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  #1  
Old April 20th, 2015, 6:50 PM
rman2222 rman2222 is offline
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Question Low at 40-50hz. Vtf15h mk2

hi,
Am in the process of setting up two VTF15h mk2s. The only practical placement is in the front corners of a sealed room 14.5 by 16.5 by 8.5 approximately with an alcove about 3.5 ft deep on about half of the back wall (under steps)
Matched the subs to each other and balanced to rest of the speakers using the receiver test tones, using 1 port open, eq1 and q .7. However when using the test cd and the frequency tones it shows very weak in the 40-50hz range (down 6-8db) while strong (matching mains) above and below, all the way down to 16hz. Phase is zero for both and that shows as correct via meter.
Any suggestions for getting a more balanced response?
They look great btw!

Thanks in advance
Joe H
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  #2  
Old April 20th, 2015, 7:44 PM
shadyJ shadyJ is offline
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That looks like it would be a room null. The only way to cure it is either to move the subwoofer to a different position (which might only move the null to another frequency) or move the listening position (risking the same consequences as moving the sub) or just getting another sub to shore up the null frequencies.
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  #3  
Old April 20th, 2015, 9:43 PM
SME SME is offline
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i wouldn't really call 6-8 db down a null, but it's hard to say had bad the dip is without more detailed measurements. In any case, I think your choice of placements should work fairly well.

Am I understanding correctly that your room is 14.5 feet from the front wall to the alcove, which extends another 3.5 feet? How far back from the front wall do you sit?
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Old April 21st, 2015, 7:04 AM
rman2222 rman2222 is offline
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SME,
correct.. the room is about 14.5 feet from the front wall to the back, and the alcove takes up maybe 1/3 of the back wall going back another 3.5 feet (tucked under a flight of stairs that basically run "behind" and parallel to the back wall. Equipment and storage racks are tucked under there.
Seating is approximately 10.5 to 11 feet back from the front wall.

Having a small room seriously constrains both sub placement and seating positions. I tried to follow some basic guidelines, (not placing seating in the middle of the room, and trying to avoid making any two dimensions multiples of each other.. I know length and height are close)

Thanks,
Joe H
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  #5  
Old April 21st, 2015, 9:47 AM
SME SME is offline
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If I had to guess, I'd say the dip in response is due to opposite side wall reflections. The bass from each sub bounces off the opposite side wall on its way to your listener positions and interferes with itself there. There's not much you can do about this, and in fact, the corner placements you are using actually minimize this effect by putting the subs as far away from those side walls as possible. You will see frequency response issues like these in any small enclosed space.

It is likely possible to correct this and achieve a flatter response using some kind of EQ. If your AVR/processor supports Audyssey MultEQ XT or XT32, it may be able to correct most of this dip. Other auto-EQ system may also do the trick.

Alternatively, you can consider purchasing an external unit that provides PEQ capabilities, which can give you a lot more control than you have with an auto-EQ system. Note that if you decide to use any kind of manual EQ, I highly recommend first investing in a more sophisticated measurement system such as a UMIK-1 microphone and the freely-available computer software REW. Another popular solution is the Dayton Audio OmniMic. Either of these tools can give you a much more detailed picture of your room response and make it easier to explore what's happening at multiple listener locations as well.

Otherwise, you can just accept what you have. A response that's only 6-8 dB down in places isn't that bad at all. Many systems in-room have peaks and nulls > +/- 20 dB.
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Old April 21st, 2015, 9:59 AM
rman2222 rman2222 is offline
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thanks for the thoughts. Would it be of any benefit to switch to EQ2 to get more low end roll off and increase the gain to raise the 40-50hz levels? Not sure if that will make it too high in the 60-70hz range, but maybe it would smooth it out better?
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  #7  
Old April 21st, 2015, 9:28 PM
SME SME is offline
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That depends on your preference and what the other measurements look like. The EQ1/EQ2 mode primarily changes the response under 30 Hz. If you have a strong rise in response below 30 Hz, then running with 1 port open and EQ2 or 2 ports open and EQ2 may help make the response flatter.

On the other hand if you listen at lower levels, having a bit more bass at the bottom may sound better because deep bass is harder to hear at those levels.
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Old April 23rd, 2015, 10:08 AM
rman2222 rman2222 is offline
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Thanks again for the help.
I lowered the Q to .5 and that did drop the lowest end a bit without lowering the 40-50hz too much and by bumping the total gain on the subwoofer in the receiver I was able to get everything within about 3db on the RS meter using the HSU test tones.

One more question. Does this sub have any protection limiting built in? For example, on the Live, Die Repeat BluRay there are some pretty low tones at the beginning. I've been chicken to play that opening scene anywhere close to reference. (kept it at about -12). If I dared to try closer to reference can I permanently damage the sub?
Thanks again for all the input and advice!
Joe H
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  #9  
Old April 23rd, 2015, 12:49 PM
SME SME is offline
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The subs have both high pass filtering and amp limiting for protection. However, that scene is unusually demanding. It consists of square waves that play down to 10 Hz at 120 dB (at reference), so I'd recommend caution.
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  #10  
Old April 23rd, 2015, 1:59 PM
shadyJ shadyJ is offline
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Hsu subs do have limiters, but they can be overidden if the signal is strong enough. The mode with the most protection would be setting the sub to 2 EQ, Q Control to .3, and plugging a port. The 1 EQ setting sets the limiter to a lower frequency, and going higher on the Q control permits more deep bass playback as well.
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