# Seeking diagnosis for rising near-field response



## tloredo (Nov 3, 2006)

Hi folks-

I'm hoping some of you acoustics experts might be able to help me diagnosis my near-field monitoring issues. I was motivated to invest in a measurement mic to take these measurements because my monitors sound brighter and harsher than I expect given their reputation as smooth and reasonably accurate budget monitors.

For monitoring computer-based audio, I have a pair of NHT SuperOne Xu's beside my video monitor. I'm facing a corner of a room that's about 11.6'W 14'L 7.25'H; wallboard walls, acoustic tile ceiling, 1 narrow window on a long wall, and doors on 3 walls to other rooms (usually open). The floor is hardwood, but mostly covered with an area rug and shelves and studio furniture. The right monitor is about 7" from the wall; the left one about 11". The depth of the triangular area bounded by the corner and the monitors is about 26" (corner to base of triangle). I suppose a drawing would have been better; sorry! The SuperOne crossover is at 2.2 kHz.

I'm taking measurements with an ECM8000 calibrated by our friends at Cross-Spectrum, with Room EQ Wizard 5b (thanks, John M!). Here is a measurement at my usual screen-facing listening position, with the speakers facing straight ahead from their rubber stands on the desk (tweeters about 4" below ear level):









Here's another measurement with the rubber stands reversed, so the monitors can be tilted up to aim at ear level at the same listening position:









The main difference between the two curves is above 8-9 kHz; evidently the tweeter is rather directional at those high frequencies.

I see two main issues with the response:

* There is a whopping low-f resonance at around 150 Hz, which was no surprise.

* There is a remarkably linear (in log-log space) rising response from 400 Hz to 13 kHz.

There is also a big dip at around 90 Hz, but I'm less concerned with that because it is very position-dependent. If I measure from my "slouch position" (a few inches further back and about 4" down, so at tweeter level when the monitors aren't tilted), the dip almost completely fills up. But the other features are substantially unchanged.

I suppose the low-f resonance might be tamed by putting bass traps along the corner; I intend to try that.

What is more puzzling for me is understanding the cause (and remedy) for the steady rise with frequency, which evidently is behind the brightness/harshness I've experienced. It's not the monitors. They have a decent reputation for accuracy even in the pro audio world. Also, I took one of them and put it in place of my home stereo L speaker in the adjacent (larger) living room. Measuring from a listening position on the couch against the opposite wall (11' from the speaker, at tweeter level) gives this response:









This one actually _falls_ somewhat with rising frequency (above 1 kHz), more so than the inexpensive Optimus speakers I usually use with that stereo (those are going soon!). So I don't think the rising response has to do with the SuperOne's.

Any insight into general causes for such rising response curves, and possible remedies, would be greatly appreciated!

Cheers,
Tom


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## bpape (Sep 14, 2006)

Listening farfield in the other room is accounting for some of the difference. I'm assuming the other room is larger which would allow the air (yes, air is a HF absorber), furniture, etc. to tame things a bit.

As you noticed, the tweeters seem to be very very directional. The first thing I would try honestly is to not aim them right at you. Try aiming them at a spot about 2' behind your head - maybe more.

Another thing that's likely happening is that the closeness to the boundaries is causing a lot of low frequency cancellations which gives the impression of the rising top end which is born out in your graphs.

Sitting facing an untreated corner is also not helping anything. If you have to face the corner, then I'd certainly want some broadband bass control in it.

Bryan


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## DanTheMan (Oct 12, 2009)

Can you do a near field polar response at say 3ft and gate the measurement at 3ms or 4ms--or before the impulse shows a boundary reflection?

Dan


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## tonyvdb (Sep 5, 2007)

Some speakers do not sound well if toed in, try simply firing them straight out into the room.


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## tloredo (Nov 3, 2006)

Thanks for the quick and helpful input so far! I appreciate all ideas.

Bryan, thanks for pointing out the possibility that the hi-f rise may really just be lots of low and mid-f cancellation. I know I need some bass treatment, so it looks like I should go ahead with that, and perhaps it will address the apparent hi-f issues indirectly.

