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PostPosted: Mon Jul 04, 2016 1:23 pm 
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Location: Del Mar, California
I had a studio built in my home (Del Mar, California) in the 80s by a contractor who had just finished a large professional studio in San Diego - I told him to build it the same way. I had a pair of JBL 4333Bs mounted in soffits (see photos). I moved out and sold the speakers 5 years ago and just unexpectedly moved back. I have been able to obtain another pair of 4333Bs :D and am preparing to mount them.

My previous contractor had them bolted with 4 lag bolts through the rear wall of the speakers to the rear of the soffits. Is this correct? I hate to drill new holes in valuable vintage speakers! And I don't think they are likely to slide off rubber pads, or vibrate, even if they are not attached!

I know there are threaded sockets on the rear of the JBLs, meant to insert fly bolts - but I cannot get inside the back of the soffits to use those. The speakers that I sold had 4 holes drilled through the back wall, matching the holes that can be seen on the rear of the soffits. I imagine this is where the beams run. Recommendations on proper attachment appreciated! Thanks!

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soffit.JPG

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PostPosted: Mon Jul 04, 2016 6:23 pm 
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Hi there Joseph, and welcome!

Quote:
My previous contractor had them bolted with 4 lag bolts through the rear wall of the speakers to the rear of the soffits. Is this correct?
Definitely not! No way. It should never be necessary to damage a speaker in order to mount it, and drilling large holes in the cabinet most certainly is damaging it! That would very likely change the acoustic performance of the speaker.

Quote:
I hate to drill new holes in valuable vintage speakers!
Absolutely! Or any other speaker for that matter.

There are two methods that are commonly used for mounting speakers in soffits. The first one is to construct a box from thick, heavy wood, exactly the same size as the outside dimensions of the speaker, so that it grips it tightly and rigidly in place. The other method is to "float" the speaker on carefully calculated resilient pads inside a rigid box, such that it is completely decoupled from the box. Both methods work, but the "float on pads" method is the one I prefer. However, you do have to do some careful math to make sure that you have the correct deflection on the pads, that they are loaded correctly, and are the correct size, thickness and durometer. The reason you have to be so careful is because the speaker plus the pads plus the box create a resonant system, and the resonance must to be tuned to at least one octave below the lowest tone that the speaker is intended to handle. I prefer to go for two octaves down. If you get the math wrong, then the speaker does not actually float correctly, and it will resonate at audible frequencies, thus distorting and "coloring" the sound.

Quote:
And I don't think they are likely to slide off rubber pads, or vibrate, even if they are not attached!
Right, but you can't just use any old rubber pads! You do need to do the math, if you want those speakers to reproduce sound accurately, without distorting it.

It would probably be possible to do that with your speakers, but it isn't an easy task. I'm not eve sure how you would manage to do that, as you need pads on all sides of the speaker to keep it centered in the enclosing box, but I can't see how you could do that: each pad need to be compressed just the right amount to ensure firstly that the speaker "floats"properly, and secondly that it doesn't fall out of the soffit, or slowly "walk" out over time, due to the vibrations.

Having said all that, there are other issues that need to be taken into account, such as ventilation for the speaker (to keep it cool, so that it does damage itself from internal overheating), and the overall faulty design of the soffit. The soffits are rather poorly designed, even by 1980's standards, and won't work all that well. It would probably be worth your while to take out those soffits and re-build them correctly, using up-to-date design criteria, and mount your speakers inside them correctly.

Here's a link to an example of how that should be done: viewtopic.php?f=2&t=20471

There are several other questionable things about that room, that could be improved greatly if you decided to go that way. A lot has changed in the world of acoustics over the last 30 years! It would be worth your while to update the studio, if you plan to use it as a commercial studio for contemporary productions.


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PostPosted: Tue Jul 05, 2016 2:55 am 
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Stuart,

Thanks so much for taking the time for such a detailed reply! Great forum! My speakers and I are relieved that we won't be drilling...

