# Thin hardwood floor over a crawlspace... huge panel absorber?



## nerml (Sep 3, 2010)

Hey all,

I've been racking my brain trying to figure out why I have a pretty wide, severe dip in the low end response of a project studio I'm building in my spare bedroom (house built in 1925, pretty bare bones construction). I've come up with a crazy idea, wanted to run it past ye olde acoustics experts that post here (thank you in advance!!). Here's what I've done to the room:

In the interest of soundproofing, I caulked around the room where the quarter-round meets the floor, as well as the windows, etc. I have 2 2' x 4', 4" thick fiberglass panels covering 3 wall-wall corners, and 10 2' wedges around the room in wall-ceiling corners. I also put 2" fiberglass panels in the window openings, leaving an airspace to try and get some free bass absorption along with my high end treatment. I have various other foam panels, blankets, etc up as well. All in all, a pretty standard DIY small room treatment.

When I fired up REW to see how I'd done, I didn't even have to see the results to know that I had some problems in the low end, for sure. Same old problem, some notes are thunderous, some whisperous. When I looked at the analysis ( I will post some graphics if y'all think it might help,) I noticed a -30 db dip from about 55 hz to about 80 hz.

:foottap:

I thought I could figure it out. I moved the listening position. I stared at mode charts. I moved the monitors. I tried some antiquated positional optimization modeling software. I messed with the switches on the monitors. At best I managed to move the entire dip 5 hz lower or higher. Still a nice big bump at about 38 hz, followed by the grand canyon, and then a +/- 30 db response all the way up ( that's a different problem!).

:dumbcrazy: So I thought...onder: and thought...:scratch:...and then: :gulp:

So, here's my big idea. By sealing the boundary around the floor, which is old oak paneling over old OLD underlayment, 1/2"...MAYBE 5/8" total thickness at best, have I not created a membrane? And if this membrane were over a big open airspace about 3' deep, followed by 7' of earth held in place by a concrete retaining wall... sounds like a room-sized resonating panel, right? Could that be why I have the dip, seemingly no matter where I put anything? 

Please discuss. 

CG


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## Moonfly (Aug 1, 2008)

I would be surprised if you had inadvertently built a huge bass trap. I would have thought the response was more a fundamental of the room. Have you plugged your room dims into a room mode calculator to see if the results it comes up with match that of your real world results? There is one online RMC here:

http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html

There is a decent article on the subject as well, which may be of interest and some use here:

http://forum.blu-ray.com/subwoofers/48286-guide-subwoofers-part-ii-standing-waves-room-modes.html

I think generally, if anything, the space under your floor is more likely to resonate and create a more boomy sound rather than suck bass out, but bass can be a funny so and so at times, and throw you a curve ball.


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## bpape (Sep 14, 2006)

I would agree with Dan on both points.

When you moved, I assume you moved primarily front to back. Did you do any experimenting left to right or up and down with the mic?

What are the room dimensions?

Bryan


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## DanTheMan (Oct 12, 2009)

And yes, graphs would be nice, and listen to what Brian ends up telling you. He got my room in order.

Dan


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## nerml (Sep 3, 2010)

_Posted via Mobile Device_The room is 13.91' long, 12.25' wide, and 8.25' tall. I did move the meter left to right as well as front to back, but not top to bottom. Same with the monitors. I will post graphs tonight when I get home. Thank you, gentlemen!

CG


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## nerml (Sep 3, 2010)

Here is a reading with the monitors and spl meter in the positions that I initially placed them, 4' from the front wall, and 3' from the side walls. They are on 39" monitor stands. The meter forms an equilateral triangle with the monitors, and is about 4' from the ground, 'bout ear level when I'm sitting down.

















Ew, gross. It's pretty bad all the way up to 100. This is not the best result I've been able to get, but I figured I'd start at the positioning I picked when I was placing everything based on what it looked like, before reality slapped me in the face with a big, ugly graph.

Please let me know if I can provide any other information.


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## bpape (Sep 14, 2006)

OK. And for those graphs you are seated where?

