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"Weirdness" at 100Hz - caused by distortion?
Old 1 week ago
  #1
Gear Addict
 

"Weirdness" at 100Hz - caused by distortion?

Hi folks. It’s a while since I’ve been here! Nice to see some familiar names still on the forum. I hope you’re all coping with the Covid situation and keeping in good health.

So… having finished my small home studio about a year ago, I decided to unpack my measurement mic the other day and take a few test readings of the room to check that the acoustics are the same as when I last measured. No reason why they wouldn’t be of course, and as you’d expect the measurements all check out as unchanged.

One thing that stood out - and has always stood out - is some “weird goings on” (technical term!) around 100Hz. You can see it in just about all of the measurements:
  • Slight dip in frequency response
  • Big spike on the group delay curve
  • Big hump on the delay plot (up to 60ms, after which it drops off rapidly)
  • Dip in the RT60 measurement
  • Strange, closely packed ‘contours’ on the waterfall
  • Weird little ‘notch’ on the spectrogram

…the cause of which, I’m guessing, can be seen on the distortion plot which shows a bump at 100Hz.

Question: am I correct in this assumption - i.e. that the pattern of issues seen throughout my measurements around 100Hz are likely caused by speaker distortion?

Thanks for any input

Link to REW file (.mdat) for anyone interested:
https://www.dropbox.com/s/cdah76m9qd...ents.mdat?dl=0

Frequency response
Slight dip at 100Hz...
"Weirdness" at 100Hz - caused by distortion?-freq-response.jpg

Waterfall
Closely packed 'contours' around 100Hz...
"Weirdness" at 100Hz - caused by distortion?-waterfall.jpg

Spectrogram
Weird little 'notch' at 100Hz...
"Weirdness" at 100Hz - caused by distortion?-spectrogram.jpg

RT60
Slight dip at 100Hz...
"Weirdness" at 100Hz - caused by distortion?-rt60.jpg

Group Delay
Big spike at 100Hz...
"Weirdness" at 100Hz - caused by distortion?-group-delay.jpg

Delay Plot
'Bloom' on the decay plot at 100Hz, which drops off rapidly after 60ms...
"Weirdness" at 100Hz - caused by distortion?-decay.jpg

Distortion
The cause? Bump in distortion around 100Hz...

Note that the distortion plot below is for both L&R speakers running together, but both speakers show a peak in distortion around 100Hz (albeit slightly more pronounced in the left speaker). To my ears, the effect of this distortion isn’t noticeable, but then my ears aren’t the most finely tuned

Speakers are a pair of Unity Audio The Rock MkII. They crossover with my sub around 80Hz.

"Weirdness" at 100Hz - caused by distortion?-distortion.jpg
Attached Thumbnails
"Weirdness" at 100Hz - caused by distortion?-decay.jpg   "Weirdness" at 100Hz - caused by distortion?-distortion.jpg   "Weirdness" at 100Hz - caused by distortion?-freq-response.jpg   "Weirdness" at 100Hz - caused by distortion?-group-delay.jpg   "Weirdness" at 100Hz - caused by distortion?-rt60.jpg  

"Weirdness" at 100Hz - caused by distortion?-spectrogram.jpg   "Weirdness" at 100Hz - caused by distortion?-waterfall.jpg  
Old 1 week ago
  #2
Lives for gear
 

i assume measurements show results including the sub - how do things look like @ 100hz without sub(s)?

what sub are you using, what x-overs settings (slope, frequency, symmetrical etc.), what position, distance to walls and tops, what alignment/filters etc.?

i vaguely remember discussing these things earlier on...
Old 1 week ago
  #3
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akebrake's Avatar
 

Quote:
Originally Posted by Skol303 View Post
One thing that stood out - and has always stood out - is some “weird goings on” (technical term!) around 100Hz. You can see it in just about all of the measurements:
Hello Skol!

Is it possible your sub is placed in mode 3.0.0 null and also suffers from the front wall SBIR? This is R spkr w Sub from your uploaded Drpbx link

Best
Attached Thumbnails
"Weirdness" at 100Hz - caused by distortion?-r-w-sub.jpg  
Old 1 week ago
  #4
Quote:
Originally Posted by akebrake View Post
Hello Skol!

Is it possible your sub is placed in mode 3.0.0 null and also suffers from the front wall SBIR? This is R spkr w Sub from your uploaded Drpbx link

Best
Just to add :

1) The attached L Filtered ETC at 100hz shows a peak before zero which is normally distortion and then the 100hz peak is delayed. Are the subs delayed at all?

