The WinISD helpfile is pretty good IMO, there are worked examples showing not just how the software works but the kind of factors you have to understand in wider box design, so I’d definitely recommend starting there.
Specifically re: entering driver data, it’s described in WinISD’s helpfile thus:
WinISD_Helpfile wrote:
The suggested procedure for entering driver parameters is following (check first that "Auto calculate unknowns" option is checked):
1. Enter Mms and Cms
This gives fs. If either is not available, then enter fs and other parameter.
2. Enter Sd, Bl and Re
Now, you should get all but Qms (and Qts), Vas. Please note that Vas may not match exactly what is specified by manufacturer, because exact value of Vas depends on environmental parameters. See FAQ.
3. Enter Rms or Qms.
Either one will do, although I tend to prefer Qms over Rms, because it can usually be measured in driver measurement procedures.
4. Enter Hc, Hg and Pe.
If Hc or Hg or either is available, then enter Xmax and optionally either Hc or Hg if available.
This procedure is most accurate. Also note that it also calculates true SPL (1W/1m) value. So it might not match the marketing SPL value, which is generally somewhat vague. Not in all cases, though. |
For designing corner or shelf vents, you need to make sure the total cross sectional area of the vents(s) is big enough to keep airspeed reasonably low when the speaker is driven to maximum excursion (or maximum power if you care less about sound quality). WinISD will do the math for you in terms of adding up the area of multiple ports and dealing with rectangular vs circular ports etc. If you want triangles, you have to convert those areas yourself. So, say you found that 4 ports each a 10x10cm square was enough, you’d need 4 triangles, each having 100 sq cm area to be the equivalent. Assuming you made corner ports with 2 45 degree angles, that would result in each of the 2 short sides of the triangle being 14.12cm long and the long side being 20cm. The other thing you need to calculate is the length as WinISD only seems to work with end correction for the backside of the vent being in free air (ie in the middle of the cabinet). Once you start placing vents against cabinet walls or in corners you end up with 2 or more of the vent walls effectively being extended by the cabinet wall – so they behave as if they’re longer than the actual vent panel. The easiest way of approximating this is by throwing the following into a spreadsheet, though in reality you may need to build a prototype box and measure actual tuning to be sure:
I’ve done a basic version here: https://drive.google.com/open?id=1tl8A4Fbkk07TmR7h2vyxVBQcDaCFOwri" rel="nofollow - https://drive.google.com/open?id=1tl8A4Fbkk07TmR7h2vyxVBQcDaCFOwri , though I never bothered adding in the conversion to triangle vents, but that’s easy to do manually anyway.
Now, there’s another thing you probably need to know about, which is Hoffman’s Iron Law. This is a principle in speaker design that ties together the overall size of a speaker enclosure, its efficiency (ie how loud it gets for a given signal input) and its low frequency extension. In essence, it’s a 3 way trade-off between these characteristics. So for example if you want to get really deep and really loud, it’s gonna take a big box. If you can afford to trade off less efficiency or less LF extension, then you can make a smaller box work.
Also, the lower you tune a reflex box, the less input you can apply in-band before it runs out of Xmax (that is, before distortion starts to rise and the driver fails to increase its output fully in proportion to any further increase in applied input).
Therefore, it’d be well worth having a look at the kind of music you want this box to play and finding out just how deep it gets. Your mention of 20Hz in particular is probably going to have to get revised, as A: very little music has lots of content down there (albeit some sub-genres of EDM definitely do); B: very few drivers are happy being pushed that low and C: see Hoffman’s Iron Law above – efficiency would decrease and/or cabinet size get huge if you really wanted to try it.
Free audio software like Audacity can do a spectral analysis for you, I’m sure there are other options here too.
The driver you’ve chosen does seem well suited to reflex use, a few examples of credible box sizes and tunings would be: 150 litres (NET internal) tuned to 40Hz for a fairly compact box with not too much extension, 200l to 35Hz for an intermediate option and 250l tuned to 31Hz at the bigger/deeper end of the scale.
HTH, David.
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