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Discussion Starter #1
I have a 13' x 29' room with a sub in the front corner and bass traps in almost all corners (RealTraps). As you can see, I still have deep troughs around 73Hz and 146Hz (wide). I can't measure any bass build-up in the room with a meter so I can't see the point of getting more traps. That probably leaves getting an equalizer or moving the sub and measure again. I have learned that changing the sub cross-over and volume has no effect on the trough frequencies and intensities (no surprise).

Would moving the sub (or the listening position) be the next step before buying more stuff?
 

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Move the sub first.
 

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Discussion Starter #3
I should have pointed out that the sub cross-over is 32 Hz. This brings up the question of which trough is the fundamental. The 73 Hz matches the width of the room (13ft) and it is doubled at 146 Hz.

It appears to me that the 73/146 set is stronger. Does that imply that the resonance is created by the mains rather than the sub?
 

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That then appears to point to the seating position and/or the distance of the speakers from the wall beside them and behind them

Bryan
 

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I should have pointed out that the sub cross-over is 32 Hz. ...
Yes it certainly suggests that your issues are with your mains. You probably want to take separate measurements of each. It could be that the wide dip around 140Hz is a combination of different problems with each front speaker.

In my room I was able to move my mains far enough from the wall that I had a dip a little below 70Hz, rather than a little above, and I then set the sub crossover to 80Hz so it covered up the hole in the mains.

Good luck,
Bill
 

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Greetings,

May I inquire as to why you have your sub crossed over at such a low frequency? There are very valid reasons why we use 80 Hz generally as a crossover, and the fact that the mains may be able to go that low is not one of them. :T

Also, placing your subwoofer in the corner is perhaps the worst thing you can do. It excites all modes evenly which is an uphill battle. There are four elements to sound quality management, and I have yet to run into a room that did not need some form of all four: Speaker placement, seating placement, treatment and electronic correction (equalization). Good luck!
 

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Discussion Starter #7
Thanks Bill, I think you may have it. The speakers are planar and it may be that the one that is in front of the corner with the sub needs to be moved a little. I may need to move the sub out of that corner and replace it with a bass trap on the floor tri-corner. Off to experiment! This is fun stuff.
 

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I agree. But, at 73 Hz, your mains are still going to contribute energy even with an 80Hz crossover, so scan the two mains separately so we can start to determine where the problem is coming from, before we go about recommending solutions. Also, a sketch of the room would be nice.
 

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Discussion Starter #9
Sure enough, the two mains are quite different and the biggest problem is the Left which is in front of equipment and a corner. The Right is in front of a opening in the room. For this graph the sub is off. The speakers are Magnapan planars (1.6R) so they are radiating front and back. In the (poor) photo you can see the two black panels (speakers), the four thin white panels (high freq. traps) and four thicker white panels (bass traps - RealTraps). The sub is in the front right corner.
 

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Wow. That left speaker is nasty from 140-175 Hz. Any chance that speaker is 2ft from the front wall?
 

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Well, this one might be a bit over my head, better left for Brian or Shawn, but I might try moving the trapping around to fill the gaps in that corner, and/or looking for a sdie/ceiling reflection that would cause a 3.4-3.9ft path difference...
 

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Wow. That left speaker is nasty from 140-175 Hz. Any chance that speaker is 2ft from the front wall?
No kidding!
Left Main is 43" from back, 38" from side. Right speaker the same.
I'm thinking along the same lines you are, Greg. If we take those as perpendicular distances to the walls, then the distance from the speaker to the corner is 4.78'. So the predicted cancellation waves would be 59.1Hz and 177.3Hz. One could match the data in the graph if the actual distances to the driver were slightly higher, i.e., if the given distances are from the corners and not from the center of the speakers. Maybe the wide dip in the 140-165 range comes from the nature of the magnaplanars, if the position of the driver is effectively dispersed.

Bill
 

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Until you either kill that corner or get the rear wave of the speaker not to fire into the corner, you're going to have those anomolies.

Also, remember that while doing individual channels is a good thing to identify issues, the only thing that matters really is both channels together - unless you listen to your speakers one at a time ;)

Bryan
 

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Discussion Starter #15
I'm thinking along the same lines you are, Greg. If we take those as perpendicular distances to the walls, then the distance from the speaker to the corner is 4.78'. So the predicted cancellation waves would be 59.1Hz and 177.3Hz. One could match the data in the graph if the actual distances to the driver were slightly higher, i.e., if the given distances are from the corners and not from the center of the speakers. Maybe the wide dip in the 140-165 range comes from the nature of the magnaplanars, if the position of the driver is effectively dispersed.

Bill
How do you get 59.1 and 177.3? I get 205Hz from 4.78ft. I am sure you are right but how? And yes the distances are perpendicular. I have taken a better photo. The result of moving the square bass trap in the tri-corner to mid-way up the wall is zero. I would have expected a poorer result. Since I have both tri-corners trapped and only the middle not covered, should the troughs be that bad?

Also when I scan the room with an SPL meter using bass frequency pink noise I don't find any other source of excess bass except in that corner (+10dB).

I guess my overall question is why if I have mostly "killed" the corner is the response so bad? Really unfortunate speaker placement?
 

