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Comb filtering
Old 3 weeks ago
  #1
Gear Head
 
Mark Alpine's Avatar
Comb filtering

Hello,

I've tried to search for ways of mitigating comb filtering that occurs when sound waves bounce off a plane surface (mixing console or studio desk) and blends with the direct sound path hitting your ears.

Some suggest tilting the desk/console by raising the feet close to the front wall, and some suggest treating the desk using the mirror method (adding acoustic panels to the surface of interest). Some also suggest adding a shield of acoustic material to the front of the desk, blocking sound from hitting the desk altogether.

Isn't a much simpler way to lower the speakers (move away from the recommended ear height) but still have the acoustical axis point to your ears? Am I overlooking something here? Which method is commonly used and why?

/Thanks, Mark
Old 3 weeks ago
  #2
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Quote:
Originally Posted by Mark Alpine View Post
Isn't a much simpler way to lower the speakers (move away from the recommended ear height) but still have the acoustical axis point to your ears? Am I overlooking something here? Which method is commonly used and why?

/Thanks, Mark
You can certainly try it, measure (and listen) to the results and see how it works for you.

The one issue with speakers being either high or low is that the folds of the ear actually filter what you hear, and the resulting filter gives you the vertical spatial cue. Not ideal for mixing to have everything filtered. Typically what I've read is +/- 6 degrees should be the max for the angle of the speaker...

One thing to keep in mind is that you always need to measure your results. The wavefront of an acoustic wave expands per inverse square law. So a reflection point isn't reflecting like a laser, it is effectively a new source (like a 2nd speaker) and has a much wider dispersion then we tend to picture.

Also it's easy to overlook diffractions. This is when a sound wave will bend around the edge of an object (EG desk, monitor, etc) rather then reflect off of it. The bending path is of course longer then the direct as well. Even a delay of less than 1ms can cause significant comb filtering- so these are important as well.

So whatever you try, get comfortable with taking a lot of measurements and comparing the ETCs of them.
Old 3 weeks ago
  #3
Gear Head
 
Mark Alpine's Avatar
Quote:
Originally Posted by RyanC View Post
You can certainly try it, measure (and listen) to the results and see how it works for you.

The one issue with speakers being either high or low is that the folds of the ear actually filter what you hear, and the resulting filter gives you the vertical spatial cue. Not ideal for mixing to have everything filtered. Typically what I've read is +/- 6 degrees should be the max for the angle of the speaker...

One thing to keep in mind is that you always need to measure your results. The wavefront of an acoustic wave expands per inverse square law. So a reflection point isn't reflecting like a laser, it is effectively a new source (like a 2nd speaker) and has a much wider dispersion then we tend to picture.

Also it's easy to overlook diffractions. This is when a sound wave will bend around the edge of an object (EG desk, monitor, etc) rather then reflect off of it. The bending path is of course longer then the direct as well. Even a delay of less than 1ms can cause significant comb filtering- so these are important as well.

So whatever you try, get comfortable with taking a lot of measurements and comparing the ETCs of them.

I guess if you mount the LCD monitor/screen low as well you'll force your head down too. Maybe I'll develop neck problems over time...

Thanks for clarifying that. I did not think about diffraction... In order to push the frequency dip up (SBIR) I've planned to mount my near-field monitors very close to the front wall. They will be surrounded by acoustic treatment panels (DIY). For the framing of these panels I've chosen quite thick wood beams (34 mm, 1.3"). Picture attached. I wonder if diffraction will be an issue here... I wonder if I'm better off putting these panels on the side wall instead (RFZ) and make some new ones without the frame next to the monitors.

I'll do plenty of measurement once the studio build is done (try different absorption strategies).

/Cheers, Mark
Attached Thumbnails
Comb filtering-img_9049.jpg  
Old 3 weeks ago
  #4
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Jason Foi's Avatar
 

Its better to angle the work surface than the speakers. If you must angle the speakers, dont go more than like 6 degrees or you really shrink your sweet spot.
Old 3 weeks ago
  #5
Gear Head
 
Mark Alpine's Avatar
Quote:
Originally Posted by Jason Foi View Post
Its better to angle the work surface than the speakers. If you must angle the speakers, dont go more than like 6 degrees or you really shrink your sweet spot.
Can you please explain that in some more detail? If my head move with this new angle of the speakers, how will that affect the sweet spot?
Old 3 weeks ago
  #6
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Jason Foi's Avatar
 

