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

Posted: Fri Mar 14, 2003 8:29 am

Wes,

> Actually, a null (deconstructive interference) will occur halfway between the floor and ceiling as Kurt said. <

When I've done sine wave tests in rooms, I'll play a tone - say 80 Hz. - and it will have a null right in front of the loudspeakers. Then I raise the frequency a bit and the null point moves toward the back of the room a little. Go higher still and it moves farther back; lower and it moves forward. So I can see how it could end up halfway between boundaries, but only at one particular frequency. If, as you suggest, there's always a null point halfway at every frequency, then all rooms would have no sound at all there! Sort of like a Bermuda triangle in every room.

That doesn't sound right. What am I missing?

--Ethan

Posted: Fri Mar 14, 2003 10:46 am

Ethan,

No no no--I'm sorry, I didn't make myself clear. It is, as you say, at one frequency (the modal frequency), and of course all the whole number multiples of that frequency, which is to say, all the harmonics of that frequency.

Keep experimenting, starting from the back wall, and see if you don't realize that the first null is always 1/4 wavelength from the boundary. It's important to test from the back wall, because the front and back walls will only act similarly at modal frequencies. The back wall is where test conditions are simplest, with a strong 1st reflection colliding with the direct sound in almost equal (6dB?) proportions. You can simplify the test conditions further by using one speaker only, and walking down the longitudinal axis of the room. I've got a great test CD with every frequency from 20Hz to 100 Hz, each frequency having the same track number as the frequency itself. Keeps me from going crazy. (Or does it?)

The next null will occur at 3/4 wavelength, with a 6dB boost in between, at 1/2 wavelength.

The points of most accurate monitoring are at 1/8, 3/8, 5/8 or 7/8 wavelength for a given frequency. So unfortunately this means that the "sweet spot" for bass moves around the room as the bass player moves up and down the neck. (I've seen you refer to this very fact in previous threads.) So tell that uppity bass player to stop playing all those fancy licks and hold down the bottom!

So how do you choose one spot for the engineer's position? You gotta pick one, then be willing to move around some.

I've found that with the engineer's spot 5/8 of the room length from the back wall, then at least the room's lowest modal frequency is fairly flat, giving the engineer the same sense of extended bass as those sitting near the back wall.

And that really is the Golden Mean spot--I'm not just getting all new-agey on you.

I wrote an article for TapeOp a couple years back, describing this bass quandary , entitled "Bass Waves in the Control Room: The Myth of the Sweet Spot".

It's always the damn bass, isn't it?

--Wes

Posted: Sat Mar 15, 2003 6:35 am

Wes,

> starting from the back wall, and see if you don't realize that the first null is always 1/4 wavelength from the boundary. <

Thanks. I'll try that when I get a chance.

> So unfortunately this means that the "sweet spot" for bass moves around the room as the bass player moves up and down the neck. <

It's worse than that, because bass instruments don't play sine wave. The timbre of every note also changes, as does the overall tone of the kick drum. That's why I point out in my FAQ that no place in the room is ever valid.

--Ethan

Posted: Sat Mar 15, 2003 7:14 am

Ethan,

>It's worse than that, because bass instruments don't play sine wave<

You got me there!

--Wes

Posted: Sat Mar 15, 2003 7:41 am

very intersting discussion,
In the above thread I listed my room demensions... kind of awkward. On the left side it's 8ft long & on the right 12ft long.
I have alot of bass build up on the left side, & alot of cancelations on the right side. Not so much front & back that iv'e noticed(not to much room to go front & back). I have placed 4" thick 703 in the 1st & 2nd reflective areas & in the corners. But it still happens left to right.
Thanks,
Ed

Posted: Sat Mar 15, 2003 9:26 pm

Ed,

Any chance you could post a sketch of the room? That list of room dimensions is boggling my mind right now. Must be those 5 cups of coffee..

--Wes

Posted: Sun Mar 16, 2003 1:47 pm

I have a jpeg of the layout. But I don't know how to post it. Any Ideas?
Thanks,
Ed

Posted: Sun Mar 16, 2003 5:35 pm

Here It is...Thanks for your help, Ed

members.bellatlantic.net/~vze2b8mh/studiolayout.jpg

Posted: Mon Mar 17, 2003 8:06 am

Ed,

> studiolayout.jpg

The picture doesn't show where you have the 703, and I'm not sure what you mean by "1st and 2nd reflective areas." And did you put the fiberglass across the corners, or flat on the walls near the corners? Also, you have five corners - did you treat them all?

Fiberglass - even four inches thick - can do only so much. You may need more substantial trapping to flatten the response even more.

--Ethan

Posted: Mon Mar 17, 2003 8:53 am

Hi Ethan,
Actually there are 7 corners. Both sides of the fire place behind the speekers too.I have alot of stuff in this room(no option) & many of the corners are occupied. So i'm in the process of getting these baffles to fit.I'm not quite done yet with placement & I have more 703 comming in.

Some one told me to find the first & second reflective areas of the tweeter/driver on both left & right walls.

To do this I sit in my chair while a friend slides a mirror on the wall until I see the drivers reflection. I do this for both speekers & both left & right walls.I then put 4" baffles at least 2" of the wall on all 4 areas.

This guy claims to be in the know & says if this is done first it will greatly help stereo image & bring out the low mids better, than I can deal with bass traps.

what do you think of this.....any merit?
I'll keep you posted.
Ed

Posted: Mon Mar 17, 2003 9:27 am

Ed,

> I sit in my chair while a friend slides a mirror on the wall until I see the drivers reflection. <

Interesting. I never heard of that, but it sounds good. Smile

--Ethan

Posted: Mon Mar 17, 2003 9:41 am

It's called the old mirror trick. I usually try to get my wife Lisa to hold the mirror, because she fairs a lot better around mirrors than I do.

--Wes

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