300hz Response dip

mp006ltk

Member
2004-10-02 5:54 pm
Hi All,

I've built something like 20 pairs of speakers and this is a first. I've attached a picture of the measurements to reference as you read this....

First is the red trace. That is the far field measurement. I'm used to seeing wierd occilations as the signal goes lower but that dip at 300hz doesn't seem to be an artifact. It apears to actually be in the response, which makes it dificult to pick a spot to merge with the nearfield (green) measurent.

When I see that on the far and not on the near, I assumed it was cabinet difracton. To test my theory I used some foam and magnets to radically change the outside shape of the cabinet. That should change the response, but no change happened. Then I upped the power of the nearfield. If there was some type of internal resonance it should become evident with more power and the signal bouncing around in there. As you can see from the green trace, it's ruler flat.

Lastly, I nearfield tested the port (white trace). Again I tested at relatively high power. The dips and falls seem to corellate to that in my far field, but I don't want to assume causal effect since it didn't show up in the nearfield of the driver at the same power....

The height of the cabinet could contribute to resonance in the 350hz region so I took the driver out, and tightly stuffed the bottom to effectively shorten the dimention. No change.

Any ideas?
 

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Dave R

Member
2006-08-09 5:30 am
My guess is reflections off the 3M cabinet might be a strong influence.
Also, what is the big flat panel behind the speaker box? Can it be removed?

Try angling your measurements set up diagonally across the room to get rid of nearby reflective surfaces.
 
My guess is reflections off the 3M cabinet might be a strong influence.
Also, what is the big flat panel behind the speaker box? Can it be removed?

Try angling your measurements set up diagonally across the room to get rid of nearby reflective surfaces.

I agree there are quite a few large hard surfaces right near that setup that could be interfering.

Two things to try...
First move the whole thing a foot or two ... does the dip change?

Second get your mic in closer and try to get as much room out of the readings as you can. Nothing says you have to test at 1 meter...