Hunting down HF reflections - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 14 Old 05-02-17, 06:11 PM Thread Starter
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Hunting down HF reflections

Hi all,

So I recently moved my system into a new/bigger room, and am in the process of treating it to improve the sound (2 channel music specifically). Basically, at high volumes the Polk A7s make my ears bleed -- these are fairly detailed monitors, but shouldn't be that bad at reference levels. I believe I'm getting reflections from the big side window and back wall that are creating this effect once I turn it up, as the other side side of the room opens to a kitchen. To note, the Polks sound great at moderate volume, and the B&K amp is warmer than average. Anyway, my plan is to get some thicker curtains for the big window (Absolute Zero Velvet Blackout panels OTW), and to put some acoustic panels on the back wall (I have 3 2'x4'x2" panels built). I also built a 2'x4'x6" panel for what it's worth (leftovers), which is currently sitting in the corner next to the sub, probably doing nothing.

So, while I'm treating the room I'd like to take proper measurements (I have a UMIK-1 mic) to see how the panels/curtains are taming the HF reflections. Questions:

---To do this, I assume the best approach is to use the UMIK-1 at 90 (pointed up) at the LP and take individual left/right speaker measurements, full range from 15hz-20khz with the sub on?

---Would taking horizontal measurements (pointing between the speakers) be better for any reason?

---If I meet REW's minimum channel level requirement, am I ok to test at that volume or do I need to crank it up to reference? I have a baby so that would limit my testing window...

---As I identify reflections in the ETC graph, how do I pinpoint where they're coming from? I read there's a calculation I can do, but I'm unable to create a loopback through the laptop->HDMI->AVR, so am I just guessing timing on that?

---For reflections I find, is there a way to determine which ones are primarily HF?

Thanks in advance and let me know if you need any additional information. I'll try to circle back with my room/LP measurements tonight (I'm at work). Basically it's a 14x18' room, facing the short wall (I have no choice), the left side opens to a kitchen, the right side is a big front window. The speakers are ~1.5' out from the wall spaced ~7' apart, and the LP is ~8' from each speaker and ~4.5' from the back wall.
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post #2 of 14 Old 05-02-17, 08:59 PM
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Re: Hunting down HF reflections

Welcome to the Forum Jon!

Quote:
Jon Ramsey wrote: View Post
Basically, at high volumes the Polk A7s make my ears bleed -- these are fairly detailed monitors, but shouldn't be that bad at reference levels. I believe I'm getting reflections from the big side window and back wall that are creating this effect once I turn it up, as the other side side of the room opens to a kitchen.
Unless your room is really “live,” what you primarily hear is the direct signal from the speaker, not the reflections. If the speakers “hurt” at higher volumes, I’m going to hazard a guess there is a peak in frequency response in the 2-4 kHz area.

I believe REW has a gating feature for frequency response measurements that will shut down the test signal before reflected signals can arrive. I’ve never used it myself, perhaps JohnM can clarify. If that’s the case, it would confirm my theory (or not).

In any event, even if gating isn’t possible, you could still get a meaningful frequency response measurement the normal way, which includes room reflections. Room surfaces are merely going to reflect what the speaker gives them. They aren’t going to amplify any particular part of the frequency spectrum, at least not in the range we’re talking about. So whatever it is causing the “bleeding” we may well be able to see regardless.


Quote:
So, while I'm treating the room I'd like to take proper measurements (I have a UMIK-1 mic) to see how the panels/curtains are taming the HF reflections. Questions:

---To do this, I assume the best approach is to use the UMIK-1 at 90 (pointed up) at the LP and take individual left/right speaker measurements, full range from 15hz-20khz with the sub on?

---Would taking horizontal measurements (pointing between the speakers) be better for any reason?
The most reliable method for taking frequency response measurements is with the mic aimed at the speaker (measure one at a time). For acoustics measurements (such as ETC) aim the mic at the ceiling. The irritation you’re hearing is in the upper frequencies, so no reason to include the sub in the measurement, other than “FYI.”


Quote:
---If I meet REW's minimum channel level requirement, am I ok to test at that volume or do I need to crank it up to reference? I have a baby so that would limit my testing window...
Measuring at 75 dB or thereabouts is perfectly fine.

Can’t help you with the questions on acoustics measurements – not in my wheelhouse. Hopefully JohnM will weigh in.

