ran low frequency test as recomended.........lots of pro

Posted: Fri Feb 14, 2003 11:26 pm

Hi Ethan,
There was a small discussion on Brad Blackwood's board the other day. You suggested running various low frequency sine waves ,Then walking arround & observing where the standing waves are.
Well....I tested 40.60.80.100.120,200,300,350hz. Of course all of them gave me a different result, but none of them were consistant through out the room.
With out getting into the exact response from every frequency, the most important thing is where im sitting there is no phase cancellation, or bass build up, it has the average ballance of the whole room. But since i don't have a simetrical :confused: room & I have alot of shelves,books, cd's cassetes, etc.. I just don't have a clue on where to put fiberglass. I have corner bass traps but that does nothing to prevent the buld up on one side & phase cancellation on the other. As stated before each frequency has it's own favorite place to "stand" & cancel. How do i approach this?
Thanks, Ed

Posted: Sat Feb 15, 2003 6:47 am

Ed,

> I tested 40.60.80.100.120,200,300,350hz. Of course all of them gave me a different result, but none of them were consistant through out the room. <

Yes, and this is typical. At one place this frequency is okay but that one isn't, and elsewhere a different frequency is okay and yet another is not.

> the most important thing is where im sitting there is no phase cancellation, or bass build up, it has the average ballance of the whole room. <

Really?! Surprised

If true, that is astonishing. Are you sure there are no nulls in the response at some bass frequencies where you sit? Try it again, focusing on the range from 60 to 120 Hz., and see if you don't get a deep null at one or more frequencies in that range. What are the room dimensions?

> I have alot of shelves,books, cd's cassetes, etc.. I just don't have a clue on where to put fiberglass. I have corner bass traps but that does nothing to prevent the buld up on one side & phase cancellation on the other. <

Exactly what type of corner bass trap? How big? Made of what?

705 rigid fiberglass is great when applied to the entire ceiling, and even better when it is suspended six or more inches so there's an air gap between the fiberglass and the ceiling. It's not as good as a real bass trap, but it can help a lot. And being on the ceiling, it's pretty much out of the way.

> As stated before each frequency has it's own favorite place to "stand" & cancel. How do i approach this? <

Bass traps, and plenty of them. This is the only solution.

--Ethan

Posted: Sat Feb 15, 2003 7:44 am

Quote:

Really?! If true, that is astonishing. Are you sure there are no nulls in the response at some bass frequencies where you sit? Try it again, focusing on the range from 60 to 120 Hz., and see if you don't get a deep null at one or more frequencies in that range. What are the room dimensions?

In comparison to many other locations I found where i sit to be the most consistant. Out of ignorance I made claims of no phase cancelation & bass build up. From where I sit, If i were to stand up with my knees bent, move to my right almost in front of the right speaker 60hz will vanish.....but not from where i'm sitting.

I'll get back to you with the exact dimentions.The room used to be 17X13 10ft high. I built a room within a room & a vestibule for the door to swing open. Unfortuantely That piece of wall extends almost halfway into the room in front of the left speaker.I have a cassette rack covering the whole area (a diffuser maybe?)& My ceiling is sloped high to low from my speakers.

Quote:

Exactly what type of corner bass trap? How big? Made of what?

Auralex LENRD Bass Traps

In the NYC area I had a hard time locating 703/705
Were do recomend even if I get it shipped to my studio.
Thanks,
Ed

Posted: Sat Feb 15, 2003 8:37 am

Ed,

> If i were to stand up with my knees bent, move to my right almost in front of the right speaker 60hz will vanish.....but not from where i'm sitting. <

I suspect other low frequencies are nulled where you sit, which is why I suggested you try again at a few other frequencies. Or maybe you're just very, very lucky! Smile

> Auralex LENRD Bass Traps <

Yeah, they'll work down to the upper bass range, but probably not much below that.