Watch your speaker setup pretty closely. You have a situation a lot of people don't think about. The distance between your speakers is double the distance from the speakers to the side walls. Try to avoid that if possible. Move the speakers say 6" farther apart so that is 7' and the distance to wall is 2.5'

4' behind speakers (if front of speaker is being used) is almost exactly 1/3 of the room width and 3x the 4' is very close to double the distance between the speakers.

Before you move anything, let me know where your ears are located in the room.

Bryan


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## nerml (Sep 3, 2010)

bpape said:


> OK. And for those graphs you are seated where?
> 
> Watch your speaker setup pretty closely. You have a situation a lot of people don't think about. The distance between your speakers is double the distance from the speakers to the side walls. Try to avoid that if possible. Move the speakers say 6" farther apart so that is 7' and the distance to wall is 2.5'
> 
> ...


Excuse me while I pick up the pieces of my blown mind... Ok. The listening position is about 60 inches from the tweeters, 4' off the ground. I did measure the distance to the back wall from the front of the speakers. How accurate should the measurements be? I can use a laser tape to make these if need be, but I am being pretty good, probably within an inch or two. Thanks!
_Posted via Mobile Device_


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## bpape (Sep 14, 2006)

Regular tape measure is fine. 

Seating - 4' from speakers to front wall. You - 5' from speakers. So, you're 9' from the front wall to your ears in a room that's approx 14' long. Ideally, you'd usually be sitting approx 8'7" to 9'2" from the front wall so you're in the ballpark. This is not a hard fast rule but usually works pretty well. I'd play with your seating position a little bit and see if anything moves in the low end.

Are you using a sub?

Overall, while you do certainly have peaks and dips to work on, the level below 100Hz seems considerably lower than from 100Hz up.

Bryan


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## nerml (Sep 3, 2010)

Thanks Bryan!

No sub, just a pair of Mackie HR824 (the old ones). Definitely thinking about a sub if I can get this beast under control, though. I'm mixing mostly modern futuristic soul and abstract hip-hop, gotta have that thump!

I have definitely played with the seating position a lot already, should I try to fix the measurements you mentioned (distance to the side wall, between monitors, etc) and then see what I come up with? Also, 4' above the floor for the listening position is half the entire height of the room, should I worry about that? Try to sit higher, maybe?

This brings up another question: How does one translate modal data about a room into information about positioning within that room? My abstract math skills are at about a 3rd grade level, so is there an easy way to look at the mode chart for your room and translate that into a map with "here there be dragons" sections on it (meaning places to avoid sitting or putting speakers)? I understand that the dimensions are directly responsible for the modes, but... nope, lost it.

Thanks Shirl... uh, I mean Bryan! You're clearly the man! Dan, you are as well, obviously.:T

CG


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## bpape (Sep 14, 2006)

I would move each speaker 6" out farther toward the side and not change anything else, then re-measure. Don't change the seat for now. One thing at a time - good old fashioned scientific method - no more than one variable.

Modal charts can sometimes get you a rough idea of problems but they depend on purely rectangular shapes, purely rigid rooms, no furniture, etc. Take them with a grain of salt.

General rules of thumb - don't sit in the middle of any dimension - except stay close on the width due to symmetry issues.

Try to keep dimensions different as I pointed out above. Try to avoid multiples. General initial setup for speakers many times works well using prime fractions (1/7th, 2/11's, etc.) of room dimensions. Same for the sub unless you have more than one.

Thump yes, boom no :hsd:


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## nerml (Sep 3, 2010)

OK, here's the best I could do.

















Here's how:

The monitors are now 21" from the side walls and 39" from the front wall. The listening position moved to 88" inches from the front wall, smack dab in the middle. I rolled off at 47hz on the monitors instead of 37hz, and turned on the "you have your monitors in the corners like a stupid" attenuation which drops everything below about 100hz by 4db. 

Moving everything closer to the front wall helped tame that nasty dip in the 55-ish range, but bought me a nice peak at 130. I was still having the big 40hz bump, and I figured it was better to roll it off drastically and get the low end at roughly the same db level. 