Unfortunately, this is less than a professional project. I was more of a performing musician than a recording engineer over the years. The studio is just a too small room in my house (11'w x 14'd x 8'h) with too-low ceilings, concrete floor. My contractor in 1980 imported what he could from the pro studio he had just built, but it was never going to be on the scale you deal with. :( The room will not be used commercially.
The rubber pads you see are what were used previously. They are on bottom and both sides. We will have to have 2 strong guys guide the speakers up, attach the wiring, and slide them in. We may be able to wedge some more rubber along the top if there is room. That is probably the best we can do. As I said, I don't think they will go anywhere. I don't play them super loud, and with their weight, I don't think the cabinets will vibrate much at all. If they don't slide out, I will be happy.

There is a false angled bass trap ceiling. I used to have some RealTrap panels spaced around as well. The mixing I did in there relied on my near field Mackies and Auratones as well as the JBLs. At the sweet spot, it was usable. There is a window, that was filled with a fiberglass frame; sound treatment was removed when we moved out 5 years ago (rented the house out). I may restore some of it.

Unless some expert in San Diego area sees this and volunteers to help out for fun, I don't foresee having resources to go further than that.

One more question while I have your expertise. Given how close my "sweet spot" has to be to the speakers (like the photo in my original post, not this one where the console has been pushed forward) , and the protruding wall/closet on the left which breaks the symmetry, will I be better off mounting the speakers with the HF drivers toward the inside, rather than further apart?

Thanks, again, Stuart!

Joe

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studio front.JPG

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studio side.JPG

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studio rear.JPG


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PostPosted: Tue Jul 05, 2016 3:58 am 
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Quote:
My contractor in 1980 imported what he could from the pro studio he had just built, ... There is a false angled bass trap ceiling.
Wait a sec: that's a COMPRESSION ceiling! :shock: I haven't seen one of those since the 1970's! Tom Hidley (renowned but slightly secretive studio designer) came up with that concept in the 70's and tried it on a couple of places he built, but later abandoned it, probably because it didn't work too well... and acoustic science today can explain why. But it's unusual to see that in such a small room! Interesting...

Quote:
but it was never going to be on the scale you deal with.
Actually, we deal on all scales! Really small to really large. John has designed entire studios to fit inside a shipping container, for example, and I've done more than just a few small studios in tight spaces. And for all budgets, too: even for a low-budget "hobby" studio, there's usually many things that can be done to make it sound reasonably good, and provide accurate acoustics. So if you ever do decide to re-do your studio, we'd certainly be here to help, regardless of the size or budget.

Quote:
That is probably the best we can do. As I said, I don't think they will go anywhere. I don't play them super loud, and with their weight, I don't think the cabinets will vibrate much at all. If they don't slide out, I will be happy.
Check them regularly with a tape measure the first few weeks, to make sure they aren't "walking"... Then again every month or so.

Quote:
There is a window, that was filled with a fiberglass frame; sound treatment was removed when we moved out 5 years ago (rented the house out). I may restore some of it.
You'll need it! If you want, once you get your speakers mounted and your DAW set up, use the free REW software package to do any acoustic analysis of the room, and post the data file here on the forum. I'll take a look at it and suggest what you need to do to deal with the worst issues.

Quote:
Unless some expert in San Diego area sees this and volunteers to help out for fun, I don't foresee having resources to go further than that.
The forum is pretty good at that: free advice is what we do mostly! (Of course, if you want a full professional re-design done for a studio, then contact John off-line for a quote). But if it's just simple guidance with basic stuff that you need, feel free to ask.

Quote:
will I be better off mounting the speakers with the HF drivers toward the inside, rather than further apart?
Neither! The correct method for setting up those speakers is vertically (tweeter above woofer).

Here's why:
Attachment:
Speakers-mounted-vertically-and-horizontally.jpg


(that comes from here: http://imgur.com/cFkzO3i )

So if you mount your speakers horizontally, you'll have comb-filtering effects going on and coloring your sound, and it will change as you move your heads left or right, even a small amount.]

