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Triticum Agricolam

6th Order Port Area Experiment

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On 8/6/2017 at 1:32 PM, WalledSonic said:

But it seems that near the tuning frequency, the large port prevents impedance rise.

Looking forward to more data!  Cool stuff man

That's what I'm seeing.


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34 minutes ago, Raptorman said:

Now, post up in-car response graphs of each. That's what really matters ;)

Although I'm down with any and all data, I dont know if in-car response curve is necessary to illustrate the point of this thread.  If the in-car response somehow affects the way the driver loads/unloads at or below tuning, then I could see how that may be helpful. 

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It's completely necessary. It's the most necessary thing to test IMO. Unless this isn't a mobile audio forum and our ears don't perceive pressure-levels and only perceive impedances lol.

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Other points to be made are "how much power you can put to a speaker" and "how low it can actually play before it launches".  I think all are valid points, really, so fair enough sir.

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1 hour ago, Raptorman said:

Now, post up in-car response graphs of each. That's what really matters ;)

I intend to. I think both in-vehicle and halfspace measurements are useful so plan on doing both as soon as I get time. 

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I got some time today to take some SPL/frequency response measurements.  I was hoping to test everything all at one, but I screwed up when I was making the lower tuned, large port configuration so it wasn't included in today's testing.  I'll get to it eventually.  For today I tested the large and small rear port, higher tuned configuration, the sealed rear chamber config, and the lower tuned small rear port config.  I was able to take measurements both outdoors and in-vehicle.  Over all I tested 48 different port configurations and power levels.  Its a lot of data but I'll try to organize it and present it as best as I can over several posts.  

So first off, here is my outdoor testing apparatus:


Mic is one meter from the port opening.  I measured at 38 volts (about 360 watts at 4 ohms) and then decreased the input signal in 4 dB increments down to -20 dB.  This should show how the frequency response changes (if any) over different power levels.  I picked 38 volts for my max power level since that's about all I dare put into my SA-12 at 15 Hz.  That should be pushing it right up to Xmax.  For the in-vehicle measurements is pretty much the same setup in my Durango.  I put the box against the tailgate with the port firing up, mic was on the passenger side dash. 


So, on to the first test results.  Here is the the plugged port configuration (effectively a 4th order):


The frequency response looks pretty consistent across different power levels (which is what we want).  To highlight differences, in the measurement software I can offset the measurements, so what I do is I offset the lower power levels so they are equal to the max output (-0 dB) measurement.  Ideally the lines will then be all stacked on top of each other.  If we are getting any compression (either port compression or power compression) that will show up as the higher power measurements being lower than the lower power ones.  Here is what we get:


All the lines are stacked up on top of each other well, this is good.  

Now the large rear port configuration:


And here they are with the measurements offset:


Overall this looks pretty good, we can start to see the effects of a little port compression between 30 - 40 Hz.  You will notice the yellow line (higher power measurement) is a bit below the lower power lines.  Probably loosing a little over a dB to port compression at that highest power level.  

Here is the small rear port config:


The offset measurements:


With the configuration we can see much more significant losses to port compression.  Here is a more zoomed in view:


Below 45 Hz you can see at each increasing power level there is a further reduced increase in output.  Comparing the highest power level measurement (the red line) to the lowest (the yellow line) there is about 2.5 dB of output that is lost to compression between 30-40 hz.  

Here is the lower tuned, small port config:


The results here are similar to the higher tuned small port config, the port compression doesn't become apparent until a lower frequency due to the lower tuning.  


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How do those four configurations compare to each other?  Here is the output of all four at the highest power level:


The yellow line is the large port config, the pink line is the plugged port (4th order) config, the red line is the small port config, and the blue-green line is the small port lower tuned config. 

So the whole reason I performed this experiment was to see how a series-tuned box will small rear port area performs compared to one with large rear port area.  Does the small rear port area config get louder?  The answer is yes, and no.  It depends on the situation.  Between about 42 - 64 hz it does get louder, above and especially below the large rear port area config beats it though.  What I find interesting is the config with the highest peak output is the plugged rear port (4th order) configuration.  It has the highest peak output of everything by a fairly significant margin.  You can also see that the large rear port config has the widest bandwidth (significantly). 

Based on what is shown here with the SPL measurements what I theorize is happening is at higher power levels the small rear port configuration gets so much port compression that it begins to perform somewhat like a 4th order bandpass.  The peak output increases, but at the expense of higher & lower frequency output (and reduced bandwidth).  Since the rear chamber still leaks into the front chamber, you don't get quite the peak in output of the 4th order config, overall the performance ends up being somewhere in between the large rear port config and the 4th order config.  If you go back and look at the impedance plots, they show pretty much the same thing with the small rear port config measuring between the large rear port and plugged rear port configs.  

Next up will be the in-vehicle measurements.  I'll try to get those up later tonight. 

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what was the difference in rise between the 3 styles?

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