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How do you get 59.1 and 177.3? I get 205Hz from 4.78ft. I am sure you are right but how? And yes the distances are perpendicular. I have taken a better photo. The result of moving the square bass trap in the tri-corner to mid-way up the wall is zero. I would have expected a poorer result. Since I have both tri-corners trapped and only the middle not covered, should the troughs be that bad?

Also when I scan the room with an SPL meter using bass frequency pink noise I don't find any other source of excess bass except in that corner (+10dB).

I guess my overall question is why if I have mostly "killed" the corner is the response so bad? Really unfortunate speaker placement?
Using the freq/distance calculator from RealTraps, it gives 4.78' as being 1/4 wavelength at 59.1Hz and 3/4 wavelength at 177.3Hz. So, if one were sitting straight out from the corner, one would have a 1/2 wave or 3/2 wave reflection at those frequencies. And being an odd half wavelength, the reflections would be exactly complementary. Of course, the analysis is a little trickier to calculate reflections off the other surfaces.

Of course, this analysis is typical of "normal" speakers, with a driver facing one way, treating the driver as a point source and the wave into the corner starting in phase with the wave out from the speaker. Magnaplanars could be different, if they are driving the air in both directions, and the sound toward the corner starts out 180 degrees out of phase. I had not considered that.

I'm sure Bryan can give more advice about traps and treatment. I've not yet gone there, but I have played a lot with placement. One can spend hours moving one speaker or the other, in one dimension or in the perpendicular dimension, to see how curves change. As Bryan suggests, one cannot forget to look at them together as well. If an anomaly appears only with the speakers together, you know it is an interference of one against the other, and is probably very location and frequency specific. But except for localized signals that would be in one channel and not the other, evenness in one speaker can cover a dip in the other.

Bill
 

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Discussion Starter #17
Thanks Bill. I am a strong advocate of statistically designed experiments (which show interactions between variables) so I appreciate the two speaker interaction.

What I am hoping is that someone here with experience in trapping and placement will give me a direction to start in based on my spectra. Changing traps that are hung on the ceiling is not trivial (and a two person job) and moving speakers is pretty tedious if done correctly.

Another question I have is whether bass traps are over-hyped. I assumed that putting traps in all the corners should have pretty well stifled that problem but look at that huge hole in my bass. All I get from RealTraps is "more traps will solve it." I can only see one space left and moving a trap in-and-out had no effect whatsoever.

An alternative is equalization (thus the thread) but I hate to potentially add expensive distortion so it is my last resort besides a new room(!).
 

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Discussion Starter #18
I can't find a place in the room or hallway that registers more than 70dB* (70dB reference at the listening position) except for 73-74 dB in the Left corner where there is no mid-wall trap. So how is energy building up to create a large trough in my room response at 150-175Hz?

Now nothing makes sense. Help please.

* low-frequency pink noise CD (RealTraps)
 

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... So how is energy building up to create a large trough in my room response at 150-175Hz? ...
You are obviously looking at the question totally different than I was.

I was suggesting that you are seeing a reflection out of the corner 180 degrees out of phase with the sound from the speaker in that range, but lower in amplitude because of absorption and dispersion over the increased distance. So this complementary wave would subtract at that range over much of the area radiating out from that corner. As it is only over a limited range, I'm not sure whether that would show up on a broad SPL measure.

When you are talking about energy buildup, you seem to be thinking of a modal frequency, that might have peaks in the corners and troughs somewhere else in the room.

If you are dealing with a simple reflection, moving the left front speaker, particular in toward or away from the corner, should change the frequency of the trough. Changing your seating position should have little effect.

If you are dealing with a modal issue, moving the speaker small distances in the same area might change the amplitude, but not the frequency. As well, changing your seating should have a significant effect on amplitude.

The traps may be having a significant effect for you. I don't know. But I would expect to see that effect in the delays in waterfall charts. That is where you would expect to see the benefits of not letting modal energy "build up", i.e., persist.

At least that's my understanding. But I am relatively new at this.

Have fun experimenting,
Bill
 

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Discussion Starter #20
I obviously understand acoustics only enough to be dangerous. My understanding is that modes are set up at multiples of room dimensions and one can "trap" them where they reflect in corners. Cancellations can be eliminated by not letting them build up. That is why RealTraps' Ethan Winer offers the pink noise file - so you can find the worst corners and put traps there. This makes sense to me although it may be wrong. I didn't find any high intensities in any corners without traps.

So it seems that I might have a node at 73Hz (doubled at 146Hz) and at 80Hz (doubled at 160Hz). My room is about 13ft wide so these two nodes could be standing waves between the side walls. My solution was to put traps in all the available corners hoping the standing waves would be reduced. It obviously isn't working.

It would seem to me that this trapping idea should work even though I am throwing in- and out-of-phase waves from the front and back of my speakers, respectively. It doesn't matter what the phase is if it is absorbed effectively.

I suppose the next thing is to measure different listening positions and measure different speaker placements and see if I can find nodes at different frequencies. I really don't like having a broad 20db hole in my mid-bass!

(Incidentally, the high frequency traps work beautifully. There are just no echoes in the room from first reflections.)
 
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