There's also the issue of timing: With the speakers set up vertically, the "flight path" for waves from the tweeter, woofer and mid-range to your ears is the same, but if they are sideways, then the flight paths are different lengths, meaning that some frequencies arrive a bit earlier or later than others. Since your brain uses slight timing differences to determine directionality, have your speaker horizontal messes up your ability to accurately determine where instruments are in the mix (left/right), and also has consequences for frequency response accuracy.

Finally, there's the issue of phasing: for the same reason as above, there will be differences in phase between the various frequencies, which once again colors the sound.

A lot of this stuff wasn't known back in the 70's and 80's, when your room and speakers were built, but is very well understood today.

Set them up vertically.


- Stuart -


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PostPosted: Tue Jul 05, 2016 5:10 am 
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Stuart,

My contractor was not an acoustical or studio contractor - he built a pro studio for a large locally owned pro sound retailer here in San Diego. They paid someone big bucks for the design plan, and my contractor just built it and then copied the techniques in my room - he was good, but he didn't really know what he was doing. I read a few books, but I didn't either. I advertised in the local paper had recorded a few clients (Tascam 80-8), mainly for fun - this was in the days when 8-track recording was still a big deal - but it was never intended to be seriously commercial.

Well - mounting vertically makes sense - but ain't gonna happen, at least not now- I'll just sit in the sweet spot and not move my head left or right... Given that limitation, HF drivers on the inside? I'm sure these are bigger compromises than you like to make. :(

REW software - didn't know about it until now, but looks great - I will definitely play around with it. I have RTA'd rooms before - but obviously there is a lot new since then. I will definitelty take you up on your offer to help! It will be a while, but I will work on getting REW data for the room and post it.

Thank you!

Joe


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PostPosted: Sat Jul 09, 2016 3:43 am 
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Hi Stuart,

So the speakers are in the soffits. One stays put, but the other does seem to want to crawl forward when playing. The speakers were placed with 1/2" rubber pads on bottom and both sides, attached to soffit, and after placing the speakers, additional pieces of 1/2" rubber were slid between the speaker and soffit along the top. The fit is close, no more than 1/16" between rubber and soffit, but not tight - obviously not tight enough. So I am thinking shims of wood or additional rubber to push the pads harder against the speakers to keep them from slipping forward. Does that sound right?

Thanks, Joe


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PostPosted: Sat Jul 09, 2016 10:41 am 
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Quote:
the other does seem to want to crawl forward when playing.


That's not such good news!

That's sort of what I was hinting at when I said: "However, you do have to do some careful math to make sure that you have the correct deflection on the pads, that they are loaded correctly, and are the correct size, thickness and durometer.". If you don't do the math right, then either the fit is too tight or too loose.

Quote:
So I am thinking shims of wood or additional rubber to push the pads harder against the speakers to keep them from slipping forward. Does that sound right?
You could do that, yes, but once again I'd urge you to do the math: Find out the characteristics of the rubber you are using (especially the durometer, Shore scale) and figure out how thick your shims need to get the correct deflection (compression) on the rubber pads.

Quote:
my contractor just built it and then copied the techniques in my room - he was good, but he didn't really know what he was doing.
That happens fairly often. There's a lot of people out there who are really good at building stuff, but don't understand the principles of acoustics, and end up doing things that just don't work. Building a studio is very different, in may ways, from building a house, shop, school, office etc. Same materials, same tools, different techniques. Things that are fine for normal construction can really mess up a studio. And it isn't even possible to copy a studio: all studios are different. So copying what worked in one studio and trying to use it in a bigger or smaller room, will very probably not work. Trying to scale it won't work either. For example, the same diffuser that works wonderfully on the back wall of a large control room could completely trash the acoustics of a smaller room.

That seems to be the case here: he might have built the pro studio very well, but I'm betting it was quite a bit larger than yours, so when he tired to do the same thing in your studio, either directly or scaled, it would have a very different effect, acoustically. Rooms are tuned, and changing the size requires re-tuning: Changing the scale of the devices does re-tune them, but in totally the wrong way.

