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In Reply to: RE: Loudspeakers in rooms - not flat, but how bad ? posted by Dave_K on December 08, 2014 at 09:17:24
Averaging locations makes no sense. I don't run around the room 30 - 150 times per second to listen to my stereo. I generally stand, sit or lay down in one place, and THAT is the measurement location I'm interested in. Further, measuring a response in one room does not provide a person with the ability to apply that data to any other room. In short, in-room measurements tell us more about the room than about the loudspeaker, unless it's a time-gated measurement. Even then, in typically-sized rooms, gated measurements are only good down to a couple hundred Hertz or so, depending on the dimensions of the room.
Averaging multiple measurements in one location DOES make sense, due to variations in the output signal.
:)
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Averaging locations makes no sense. I don't run around the room 30 - 150 times per second to listen to my stereo. I generally stand, sit or lay down in one place, and THAT is the measurement location I'm interested in.
Stereophile's measurements are taken from the listening area, not all around the room. I don't remember how big of a listening area they try to cover, maybe a sofa's worth.
I have found that averaging multiple measurements taken from around the vicinity of the sweet spot produces a response that correlates better with what I hear than a single measurement. I usually confine the measurements to an area about 2 or 3 feet wide, 1 or 2 feet deep, and 1 foot high centered on my head position when seated in the sweet spot.
Single-point measurements are full of peaks and nulls and most of them are highly location specific and shift in frequency as you move your head around. Spatial averaging will tend to eliminate peaks and nulls that are highly location specific and tend to be inaudible, leaving the ones that are consistent across the area around the sweet spot which are typically audible.
Smoothing a single point measurement using a fixed-octave filter is effective at eliminating the high frequency comb filter artifacts, but isn't as effective at sorting out which peaks and nulls are most audible in the bass and lower midrange. Also, if you first average the measurements from multiple locations and then filter the averaged result, you can get by with a narrower filter bandwidth so you see more detail.
Further, measuring a response in one room does not provide a person with the ability to apply that data to any other room. In short, in-room measurements tell us more about the room than about the loudspeaker, unless it's a time-gated measurement.
I generally agree, but I still find it helpful when Stereophile compares the measurements from different speakers in the same room. Sometimes the comparison reveals something that wasn't obvious when looking at the pseudo-anechoic measurements. I also find that their in-room measurements help to shed light on the power response. For example, when there is a suckout in one axis accompanied by a flare at the same frequency range in another it's hard to tell what to expect at the listening position without an in-room measurement.
Room measurements can be averaged within a small area, and that will obviously yield a smoother curve for that area, which may or may not be adequate, depending upon the measurement goals. But to really understand a speaker's performance in a particular room, and the room itself, averaging - any amount of averaging - blurs essential details which may be needed in order to improve the room or the speaker positioning.
In a home playback environment, averaging the response over several feet, and producing a single "average" curve, may be fine. In a tracking/mixing/mastering control room, not so much. There, details matter.
:)
On the contrary, in a home environment it might be "good enough" to take a single measurement, but in a control room I think it's more important to make measurements over the full listening area. First, because performance is more critical. Second, because a working engineer is not stationary; they need to move around a bit especially in front of a big console.
Even if you lock your head in a vice at the sweet spot, your ears aren't in the same location. They are ~7" apart and if you place the microphone at the location of your two ear openings, there will be differences between the two responses which are visible on plots of amplitude and phase vs. frequency. These differences are meaningless. And in every system I've listened to, with the possible exception of extreme nearfield setups, I can move my head left and right a foot and the only significant changes in the sound are a shift in the center of the soundstage because you're not equidistant between the speakers, and subtle shifts in treble balance as the effective toe-in angle changes. The measured responses at the left-of-center and right-of-center positions will show changes in the pattern of peaks and nulls, but these differences aren't audible.
My point is that a single unsmoothed, unaveraged in-room measurement is dominated by features which are inaudible. These aren't details, they are 'noise' obscuring the things that are actually important. You want to suppress this 'noise' so that the 'signal' is more apparent. That requires smoothing the response with a filter, or averaging multiple measurements from different locations around the listening position, or a combination of both. Spatial averaging helps separate the response deviations that are really audible from those that aren't, and it also allows you to use less smoothing. A spatially averaged result with 1/12 octave or even 1/24 octave smoothing will reveal more meaningful detail than a single point measurement with 1/3 or 1/6 octave smoothing.
