Subs in new room ... huge null at 40hz? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 14 Old 08-20-2017, 12:26 PM - Thread Starter
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Subs in new room ... huge null at 40hz?

Hey guys, been a while. Still using my two dual opposed AV15H's along with my Funk Audio 2400x2 external amp. Moved into a new house a while ago and finally able to do some measurements. Picked up an Anthem AVM 60 that has ARC, and noticed my subs seem neutered. Now, I didn't expect to have the same level of bass in a 5,000cubic foot room that I had in my old 1700 cubed family room, but something was amiss.

Looks like I have a huge null around 40 hz. Room size is about 28x18x10, but it's also open to a kitchen about the same size.

I've tried positioning the dual opposed facing the wall vs facing out, moved them a couple feet forward, back, away from the wall. Changed distance from 0 feet to 30 feet. But nothing seems to help. Also checked each subs individual measurements and they both show a similar suck out. One actually has two nulls (around 25 hz then again around 40hz).

Anything else I can do?

ARC measurement below.
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File Type: jpg ARC1.JPG (41.2 KB, 89 views)

 

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post #2 of 14 Old 08-20-2017, 12:33 PM - Thread Starter
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Attached is a rendering of the room as well that I did in Sketchup. The subs aren't up/down firing as they are in this pic (was going to build that but held off as I wanted to run measurements first).
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post #3 of 14 Old 08-20-2017, 12:38 PM
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Did you try relocating the subs, or one of them?

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post #4 of 14 Old 08-20-2017, 12:48 PM - Thread Starter
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Moved them around a 5 foot radius or so, don't have many options in the room.

 

My DIY Subs ... http://www.avsforum.com/avs-vb/showthread.php?t=1233892

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post #5 of 14 Old 08-20-2017, 03:32 PM
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I would try moving the sub on the right to the other end of the couch. Right in between the two couches and use it as a end table. I've had good luck with my last house doing this. But point it to the center of the room


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post #6 of 14 Old 08-20-2017, 03:43 PM
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To check if the null might be room related, divide the speed of sound by the problem frequency to see if you end up with a room dimension (or multiple thereof). 1130 ÷ 40 = 28.25 (close enough to your room length). Try temporarily moving the subs so that they are centered 7 feet from the front wall and re-measure to see if the null goes away. Make sure that none of the speakers are playing (only subs) and that both subs have the same delay setting (not individually time aligned).

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post #7 of 14 Old 08-21-2017, 04:16 PM
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Have you messed around with REW room simulator at all? It looks it is your seating location and subwoofer placement.

Using your room dimension it looks like moving the MLP forward will help quite a bit as will moving 1 of the subwoofers to the back of the room. Other than that you will have to live with it if you don't want to do either of those from the little time I spent looking at it in REW room simulator.

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post #8 of 14 Old 08-30-2017, 09:28 AM - Thread Starter
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Thanks, will try moving the MIC to see what location works. Given it's a family room, there's not much choice as to MLP or sub positioning unfortunately! Hate living with that null though.

 

My DIY Subs ... http://www.avsforum.com/avs-vb/showthread.php?t=1233892

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J Dunlavy:.. if you stop to think about it, no loudspeaker can sound more accurate than it measures.

 

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post #9 of 14 Old 08-31-2017, 11:34 AM
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Try decoupling the symmetry. Sometimes it's best when you have two subs to keep them at different distances to MLP and various areas. I had limited space to move my subs but I was eventually able to find a good spot.

Also to note, I remember reading somewhere when the room is shorter than the wavelength you need room pressurization to get that frequency since the wavelength can't fit. 40 Hz is 28 feet. Maybe this could be related but I don't know. I found that sealing off my room did little to my low frequency response and only changed the way the room reacted to the higher bass frequencies.
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post #10 of 14 Old 08-31-2017, 12:36 PM
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Originally Posted by Saber008 View Post
I remember reading somewhere when the room is shorter than the wavelength you need room pressurization to get that frequency since the wavelength can't fit. 40 Hz is 28 feet.
If that was true, you shouldn't be able to hear 40Hz through headphones.
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post #11 of 14 Old 08-31-2017, 04:08 PM
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Originally Posted by sdurani View Post
If that was true, you shouldn't be able to hear 40Hz through headphones.
Not sure how to respond to this. Perhaps I misinterpret what I read or it was plain wrong. I suppose I'll try to dig it up in AVS.

EDIT: I was referring to this passage from Mr. Ethan Winer:

"Pressurize refers to frequency versus room dimensions, and the term is often tossed around incorrectly. At very low frequencies, where a single wavelength cannot fit in the longest dimension, the frequency can still be heard. But instead of being a true wave it's a change in pressure. For example, a 40 Hz wave is 28'3" long, so playing 40 Hz in a room shorter than that will "pressurize" the room. That's an overly simple explanation, but hopefully makes the point."

And what I took from this is that if your back wall is shorter than the frequency we are interested in and your area opens up to the rest of the house then getting those frequencies can be difficult. Of course in my testing it didn't seem to affect much and in his Frequency Response graph it's obvious that he has decent sound pressure levels at even lower frequencies. I'm open for hearing what you guys have to say. I'm just trying to learn!

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post #12 of 14 Old 08-31-2017, 04:38 PM
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Quote:
Originally Posted by Saber008 View Post
I was referring to this passage from Mr. Ethan Winer:
Seems he's talking about room gain. You already know that bass waves bounce around a room and combine in different ways, sometimes reinforcing each other (creating peaks) or cancelling each other (creating nulls). If you measure the length of the longest dimension in a room and figure out the frequency for that wavelength, you'll notice that frequencies below that wavelength don't create peaks & nulls; they just build up in pressure. This is called room gain or cabin gain.

But that has nothing to do with which frequencies are audible. You hear a 40Hz frequency when something vibrates 40 times a second. If I stuffed you into a phone booth with a speaker that vibrated 40 times a second, you'd hear a 40Hz tone. Same with sending a 40Hz signal to a pair of ear buds plugged into your ears. So the size of the room, whether a phone booth or your ear canal (talk about a small room), doesn't prevent you from hearing bass notes whose wavelengths are tens of feet long.
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post #13 of 14 Old 09-01-2017, 07:04 AM
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Originally Posted by pbc View Post
Attached is a rendering of the room as well that I did in Sketchup. The subs aren't up/down firing as they are in this pic (was going to build that but held off as I wanted to run measurements first).
I have a very similar situation and multiple subs at the back of the room does help a lot. Isn't your room open at the bottom? If so you will find that the overall dimension is more determining of the response. Whats the long dimesion of the overall space? Also it helps to visualize the overall area including adjacent rooms with large openings. For example, if your room is open at the bottom and you place two subs in those corners (roughly what I did) they may be more at the midpoint of the overall space, not really in corners. Make sense?

In my case the main sub is at the front center and the flanking subs are roughly midway of the overall space. I'm working on a sketchup diagram so it will make more sense.

You might consider small high excursion subs at the back corners (two 12" in tiny boxes or two 15 but boxes wont be tiny) to even the response. In my room the effect at 40hz is tremendous at the expense of new issues around 80hz where the crossover also is. Still working on it. I'd rather be working on an 80hz issue though because there's more headroom there to eq etc.

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post #14 of 14 Old 09-01-2017, 07:16 AM - Thread Starter
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Yes, family room (18 wide x 26 long x 10 height) opens up to a large kitchen that is 29 x 15 x 10 high. Here's a rendering of the layout.
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My DIY Subs ... http://www.avsforum.com/avs-vb/showthread.php?t=1233892

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