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-   -   WinISD - Vent Mach Calc Correct? (http://www.diyaudio.com/forums/multi-way/127464-winisd-vent-mach-calc-correct.html)

BlueWizard 2nd August 2008 10:29 PM

WinISD - Vent Mach Calc Correct?
 
I'm dabbling in WinISD and simulating the Dayton 12" Classic Woofer (DC300-8) using the data from the Dayton Spec Sheet.

I'm running the simulation with a 2.3 cu.ft cabinet with a single 2" port and a cabinet resonance between 25hz and 35hz.

With a resonance of 28hz, I get a port length of 3.14 inches and a VENT MACH of 0.24.

But when I go to Linear Team's website, and do the same calculations, I get a port length of 3.22 inches; that's not a problem. The problem is, I use the Acoustic Power calculator, which comes up with 0.38. I then plug that into the Port Length Calculator which comes up with a Vent Mach of 0.04.

Port Length Calculation -
http://www.linearteam.dk/default.asp...ventcalculator

Acoustic Power Calculation -
http://www.linearteam.dk/default.aspx?pageid=acousticpc

Dayton Speaker Spec Sheet -
http://partsexpress.com/pdf/DC300-8_specsheet.pdf

2.3 Cu.Ft.
28hz required resonance
ONE 2" Diameter Port

I know there are some problems with the current Beta version of WinISD. Does anyone know if one of the problems is an incorrect Vent Mach calculation.

According to sources, the Vent Mach should be below 0.16 or 0.17.

I've seen some calculators for Vent Mach, but they all require Acoustical Power, but no definition of how to calculate acoustical power. Also, the formulas I saw, use very different parameters than the Team Linear Acoustic Power calculator.

Any idea which of Linear Team's Vent Mach calculations is correct?

Thought I should work out this detail before I go cutting holes in my cabinet.

steve/bluewizard

Brett 2nd August 2008 10:39 PM

With WinISD what power setting are you using in the test. Default is 1W from memory. Set it to the driver's rated value, or the max power you can input before Xmax and see what you come up with.

richie00boy 2nd August 2008 10:40 PM

2 inch vent diameter is simply far too small for a 12 inch woofer at that frequency, you need to be looking at 3 inch absolute minimum. 4 ideally.

pjpoes 3rd August 2008 12:36 AM

I would agree 2" is going to be a bit small causing noticeable port chuffing. The way I figure out what size port to use is to go by output rather than xmax or power rating. Of course both of those must be taken into account, but as an example, my current subwoofers use TC3000 drivers capable of over 30mm of xmax at 2000 watts power handeling (These aren't normal 3000's they were one offs, er two offs for me). If I was to design around the limits of the drivers, I would end up with an unmanageably large port, but it would be unneeded. Who honestly listens to bass frequencies in excess of 120db's for a long enough duration for chuffing to be noticeable. In the case of my TC drivers I would need a 13" port that is 57 inches long (Using a round port) to keep air velocity that low at max power (4000 watts). First off I dont have 4000 watts going to them, and second its not even close to hitting xmax at 4000 watts according to Winisd. My port ended up being rectangular (Slot) and was roughly 4"x13" and while it would have noticeable chuff if fed all the power it could handle, it would be playing so loud I don't think port noise would be my biggest concern.

I have a few other programs which calculate port noise and they are all consistent with Winisd (Close enough), so I would use it as a rough guide.

Also, the port should be flared on both ends. If you can't flare it at both ends, then cut the port at an angle roughly 45 degrees, where its center bisection is the desired length.

Ron E 3rd August 2008 04:14 AM

Re: WinISD - Vent Mach Calc Correct?
 
Quote:

Originally posted by BlueWizard
According to sources, the Vent Mach should be below 0.16 or 0.17.
This woofer really needs almost twice the volume you want to give it.
You are only getting ~3dB more than a sealed box when cutting the vented box size in half.

Vent air speed is traditionally held at 5% of the speed of sound (Mach 0.05), and it has been tested that you can go with double this value if both ends are flared.

With your parameters and box size, I get 0.2 mach at 80 watts input.

Use a 3" or 4" diameter port, if possible.