2) Positive peaks you don't see in the minimum group delay but in the excess window are normally some kind of SBIR.

So Akebrake might be correct on that one.
Attached Thumbnails
"Weirdness" at 100Hz - caused by distortion?-screen-shot-2020-06-24-4.19.01-pm.jpg   "Weirdness" at 100Hz - caused by distortion?-screen-shot-2020-06-24-4.29.19-pm.png  
Old 1 week ago
  #5
Gear Addict
 

Quote:
Originally Posted by deedeeyeah View Post
i assume measurements show results including the sub - how do things look like @ 100hz without sub(s)?

what sub are you using, what x-overs settings (slope, frequency, symmetrical etc.), what position, distance to walls and tops, what alignment/filters etc.?

i vaguely remember discussing these things earlier on...
Hi deedee, nice to hear from you! Yeah, you helped me a lot with my sub set up in the past - thanks again for that - so don’t worry, there’s no need for you to go over that again here in this thread

To clarify: yes, all of these measurements were taken with the sub running in addition to the left/right speakers. Your question on that prompted me to find some older measurements, where I had the sub and (importantly) my DSP box switched off. Here’s what I noticed…

With the sub and DSP off there’s much less distortion at 100Hz and it seems very likely that the DSP is causing the distortion there (probably trying too hard to fill in a null at that frequency). There’s also much less decay around 100Hz - no ‘bloom’ on the decay plot or weird, tightly packed ‘contours’ on the waterfall.

The same effects are still present in other measurements, however:
  • Group delay still shows a big spike at 100Hz
  • RT60 still shows a dip at that frequency (which seems odd…)
  • Spectrogram still show at little ‘notch’ at 100Hz, same as the image above

So it seems the speaker distortion, caused by the DSP, is impacting on the decay around 100Hz. But perhaps some other factor is causing the other ‘little oddities’ at that same frequency.

That’s actually gone a long way to answering my initial question… so thanks for your input, which set me thinking along the right track

Quote:
Originally Posted by akebrake View Post
Hello Skol!

Is it possible your sub is placed in mode 3.0.0 null and also suffers from the front wall SBIR? This is R spkr w Sub from your uploaded Drpbx link

Best
Ake! Great to see you too. Glad you’re still here and rockin’ the wavelets

Nice suggestion re. the sub being in a null. Here’s a wavelet from an older measurement - same right-side speaker with sub and DSP switched off.

A very similar result. That's not to say it isn't SBIR causing the issues at 100Hz, just that if it is, then it's affecting my main monitors also.

"Weirdness" at 100Hz - caused by distortion?-right-speaker.jpg

Quote:
Originally Posted by thethrillfactor View Post
Just to add :

1) The attached L Filtered ETC at 100hz shows a peak before zero which is normally distortion and then the 100hz peak is delayed. Are the subs delayed at all?

2) Positive peaks you don't see in the minimum group delay but in the excess window are normally some kind of SBIR.

So Akebrake might be correct on that one.
Thanks for the input, appreciated.

No delay on the sub (at least not intentionally dialed-in to my DSP).
Attached Thumbnails
"Weirdness" at 100Hz - caused by distortion?-right-speaker.jpg  
Old 1 week ago
  #6
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akebrake's Avatar
 

Quote:
Originally Posted by Skol303 View Post
...Ake! Great to see you too. Glad you’re still here and rockin’ the wavelets
My pleasure

Unfortunately the impulse from your latest uploaded (Dropbox) files looks strange.(pic1)
Distortion may be?

Check if a lower send-level (while measuring) might improve it.
Clocking error? Try a 256k sweep in stead of 1M

Best

BTW
RE Modes and Room dimensions.
I did a sketch of your room some time ago. Note: Narrow room. (pic 2)
(Only metric)
Attached Thumbnails
"Weirdness" at 100Hz - caused by distortion?-r-sub-imp-zoom.jpg   "Weirdness" at 100Hz - caused by distortion?-skol-plan-elev.jpg  
Old 1 week ago
  #7
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yep, still around (albeit a lot less active in this forum for various reasons)...

anyway, looking at the sketches akebrake did (what a nice chap!) and assuming that they largely reflect your setup, i'm convinced that
- the positions of your top speakers are not ideal
- their current positions are part of the issues you experience
- we should beat you up for pretending you already found optimum position...
- you can get better results by also adjusting external x-overs

