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Custom Reflex for P.D.18"

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RkH View Drop Down
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    Posted: 30 September 2019 at 7:35pm
Hi, i have a PD model 18BR40, i would like to know if someone can help me with WinISD, so i can do a custom bass reflex, as this loudspeakers is suited for that.

I want to build this in 18mm birch plywood.

I think i need to know first what are my technical specifications

Also, I want to know how I calculate an optimate port lenght for this, its gonna play mostly reggae/roots/dancehall and dub...

If someone can give me some references would be amazing! and i'll keep my question on this post.
Thank you guys!!!


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Pinyorouk View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pinyorouk Quote  Post ReplyReply Direct Link To This Post Posted: 01 October 2019 at 6:39am
There are plenty of tutorials,.even on YouTube.

Edited by Pinyorouk - 01 October 2019 at 6:39am
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JonB67 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 01 October 2019 at 8:29am
Optimum depends on what you want, spl, low frequencies or compact size? You have to choose the compromise that fits your need.  

On the win isd front, i think the best tip i was given was that you don't have to enter all the ts params, just some and let the rest self calculate.  Cant remember which ones off the top of my head so you'll have to goggle for it. I found a thread on one of the forums.

There's loads of tutorials out there so little point in someone reinventing the wheel here. Get stuck in and have a go and come back if you're stuck with a specific thing or have more specific questions and people can then help you better 

Good luck. I found it a really satisfying process!
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RkH View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RkH Quote  Post ReplyReply Direct Link To This Post Posted: 01 October 2019 at 9:56am
Thank you for the response.

By the time im starting im going to build a bass reflex that give me enough SPL to put just one box and feel some pressure. Now is gonna work from 20 to 150hz but i will add a scopp later for the 20 to 90 band. I think i prefer not so lower Freqs. because i know its not going to give enough SPL for that.

If you can recommend another software for this aplication that is not WinISD, would be cool too! So could compare different parameters and calculations made by the software.

Also, i got some question about the port calculations in WinISD, how is it supposed to calculate it if I design the ports to be on the corners? Is it supposed to be half square? Its the same the port and the vent? If so, how do i change it from diameter to a rectangular-sized-vent that matches my box?
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DMorison View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 02 October 2019 at 11:32am
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:

Originally posted by WinISD_Helpfile 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, 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote RkH Quote  Post ReplyReply Direct Link To This Post Posted: 02 October 2019 at 6:44pm
Thank you for the extensive message, i'm not so lost as i was thinking, but i have to read again the 'port' explanations you give. We are suppossing that WinISD calculates the port lenght centered on the box. Something like this 
https://www.supravox.fr/wp-content/uploads/2018/02/Supravox-produit-1050x1400-KIT-135LB-01-1.jpg

But i want that the cab looks like this:
http://www.lean-business.co.uk/SPEAKER_PLANS/CIARE%2015.00%20SW%20BASS%20REFLEX.gif

When converting it, is it supposed that the vent changes its dimensions when changing the position on the box?

Talking about other think, I said a freq. like 20hz, but no for port tunning, just that is going to play 20 to 150 at the beggining. Later i will add a scoop bin so I can separate for lower-end frequencies without distortion. That's why i think i don't whant to tune it so low, because in the future is gonna run higher frequecies like 90 to 250 or something like that. Music is gonna be black music from 60s to 90s. There are a lot of songs with upper bass and also lot of them have subharmonics on the bassline, above all in 80s and 90s songs.

So I suppose that my project needs a smaller box, but the prototype its true, it has to be usable for smaller and mid rooms like restaurants and cafes. 
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Post Options Post Options   Thanks (1) Thanks(1)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 03 October 2019 at 12:59pm
Originally posted by RkH RkH wrote:

Thank you for the extensive message, i'm not so lost as i was thinking, but i have to read again the 'port' explanations you give. We are suppossing that WinISD calculates the port lenght centered on the box. Something like this 
https://www.supravox.fr/wp-content/uploads/2018/02/Supravox-produit-1050x1400-KIT-135LB-01-1.jpg

But i want that the cab looks like this:
http://www.lean-business.co.uk/SPEAKER_PLANS/CIARE%2015.00%20SW%20BASS%20REFLEX.gif

When converting it, is it supposed that the vent changes its dimensions when changing the position on the box?

Yes, that is correct. When the vent is built in a corner or along the bottom of the cabinet like you want, you can make it shorter than predicted by WinISD and still get the correct tuning frequency.

Quote Talking about other think, I said a freq. like 20hz, but no for port tunning, just that is going to play 20 to 150 at the beggining. Later i will add a scoop bin so I can separate for lower-end frequencies without distortion. That's why i think i don't whant to tune it so low, because in the future is gonna run higher frequecies like 90 to 250 or something like that. Music is gonna be black music from 60s to 90s. There are a lot of songs with upper bass and also lot of them have subharmonics on the bassline, above all in 80s and 90s songs.

So I suppose that my project needs a smaller box, but the prototype its true, it has to be usable for smaller and mid rooms like restaurants and cafes. 

I think for that use you will be OK with the smallest and highest tuned of the options I suggested, 150litres tuned to 40Hz.
You can always use an analyser like in Audacity to check your music to be sure.

Good luck,
David.
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RkH View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RkH Quote  Post ReplyReply Direct Link To This Post Posted: 03 October 2019 at 9:40pm
Thank you for these clarifications.

How much shorter should it be? How can I know it?

Would it viable to tune it to 60hz? I don't matter to port it to lower frequencies because I will add a Scoop later for that band.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 03 October 2019 at 11:19pm
Have you simmed it in winisd?

You're at the point where you're going to start answering your own questions when you see the graphs.  
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Post Options Post Options   Thanks (1) Thanks(1)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 04 October 2019 at 1:15pm
Originally posted by RkH RkH wrote:

Thank you for these clarifications.

How much shorter should it be? How can I know it?

That's what the spreadsheet is for ;)

If you have any problems with it after trying things out, post back what you're struggling with and I'll see if I can help.

Quote Would it viable to tune it to 60hz? I don't matter to port it to lower frequencies because I will add a Scoop later for that band.

IMO, no, that's too high - you'll get an ugly sounding peak around 70-80Hz, possibly compromise the time response (ie how "fast" it sounds) and loose a lot of low end, so you'll be disappointed when you use it on it's own before getting the scoop sorted.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RkH Quote  Post ReplyReply Direct Link To This Post Posted: 09 October 2019 at 8:27am
Originally posted by JonB67 JonB67 wrote:

Have you simmed it in winisd?

You're at the point where you're going to start answering your own questions when you see the graphs.  


Not yet, I will do soon. I will keep you updated. Thank you
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