BlueWizard 3rd August 2008 06:39 AM

The box is already built and has a CTS woofer, however, while still in excellent shape, I'm looking for a potential woofer to replace the CTS. The Dayton Classic looked pretty close to the limited parameters I have available for the CTS.

When I built the cabinets, I had an excellent wood shop at my disposal. Now, I'm working with extremely limited resources. Ideally, I would like to use PVC pipe and simply use a router to flair the ends, but I don't have a router or a place to work.

So, I ordered ready-made flared-end ports from Part Express. Unfortunately, the large flairs require a 4" diameter hole, and even a hole that size is going to be an exceptionally tight fit.

So, I was modeling the Dayton woofer in my existing cabinet to see how it would work. It didn't look that bad until I started looking into the Vent Mach.

Part of the confusion, is the Linear Team's website says one thing and their software says another. Using the website parameters I'm well within the safe zone, but using the data from the software, I'm a little on the high side. So, I don't know which one to believe. And I can't really find a second source for the calculations. Though several here have confirmed that the WinISD is in the ballpark.

The speaker is rated at 80watts, I've got a 50 watt amp. Viewing the SPL graph in WinISD; 1 watt gives me 92db at one meter. TEN watts give me 102db, and anything over 10 watts is off the chart.

If I back out to 10 feet (3 Meters), 1 watt is 82db, 10 watts is 92db, and 25 watts is 96db. That's plenty loud enough.

As to port size, if I could go as large as 2.5" the Vent Mach in WinISD drops down to 0.15. But when, where, and how do I get a flared 2.5" x 5.36" port that only requires a minimum of tools to install?

Comparing a sealed box to the described vented box, I gain 10hz on the low end, going from roughly 43hz sealed down to 33hz vented.

Further complicating matters is the fact that when the port flares are placed end-to-end, no tube, it is functionally about 2-7/8". That length corresponds to a tuning of 29hz and a Vent Mach of 0.23.

At this stage I'm just trying to see what the possibilities are, and to learn to use the WinISD sofware. I'm sure there is a lot more to the software than what I have found do far.

Thanks for the replies.

steve/bluewizard

BlueWizard 3rd August 2008 07:19 PM

Not that anyone is still listening, but I thought I should make an amendment to the convoluted mess above.

I am rebuilding the CTS configuration to correct a ridiculous error in the porting. The original design book I used suggested a port of 2"x 4" for my cabinet volume. But this was taken off a chart; duct length on the Y-axis and Tuning Frequency on the X-axis with cabinet volumes in the center of the graph. Follow tuning frequency up until it intersects with the cabinet volume then follow it over to the port length on the Y-axis.

No, not very sophisticated or accurate. Though on-line port length calculators confirm that is in the ball park.

The speakers are ancient (1985), but in excellent shape, and no published parameters are available, and I don't currently have the resources to set up test equipment.

But I thought about the possibility of replacing the CTS with speakers with known and established T/S values; hence, the Dayton 12" Classic which are vaguely similar to the CTS.

I'm well aware that this is all hack-chop-hope construction, but with my current resources, it's the best I can do.

So, patch up the CTS for now, and see where the future takes me.

Using WinISD, I did determined that TWO 2" ports would give a nice response from the Daytons with a Vent Mach of 0.06 and a vent length of 6.56". That might actually be do-able given my available space, if I ever get around to it.

In the meantime, I will model a few other speakers, but the Dayton's in general appeal to me.

Again, thanks for the responses and Vent Mach confirmation to what is otherwise a very rookie question.

Steve/bluewizard

Conrad Hoffman 3rd August 2008 11:02 PM

IMO you're not that far off according to my old software. Though the driver will do far better in something like 5.8 ft^3 and 24.7 Hz tuning, that's just not practical for most. A sealed box between 1.4 and 2.76 ft^3 gives Qs of .9 to .7 and an F3 of about 43 Hz. Too much roll off for my tastes, but it could be equalized with various circuits- Marchand also makes a good equalizer for this. That way you avoid the mach problem, phase issues, and other complaints about vented systems. All you need is power. 2.3 ft^3 tuned in the region you state won't be too bad, but I wonder if there's a third alternative? Though I don't know how to calculate it, how about an aperiodic system- a stuffed vent? I've built a few that were good, and always liked my old Dynacos. Can anybody offer some thoughts in that direction?


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