> In the NYC area I had a hard time locating 703/705 Were do recomend even if I get it shipped to my studio. <

Try Kamco. Here's a list of their NY and CT locations:

www.kamco.com/locations.html

--Ethan

Posted: Sat Feb 15, 2003 10:23 am

Ed,
Where you are experiencing that null is, I would guess, at about five feet high, correct? Nulls occur at the half way point of the ceiling height. 10' ceiling, nulls at 5'!. Fats
------------------------------------------------------------------------
Tannoy, Dynaudio, Blue Sky, JBL, Earthworks, Westlake, NS 10's Very Happy

, Genelec, Hafler, KRK, and PMC
Those are good. …………………….. Pick one.
------------------------------------------------------------------------

Posted: Sat Feb 15, 2003 12:36 pm

Fats,

> Nulls occur at the half way point of the ceiling height. 10' ceiling, nulls at 5'!. <

Nulls occur at half the wavelength, which varies with frequency. Again, with sine waves you can see how the null locations vary along the length of the room at different frequencies. The following assumes a room that is reasonably long from front to back - like 15 or more feet - and that the speakers are in the front aimed toward the rear.

Start with 50 Hz., stand right in front of the speakers, then walk slowly toward the back of the room. Somewhere along that path the volume will dip very low as you hit a null. Let's say in your room with its dimensions that happens 6 feet in front of the speakers. Now raise the frequency from 50 Hz. to 70 Hz. and again walk from front to back. Now the null is closer to the back of the room, because the shorter wavelength completes a cycle in a shorter distance.

As you continue to raise the frequency the null will move ever farther back. Note that the phase shift at the null point is not necessarily 180 degrees. It could well be 180 + 360 degrees at higher frequencies if the room is long enough to complete a full cycle plus the extra half cycle that causes the null. In fact, you can have these cancellations at much higher frequencies where the phase shift is many times 360 degrees.

--Ethan

Posted: Sun Feb 16, 2003 2:28 am

I found a null at 40hz just left of the right speaker at the top of the speaker (4-1/2ft)& 60hz about the same hight but 4 ft in front of the speaker.
Now here are the exact dimentions of my room.

The way i'm facing the speakers, the room is 16ft wide. on the right side the first 4ft inward is 12-1/2ft long, the next 5ft 10-1/2 ft long, the next 3ft 7ft long, the last 4ft 8-1/2 ft long.The ceiling from speakers to back is slanted starting at 9'5"ft to 8'10"ft.
my speakers are on both sides of the fireplace mantelin the center of the room, Dunlavy sciiia
(4-/1/2ft tall).

One of the reasons for these variations in length is that there is a fire place mantel in the center that protrudes in to the room about 2ft.The other variations accure as we go toward the left of the room due to an extention in the room from the back because there's a vestebule to accomodate the second door to the sound proof room(a room within a room) Can you picture it?

I don't know if this really hurts the situation more than helps it, but I have to work with it.

I found out today a friend of mine has about 30 extra 703 1" boards that i can get from him. I cant really afford the extra space that you recomend between the wall & baffle, so I'm planning to use the Rotofast anchors direct to the wall. http://www.rotofast.com/home.htm

I messured that i can fit 16 on the ceiling, 8 on the front wall next to the fire place, both sides, 1 in the center of the fire place(it's pluged up)4 on the right wall & 2 on the left wall.

Is it more important to cover more area with the 1" baffles or should i concentrate in a specific place doubling them up to 2"?

also what is a recomended fabric to cover the baffles with thats not expensive & looks good in white?
Thanks so much for your advise!
Ed

Posted: Sun Feb 16, 2003 9:01 am

Ed,

> I found a null at 40hz just left of the right speaker at the top of the speaker (4-1/2ft)& 60hz about the same hight but 4 ft in front of the speaker. <

That's more like it! Smile

But you're not going to treat those low frequencies with anything but real bass traps. Thick bundles of 703 will help a little at the higher bass frequencies, but not at all at 60 Hz. and below.

> a friend of mine has about 30 extra 703 1" boards that i can get from him ... Is it more important to cover more area with the 1" baffles or should i concentrate in a specific place doubling them up to 2"? <

You should stack four pieces together to make the bundles four inches thick, and put them across the corners. [With 30 pieces you can have six bundles four thick, and two only three thick.] Attach them so they straddle the room corners, like this [ignore the periods]:

_______________
|........../
|......../
|....../
|..../
|../
|/
|
|

The "drawing" above is looking down from the ceiling into the corner. The goal is to have fiberglass as thick as possible, and with a substantial air gap behind it.