Quote:
REW software - didn't know about it until now, but looks great - I will definitely play around with it.
Post the data files here, so we can help you identify the issues with your room, and suggest how you can fix them.

Quote:
I have RTA'd rooms before - but obviously there is a lot new since then.
Yep. RTA's are actually not much use at all for room tuning, since they are far too broad and imprecise. Even a 31 band graphic equalizer (GEQ) falls far short. Fore example, if you have a modal issue at 70 Hz, there's nothing you can do about it with a GEQ. 63 Hz is way too low and 80 Hz is way too high: Modes are very narrow band, just a couple of Hz wide, and GEQ is very, very broad, so even trying is futile. You might reduce the modal response slightly be pulling down both 63 and 80, but then you are also pulling down a whole bunch of other frequencies that do not need to be touched.

And of course, "fixing" an acoustic problem with GEQ only works for one specific location in the room, at the expense of all other locations in the room. So you might be able to make it sound flatter at the mix position, but everywhere else that you stand or sit in the room will now be worse.

Plus, you cannot change time-domain issues with EQ: all you can do is reduce the intensity a little, but the ringing will carry on exactly as before. And you cannot fill in a null either, because nulls are related to phase cancellation at specific points in the room. If you have a null at 100 Hz, for example (very common in small rooms), then boosting the GEQ fader for 100 Hz will actually make the null deeper, not shallower, because you are providing even more energy at that frequency to cancel itself even better.

So RTA and ordinary GEQ is not a useful option. It used to be used commonly a few decades back, in the mistaken belief that it was helping, but in fact it mostly does the opposite.

High-precision parametric equalizers can help a bit, but even then there are limits, and in order to be able to use it at all, the room first has to be treated as well as possible acoustically. It makes little sense to even try if the room has no treatment, or insufficient treatment, or incorrect treatment.

Quote:
I will work on getting REW data for the room and post it.
Cool! I'm looking forward to seeing that!


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PostPosted: Sat Jul 09, 2016 1:56 pm 
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Quote:
You could do that, yes, but once again I'd urge you to do the math: Find out the characteristics of the rubber you are using (especially the durometer, Shore scale) and figure out how thick your shims need to get the correct deflection (compression) on the rubber pads.
Stuart, I'm afraid the math is I spent $10 on wood shims from Amazon, wedged them between the top soffit wall and rubber pads along the top of the speakers, pushing the speakers against the pads on the bottom - and it works great- no more slipping around. More math than that is way over my head in this situation I'm afraid...
Quote:
There's a lot of people out there who are really good at building stuff, but don't understand the principles of acoustics...
For sure. But again, I wasn't going for a commercial situation, just an awesome musician's man-cave. That said, I am willing to learn - I am very glad that I learned I shouldn't drill my speakers! - and I hope there might be some simple (in implementation at least) things I can do to improve my impossible room's acoustics. But no worries, it sounds great to me already.
Quote:
Post the data files here, so we can help you identify the issues with your room, and suggest how you can fix them.
. Should be able to get some data in the coming week. Thanks again!


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PostPosted: Wed Jul 13, 2016 3:56 am 
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OK, here is my first attempt at REW data. I have a feeling I should run away as fast as I can...

Calibrated UMIK mic at approximately sweet spot, on a stand at ear level, full range pink noise at 76db SPL through both speakers.

Bad as it looks?

Joe


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PostPosted: Wed Jul 13, 2016 4:00 am 
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OK, here is my first attempt at REW data. I have a feeling I should run away as fast as I can...

Calibrated UMIK mic at approximately sweet spot, on a stand at ear level, full range pink noise at 76db SPL through both speakers.

Bad as it looks?

Joe

(why is file extension .mdat "not allowed?" I tried changing it to .data - did you get it? If so, change it back.)


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PostPosted: Wed Jul 13, 2016 4:41 am 
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Quote:
(why is file extension .mdat "not allowed?"
The forum software doesn't want to allow that at present, and there's also an issue with file sizes being too big: MDAT files are large. John and I are working on that restriction, and hopefully should have it fixed in a few weeks.

