Suggestions on OB problems

[maoumaou]

Hi
I recently decided to test a couple of seas P17REX/P on a small OB considering a future hybrid WMT with the woofer on a separate BR box.

This is the pdf for the seas driver:
http://www.seas.no/seas_line/woofers/H602.PDF

I bought them a few years ago having heard good things about them and also listened to them on a commercial product. They sounded really nice (well, maybe they were customized for the particular manufacturer).


I built a test OB for measuring purposes. You can see the front and rear images with the mid-woofer installed in the following pictures.

http://rapidshare.com/files/34086692/Front.JPG.html
http://rapidshare.com/files/34087520/Rear.JPG.html

Please let me know if there is a problem looking a pics through rapidshare.

Originally the baffle didn't have the bracing on the sides but I decided to built one since the baffle was vibrating too much. After installing the bracing there was still some vibration mainly on the top side of the baffle but I haven't done anything about it yet.

Attached you can see the FR 100-10000Hz for the particular drivers from approx 25cm using behringer equipment.
The first is one of the drivers on a small BR box with a PC XO applied.
The second and the third are the FR of either driver on the OB without XO.

From my limited knowledge on OB speakers I believe the bump around 500Hz is something to be expected and it has to be equalized.
However, in the region between 1-1.5kHz there are major cancellations. I attached the BR box FR just to point out that it's not the driver itself. I also measured using different distances between the speaker and the microphone and the result is pretty much the same. I haven't measure in 30 or 60 degrees, neither the back of the baffle. I didn't see the point to be honest.

Does anyone have any idea to what the cause of such an odd FR might be? Do you think it's the cavity in the back? The baffle is made from 2.5cm thick mdf. Could it be the poor bracing-resonances of the baffle? Any suggestions will be highly appreciated.

Regards

[Pano]

Hi,
Not sure why you are having this problem, but I am having the opposite problem. Peaking at about 1.25KHz.

I have a very different OB setup from yours, but maybe the problems are related?

Let's hope someone here can shes some light one the peaks and nulls.

BTW, I could only see the rear photo of your OB. Seems the download limit has been reached on your host. Here is the rear photo for others to see.

[Pano]

Got the front photo, here it is:

[maoumaou]

Hi
I'd like to see the baffles you are talking about.
Peaks IMO appear when stored energy is released in phase with the reproduced frequency range adding energy to that range. Also resonances of the materials play a big role and maybe that's what is happening to me too. What materials are you using?

Have you measured the driver on a different baffle/box?

Some photos would be nice so that others with more experience could share some suggestions.

Regards

[Pano]

Originally posted by maoumaou
I'd like to see the baffles you are talking about.

Here they are. The Lounge Lizards.
Front side in this post, back in the next.

Gotta take some time to chase down where to peak is coming from. Could be the horns, could be the crossover, could be the baffle. Or even the room.

Looks like you do not have the room or crossover to worry about. Your measurements were done outside, right?

Have you tried simulating your baffles with The Edge? It may show you what is causing your dip, if it is baffle related.

[Pano]

Here is the back before side wings. Baffle now has side wings that are full hight and go straight back. 8" wide on one side, 11" wide on the other.

The vertical pole is now tied to the back of the horn. It's a press fit.

[Rudolf]

Originally posted by maoumaou
Does anyone have any idea to what the cause of such an odd FR might be? Do you think it's the cavity in the back? The baffle is made from 2.5cm thick mdf. Could it be the poor bracing-resonances of the baffle? Any suggestions will be highly appreciated.

The general FR seems to be what one would expect with such a symmetrical position on the baffle. Look at the EDGE simulation. Green line is the FR for your open baffle, red for a CB with the same baffle.
Regarding the small ripple: It is most present in the range from 1-2 kHz, where front and back radiation usually are most out of phase:
Front and rear radiation of the Ciare HX132 .
IMHO this is indicative of front/back interference.

[Pano]

By Jove, Rudolf. I think you've found it! It's a baffle null.
Anything Maoumaou can do about it short of rebuilding the baffle?

And here is my problem as seen in The Edge with the mic 2M away. Only the front baffle is simulated, I don't know what the straight back wings are doing to the peak.

My peak sims a little higher than I hear it, but it's close - 1350 vs. 1670 Hz.

Now we have to figure out what to do to fix the problems.

BTW, Rudolf has a great OB site all in German.

[maoumaou]

Hey guys, thanks for the replies.

Nice work panomaniac. What's your crossover frequency/order?

I'm doing most of the measurements inside cause I live in a very busy street. I'll install edge and rebuild the test baffle probably during the weekend. I'll post back any results I get.
By the way, here's what I was thinking of building if everything turns out well.

Thanks again.

[Rudolf]

I would like to give some hints regarding the best position of a driver on an open baffle:

This is essentially a mid-driver issue, because the bass driver would (for the most part) work in the 6 dB dipole slope region anyway, where driver position on the baffle does not matter much. And for the tweeter frequencies most baffles will already look like infinite.

The best strategy for getting an even frequency response on an OB is to make the distancies from the mid-driver centre to the baffle edge as variable as possible. This will be achieved by positioning the driver as close to one baffle edge (not corner!) as possible. But what baffle edge? Moving to the left or right edge will result in a very different radiation pattern to the left and right, which in turn will compromise the stereo imaging. Moving to the bottom/floor obviously is no alternative. This leaves us with the top edge. Surely this will compromise the vertical radiation pattern, but this is already a mess because of the floor influence.

In EDGE you will see, how positioning the mid-driver at the upper edge of the baffle straightens the FR very efficiently. What then is left of your 1-2 KHz dip combined with a hump below can be avoided by moving the driver slightly away from the symmetrical (middle) position to the inside of the baffle. An aspect ratio of 3:4 or 4:5 is recommended.
If you are adventurous you could even position the tweeter below the mid driver.

I believe that this kind of "acoustical" equalizing should be preferred to electronic equalizing. But what do I know.