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Discussion Starter #1
Long time lurker. My HT is finally functional so it's time to post.

2.5 of the 4 eventual bass traps are in (superchunks in front left/right, 2'x3'x5'tall rectangular cotton stack near right rear), 1 and 2" wall treatments are not yet up.

16' wide x 24' long room, front 3' is behind an acoustically transparent SMX screen.

Two HSU vtf3-mk3 subs, one center front (behind screen), the other center back. The rear sub is 14" above main floor level on a riser, the front sub is 10" above main floor level on the stage. Both subs have one port plugged, no turbo extension.

I've got a tower PC in the room, using the built in sound card (realtek) for measurements for now. Radio shack analog meter (maybe 8 years old, new battery). I downloaded and applied the SPL meter calibration file, and I calibrated the sound card (right channel out to right channel in). I now have the right channel out connected to the multi-channel subwoofer pre-in of my Pioneer Elite vsx01-txh. The right channel input is connected to the SPL on a tripod near the right ear position in the primary listening chair.

No BFD yet. When I hooked things up to start, I did run the Pioneer auto-MCACC.

For now, I'm just trying to get a feel for where I'm at, how to measure, and how to interpret.

Graphs, Front sub only:


Rear sub only:


Both subs, set out of phase (rear 0, front 180):


Both subs, set in phase (both 0):


I'm attracted to the in-phase behavior below 50Hz. Nice and smooth and easy to correct. However the dips in the upper 50s and at 85 seem like they would be tough to deal with. The out of phase behavior avoids all the extremely sharp spikes and dips, at the expense of being generally more noisy.


My next steps:
1) don't fret it to much until I have at least the rest of the bass traps in (2'x2'x4' above the doorway and a big pile of cotton behind the rack in the right rear). Hope the traps or some minor sub movement tame the dips in the in-phase option.

2) Try to remove the receiver auto-equalization (since I have no idea what it is doing).

3) Run through both L/R receiver input rather than the sub in to see how it looks with the front mains in as well.

Any thoughts? If I'm posting insufficient info, please let me know. I'll post more as I go.

Paul Meyer
 
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Very cool, my first direct evidence that bass traps help. (I know, it's 'common knowledge', but seeing it in my own measurements is different.)

I have a bunch of dead space in the right rear corner that is behind acoustically transparent frames. This is essentially dead space behind/around my rack. I have my current right rear bass trap close to but not quite in that corner. I planned to just fill in the corner with cotton insulation as I have room. This extra insulation was not in during my first measurements. For the of it, I rolled the cotton insulation roll in and squeezed it back there, essentially filling that low corner with a floor to ceiling 3' x 3' loose cotton bass trap.

It helped measurably! Pre and post insulation measurements, both subs, in phase:



Green is pre bass-trap addition, purple is post. The magnitude is different as I had adjusted the volume between measurements (a couple of hours apart). The mic hadn't moved.

The 85 Hz dip is pretty much unchanged, but the 55 Hz dip is widened, and is about 10 db less deep! I'll keep plugging away at room treatments before worrying about eq too much.

Is it fair to say that my 1" (side walls) and 2" panels (all of front wall, all of rear wall) aren't going to affect bass much? My plan is to get my bass traps in and reassess at that point, but the wall panels will take a bit longer.
 

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Is it fair to say that my 1" (side walls) and 2" panels (all of front wall, all of rear wall) aren't going to affect bass much? My plan is to get my bass traps in and reassess at that point, but the wall panels will take a bit longer.
You're correct, I recall that 1"-2" panels help with high frequencies most of the time, to dela with lower frequencies you need to use at least 6" :yes:
 

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Welcome to the Forum, Paul!

In your first group of graphs, did you calibrate for 75 dB? The overall levels are really low…

While bass traps can reduce the spread between the peaks and valleys, their main benefit is to reduce the time it takes the low frequency signals to fade away (aka “ringing”). The audible benefit is that the bass sounds “tighter.”

So, you might want to generate some waterfall graphs for your “before and after” readings. Just click the “Waterfall” tab above the graph. Try looking at a relatively short 300 ms window, and also a longer 600 ms window.

Regards,
Wayne
 
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I remember the 'calibrate your SPL' step, where it asked me to enter exactly what my SPL read. That box started at 75, and I think my reading was within 3db or so of that. I did not adjust volume at that point to bring the SPL reading to 75db, I just entered what the SPL read. I actually thought the SPL read slightly higher (77?).