Dan, I don't know what you mean by polar response; do you mean measure at several points around a 3' circle? I measured at tweeter height at the mid-plane, 3' back, and let REW do it's impulse response FFTs with the default window (-125 + 500 ms), and with a (-4, +4 ms) window:









The short window corresponds to the green curve (obviously!). This seems to me to indicate that reflections from surfaces beyond a few feet are not playing a role in the rising response.

Tony, regarding the toe-in, they were only slightly toed-in for the "no-tilt" plot (but more so for the tilted plot). NHT recommends a slight toe-in for the SuperOnes. But I tried firing them straight, and measured from listening position (about 4" above tweeter level), and loe and behold:









The general rise is now largely gone, but there is now quite a bit more raggedness in the curve above 1 kHz. It doesn't go away if I window to +- 4ms; presumably I have some earlier reflections to tame. Now I'm going to do some subjective listening like this. I would not have guessed the slight toe-in I had (aiming a coupe feet behind my head) would have had this strong of a broad-band effect. Thanks for that suggestion!

-Tom


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## tonyvdb (Sep 5, 2007)

tloredo said:


> Tony, regarding the toe-in, they were only slightly toed-in for the "no-tilt" plot (but more so for the tilted plot). NHT recommends a slight toe-in for the SuperOnes. But I tried firing them straight, and measured from listening position (about 4" above tweeter level), and loe and behold:
> 
> The general rise is now largely gone, but there is now quite a bit more raggedness in the curve above 1 kHz. It doesn't go away if I window to +- 4ms; presumably I have some earlier reflections to tame. Now I'm going to do some subjective listening like this. I would not have guessed the slight toe-in I had (aiming a coupe feet behind my head) would have had this strong of a broad-band effect. Thanks for that suggestion!
> 
> -Tom


Thats good to hear that some of the issue was solved, What mic are you using with REW? if its anything but the Behringer ECM8000 the readings above 5K wont be very accurate. I suspect the "raggedness" is more just the result of reflections off walls and can be tamed with some well placed acoustic panels.


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## bpape (Sep 14, 2006)

That could be part of it along with just the tweeter not being terribly linear off axis (which I suspect is why they recommend a little toe in. Almost all tweeters become more non-linear off axis. The trick is finding that magic spot where you can balance the general rising tendency and the non-linearity.

Bryan


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## tloredo (Nov 3, 2006)

Thanks again for the continued help!

Tony, I'm using an ECM8000 calibrated by Cross-Spectrum. I just got it this week, and I've loaded the calibration file into REW. So presumably everything measured up to about 20 kHz is "real," though the mic does have a big bump spanning 7-15 kHz (at which point the response tails off). If you have any specific recommendations for (affordable and not-too-ugly!) acoustic panel good for this kind of reflection taming, I'm all ears!

Bryan, when you note that many tweeters are not linear off-axis, are you referring to their frequency response, or nonlinear distortion? If the former, is that a property of many tweeters per se, or does it have more to do with their interaction with the cabinet (e.g., presence/absence of waveguides, edges)? Just curious; I don't have much background yet with speaker design.

Part of my pursuing these measurements is to judge whether I should be investing in better monitors as well as room treatment, so that's partly why I'd like to understand how much of this may be an issue with the SuperOnes.

-Tom


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## bpape (Sep 14, 2006)

Actually, it's a bit of both. Excellent off axis power response is difficult to achieve. 

Bryan


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## tloredo (Nov 3, 2006)

Hi again-

A few more measurements....

Following up on the suggestion to look at polar response, motivated by the effect of the toe-in, I measured the left (L) speaker only, on-axis, and then at 10, 20, and 30 deg in, at 28". 28" at 30 deg is about at my listening position (if I move up about 4" above the tweeter). I also repeated the 30 deg measurement, moving up those 4" (it was not dramatically different from tweeter-level, though some hi-f dips & peaks swapped places). Here are some of the curves (in the legend, B=blue, Y=yellow(ish!), P=purple):









Interestingly, the rising response is present at all angles. Then I switched on the right (R) speaker, and the flattening became apparent (green=L @ 30 deg, tweeter-level (4" below P curve above); red=same, L+R):









(The dip just above 3 kHz makes sense; in this geometry L is at about 28" and R at about 30", and the difference is about half a wavelength at 3+ kHz.)

