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First measurement of HT in construction - Any tips?

3K views 18 replies 5 participants last post by  Wayne A. Pflughaupt 
#1 ·
Hi,

this is my first measurements of my home theater that I am building. I am going to build new walls ontop of the existing ones. I can basicly do anything with the room (that isn't very expensive). Any ideas?

I seem to have a strange problem with 65-100 Hz just disapearing? Is my current plasterboard walls eating my base?
Also, do I need more absorbtion? Please look at my profile for Room dimensions/threatment.

Misc info: Calibrated Behringer EMC8000 pointing upwards at listening position.
 

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#3 · (Edited)
Here is the room mode calculations for my room.
I have verified a destict room mode at 45Hz and one at 65? hz (strange because I expected 60 Hz).
What I cant really understand is that 60-100 Hz span is so low.

Freq % Wavelength, 1/2, 1/4 p,q,r Mode Group Weighting
44.7 hz 7.71 : 3.85 : 1.93 (1,0,0 Axial) Start iso, End iso
60.4 hz 25.9% 5.7 : 2.85 : 1.43 (0,1,0 Axial) Start iso, End iso
75.2 hz 19.6% 4.58 : 2.29 : 1.15 (1,1,0 Tangential)
76.5 hz 1.6% 4.5 : 2.25 : 1.13 (0,0,1 Axial) Start iso, End iso
88.7 hz 13.7% 3.88 : 1.94 : 0.97 (1,0,1 Tangential)
89.5 hz 0.8% 3.85 : 1.92 : 0.96 (2,0,0 Axial) Start iso, Near
97.5 hz 8.2% 3.53 : 1.77 : 0.88 (0,1,1 Tangential)
107.3 hz 9.1% 3.21 : 1.6 : 0.8 (1,1,1 Oblique)
108.0 hz 0.6% 3.19 : 1.59 : 0.8 (2,1,0 Tangential) Near
117.7 hz 8.2% 2.93 : 1.46 : 0.73 (2,0,1 Tangential)
120.9 hz 2.6% 2.85 : 1.42 : 0.71 (0,2,0 Axial)

I have tried multiple sub positions with both 1 and 2 subs. This measurement is with 2 subs and is the best placement I found.
 
#11 ·
Don't ever put your sub at corner. Sub woofer need a lot of air to kick. Put it in the corner will create sound wave turbulence that will suck certain freq creating dip in freq response. If you want to experiment. Put your sub in the corner 45 degre to each of the wall. Measure it from the close range around 1.5 mtr. You will see dip in the freq response. Now cut thick large card board with a hole to fit your sub. Put the card board flush to the front of your sub to cover the side and top part of your sub then measure it once again. You will see what I mean.
 
#12 ·
My HT is designed for 2 people. Unfourtunally this means listening positions is ¼ length form back wall and ¼ length from side walls.

And as usual height is about ½.

New meassurement shows the same thing big peek at 45 Hz for the length mode. The other modes is not so distinct as 45 Hz one. I thinking of buiding a panel absorber tuned for 45 Hz and placing this behind the projector screen. Is this a good solution?
 
#14 ·

New meassurement shows the same thing big peek at 45 Hz for the length mode. The other modes is not so distinct as 45 Hz one. I thinking of buiding a panel absorber tuned for 45 Hz and placing this behind the projector screen. Is this a good solution?
The problem shown in your first post is not a peak at 45 Hz, but a big hole in response between 60-120 Hz.

I’ve been helping people sort out their subwoofer frequency response problems for going on 15 years now, and I can probably count on one hand the number of cases I’ve seen where corner placement didn’t get the best baseline response – that is, one that could be easily equalized.

That said, you have not supplied us with much info to help your situation – like, how many subs you were operating for the measurement graphs provided (e.g. one of them or both), or their location in the room. Typically these issues can be largely resolved with proper placement, and cleaned up with parametric equalization.

Regards,
Wayne
 
#13 ·
Ideal listening point is 38% of the length of the room, avoid quarter or half. 45hz dip means you have to move your measurement for more than 3m half wave length to take effect. Place an absorber behind the sub is a good idea, try the card board technique to see if anything is sucking the freq behind the sub. Using a plaster panel instead of card board is better.
 
