improving my subwoofer enclosure

I have a few question regarding vented subwoofer enclosures. First of all, I am having trouble making sense out of the enclosure I currently have. Here is a link to the enclosure: https://www.diamondaudio.com/PDF/current/WiringEnclosures/Single10VentedEnclosure.pdf.

Does the volume of the enclosure (as given by winisd) include the volume of the vent or is it only the volume where the back of the subwoofer is in? WinIsd seems to provide a pretty good response for my woofer (D910D4) at a box volume of around 1.35 ft^3 and a tuning frequency of ~27Hz. Would this 1.35 ft^3 include the volume occupied by the vent?

    I've tried calculating the volume of the box I currently have with and without the vent volume and I can't seem to match up the tuning frequency between WinIsd and the enclosure specs with the same vent (2" X 12" X 44"). Namely, when I include the vent volume I get a tuning frequency much lower (~33Hz) from WinIsd than what is specified on the specs. When I don't include the volume of the vent I get around 42Hz.

   The volume of the box I have without the vent volume included (which gives me Fb~42Hz) is only about 0.8 ft^3. The response using WinIsd seems to be kind of peaky with that small of a volume so I'm thinking about building a different enclosure. I'm more into SQ and like my bass to be as deep as possible. I generally keep my Low Pass crossover on my amplifier at ~60Hz.

   So, all of that for my main question: would it be worth it for me to build another box with a volume of 1.35 ft^3 tuned to 27 Hz?
I know this is a rather subjective question, but please keep in mind that it's SQ and deep bass that I'm after.

Here are a couple of plots from WinIsd. The first one is of the setup I am currently running and the second is of the setup I am considering changing to:

Read https://www.the12volt.com/installbay/forum_posts.asp~TID~104086~PN~2

Make sure you deduct the entire port structure as displacement, not just the port airspace.

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stevdart wrote:

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Read https://www.the12volt.com/installbay/forum_posts.asp~TID~104086~PN~2

Make sure you deduct the entire port structure as displacement, not just the port airspace.

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Thanks for the link. I must have just missed that thread with it being so recent and all.

So, I have calculated the box volume correctly for the first WinIsd graph displayed above. The main question still stands. Should I change enclosures?

Unless you're looking to build an SPL comp vehicle, the second graph will produce far more usable results.

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Ok then, I will build a new enclosure ... eventually. I won't be able to do it until the end of summer when I go back home (where all my dad's woodworking tools are) from school. So that gives me a good amount of time to make plans for the enclosure.

Here's another question: how much group delay is acceptable?

In general, you want to keep in-room or in-car group delay under 20 msec  at 20Hz and under 10 msec at 40 Hz.  Software will tell you the group delay based on the plotted frequency response and system phase but ths will change once the system is installed.

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DYohn] wrote:

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In general, you want to keep in-room or in-car group delay under 20 msec  at 20Hz and under 10 msec at 40 Hz.  Software will tell you the group delay based on the plotted frequency response and system phase but ths will change once the system is installed.

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Question for you, DYohn:  can you just take it for granted that the group delay always gets worse, or can it actually get better in the installation than it shows in the modeling?  (You said it "will change".)

Do you just model for the best group delay that you can get and hope for the best when it's installed?  Or is it, in reality, a minor factor and of little concern in the modeling phase because it is known to change once installed?

(For these questions let's assume we have absolutely determined that the enclosure will be vented, not sealed.)

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Yes, it can get better in the car.  Group delay is determined by the phase delay at a specific frequency and in car (or in room) the phase delay will vary depending on the size and geometry of the space and how many reflections are generated.  In a car, which is relatively small, the group delay at higher frequencies tends to be worse and at lower frequencies tends to be pretty much what gets modelled, or can even be better than what's modeled because the space is too small for low frequency waves to fully develop.  Determine the standing wave frequency of the space as group delay at this frequency will be increased significantly and can lead to significant SPL nulls.  You want it to be as low as possible at and above the standing wave frequency even if it means lower frequencies are higher.  Indeed, it becomes harder and harder to hear group delay effects at frequencies below 40Hz, unless it is very severe.

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Here's a plot of the group delay for the box I am currently modeling:




It is a little above 20ms @ 20Hz and is below 10ms @ 40Hz. However, as you can see, there is a big spike at about 24Hz. Would this group delay be ok?

That's your port tuning frequency, right?  It is pretty severe, although I bet you won't be able to hear it.

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jmalbright, very well written and supported topic;  I commend you.

I believe the intent of DYohn's post "in-car group delay under 20 msec  at 20Hz and under 10 msec at 40 Hz" was referring to one gradual slope, that is, the slope on the upside of the peak.  You are looking for a sloping change from better to worse as you look at the response down through the frequency range.  In this case, the peak has already ocurred at 24 Hz before the group delay becomes "acceptable" at 20 Hz.  The reason for acceptable response below 24 Hz, in this chart, is because of lack of output;  there is greatly diminished sound below 24 Hz.