"On the contrary, in a home environment it might be "good enough" to take a single measurement, but in a control room I think it's more important to make measurements over the full listening area. First, because performance is more critical. Second, because a working engineer is not stationary; they need to move around a bit especially in front of a big console."
WTF? I already said that detailed measurements in a control room are essential, and that averaging multiple locations doesn't produce better data. Did you even read or understand what I wrote?
nt
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: 12/11/14
-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.
aneoichic measurments for speakers will tell you a lot about the speakers and is very useful.
but once in your room, if you have a bad room and you place stupidly your speakers, no matter how flat the speaker were, it wont be in your room.
therefore, its very important to place your speakers at a good place, ect.
bass response is what good placement and room treatment really help
Regarding the anechoic measurements, I was mainly trying to note that it can be difficult to infer the summed power response of the speaker from off-axis anechoic measurements. A case in point is the Zu speaker measured by Stereophile ( link to Zu measurements ). There is a massive -15 dB null in the on-axis response centered at 4.5 Khz, and a corresponding flare at the same frequency in the off-axis responses in the horizontal and vertical plane. It's really hard to guess how these opposing features will counteract in the summed power response, so having the two in-room measurements with commentary by JA is essential to understand how this speaker will sound in a real room and how it should be set up. Conversely, for Stereophile's measurements of the KEF R700 ( link to KEF measurements ), the anechoic measurements tell me everything I need to know.
terribly bad honestly.
with speaker placement, you can deal with this. but honetly, any speaker that Ive measured which were closer to 2 feet to the back wall had very bad peaks and dips in the bass region up to 300hz. up to the god damn middle C.
its so important to try to control the effect of the room.
I have measured all my speaker, and as soon as they are too close to the back wall, for get it, you get magnificient 15 db dips. its almost certain.
then, your listening position will also add to the party. when I move my mic, I see huge change. ideally, you must be far from the back wall also as each time I measured with the listening position to close to the back wall, dips and peaks appears automatically.
I sit 3.5 feet away from the back wall and my speakers are 2 feet away from the front wall.
I also have my room well treated (bass traps and first reflection poiints), which reduced the peaks and dips another 4-5 db. I now have +- 4-5 db from flat from 40hz to 250 hz, then 2-3 db flat from 250hz to 20khz.
without treatment and placement, you will easily have a +-10db to 15 db from flat from 40hz to 250hz.
its a very serious issue. very serious. no matter the speaker, you hear your room first and foremost.
in my experience, the rule seem to be that 15 db peaks and dips are certain if you dont place the speakers correctly.
if placed correctly, as you need to place your speaker in a place where the room mode are not proeminent.
the speaker placement is the most important as its the source and it triggers the room modes automatically. listening position is important but to a much lesser degree.when you take measurments, it will be evident. place your mic at your listening position, then move your speaker around, you will clearly see where your speaker response is less affected by the position. room modes works like that: if your room is 100 feet by 10 feet. if you place your speaker right in the middle, terrible response, if you place it at 2.5 feet away from the back wall (always measuring from the front baffle), terrible response. if you place it around 3.5 feet, it should be much better. as the room modes seem to act like in half measures: for a 10 feet by 10 feet room, if your listening position or your speaker placement is at the 2.5feet or 5 feet, you will get bad result. so, if you have a 12 by 12 room, dont place your speaker at the 3 feet or 6 feet position. that seems to be a crude but overall true and right generalisation to do. as everything, measurments is necessary as every room is different.
once you find the right speaker placement, you should be able to have no more 15 and more db peaks and dips, it should be around 10 db peaks. so thats much better. Its very evident. you move your speaker one feet at a time and you will see clear sometime troubling difference in the response. its very fun to do and easy to determine your room modes. Its a sure thing. I have been able every time to reduce those mad 15 to 20 db dips and peaks every time simply by room placement.
I guess theres room thhat could be very difficult, but in general;, its doable.
in my experience, speaker position is more important then listening position.
then, if you again place your listening position in a room mode free zone, you should again reduce the peaks and dips another 2-3-4 db. in my experience, this is as good as I ever been able to get without room treatment.
so you pass from 15 db dips to 10db with speaker placement, then from 10 db to at least a 7-8 db peaks and dips. MUCH better only with proper speaker placement and understanding of your room modes.
then, treatment helps to win the last 2-3 db.
4-5 db from flat in the bass region is okay and doesnt effect my pleasure or the clarity of the bass ime.
Edits: 12/09/14 12/09/14
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