink noise or with periodic pink noise, and yields the same amount of detail
per frequency band, i.e.
per octave, or
per smoothing band. I prefer periodic pink noise because of its "steady" nature for readings, especially SPL readings with the smaller block size. The SPL reading will hardly vary, if at all. Whatever settings you use, the block size in the generator and in the analyzer must be the same, or you can get insane results. With REW (later versions), the FFT block size in the generator automatically tracks the setting you have entered in the analyzer.
The spectrum analyzer mode will generally give a downward-tilted plot toward higher frequencies, depending on the source signal. It is not used nearly as much in audio, in my experience. Others may disagree. RTA with pink noise and selected smoothing and the desired number of averages gives tremendous flexibility.
There is no
right, only what is
most useful. Use 1-oct smoothing to see the over freq balance, 1/48-oct to see the fine detail that is "hidden" by the smoothing. 12th-oct or 1/6th-oct is a good setting for seeing what most listeners are likely to be able to hear. For automatic EQing, it is usually wise to use more smoothing, 1/6-oct, maybe even 1/3-oct, so you don't get a crazy number of filter values and/or a sterile sound (be careful not to over-EQ), or VAR smoothing, which gives more detail at LF, where modal EQing prevails, and less at HF, where broader EQing of
trends is more common. The newer Psychoacoustic and ERB smoothing modes are useful in getting more detail of what the ear actually perceives. Both put more emphasis on peaks and less on dips in the response, as the ear does. I find myself using the Psychoacoustic most of the time any more.
Averages (0,2,4,8...) allow you to quiet down all the motion in the plot, or see the raw analysis detail with full motion, all depending on what you are looking for. As with any kind of averaging, detail is lost. But the brain sees the result more easily. Periodic PN helps with this too.
Just move the mic around very slowly to get the averages around the headspace.
The moving mic method (MMM) is a useful way to get a picture of the whole headspace in one plot. Try this: plug two mouse receivers into your PC, use one mouse for regular navigation, the other for a remote. Put a piece of black tape over the laser of the "remote" mouse, and hold it in one hand while holding the mic in the other. Now you can start/stop the analysis when you want to. Set averaging to "forever," hold the mic at the starting point, click the left mouse button on your "remote," move the mic around slowly and randomly for 30 seconds, and
Voila! you have an acoustically well-averaged plot of the FR of your headspace!
I've seen some big differences and some not so big differences when people measured both ways.
Probably due to user error, i.e., amount of MMM movement, randomness, speed, etc. It is not a perfect method, there are ways to mess it up. But it is very useful and pretty easy to master.
Ah, I just got what you meant. It is probable there will be significant difference between a MMM plot and any other single-position or single-measurement method. The spacial averaging of the MMM will de-emphasize - as the ears and brain tend to do automatically - the modal variations at low frequencies, which slow up big time in a single-measurement plot.
I've always used the uncorrelated pink noise and tuned for a nice downward curve and tuned by ear after.
"tuned for a nice downward curve" - sounds like a combination of personal preference, tweeter evasion, and "too much HF stuff bouncing around."
I'm interesting in trying the Pink PN with the RTA Function. I see he mentions to use the same FFT for the RTA and the Pink PN.
Do or die. Actually, it will hurt no one, but the results will not make sense if you do not match the block sizes. As mentioned, this is automatic in later versions, the generator setting will automatically track the analyzer setting (but not the other way around).
65536 FFT with a 1/48 smoothing seems to be the recommendation. Although, I like the idea of trying it at 1/12 and going from there.
see suggestions above
FFT block size, while the math is beyond most of us (except John), is gives another way of getting more detail at the sacrifice of time response. More averaging is a result of the analysis, so the averaging setting is usually left at 0 with longer FFT block sizes.
Bigger block size > More LF detail and slower response
Now, with uncorrelated pink noise you get a nice downward curve - not flat. With using the Pink PN, would I see the same trend?
A "nice downward curve" is a subjective choice, probably fairly common in car work because of the constraints of car acoustics, and because a lot of car audio people like bass response that measures on the Richter scale:surprise:. I am not sure why that would automatically be the result with the conditions you mention. Uncorrelated pink noise is a necessity for FR measurements with multiple channels at once, vs channel balancing with correlated noise.
Periodic PN gives a nice, steady result because of its repetitive nature and low crest factor (as previously mentioned). It is VERY convenient to work with.
It should give a similar result and be easier to read.
Will I see the same 10db filter as with the uncorrelated? Thanks!
Not sure I understand this question.
By the way, welcome!
onder:
