Questions about new build

knightfly · Post by **knightfly** » Thu Mar 09, 2006 11:38 am

Your two walls of the booth are splayed enough so modes shouldn't bunch up too much; I'd have framed just the duct and filled the "hard corner" with triangular cross-section rockwool in order to maintain more ceiling height, but if it's done it's done. I wouldn't obsess over it.

Where the HVAC is in the other room I'd go with the triangle-shaped wool for sure... Steve

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Fri Mar 10, 2006 9:04 am

Framing inspector came by today..

No issues.

Talked to him about all that I would like to do and he had no problems.

He was even helpful... turns out a buddy of his is into music and he helped with his place.

I asked him about a place in my drum room where I have a whirlpool tub right above it. By code, I need access to the motor and the bathroom has no access.. so you guessed it, access would need to be from the ceiling of the drum room.

HOWEVER (I did not know this), the inspector said there doesn't actually have to BE a hole... the hole just has to be located. He said I could go ahead and seal up the ceiling and just glue some framing around the area that would ever need to be cut away for emergency access to the tub motor.

I would have spent a long time trying to seal up a hole there and make it removeable had I not been told that by him... so now life is MUCH easier there!

Jim

knightfly · Post by **knightfly** » Fri Mar 10, 2006 4:39 pm

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Mon Mar 13, 2006 8:40 am

I'm going to have some more questions coming up before too long about the build, but I've got a simple one (hopefully) about the soffits...

John's drawing shows 5/8 plywood... can these be done with MDF? Any advantage or disadvantage?

The reason I ask, is that the framers used MDF to soffit around the HVAC and plumbing, and I have plenty left over... enough to easily build the actual speaker boxes.

Just curious.

Thanks,
Jim

knightfly · Post by **knightfly** » Mon Mar 13, 2006 8:46 am

MDF is heavier than plywood, therefore lower resonance frequency. If you're serious about optimizing, you can't go too heavy on your soffit baffles; even two layers of 3/4" MDF isn't overkill for this, as long as your room can support the weight.

You still need to keep your speaker boxes physically isolated from these baffle extensions though... STeve

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Mon Mar 13, 2006 8:49 am

Steve,

Ok... I'm confused now...

I was planning on following this design almost to the T... (minus the vents behind, as my speakers are passive).. But I don't see the baffles as being isolated from the speaker box.

Am I missing something?

Also... is it smart/wise to put stringers/firestops in all the studio walls? I know that I don't need them for code, but I wasn't sure if it would help stiffen up the wall a little and create a little less vibration.

Thanks,
Jim

knightfly · Post by **knightfly** » Tue Mar 14, 2006 8:36 am

That design is one of John's from the older SAE site - it's become more common knowledge since, that NOT having speaker boxes isolated from structure can cause sound to "flank" through the structure and into mix desks, re-radiating at the mix position BEFORE the airborne, direct sound from the speakers gets to you - this, because sound travels only about 1130 feet per second in air at normal altitudes and temperatures, but in various WOODS it can travel at speeds between 10-16 THOUSAND feet per second

http://www.uk-piano.org/sound.html

Sooo, for all practical purposes sound can get to your desk thru solids nearly INSTANTANEOUSLY, but takes about 5.3 milliseconds to travel 6 feet in AIR - this time delay can/will cause comb filtering problems - in fact, at higher frequencies the phase change made by even a half-inch movement in one mic can be noticed (although usually not unless you're listening while it's moved) -

For this reason, you don't want your speaker box hard-coupled to the baffle extension or to your room, but more like Barefoot recommended here

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=718

As to the lower section being absorbent, that would depend on the reflections mainly from the side of your desk closest to the speakers, and whether you're building a stereo CR or one that also does surround.

If you're only doing stereo, the entire front surface of a soffited setup can be hard, and NEEDS to be exactly FLUSH with the front surface of the speaker box for typically 4 woofer diameters in every direction if possible - in that case you would need to put fairly heavy absorption on the front side (toward speakers) of the desk to avoid reflections going from speaker to desk to front wall to listening position.

If you intend to do surround, check out the link to official specs in the Design forum - basically, you want most surfaces in the CR to be absorbent so your surrounds don't provide early reflection paths back to mix position; you then "make up" for lost ambience by adding reverb to the monitors til it sounds right (on good, commercially released CD's for example)

Speakers sitting on pedestals, etc, also conform to the "mass-spring-mass" principle, and as such they need isolation mounts somewhere; it's also better if this m-s-m resonance is sub-audible in frequency, so the speaker box needs to be heavier if your isolation mount is right under the box; this can be done with a heavy steel plate on top of the speaker - this would increase the mass of the upper element in the m-s-m equation; another way would be to make the pedestal itself out of concrete block, filled with sand or grout, and float the entire pedestal and speaker. This makes the entire mass the UPPPER "m" in the m-s-m formula, lowering resonance dramatically.

If you were to use sand-filled blocks as your pedestal, leakage can become a problem - one way around this is to use a solid 2" thick "paver" for the base, rested on your isolation mounts; then mortar the blocks together as you go and fill with sand, finally using another "paver" for the top. you would then use only thick enough pad so the speaker doesn't shift around on the pedestal, otherwise you'd be getting into a double-spring situation, much like the 3-leaf walls we warn against.

This is like a lot of things, in that there are many ways to do the same basic job - but keep in mind that the worst implementation of flush mounting is going to make your speakers sound better than the best free-standing mounts, especially if you don't use copious quantities of absorption behind your speakers... Steve

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Tue Mar 14, 2006 10:31 am

Steve,

Ok... that makes more sense.

In Barefoot's design, what is the back framing connected to? I would assume that if the front baffle is attached to the floor and ceiling... and the back framing (where the speaker is mounted) is also attached to the floor and ceiling, that the flanking effect would still be there?

So now its looking like speaker stands are the best way to go installed inside of the front soffits.

Should these be connected to the concrete (my floor) or are they best free standing? - Assuming I will use a MoPad or something to isolate the speaker from the stand.

I am intending stereo only, but the room is small, so I assumed the front should be dead with the exception of the baffle area directly around the speakers.

Leftover question from my last post.... should I put stringers in the framing of the walls to help stiffen them up? I've got a significant amount of leftoever wood from the framing that would be perfect to chop up and use for stringers if it helps the end result.

knightfly · Post by **knightfly** » Tue Mar 14, 2006 2:25 pm

I had kind of assumed that the inner frame in Barefoot's design was floated; otherwise, the speaker box would need to be resting on something like a MoPad to get much isolation from structure. But for simplicity's sake, using stands and mopads is more sensible. Again, be sure to keep your woofer centers AWAY from the midpoint between floor and ceiling by at least 6" - use the Harmon mode calculator to find good heights for woofers, they should be neither at peaks or nulls if at all possible; and if not, choose the highest (3rd or 4th) harmonic to allow at a null, not the first or second.

Stringers/fire blocks - if not needed for code, don't use 'em. The floppier your framing, the better the isolation. That's why walls using 24" centers work better than the same materials using 16" centers... Steve

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Tue Mar 14, 2006 4:34 pm

Wow Steve... seriously... you keep making this easier and easier for me

If I use stands and MoPads, do I need weight on top of the speakers to maximize the MSM properties and do the speakers still need to be in a box of some sort, or can they simply be on top of the stands sitting flush (but not touching) with the front baffle?

Thanks again.. you are a wonderful help!!

Jim

knightfly · Post by **knightfly** » Tue Mar 14, 2006 9:36 pm

You can experiment with weights on top, they WILL lower the m-s-m of the assembly. And if you can set the speakers so they stick over the front of your monitor stands enough so that the face of the speaker box is flush with your soffit WITHOUT the speaker or stand actually TOUCHING the soffit wall, that's just right. You want to cut the holes for the speakers less than 1/4" bigger all the way around than the actual size of the speaker box.

If your speaker boxes have radiused front corners, you should make it even tighter fit to compensate for the larger gap caused by radiused corners (quite a few nearfields do this to minimise diffraction and point source re-radiation caused by sharp edges; this is a good idea for free-standing speakers, but NOT as good for flush mounting; although it's mainly the longer wavelengths reproduced by woofers that care about this, since tweets are more directional.

Just remember though, that you do NOT want the speaker box in physical contact with the soffit wall (aka the baffle extension)

As to SIZE of your baffle extension - I posted the formula for this in another thread about a week ago -

http://www.johnlsayers.com/phpBB2/viewt ... c&start=15

My second post on that page has the formula for ideal baffle size, measured from center of woofer to edge of baffle... Steve

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Wed Mar 15, 2006 4:12 am

OK... this is starting to make sense.

Would the low frequency cut-off in the equation be the same as the lowest range of the speaker?

In my case I am dealing with passive Event 20/20's, so they have a response of 38Hz to 20kHz.

By the equation, I'd be looking at 4560/38 = 120 inches... or 10 feet

I am still trying to wrap my mind around what would make a practical speaker stand... If I build one out of 2x4's, it would be fairly light and therefore have a fairly high resonant frequency. Concrete pillars aren't very practical and would be tipsy.

Jim

knightfly · Post by **knightfly** » Wed Mar 15, 2006 7:32 am

Yup, 10 feet would be total baffle width (per speaker)

Realistically, make 'em as wide as you can - just be aware that if you make 'em smaller, and the edges then become conVEX (and they probably will) that it can cause somewhat of a horn loading effect, narrowing the sweet spot a bit. This can take away from stereo effect the further from the mix position you get (like the Producer's couch) but this can't be helped in a room that's too small for ideal baffle size, and probably won't be noticed by most people that'd sit there anyway

Speaker stands - since you're out in AG country, check your local irrigation supply for 6" PVC irrigation pipe and fittings - use two flanges one on each end of a section of pipe, tops and bottoms of 1" MDF or 1-1/4 ply, glue together, drill a hole in top shelf, fill pipe with dry sand. Offset top plate enough so bottom one doesn't hit your soffit frame, screw to the floor, add mopads and speakers... Steve

Velvet Elvis 2 · Post by **Velvet Elvis 2** » Wed Mar 15, 2006 7:43 am

Cool on the speaker stands... I can do that.

Per the horn loading effect... I should still assume that "worst implementation of flush mounting is going to make your speakers sound better than the best free-standing mounts" right?

Meaning... this is all worth the trouble, right?

Jim

knightfly · Post by **knightfly** » Wed Mar 15, 2006 7:52 am

Absolutely. Positively. Totally. Maybe. (OK, I'm kidding about the maybe part...

)