(setup/alignment is the area i keep focusing on and in which i have considerable knowledge while other folks are much better in room properties, strategy to optimized in terms of absorption, construction, rew-analysis etc.)


so, here's my point:

since you seem to have an issue at 100hz, i suggest that you move your speakers apart until the center of the lf woofers are exactly at the distance of half a wavelength of the frequency in question, meaning 170cm in your case; the idea is to get max. phase offset between the two at 100hz...

i'm quite serious about that! give it a try - if it doesn't yield any advances, i'll owe you a beer and will shut up!

the other thing i recommend is that (although i vaguely remember that you're not fond of running the x-overs too high - correct me if i'm wrong) you do exactly this:
run them higher than your problem frequency and let the sub(s) even out room issues! don't let the tops add to the issue by running the tops down into the problem area...

finally, i also remember that you setteled for a sub position which i thought was questionable (sorry again if i'm insinuating things which you have long addressed): imo there are about three (to four) position in most any rectangular room which i consider worth trying and i'm not sure you have checked them all out...
Old 1 week ago
  #8
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Fay Smearing's Avatar
 

Quote:
Originally Posted by Skol303 View Post

With the sub and DSP off there’s much less distortion at 100Hz and it seems very likely that the DSP is causing the distortion there (probably trying too hard to fill in a null at that frequency). There’s also much less decay around 100Hz - no ‘bloom’ on the decay plot or weird, tightly packed ‘contours’ on the waterfall.

The same effects are still present in other measurements, however:
  • Group delay still shows a big spike at 100Hz
  • RT60 still shows a dip at that frequency (which seems odd…)
  • Spectrogram still show at little ‘notch’ at 100Hz, same as the image above

So it seems the speaker distortion, caused by the DSP, is impacting on the decay around 100Hz. But perhaps some other factor is causing the other ‘little oddities’ at that same frequency.
You have EQ boost there trying to fix a null? If so, how much? That a possible candidate for some phase issues, and it's in the crossing-over range.

Is it being applied to the mains and sub, or just mains? Because if possible, you might get a better result from taking it off the mains and filling the dip with the requisite boost there to just the sub.

Also, does the crossover being at "about 80" mean it's 80 or not. Some time alignment issues between mains and sub in the crossover region might confound the troubleshooting somewhat, and looking at an older thread, you say the crossover is at 100, which would narrow the search. A couple of milliseconds delay, with any EQ applied there removed, to either mains or sub (depending on whichever is running late) could solve that, were it the case.
Old 1 week ago
  #9
Gear Addict
 

Thanks for the input everyone, really appreciated. I think some of you know my room even better than I do

I also feel like I've made you revisit some issues we've previously discussed with this room - so sorry about that. I should've been clearer from the start and made more effort to check back through old threads before starting this one. My bad.

So to clarify some things:

ETC plot: Ake, you’re absolutely right about the clocking error! I’d forgotten that I have a mis-match in clock rate between my measurement mic and audio interface Here's an older shot of the ETC plot taken at 256k, which looks a bit more 'normal'.



Sub and speaker position: Deedee, thanks for your advice here. Unfortunately, I've run out of options for repositioning my main speakers and sub (see Sketchups below). My room is relatively small/narrow and I'm restricted by the location of corner treatment, desk, etc. I have tried to rearrange my speakers/sub in the past, but the results have always been worse. So my set up ended up being the 'best compromise', if not the ideal solution.




DSP and sub crossover: thanks for the input here Fay. I use Dirac Live as my DSP, which uses an algorithm to EQ the speaker response in relation to the room (amongst other things). I use the 2-channel version of the software, so it treats my mains and sub together as one system (yeah I know... not ideal). The software is 'trained' to avoid over-boosting SBIR nulls, but my guess is that it's trying too hard at 100Hz, resulting in the distortion plot above. The software allows the EQ curve to be fully adjusted, so I might try reducing it a little around 100Hz and see if that helps at all.

Oh yeah and my crossover is at 100Hz, not 80 as I'd guessed (again, you all seem to know my room better than I do!). I previously rented a speaker delay unit and experimented with a whole range of delay settings on the sub - using wavelets and group delay as a guide - but was unable to find any worthwhile improvements.