A slight movement forward or backward will give larger FR changes with a vertical tilt. Lots of speakers dont have nearly as wide directivity vertically as they do horizontally. Read through the following threads or just google it for yourself and read up on sound reproduction system setup for critical listening environmemts.

http://www.johnlsayers.com/phpBB2/vi...hp?f=1&t=16419

https://www.google.com/url?q=http://...7vJ6tASq3vy1dy

https://johnlsayers.com/phpBB2/viewt...hp?f=1&t=20757
Old 3 weeks ago
  #7
Lives for gear
Quote:
Originally Posted by Mark Alpine View Post
Thanks for clarifying that. I did not think about diffraction... In order to push the frequency dip up (SBIR) I've planned to mount my near-field monitors very close to the front wall. They will be surrounded by acoustic treatment panels (DIY). For the framing of these panels I've chosen quite thick wood beams (34 mm, 1.3"). Picture attached. I wonder if diffraction will be an issue here... I wonder if I'm better off putting these panels on the side wall instead (RFZ) and make some new ones without the frame next to the monitors.
I think you should be OK in that size, but it would depend on the overall geometry of everything. Something that size would only diffract relatively high frequencies- my understanding is this is into quarter-wave territory so a wavelength of 136mm would diffract a tiny amount, and it would go up to the full amount at the wavelength of 34mm. This is pretty high frequencies. It's not likely that your speakers have very wide dispersion in this range, but that would depend on placement of everything.

Maybe someone with more knowledge on that can chime in to either correct me there or refine that!
Old 3 weeks ago
  #8
Gear Head
 
Mark Alpine's Avatar
Quote:
Originally Posted by Jason Foi View Post
A slight movement forward or backward will give larger FR changes with a vertical tilt. Lots of speakers dont have nearly as wide directivity vertically as they do horizontally. Read through the following threads or just google it for yourself and read up on sound reproduction system setup for critical listening environmemts.

http://www.johnlsayers.com/phpBB2/vi...hp?f=1&t=16419

https://www.google.com/url?q=http://...7vJ6tASq3vy1dy

https://johnlsayers.com/phpBB2/viewt...hp?f=1&t=20757
Yes I understand that speakers have different vertical and horizontal directivity. However, if you move the speakers down and at the same time tilt them AND your head you haven't really changed a thing (you and your ears are still pointing towards the speakers the same as before). Basically consider the speakers and head as a plane and rotate this plane down facing the front wall (rotation centred around your head).

In the first link, a guy asks about this but does not really get an answer.

Are there other effects at play here? Wall bounce, speaker to "front of desk edge" diffraction, or other artefacts since the speakers are now slightly pointing up toward the ceiling?

Edit: Ok I see your point now. With a tilt, since you are still bound to the chair/floor, any head movement forward/backward will move you out of the sweet spot (not only horizontally but vertically too).


/Thanks

Last edited by Mark Alpine; 3 weeks ago at 10:55 PM.. Reason: Got it, thanks!
Old 3 weeks ago
  #9
Lives for gear
Quote:
Originally Posted by Mark Alpine View Post
Edit: Ok I see your point now. With a tilt, since you are still bound to the chair/floor, any head movement forward/backward will move you out of the sweet spot (not only horizontally but vertically too).
This is another area where measuring is key. The two factors at play are how wide the dispersion is in the top octave or so, and the design of the crossover coupled with the spacing of the woofer to tweeter.

For the first, there are 15" woofers that can play very cleanly up to ~20k, but the problem with them is the treble coming out is like a laser, very narrow dispersion. So we crossover to a tweeter which has wide dispersion and then again narrows as it goes up in freq.

So you want to make sure that you have a relatively large area you can move forward and back in without loosing the top octave or so.

The other is crossover overlap region. Where you have a typical tweeter over woofer design, as you move off the vertical axis, the flight path from the woofer to the tweeter is no longer the same, eventually enough that what sums on axis, will null off axis. The greater the distance between woofer and tweeter relative to the wavelength of the crossover freq, the more off-axis lobing there is.

If the woofer and tweeter center to center location is 1/4 the wavelength of the crossover frequency or less there is no off axis lobing, this is rare as it takes a tweeter that can play very low. But concentric speakers don't have this problem.

With most modern tweeter over woofer speakers the problem is also mitigated with steeper crossover slopes. This creates a very small crossover region and widens the angle of the lobe free window at the cost of having deeper nulls that are higher in q as you eventually move far enough off axis.

IME, this is a lot less likely to be a major factor in the real world as long as you aren't moving more than +/-10 degrees or so, but that is very dependant on your speakers.
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