On the second graph, I also calibrated the sound card, but the reading was different. I'll take better notes next run...

Mostly I was looking for the relative shape, and wasn't too worried about the absolute levels at this point. My assumption was that the frequency and relative magnitude of spikes/dips wouldn't vary much with absolute volume.

How do I calibrate for 75db?
- Adjust the AVR volume during the calibrating the SPL step?
- Adjusting to some level during the 'checking levels step'?
- Running test measurements and adjusting volume to get the overall 'average' around 75db?

I've read enough of these threads to be embarrassed with the newbie questions, so I apologize in advance...

Paul
 

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How do I calibrate for 75db?
- Adjust the AVR volume during the calibrating the SPL step?
Yeah, it's set during the Calibrate Levels routine (Settings icon, Soundcard tab) before taking a reading. The SPL figure is entered after that (Mic/Meter tab).

The reason I ask is, most of your graphs look kind of “backwards,” with levels hot in the upper range and reduced in the lower range. That’s a bit unusual, especially with subs as capable as your Hsus. I expect it may be the result of less-than-optimal placement. You might want to experiment with other locations. I tried various locations in a room with shoe-box dimensions a number of years ago, and IIR, center-of-wall placement got me some of the worst readings.

Also note that you’re not getting any appreciable gain adding the second sub.

Regards,
Wayne
 
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Discussion Starter #8
I'll calibrate to 75db for future runs.

I went with front/back center for the subs based on two things: 1) the advice of my acoustic treatment plan designer Bryan Pape, and 2) the Harmon Kardon white paper on best placement of multiple subwoofers. The HD paper was optimizing for even bass distribution across the seating area, as opposed to flat distribution for a single listening position, if I remember correctly. I believe their assumption was that if you got the same bass across the seating (and hopefully avoided extreme slopes) EQ could take care of the rest.

My flexibility on sub placement is somewhat limited. I can shift each sub left-right up to maybe 5 ft without major problems, and I could bring the front sub out from behind the screen, but I would really rather not if I can avoid it. I'll try shifting the subs around only once I've finished my first pass treatments, however. I'm taking a slow, long term approach to this. No hurry. I've been working on this for two years, I've learned patience...

There is plenty of headroom left on the subs. They are currently cranked way down (maybe 20% on the volume knob) because my Pioneer Elite vsx01-thx wouldn't allow its auto-cal to proceed with the volume turned any higher. Again, as I figure out how to use my new receiver I should be able to pick and chose the parts of the auto-cal I want to use and remove this limitation.

As for bass traps, my understanding (limited to reading 'Master Handbook of Acoustics' and lots of forum dabbling) was that well positioned thick bass traps were helpful in lessening/damping standing waves, and therefore were useful for 'softening' positional peaks/nulls. Not arguing with the 'tight bass' advantage, that makes sense. Since I haven't tried measuring multiple seating positions yet, I can't tell if my worst dips in my graphs were positional or not.

For now, I've got a few parallel tracks going:

1) try to dig up my SB Live card so I can get off of the motherboard audio. I'm getting a hum from the MB soundcard that I think I didn't have with the SB card.

2) finish my last bass trap (over the door, so a bit of a pain)

3) get the AVR out of the equation...

That'll give me a better baseline to start with. I'll also watch for good prices on a BFD.

Do people with two subs use one BFD channel for each sub?
 

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Do people with two subs use one BFD channel for each sub?
Usually one channel is fine when the subs are co-located, but for multi-position it would be neccessary to use both channels to allow different filters.

brucek
 
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Usually one channel is fine when the subs are co-located, but for multi-position it would be neccessary to use both channels to allow different filters.

brucek
Makes sense. I can see this is going to be a very complex optimization! Each sub can be shifted, each sub can get different filters. Ideally, I could just figure the dips/peaks of each sub and then filter them to sort of add up to flat. Unfortunately, I expect the two subs together will not add in that nice linear fashion.

I foresee a lot of playing around to get a feel for how the subs add/average/interfere with each other before I get a feel for how to optimize this... I'll have to figure out a way to organize my measurements and take good notes.
 

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I expect the two subs together will not add in that nice linear fashion
That's an understatement for sure. :)

No matter what the white papers say, it's a lot harder to get two subs in different locations to behave. Then, once you have them co-operating, you add the mains and it's time to start all over. Co-locating is really the easiest way to use two subs...

brucek
 
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