You may not be able to improve this response and still build an enclosure that will fit the car and give you everything else you're looking for.  It's one of the tradeoffs you'll be balancing as you complete the design.  And keep real life in your vision as you do these models;  how much of the material you will listen to will fall into the below-30Hz realm?

DYohn will correct me if I erred in this response.  And DYohn, thanks for clarifying for me.  Standing waves have always made my head hurt.  I'm trying now to find out how I would determine the standing wave frequency of a given room, per your instruction "Determine the standing wave frequency of the space".

Believe me, people:  there are a lot of things that come easy to me, but physics is not one of them!

https://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/waves/swf.html

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Standing Wave calculator

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  Thank you.  And I assume if we're dealing with an automobile that it is trial and error, and experience.

As Mr. James Dyson, aka Dr. Bose Jr., says in his TV ad about the vacuum cleaner (use a British accent as you lip-read this):  "I made 5000 prototypes - that's 5000 mistakes - before I got it perfected."

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

DYohn] wrote:

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That's your port tuning frequency, right?

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     Yeah, the tuning frequency is approximately 27Hz.

stevdart wrote:

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jmalbright, very well written and supported topic; I commend you.

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     Thank you. I have found that the more concisely you pose your questions, the clearer the answer will be to them.

stevdart wrote:

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And keep real life in your vision as you do these models; how much of the material you will listen to will fall into the below-30Hz realm?

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     Probably not a lot from what I understand. Furthermore, I was planning on using a subsonic filter at around 25Hz because that's where my subwoofer is predicted to start to enter the realm of over-excursion.

     On another note, however, when I modeled a 2nd order, butterworth, 25Hz highpass filter my group delay really went to hell in a handbasket. Now, I'm not sure what to do.

jmalbright wrote:

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the realm of over-excursion

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Nice.

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WinISD can only go so far.

I used to run a single 15 inch JL 15W4 in 4.3 cu ft ported at 40Hz with 112.5 sq inches of port area.

The excursion graph on WinISD showed major excursion problems...in real life though with the sub receiving 3 times the rated power handling the sub barely even looked like it was moving much at all...yet was loud as hell. Never even used a subsonic filter....didn't have any problems at alal. The sub moved MUCH more in a sealed enclosure.

aznboi3644 wrote:

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WinISD can only go so far.

I used to run a single 15 inch JL 15W4 in 4.3 cu ft ported at 40Hz with 112.5 sq inches of port area.

The excursion graph on WinISD showed major excursion problems...in real life though with the sub receiving 3 times the rated power handling the sub barely even looked like it was moving much at all...yet was loud as hell. Never even used a subsonic filter....didn't have any problems at alal. The sub moved MUCH more in a sealed enclosure.

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Sure, I understand that the picture WinIsd paints is only a model, but it's the best I have right now.

I have a couple of questions about ports.
1) Does it matter where the port originates and terminates? For example, say I had a fairly large enclosure. Would it matter if the port inlet is located at the upper left corner as opposed to the lower right corner of the enclosure? Similarly for the port exit.

2) Would PVC pipe work well for a port? I was thinking about using 5"-6" PVC because it would be an easy way to keep a very consistent port diameter. Also, my port is going to have to be fairly long (~40") so I was thinking that with the right elbows and such PVC would be easy to snake around my enclosure so I could get the correct length. I'm not sure how PVC will perform as a port material though.

1.  In general, no, although subwoofers tend to be more predictable with the port opening on the same baffle as the driver.

2.  PVC pipe is what I usually use.

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40 inch long port at 5 or 6 inches diameter??  Ewwww...that's a lot of port.  Should that box have been better sealed instead?  Did I look at this (sounds almost familiar, in an almost familiar kind of way)?

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

stevdart wrote:

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40 inch long port at 5 or 6 inches diameter?? Ewwww...that's a lot of port. Should that box have been better sealed instead? Did I look at this (sounds almost familiar, in an almost familiar kind of way)?

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My EBP was right around 60 so I probably could have gone sealed. However, the manufacturer recommends a vented enclosure for the subwoofer I had. Also, I have had all sealed subs in the past and I was looking for a little more output than what sealed offered, so I really want to go vented.

DYohn] wrote:

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1. In general, no, although subwoofers tend to be more predictable with the port opening on the same baffle as the driver.

2. PVC pipe is what I usually use.

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Excellent.

A couple more port related questions:

1) What exactly is the "end correction" on WinIsd?

2) Considering that my subwoofer will be in my trunk, is the port velocity that critical as long as I keep it within reasonable values?

I like using numbers.  Makes it so easy to ask and answer questions!

1) End correction is so the program can take into account the effects of the entrance and/or exit of your port being flush, flared, etc.

2) Not nearly as critical, no.  Also, if the high velocity frequency is very low (like ~24Hz or lower) you generally won't hear it anyway.

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re:  end correction in WinISD Pro:

Look for this diagram and the accompanying information in the Help files.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.