Problems tuning Passive Radiators with Built Box

[Blueeyedfrog]

I'm having problems tuning my two 12" XLS Peerless passive radiators.

I have one 12" XLS driver in a small 50.78 ltr box (1.79 ft^3) with a 240 (8ohm) amp.

My question is, how critical is it to have the PRs tuned to exactly the design fb (in my case 20hz) once the box has been built? Specifically, is it ok to be within the ballpark of the design fb +/- 0.5 - 1 hz (i.e. within 19-21hz range)? If not, what are the possible drawbacks if my tuned fb is not exactly right?

I built my sub using a DIY design that recommends adding 365gms of mass to each the PRs. However, I have also found a spreadsheet for another forum which assists in calculating the approximate starting amounts - this spreadsheet recommends 338grms for each radiator but also to start at about 10% less than the recommended amount and work up to the suggested mass.

To measure my sub's Fb I am using a RS meter and Room EQ wizard. I have the meter less than a 1/2 inch away from the cone and set the measurement level for 76db (cal corrections file for meter is loaded into RoomEQ). Sub is 3 metres away from any walls.

I have a definite low point/drop near my target Fb of 20hz, However, it is in the form of a 'u' shape rather than a 'v' (see attached photos). Is this a problem?

With the radiators massed at 307gms each, the lowest db is at 20.5hz (top graph). At 335gms two low points appear in a flat horizontal line from 20.14 - 19.78 (middle graph). At 350gms the lowest point is at 19.8 (bottom graph). I have also noticed that the slope of the bottom of the 'u' shape changes as I add mass. At 307gms it slopes down from left to right. At 335gms the slope is flat. At 350gms the slope drops from right to left. Is this normal? Could it be that I have not got the PRs weighted exactly the same (I don't have electronic scales - just cheapo cooking ones)?

Which mass should I use? A present I have decided to use the 335gms which give the 'flat' response across the Fb of 20hz. Is that correct or should I use one of the others?

Suggestions please, I have been changing weights for days!

Cheers
Blue

[imbeaujp]

Hello, I think first that your spl meter should be at the listening position, not so close to the driver. Am I wrong ?

[Geoff St. Germain]

To measure the tuning frequency you want the mic as close to the driver as possible so that you only measure the output of the driver and not of the PRs in this case.

Blue, part of the problem you are running into with what you are doing is related to the resolution limits of the measurements. The minimas look like a "U" because the resolution isn't high enough to resolve the bottom of the minima so you get a smoothed over look to the measurement. For all three of your measurements the actual minima looks like it is probably near halfway between the two points forming the bottom of the "U". The two bottom graphs show an almost identical tuning frequency, likely within .1 Hz, which makes sense since there is only a 15g difference in the mass added to the PRs. I'd go with either of the bottom ones (335g or 350g) and be happy. It is unlikely you'll be able to hear a difference between the two.

Got any pictures of your enclosure? I've been toying with the idea of picking up an XLS and a couple of the PRs and doing a small subwoofer for the bedroom.

[Blueeyedfrog]

Many thanks Geoff. I feel much more at ease now! Please find attached photos. The design can be found here: http://www.geocities.com/adrian_mack/

I'm really pleased with the finish however, I did rush things a bit - the finish is contact vinyl and I didn't wait long enough for the filler to dry over the screws before covering the box - so I ended up with a few raises and dips. However, I found two layers of contact film made for a better finish, hiding some of the imperfections. Still, when it's sat in the corner it looks great from the listening position (keep having to remind myself to watch the move sometimes...).

I changed the design slightly and put an internal vertical brace in because the top was only 25mm think and I was worried about flexing. I also changed the dimensions slightly so I could use some offcuts from the timber suppliers rather than buy an entire sheet - saved about $100 Aust. Only problem was that I couldn't fit the radio control unit on the front however, I prefer it in the back because the sub looks cleaner from the front. I also don't know if I changed from the 'golden dimensions' (if the design even used those dimensions).

It's an excellent small sized box with punch.

Cheers

[Blueeyedfrog]

Photos...

[Blueeyedfrog]

More photos...