In the meantime, please load your MDAT file to a free file-sharing service, such as DropBox, then post the link here on the forum..

Looking forward to seeing the results!


- Stuart -

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PostPosted: Wed Jul 13, 2016 7:01 am 
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Hey Stuart,

Here is link to my file: https://we.tl/zE4AP48mS6

I'm bracing myself! lol

Joe


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PostPosted: Wed Jul 13, 2016 10:31 am 
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OK, I got it fine. And it looks like the data is valid.

So here's the rough analysis:

First, the overall full-spectrum frequency response is not too bad:

Attachment:
josephwit--fr-20-20k--1..48.jpg


You are within +/- 12.5 dB, which is sort of almost acceptable for a basic home studio. The minimum goal is +/-10 dB. A top home studio might be around +/- 6 dB. The spec for pro studios are +/- 3dB.

But that's pure frequency domain. Now for the time-domain (how the levels decay over time, sometimes also called the "RT-60" graph, which isn't strictly correct):

Attachment:
josephwit--rt-20-20k.jpg

That shows how each 1/3 octave frequency band decays over time.

Not so good. Specs call for the decay rate to be roughly the same for adjacent bands, but yours are all over the place. The overall decay rate is too low for that room, and skewed towards the lows. You can see that in the high end it's close 150 ms, while at the low end it's more like 400 ms. That means there's too much absorption in the room for high frequencies, and not enough for the lows.

Next up, the waterfall plot, and concentrating on the part that matters most: below 500 Hz...

Attachment:
josephwit--wf--17-500.jpg


That shows the decay rates in a much more detailed manner. In that graph, the z-axis is time (coming out of the screen, towards you), so you can see how the sound intensity drops over time for each frequency. Clearly visible there are the modal issues in the room, which are the spiky peaks that stay up for a long time. 28 Hz, 41 Hz, 80 Hz, 90 Hz, etc. Those are all modal.

You can also see the "holes" which are likely due to SBIR: 65 Hz, 95 hz, 160 Hz, etc. Those are related to room boundaries. And you can see the overall unevenness.

I'm not at all convinced that the very low end is your speakers at all: they don't really go that low. It might be something external that is getting into your room.

The Spectrogram shows you something similar, but from a different point of view. It shows how the sound pressure levels change, but color coded, with time on the Y-axis.

Attachment:
josephwit--sp--17-500.jpg


You can more clearly see where the modes and SBIR issues are there. Once again, it focuses on just the low frequencies, below 500 Hz.

To go along with that one is the close-up view of the frequency response for the same low-end part of the spectrum:
Attachment:
josephwit--fr--17-500.jpg



And finally, what is perhaps one of the most important graphs of all, but also not so easy to interpret: the Impulse Response graph:
Attachment:
josephwit--ir-150ms.jpg


This is purely time and intensity. It shows how the room responds to a sudden extremely short pulse of acoustic energy, and is the most revealing of all.

You can basically think of it as showing a bunch of other impulses that are the reflections of the original pulse, coming back from different parts of the room. (Well... sort of... kind of...)

So the tall spike you see at time=0, that hits 0 dB, is the actual impulse, followed by a train of reflections of that impulse. The time difference after the original impulse tells you how far away that reflection came from, and the height tells you how intense it is.

What you SHOULD see on that graph, is nothing at all above -20 dB. All of the echos should be at least 20 dB down. At the very least, they should be below -20 dB for the first 20 ms, and then they could rise up a bit, to give you a low level ambient or reverberant sound field.

That's not what your graph is showing.

The reason is simple: any "copies" of the original direct signal that reach your ears inside of 20 ms, and higher than -20 dB, mess up the way your brain interprets what your ears are hearing. It skews your perception of frequency response, and it messes up directionality. In other words, your brain thinks the real frequencies that it heard were actually different frequencies coming from different directions.