TL/DR: I think my room is as about as good as it can get - i.e. bearing in mind the limitations of space that I have to work with; and without ripping everything out and completely re-building from scratch (which I'm not prepared to do). I've seen better, and a lot worse. So I'll just have to live with it

Back to my original question to finish... "is the pattern of issues seen throughout my measurements around 100Hz likely caused by speaker distortion?" I think you're all absolutely right, that the issues around 100Hz are caused by SBIR - and the speaker distortion is a result of my DSP trying hard to "fix" the SBIR null, resulting in an EQ boost at that frequency, and hence the 'bump' in the distortion. So the distortion is a by-product and not the cause of the issues seen in my measurements around 100Hz.

I can't easily fix this issue, but I think I understand it better now. So thanks all for your help with that
Attached Thumbnails
"Weirdness" at 100Hz - caused by distortion?-256k.jpg   "Weirdness" at 100Hz - caused by distortion?-listening-position.jpg   "Weirdness" at 100Hz - caused by distortion?-plan-view.jpg  
Old 6 days ago
  #10
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Quote:
Originally Posted by Skol303 View Post
(...) Unfortunately, I've run out of options for repositioning my main speakers and sub (see Sketchups below). My room is relatively small/narrow and I'm restricted by the location of corner treatment, desk, etc. I have tried to rearrange my speakers/sub in the past, but the results have always been worse. So my set up ended up being the 'best compromise', if not the ideal solution (...)
your case could be used for an interesting study: under which conditions do measures that are actually meant to improve the acoustic behaviour of a room become obstacles at the same time?!

others may disagree (and for good reason/backed up by good arguments) but imo you approached the problem from the wrong side:

unless we're talking about a design which starts from the ground, optimum setup of playback system comes first, acoustic treatment comes second.

of course they depend on each other but one can learn to deal with subpar treatment while no amount of treatment or dsp can cure subpar positioning...
Old 5 days ago
  #11
Gear Addict
 

Thanks Deedee, I totally agree! Positioning/placement should come first, followed by acoustic treatment.

That said, I think for a lot of small home studios, the options for placement are often limited by factors such as size/shape of room; the necessity of furniture; space occupied by acoustic treatment, etc. I think for small rooms, it’s always a matter of compromise.

In my own room, I can’t position the main speakers any wider without removing the front wall corner treatment. Would widening the position of my speakers produce a better result than having the deep corner treatment? I’d have to measure and find out... but based on my (albeit limited!) experience, I’m not sure it would.

I had this same issue when trying different positions for my sub. I moved the sub within the maximum area possible (limited by my room furniture): an area of about 2m x 1m. Tried various speaker delay settings too. None of the results justified a major re-think of my set up. I think the close proximity of modal peaks/nulls in a small room such as mine makes these adjustments very difficult/limiting, especially when you factor in practicalities such as furniture. Or maybe I’m just making excuses!...

Ideas I have saved for future consideration include:

1) Soffit-mounting my main speakers in the front corners.

2) Replacing my large desk (ie. the table, not my modest size mixer!) with a smaller one, allowing more options for sub placement and also mixing position.

...but those ideas can wait for another day.

In the meantime - and despite all of these compromises - I’m actually very happy with my room measurements and importantly, how the room sounds. The frequency response and decay times are better than a lot of similar-sized rooms I see on here; better than some rooms that claim to be professional studios, even. And whilst acoustics should never be a “dick swinging contest”, I like to think that my room can swing with the best of them. At least for its small size
Old 5 days ago
  #12
Lives for gear
 

could well be that you've approached the threshold of diminishing returns...

...although a smaller desk, an addtional sub or then an in-wall design would certainly have an impact!

___


back to the sub: i'm convinced there's a better position!

from the sketch, it looks like the table is a large enough boundary to mess with its radiation pattern, bouncing back some energy and make the waves recombine with the direct sound in bad ways (meaning out of phase)...

i'd make sure that except for the floor, there is no other hard surface closer than ca. 1m of the sub for unobstructed radiation behaviour or then (and with a single sub), the woofer goes as close as possible to two or better three boundaries for maximum spl and becoming better at even out sbir - none of these essential criteria* are matched with your current setup...


* there are a few more criteria but they don't come (much) into play with your setup
Old 5 days ago
  #13
Gear Addict
 

Quote:
Originally Posted by deedeeyeah View Post
i'd make sure that except for the floor, there is no other hard surface closer than ca. 1m of the sub for unobstructed radiation behaviour...
Thanks again Dee. Unfortunately that’s not possible with my current table/desk... and not helped because the upright sides of the desk are solid (not 4 legs but solid like the table top). So there are even more boundaries to mess with the sub’s radiation pattern as you say.

Not something I can fix without replacing the desk. But it’s on my list of options to explore when I decide to downsize it
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