> what is a recomended fabric to cover the baffles with thats not expensive & looks good in white? <

Died burlap is good, or any other fabric that is reasonably porous and not so shiny it reflects high frequencies. The standard test is to put the fabric to your mouth and blow through it. If you can blow through it easily, it will be good for covering fiberglass.

--Ethan

Posted: Sun Feb 16, 2003 9:40 am

Quote:

Originally posted by Ethan Winer:
_______________
|........../
|......../
|....../
|..../
|../
|/
|
|

The "drawing" above is looking down from the ceiling into the corner.
Dried burlap is good...

Ethan, is that the 703 suspended with space between, or is it suspended 6" batts?

ED, go to the fabric or crafts store, ask the clerk for "decorator burlap" comes in brown, gold, blue, or white. The white is really Tan in color, it is about $3.50 a yard, not square yard, but fabric yard, a larger amount. This can be stretched over a wood frame, if it is a strong frame, use a mist spray bottle and lightly moisten the surface when attaching to frame. When it dries out, it will stretch tighter on the frame.
--Rick

Posted: Sun Feb 16, 2003 10:38 am

Rick,

> is that the 703 suspended with space between, or is it suspended 6" batts? <

It is rigid fiberglass. Note that 705 is twice as dense as 703, and is therefore twice as effective at low frequencies. The surface of 705 is a little "harder" than 703 so it doesn't absorb the highest frequencies quite as well. But since the goal of fiberglass in a corner is to trap low frequencies, 705 makes the most sense there.

--Ethan

Posted: Sun Feb 16, 2003 11:14 am

Great! Now I'm starting to get the picture.

Quote:

That's more like it! But you're not going to treat those low frequencies with anything but real bass traps. Thick bundles of 703 will help a little at the higher bass frequencies, but not at all at 60 Hz. and below.

The nodes are there I was just noticing that from where i sit it's not extreme either way. Some frequencies do drop but not null. It seems like it's an average response.

What is the best way to affix baffles for this angle?

I have some challenges to the room as far as corners goes. On the left side of the fireplace ..no problem, On the right sidewall i have an 2nd window(inner room)that comes about 2" from the front wall(i can't disable the window)by covering it with these baffles :confused: , Although
there is about 3ft to the floor below the window, maybe that will work. Also Because of the size of the room the rest of my gear, the back wall corners wont fit the full pannel 2X4ft long.

i just don't have space to my back wall corners. I can fit smaller pieces in spots in the back. As long as there is air behind the panel even if it's a small amount Will that help?
Thanks,
Ed

Posted: Sun Feb 16, 2003 11:59 am

Ed,

> What is the best way to affix baffles for this angle? <

I am not an interior decorator! Rolling Eyes

Nails? Glue? Build a wooden frame? You're on your own there.

> I can fit smaller pieces in spots in the back. As long as there is air behind the panel even if it's a small amount Will that help? <

Yes, covering the corners as I showed is ideal. But if you can't do that exactly, just put as much as you can where you can.

--Ethan

Posted: Sun Feb 16, 2003 1:10 pm

Got it! I didn't know if there was a "proper effective way".
i'll keep you posted.
thanks,
for your help.
Ed

Posted: Thu Mar 13, 2003 6:28 pm

Hi Ethan and Kurt,

Found this old thread, had to comment.

Kurt said

Quote:

Nulls occur at the half way point of the ceiling height.

Ethan replied

Quote:

Nulls occur at half the wavelength

.

Actually, a null (deconstructive interference) will occur halfway between the floor and ceiling as Kurt said. This is because a null occurs one quarter wavelength from every surface in the room. The first mode of any dimension has a wavelength twice the length of the dimension, which means halfway between the surfaces equals one quarter wavelength. So Kurt is right.

On the other hand, after the first null one quarter wavelength from the surface, more nulls occur every half wavelength after that. In order not to go into a mental feedback loop, it helps to visualize a higher harmonic with a wavelength much smaller than the room dimension. But the same thing happens at the fundamental frequency.

Why a quarter wavelength from the wall, then one half wavelength after that? It's a symmetry thing. Picture the wavelength bending around the corner, so that the distance between nulls is still really one half wavelength.

So you're both right.

--Wes

Posted: Thu Mar 13, 2003 6:31 pm

Wes,
Thanks for that. It's nice to know I wasn't wrong about everything.. Very Happy

Kurt

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