#15 ·
Hi,

first of all. Thanks for all help!
I made some new meassurements that will hopefully give more information.
Room dimentions is 3,87 x 2,85 x 2,25m. All walls and celling is plasterboard. Floor is plastic carpet.

My listening position is 3m form front wall and 1 meter from side wall (See pic)

Audio Pro B2.27 sub has two 6,5" elements and implements ACE tecnology. (should play 26-100Hz)
Dali Trio AW 8 has one 8" element. (should play 40-150 Hz)
No EQ has been used. No other speakers than subs has been connected.

I hope this gives more good information about my problem.
 

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#16 ·

Thanks for the info, Neomius. Now we have something to work with. :T

First of all, two different subs in corners of a perfectly symmetrical room like yours should show virtually the same frequency response, except for extension. This is because measured response is mostly a function of the room, while extension is a function of the sub itself.

The fact that your two subs are exhibiting such drastic differences in response suggests that one of them has issues. Part of the problem might be that the Audio Pro sub’s highest crossover setting is only 100 Hz. So if you’re using your receiver’s internal crossover for the system (as you should be), which are typically set in the 80-100 Hz range, it’s so close to the Audio Pro sub’s crossover frequency that they could be combining with each other. For instance, if both crossovers have slopes of 24 dB/octave, the result of both crossovers combined would be a roll-out of 48 dB/octave.

If, however, you got these measurement by sending the signal directly to the subs from REW – i.e., no AVR in front of them – then response from the Audio Pro sub is definitely and really whacked. There should not be that huge hole at 95 Hz while other sub shows no such problem.

In addition, we see evidence supporting the theory that it’s a bad idea to use mismatched subwoofers. Inevitably one performs better than the other, and response of the two combined will ultimately drag down overall performance to that of the lesser sub. More on that here.

Looking at the full range graph you gave us in the first post we see another possible issue, that your main speakers appear to be “dead in the water” below ~120 Hz. Are you using speakers with small woofers? The main speaker’s extension should at least an octave below the crossover frequency used. If not, you’ll have a hole in overall response, like we see here, between the point where they roll out and the subwoofers take over.




Laying aside the main-speakers issue, as far as a solution to the subwoofer problem, I’m not sure there’s much you can do with your current equipment. EQ is not a viable option for subs with tiny drivers – they will overload and bottom out in nothing flat. Absorption mainly has to do with reducing signal decay times in the room – it’s not going to eliminate a huge hole in response. That money would be better spent upgrading your subwoofers or (if need be) your main speakers.

Regards,
Wayne

 
#17 ·
Hi Wayne,

My main speakers in a pair of very old(80's) B&O Beovox Penta they have 4 units 13 cm as woofers each. And might not work properly since they are so old.

I going to buy
Q Acoustics 2000i 5.1 system to replace all my speakers. (http://www.whathifi.com/review/q-acoustics-2000i-51-pack)
or
Q Acoustics 2000i 5.0 and 1 or 2 new subs. (Iv'e heard that 2 subs is superior?)
Which one of these alternatives do you think is best for a small room like mine?

My Dali sub is of low quality. And the Audio Pro might have a problem with 70+ Hz because of it's ACE tecnology.
I am thinking of replacing them with 2 XTZ 99 W10.16. (http://www.xtzsound.com/en/products/subwoofers/99w10-16-matt#)

So I am going to rebuild walls and roof in my theater. Is there anything in the graphs that indicates problems with the room? Now is the time to correct it.

I probably try to build a front wall to absorb 45 Hz and add absobtion (rockwool) in corners and first relection points. Diffusion where ever there is any room left.

Any suggestions?
 
#18 · (Edited)
For the messurements with two subs I used 200 Hz CrossOver in there receiver. Unfortunally I could not use the "Pure Direct" Setting in my Yamaha RX-A730 because it won't use subs if I do. But I made sure that the equalizing was off.

Since I used 200 Hz as XO in the reciever that explains why my main speakers dont play low frequencies.
 
#19 ·

Okay, good that you figured out what was causing the big hole. :T

The XTZ subs looks like a decent option, although the 99W12 would be an even better choice than the 99W10. Sorry, can’t advise you on room construction for the purposes of acoustics – we have a separate forum for that where you can get some informed opinions.

http://www.hometheatershack.com/forums/home-theater-installation-systems/

Regards,
Wayne
 
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