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In Reply to: RE: Averaging of multiple locations posted by Dave_K on December 11, 2014 at 08:57:35
Good points on measuring -But two things were and (still might be) limitations:
-Swept tones and pink noise generate "reflections" and (were) corrupting the data. Does this still happen ?
-Testers were using 1/3 octave smoothing - which was low-resolution. Are they doing higher-rez now ?
Edits: 12/11/14Follow Ups:
-Swept tones and pink noise generate "reflections" and (were) corrupting the data. Does this still happen?
If you're trying to measure and optimize in-room response, the reflections ARE the data. If you're trying to measure the speaker response alone, e.g. if you're designing speakers, then you don't want the reflections.
Anyway, using pink noise and a real-time analyzer to make frequency response measurements is not a good way to go. What you want to do is measure the impulse response from a full range sine sweep or maximum length sequence (MLS). From the impulse response, you can derive lots of things including magnitude and phase response vs. frequency, RT60, waterfall plots, spectrograms, etc. This is how most room and speaker measurement software works (e.g. REW, R+D, XTZ, OmniMic).
If you have the impulse response, you can time gate (truncate) it to remove some or all reflections. However, gating limits the lowest frequency, the resolution, and signal-to-noise ratio of the response. If you want to remove ALL reflections, including floor bounce, then you need a very short gate, typically <= 1msec for measurements at the listening position, which limits the response to 1 KHz and above. Or you could choose a gate length that includes the first arrival + early reflections, but excludes reverberation & modal ringing. That would likely yield a response from ~150 Hz and above.
But if you want to look at the bass region, then you can't use time gated measurements. There's no way to separate the direct & reflected sound in the bass because the wavelengths are long. Nor would you want to, because you don't hear them separately.
So the usefulness of time gating depends on what you are making the measurements for. If you're doing it to optimize the location of speakers and/or the listening position, you usually shouldn't use any time gating. An exception might be if you're only adjusting toe-in, but I generally do that by ear. On the other hand, if you're making measurements to generate EQ settings for the midrange or treble, or to optimize crossovers, then you absolutely should time gate.
-Testers were using 1/3 octave smoothing - which was low-resolution. Are they doing higher-rez now ?
Any halfway decent software will have options for smoothing, ranging from no smoothing at all up to 1/1. I usually prefer no smoothing when looking at data below 100 Hz. When looking at full range frequency response plots, I use 1/6 octave for single measurements and 1/12 octave for spatially averaged measurements.
Thanks for this -I guess Stereophile's published data - if high-resolution, is not so bad after all.
My gripe (now) would be that no magazine indicates *internal* problems of loudspeakers - like voice-coil inductance (motor distortion) and then, system noise below signal (in db, like 30-40 which many are).
Probably too time consuming - but the noise (I would think) they can do...
Edits: 12/12/14 12/12/14 12/12/14
And yes, it's common to see THD+N only 30-40 dB down, sometimes as little as 20 dB down in the bass.
The measurement I would like to see is dynamic compression.
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