So... everything LOOKS great. The music sounds nice and balanced, clear, detailed, wide stereo field. But it has no PUNCH!! :boxer: I understand that what I'm looking for lives in the range that I just finished neutering. But accuracy is the most important thing when you're mixing, right?

Would adding a sub help, or would it be like turning the volume back up on all the problems I just spent my entire day trying to fix?

(sigh) 

CG


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## DanTheMan (Oct 12, 2009)

My experience with the "there be the dragons" has been "it's complicated". Measure the out of the room would be my advice. Like block it off foot by foot and map it out even at different heights. Have you thought more about room treatments? Somethings I've noticed in my room and it would be interesting to see your experience if you should choose to try: 1) the closer your listening position to the monitors, the more the sound field varies above the modal region with head movement and the more the graphs vary from place to place. The further into the reverberant field, the less the sound varies above the modal region. 2) the closer you get to the subs, the smoother the bass gets subjectively and graphically. Of course you don't have subs yet, but you'll need them for mixing/monitoring. 

I'm not so sure about the need to sit near field for monitoring, but I do believe accuracy is more important than most people know. I'm less sure about the suitability of the Mackie monitors for the near field though I know they are touted for it. I have the 624 mkII, and their vertical pattern really isn't suitable for it unless you are very good with placement and still with your head. The old models may be different as they look very much like my Behringer B2031s(many say these are the copy of those). If so, their vertical pattern may be better than the new versions as the Behringer's are. 

Oh, from that last graph it looks like you'd still be better off with kicking you bass level well south of where they are set. Your mixes if EQed for your liking now will probably sound a bit weak in the Mid Bass and heavy in the Deep bass. You're just going to need subs IMO. I think everyone does.

Dan


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## bpape (Sep 14, 2006)

Turn off the corner compensation. 

This graph is not a one area problem. The entire range below 100Hz is low. Easy enough to try and re-plot.

Bryan


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## nerml (Sep 3, 2010)

Bryan: I turned off the compensation, haven't had a chance to graph it yet, but I'll do so soon. It definitely sounds more balanced, but I'm curious about that dreaded 40hz. As far as the narrow, deep nulls (that's a null, right?) around 320, 500, 580, etc... Those are related to the positioning of the monitors as well, right? It seems like whenever I position them to solve one problem, another one crops up, like whack-a-mole! Grrr. 

Dan: Not sure what you mean by "above the modal region". Does that mean that your head, when sitting down, is in a plane that is affected by room modes more than if you were standing up? If so, then yes, absolutely. If I'm sitting at the mix position and then I stand up, I get all kinds of bass resonance that I didn't have before. It's well documented all over this forum that the so-called "bass vortex" exists towards the middle of the room, and I know this to be true now! Unfortunately, it's the only way I have to get any sort of even response in that frequency range. I really lose all perception of low bass pitch, like tuned sine hits and low bass lines, etc. This aggression will not stand!



> looks like you'd still be better off with kicking you bass level well south of where they are set


Do you mean that I should be turning the bass down even more? :huh: Er... no way! Or just that I should roll it all off and get subs... Hmmm... I kinda like that! I'm just worried that I'll spend a grand for a really efficient way to pump more bass into a room that can't handle what I already have...

CG


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## bpape (Sep 14, 2006)

I think you'll be OK. 

I do believe the higher up issues are positional in nature.

Getting a sub will allow you to move IT for best bass response without incurring the other changes that come from moving the monitors.

Bryan


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## DanTheMan (Oct 12, 2009)

By "above the modal region" I meant high enough in frequency where there are too many modes for them to be a factor in the response. This is typically above 200Hz. You can see in your graph that about 110Hz or so, you've got too much output until 500-600Hz. Turning "corner compensation" may well help smooth that out. Yes, your deep bass will suffer as well, but that's what subs are made for. That's what I was getting at. A decent completed sub can be had for under $200. The more the merrier however. I personally just have 2, but am on the look-out.

Dan


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