You can see that there in your room there are several very strong echos coming back very soon after the initial impulse. Here's a close-up of just the first 25 ms:
Attachment:
josephwit--ir-25ms.jpg

The first two big spikes are just a couple of dB below the direct sound, and that's a big problem. Those are very strong reflections coming off some hard, solid surface in the room, perhaps the ceiling, the floor, a wall, or the desk. The time difference there shows that there's a difference in path length of 1.4 feet for the first reflection, and 2.4 feet for the second one. There's also about a dozen other reflections within the critical first 20 ms.

That's not so good at all.

So, what I'd suggest for your room is to add some serious bass trapping that does not harm high frequencies, remove or modify whatever it is in there that is sucking out the high end, and treat those first reflection points that are killing your clarity and directionality.

I think that if you do those three things, you'll notice a pretty significant improvement in imaging, clarity, bass tightness, and overall warmth.

- Stuart -


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PostPosted: Wed Jul 13, 2016 10:58 am 
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Stuart,

Fascinating! Thanks for sharing your expertise!

Well - I have some guesses as to the reasons for the problems.

1. The immediate strong reflections: If you look at the photos, you will see that there is a fairly large glass mirror hanging between the speakers. Very helpful for esthetics when we had an 8 piece band rehearsing in the room, facing front..... Behind it is the soffit wall; surface is somewhat rough wood panel with fiberglass-filled cavity behind, meant to do some bass trapping. Probably reflective as well, but not like the mirror. There is also the 3 x 6 ft glass window by the right speaker, right above the keyboards, which used to be covered (soundboard/fiberglass tightly fitted into the window frame in front of the glass) - I was planning to re-do that.

2. The general loss of highs - the dark material on the walls is cork tile, about 1/4" thick. The entire room is about 3ft below ground level, so the cork is basically against concrete for the lowest few feet. The cork was not part of the studio design - it was already there. It would be difficult to remove it, but I could affix some more reflective surface in front of it in some areas. Not sure how to guess where...

3. Bass trapping - I actually have around 5 or 6 RealTraps mini traps, 2 ft x 4 ft, that used to be in the studio. I can get them back. The room is only 1200 sq ft, and there is not a lot of room, especially if the goal is to bring them outward from the walls to get lower frequencies. But I do have them to play with.

So what do you think? Any suggestions as to how to implement these changes, and whether they will help?

Thanks!!

Joe


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PostPosted: Mon Jul 18, 2016 4:41 pm 
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Well - I have some guesses as to the reasons for the problems.
My guess would be the ceiling. :) The reflection is too intense and too marked to be caused by something way off axis. The ceiling is not far off-axis.

You can easily y check this, with a piece of string. Or two pieces of string.

Since we know that there is a difference in path length of 1.4 feet, all you need to do is to take measure the distance from the center of the speaker to the mic, and add 17 inches to that (1.4 feet). Now get a piece of string and make two marks on it that are exactly "distance + 17 inches" apart. Tape the string up so that one mark is on the front baffle of the speaker, and the other mark is exactly where the tip of the mic was (tape it to a mic stand). So you'll have a piece of string that sags in the middle. That string represents the exact path that the first reflection is taking.

Now carefully move the middle part of that string around, to see what surfaces in the room it can touch. You will find that there's only one surface that it touches.... that's your culprit. My money is on the ceiling...

Then repeat the same procedure, but this time placing the two marks "distance + 29 inches" apart. That's the second big reflection path, 2.4 feet longer than the direct path. My guess would be the side walls....

Quote:
3. Bass trapping - I actually have around 5 or 6 RealTraps mini traps, 2 ft x 4 ft, that used to be in the studio. I can get them back.
... or you could build something more effective yourself, from OC-703 insulation...

Quote:
But I do have them to play with.
If you have them, then it's worth a try! Set them up straddling across the rear corners of the room, diagonally, and run a REW test like that.

Quote:
The room is only 1200 sq ft, and there is not a lot of room,
1200 square feet??? It looks more like about 120 to me! Maybe there's an extra zero on there...?


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