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