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#169090 - 06/17/07 09:40 AM Re: Graph testing again, I'm confused, so is mojo [Re: bridgman]
a401classic Offline
connoisseur

Registered: 11/29/06
Posts: 1132
Loc: Alpharetta, GA
Quote:

>>John, my thorough scientific analysis of Scott's graph would indicate to me that it results from room dimensions of 8'x 20'x 24(just kidding; in the past that's what he said the dimensions were).

Once again I bow to your wisdom. That makes sense then -- a 28 Hz peak (plus 56, 84 etc..) from the 20 foot dimension and 23, 46, 69 hz peaks from the 24 foot dimension. The 8 foot dimension would make its first appearance around 70 Hz.

Dips would be 14, 42, 70 from the 20 foot dimension, 12, 35, 58 hz from the 24 foot dimension, and 35, 100, 175 from the 8 foot dimension.

The good news is that the 80 hz dip really does look to be related to crossover not room acoustics, so give that 'ol phase switch a flip.

Nuts, I need to submit this before I can look at the graph and see if it fits

EDIT - Whew, it fits. That's actually a pretty good set of room dimensions, have to remember them.

One last question -- where is the primary seating position in the room relative to the walls, ie how far away are you from the 20 foot and 24 foot walls ? My recollection was that you were pretty squarely in the middle of the room -- which argues for moving the sub out of the corner.




Gonna have to go back and pull out the college physics books - "565/dimension" - I don't recall that short formula when we were going thru the sound section.

Actual wall to wall dimensions are 23'0" x 19'3" - wall unit is right of center on long wall 10'10" from right. Mains are 2'9" from wall to drivers and 13' apart (center of drivers). Sweet spot listening position is 13' from mains, 12' from center and 5' from back wall - back surrounds semetrically placed 7' from listening position, side surrounds not-so-sematrically positioned, but just behind listening position.



I'll try the phase switch and see what happens below ~140...

For those who have not gone thru this test I do recommend a set of ear plugs as certain frequencies can get pretty intense.

Thanks again for all who have chimed in here, I really appreciate the help. Theo is scheduled to come over next Sat for a demo and I'd like to do my best to get the room out of the equation. He's looking at a set of M80's to replace a set of Bose cubes. Not a tough decision, I'm thinking

Scott
_________________________
Scott

My HT

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#169091 - 06/17/07 06:39 PM Re: Graph testing again, I'm confused, so is mojo [Re: a401classic]
bridgman Offline
axiomite

Registered: 08/25/04
Posts: 5256
Loc: Bowmanville, Ontario, Canada
The "565/dimension" shortcut comes from the fact that you get room reinforcement at the frequency where the distance between walls is a half wavelength -- half wavelength between walls = 1 wavelength round trip. You also get reinforcement at 2x the frequency, 3x, 4x etc..

If speed of sound is 1130 feet per second (can you check that while you're at it ?) then the frequency where you have a full wavelength between walls would be at freq = 1130/distance and you would have a half wavelength between walls at 565/distance.

I'm relying on JohnK to step in if I have this wrong


Edited by bridgman (06/17/07 06:46 PM)

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#169092 - 06/17/07 10:03 PM Re: Graph testing again, I'm confused, so is mojo [Re: bridgman]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10305
Yes, the first room modes can be calculated by dividing the various dimensions into 1/2 the speed of sound(use 560 or 565 ft./sec.), and the other effects besides the room modes are the cancellations caused by bouncing off a room surface about 1/4 wavelength away or by another speaker about 1/2 wavelength away playing the same sound.
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#169093 - 06/17/07 10:20 PM Re: Graph testing again, I'm confused, so is mojo [Re: bridgman]
a401classic Offline
connoisseur

Registered: 11/29/06
Posts: 1132
Loc: Alpharetta, GA
Now I know why it didn't look familiar - we did everything in metric!

Thanks for the physics refresher course.

Scott
_________________________
Scott

My HT

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#169094 - 06/17/07 10:58 PM Re: Graph testing again, I'm confused, so is mojo [Re: Wid]
Mojo Offline
connoisseur

Registered: 01/21/07
Posts: 3292
Quote:

There is no peak in the 70Hz to 80Hz range with the M80s. There is what seems to be a dip starting at the 100Hz to 150Hz range but as Ian said before " In reference to the little dip and rise at 150 Hz in the M80 compared to the M60; it is just the microphone positioning being up right under the tweeter and only 2 meters back in the anechoic chamber giving it an increased angle of attack on the very tall M80s."




Thanks, Wid. I didn't know that.

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#169095 - 06/18/07 12:27 AM Re: Graph testing again, I'm confused, so is mojo [Re: a401classic]
bridgman Offline
axiomite

Registered: 08/25/04
Posts: 5256
Loc: Bowmanville, Ontario, Canada
Geez, they don't use metric in the US do they ?

I settled on what seems to be "best of both worlds" in my early teens and successfully resisted any attempts to teach me anything different. Metric for liquid measure, feet and miles for distances, and pounds for weight.

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#169096 - 06/18/07 01:37 AM Re: Graph testing again, I'm confused, so is mojo [Re: bridgman]
a401classic Offline
connoisseur

Registered: 11/29/06
Posts: 1132
Loc: Alpharetta, GA
John,

I am hopelessly stuck in two worlds. As a chemist I work with liters (litres for the folks North of here ) for liquid measurement, grams, kilograms and metric ton for weight, psig for positive pressure, psia or mm Hg for vacuum, C and K for temperature, and I don't deal with diatance too much.
When I leave work I want to see the bank clock in F, fill my gas tank with gallons, and read my weight in pounds.
I just can't seem to separate the two, although I can convert back and forth at will. And being part engineer, I do it all on an HP calculator - once you go RPN you can't go back.
And considering I brew beer in relatively large quantity here

I guess that means my life is a really big waste

Scott
_________________________
Scott

My HT

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#169097 - 06/18/07 02:27 AM Re: Graph testing again, I'm confused, so is mojo [Re: a401classic]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10305
Ah, I wondered what field of chemistry it was(my undergraduate degree is a B.S. in Chemistry). You'll fit in just fine here!
_________________________
-----------------------------------

Enjoy the music, not the equipment.



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#169098 - 06/18/07 07:38 AM Re: Graph testing again, I'm confused, so is mojo [Re: JohnK]
Ajax Offline
axiomite

Registered: 12/30/03
Posts: 6212
Loc: Cleveland, Ohio
And you've been BSing ever since. (couldn't resist )
_________________________
Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton

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#169099 - 06/18/07 09:23 AM Re: Graph testing again, I'm confused, so is mojo [Re: a401classic]
NOLAGT Offline
veteran

Registered: 03/23/07
Posts: 189
Loc: Nawlins LA
Quote:

John,

I am hopelessly stuck in two worlds. As a chemist I work with liters (litres for the folks North of here ) for liquid measurement, grams, kilograms and metric ton for weight, psig for positive pressure, psia or mm Hg for vacuum, C and K for temperature, and I don't deal with diatance too much.
When I leave work I want to see the bank clock in F, fill my gas tank with gallons, and read my weight in pounds.
I just can't seem to separate the two, although I can convert back and forth at will. And being part engineer, I do it all on an HP calculator - once you go RPN you can't go back.
And considering I brew beer in relatively large quantity here

I guess that means my life is a really big waste

Scott




2 things....yall make my head HURT becasue what yall talk about is way over my head...2nd thing is SOMABICH thats a lotta beer!!!!! Is that at your house???

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