Originally Posted by
RMC
Hi Goldjazz,
In his "Bullock on boxes" book, Bullock says that cabinet air tightness is the single most important issue for bass and that this cannot be overstressed...
If you model in speaker design software some boxes but with different QL numbers (box losses) as can be done in Win ISD Pro 2016 edition, you can see easily the effect of air losses on bass response. Model a design starting with the usual QL 7 number, keep all parameters the same but then change only the QL to 5 or even 3 (more lossy boxes) and you'll see the bass response dropping. On the other hand, increase only QL to 10 or more (more air-tight boxes) and you'll see the bass response increase. So, box tight seal IS really important for low-frequency performance you are expecting from the software model.
Win ISD uses as default QL 10 which is overly optimistic. To change that number when your box is designed, click on Box tab then at the bottom of the screen click on Advanced, click QL, a window opens with QL 10 and change that number for 7 and close that window. The response on the graph should have changed. To make sure you did it right, re-do the same path up to QL number window opening to see if it shows QL 7 and if yes just close that window you're done for that and if not change the number again. SINCE MANY FORGET ABOUT THIS, ONE MUST REMEMBER TO RESET THIS EVERY TIME OPENING WIN ISD PRO SINCE THE DEFAULT IS ALWAYS QL 10. Its not over yet.
Because your on screen response design has changed a little re QL number change, then you have to re-work a little cabinet volume and/or tuning frequency to get back to the response curve you wanted initially. Don't forget that, according to that software's designer- see help files in the WIN ISD PRO software- and to John Eargle of JBL (Loudspeaker Handbook and Handbook of Sound System Design) the response modeling in software already assumes or includes ONE boundary speaker placement. So, low-frequency response seen on the screen is valid for ground/floor OR large wall placement.
In theory, this means air-tight boxes can reach similar LF response in a smaller cabinet since it makes better use of the air Inside (no losses). In practice this remains difficult to achieve as each and every POSSIBLE air leak must be dealt with, a monk's job... Since I built my boxes meticulously and chase all possible air leaks I tend to think I may reach QL 10.
Regards,
Richard