So evidently the rising trend is mostly due to some features of the room & placement, and adding L+R by luck appears to somewhat ameliorate those effects, in part due to the way the polar responses of L+R interact. Maybe!

-Tom


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## bpape (Sep 14, 2006)

Just for giggles, swap the left and right speaker and see if the dip around 3k stays on that side or with the speaker. That's a little high but in the general area of the xover. If the drivers in that speaker are wired out of phase to the xover, that would account for the dip. Not likely but possible.

Bryan


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## DanTheMan (Oct 12, 2009)

For my polar plots I usually do about 3ft(building a better measurement rig today so this may change) from the point midway between the woofer and tweeter and rotate the speaker by 11.25 degrees and measure from on axis to 90 degrees. I also like to do it with the speaker on its side to see what's going on with the vertical lobe. Check my blog http://dtmblabber.blogspot.com/ or here: http://www.hometheatershack.com/for...o-monitors-2030-others-active-passive-13.html
For examples. I set the gate for wherever the impulse tells me it can go. If the woofer is operating in its break up region, this will be hard to determine. Here's an overlay of 9 impulses taken from a polar response:









You can see the floor reflection at about 4ms. That means I have to close the window at 3ms to keep that from contaminating the measurement. This will limit your resolution to about 250Hz, but it's enough to let you know what's going on. I personally start my gate at 1ms because for some reason it gives me higher resolution that way.

Dan


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## tloredo (Nov 3, 2006)

Dan,

Thanks for the further tips re: polar response. I looked at your examples---the B1030A response was remarkably flat. What kind of room and treatment was that measurement in? Where were the speakers placed?

-Tom


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## tloredo (Nov 3, 2006)

Bryan,

Clever idea, but the dip behavior is unaffected by swapping speakers, and it fills in if I move the mic to be equally spaced from the pair, so my earlier explanation that it was due to interference appears to be right. I'll keep that trick in mind for checking future speakers, though!

-Tom


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## DanTheMan (Oct 12, 2009)

The beauty of gating is that you don't need room treatments. You knock out the room reflections with your gate. To make the measurements I place all the furniture against the walls and place the speaker and mic near the middle of the room. Imagine a giant oval stretching across the room catacorner across the room. From the left front to the right rear like a giant football. Place the speaker and the mic each in an imaginary foci. The speaker is always on a stand. This will cause the first reflection to be the floor. The taller you make the stand, the better. Well, to a point I suppose. Until the ceiling becomes the first reflection. Place the stand on a board with a circle marked in whatever increments you want to take the measurements in. Then just take a measurement, rotate, and repeat.

Dan


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## tloredo (Nov 3, 2006)

Thanks for the measurement tips. I was hoping what you were measuring is what you were hearing in actual use, in which case you must have found pretty magical treatment and/or placement! Wishful thinking, I realize....

-Tom


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## DanTheMan (Oct 12, 2009)

Yes, those graphs would be an insane room response we could only have in an anechoic chamber--not a place you'd likely want to listen. That said, there's a lot of evidence to suggest that the polar response does correlate with what we hear above the modal region. That's certainly not to say that room treatments are silly above that.

Dan


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## Wayne A. Pflughaupt (Apr 13, 2006)

tloredo said:


> For monitoring computer-based audio, I have a pair of NHT SuperOne Xu's beside my video monitor.
> 
> I see two main issues with the response:
> 
> ...


 It’s perfectly natural for speakers to sound brighter when you’re sitting close to them, and that’s why you’re getting the near field measurements you are for the NHTs. 

I found a quote from NHT on another Forum describing the SuperOne XUs, and nothing in the quote indicated that they were intended for near field use. That’s what the rising-response problem is, and that’s why they measure great at a normal listening position (the third graph in your opening post).

Speakers designed for near-field will have the highs attenuated so that response is balanced from a distance of a few feet. They also don’t use tweeters that are highly directional.

Regards,
Wayne


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## lcaillo (May 2, 2006)

Those were my thoughts, exactly, Wayne. I just got around to reading this thread. The old Rogers LS35a comes to mind as a great monitoring speaker that had nice near-field response and superb dispersion.


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