Regards,
Wayne



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post #3 of 14 Old 05-03-17, 09:44 AM
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Re: Hunting down HF reflections

Quote:
Jon Ramsey wrote: View Post
Basically, at high volumes the Polk A7s make my ears bleed -- these are fairly detailed monitors, but shouldn't be that bad at reference levels. I believe I'm getting reflections from the big side window and back wall that are creating this effect once I turn it up, as the other side side of the room opens to a kitchen. To note, the Polks sound great at moderate volume, and the B&K amp is warmer than average. Anyway, my plan is to get some thicker curtains for the big window (Absolute Zero Velvet Blackout panels OTW), and to put some acoustic panels on the back wall (I have 3 2'x4'x2" panels built). I also built a 2'x4'x6" panel for what it's worth (leftovers), which is currently sitting in the corner next to the sub, probably doing nothing.
Could it be that the offensive sound is due to amplifier clipping rather than runaway reflections? At those levels, you may just be experiencing distortion at the onset of clipping, causing a "hardening" of the sound. Another culprit could be listening/speaker positions. Extreme toe-in emphasizes high frequencies, as you're sitting on-axis. Ear height can also play a role, as can source material. Finally, the speakers themselves are known to have a forward presentation (Ref: Hometheaterreview.com). I'm not personally familiar with the A7's, but "forward" and "in-your-face" become synonymous with "sounds-bad-loud" (IMO).

Quote:
Jon Ramsey wrote: View Post
---As I identify reflections in the ETC graph, how do I pinpoint where they're coming from? I read there's a calculation I can do, but I'm unable to create a loopback through the laptop->HDMI->AVR, so am I just guessing timing on that?
Try this step-by-step procedure on pg. 82 of this REW Guide

.
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post #4 of 14 Old 05-03-17, 01:17 PM
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Re: Hunting down HF reflections

Can you post a frequency response graph with 1/3-octave smoothing? That will give us a better idea of what you’re actually hearing.

Regards,
Wayne



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post #5 of 14 Old 05-03-17, 02:50 PM Thread Starter
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Re: Hunting down HF reflections

Hi guys -- I've tried to share graphs twice now. Says they need mod approval (I assume because of the attachments). Guess I need to wait?
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post #6 of 14 Old 05-03-17, 04:02 PM Thread Starter
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Re: Hunting down HF reflections

Looks like it's working now! Regarding the suggestions -- I doubt it's clipping/distortion because the EX-442 is 200wpc (of quality power) and they're crossed over at 60-80hz. I can share my toe-in angles, but they're not that extreme (pointing at shoulders, essentially)...

Thanks both for the responses/suggestions. I'll try the gating and/or string test this week.

In the meantime, below are graphs. I'm thinking my room needs a lot of work... and maybe I can EQ some of this, though Audyssey is already supposed to be rolling off HF...

Direct speaker measurements (horizontal/direct angle, both speakers Large):
Hunting down HF reflections-direct-speaker-measures.jpg

ETC measurements, showing the reflections the acoustic panels impacted:
Hunting down HF reflections-panels-off.jpg

90 degree ETC measurements (both speakers at Large):
Hunting down HF reflections-90-both-speakers-etc.jpg

90 degree SPL measurements (both speakers Large):
Hunting down HF reflections-90-full-speakers-spl.jpg

*Sorry for the double/triple posts! As mentioned they weren't showing up...*
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post #7 of 14 Old 05-04-17, 07:07 AM
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Re: Hunting down HF reflections

After you complete applying Acoustic treatments in the room, you can try equalization of the high frequencies with a roll-off of 6 db/oct starting from about 1.5Khz.
See my current SPL curve below that achievs excellent natural sound with out to mach High Frequencies.Hunting down HF reflections-spl-sub.jpg
Let me know if it helped.
Regards,
Issi Geier
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post #8 of 14 Old 05-04-17, 09:34 AM
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Re: Hunting down HF reflections

Quote:
Jon Ramsey wrote: View Post

Direct speaker measurements (horizontal/direct angle, both speakers Large):
Attachment 141890
You appear to have a bit of peaking around 8k. This will make Sss sounds sibilant and can result in "harshness" at higher volumes.
Not sure of what to make of dip in XO region. A search of that particular model Polk found some folks complaining about "brightness".
You may need a treble control ;-)

cheers

AJ

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post #9 of 14 Old 05-04-17, 11:59 AM Thread Starter
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Re: Hunting down HF reflections

Thanks, all. I don't think any additional acoustic treatments (other than the drapes that are coming today) will pass the WAF in the short-term (I had to bribe her for the back panels), so EQ may be my only option.

I'll take additional measurements with Audyssey off tonight, as that's essentially what I'll be starting from (well, Audyssey flat technically) if I try to manually EQ the high end. Who knows, maybe Audyssey is creating a bad curve for music? I guess I should have taken those direct measurements with in "direct."
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post #10 of 14 Old 05-04-17, 12:45 PM
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Re: Hunting down HF reflections

Quote:
Jon Ramsey wrote: View Post
Quote:
Wayne A. Pflughaupt wrote: View Post
Can you post a frequency response graph with 1/3-octave smoothing? That will give us a better idea of what you’re actually hearing.

Regards,
Wayne
In the meantime, let me know if these graphs provide enough info.

That’s not 1/3-octave smoothing. It’s the same as in the previous post.

Regards,
Wayne



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