This is most likely going to be a pretty long thread. I got a lot of info/observations.. Plus I like to ramble. So buckle in.
Yesterday I decided to pull my subs and box out to add some toolmaker gear and pull some plastics out. Well I thought this would be the perfect time to do some testing on my box with my MiniDSP umik-1 RTA meter and REW on my laptop. Going to try to not say anything definitive, as with most things in the world what we observe might just be coincidence rather than fact. I will Just merely make my own observations and you can draw from that what you will.
Oh and people give you really weird looks when you have this set up in your driveway:
We will start with my box specs.
I'm sure most of you haven't seen it before so here's a pic:
So with my daily port the net volume comes out to be 18.451 cubic feet. The port itself is 15" in diameter and 9.5" long and according to a common port tuning calculator my tuning is at 35.5hz.
However. My port is 1/2 diameter away from the bottom of the box. Which will in fact lengthen the port. I won't get into the laws of this but anytime your port is closer to a wall then the smallest dimension it will lower tuning and extend the port.
Let's just say that my port is now 8.5" longer. (the bottom ring of my port has an OD of 19" hence the port being 8.5" longer vs 7.5" longer which is half port distance.) This will also change the net to roughly 17.467 cubes. Now with the same port but factoring in the "new" length of 18" it is tuned to 30.7hz.
I'm aware that due to my port configuration being adjacent to two walls and half diameter away it's not exactly going to behave like a 18" long port but for the sake of this thread we will just say it is.
Now I guess the first REW measurement we will look at is in the port:
That peak is centered right about at 27.5hz. Now this is peak spl measured in the port. So this is going to be very close to port tuning where the port is resonating. What we can gather from this is that indeed the port being half diameter from the bottom of the enclosure has made it behave more like 18" port rather than the 9.5" port. Something to consider when building boxes. (let it be known that this is roughly 10 square inches of port per cube and I have observed zero port noise even down to 19hz full tilt.)
This his how the mic was oriented. Centered in the port:
**It's important in this write-up to not focus on the actual spl levels but relative levels. As I had to adjust volume on different measurements to avoid clipping the microphone. So you should really be concerned with the shape of the response rather how loud it is.
Let's move onto the box itself.
Here are three measurements taken all in "half space" or outside:
The Green is no port (Or a 20" opening by .75")
The Blue is daily port in( 15" diameter)
The Gold is sealed. (just took a board and covered the port)
Now what should we gather from this?
Well first off the Green line has a peak at 56 ish hz. If we run the tuning for the box with just a hole it comes out to roughly 53hz tuning. Interesting. Keep in mind though that this is the box as a whole now. Not just an in port measurement. But I'm sure if I put the mic in the open port hole opening it would peak close to 53hz. You should also notice how peaky this response is. Big high tuned ports will always make boxes peaky and really kills your low end.
Now the blue line shows the response of the whole enclosure with the daily port. Notice less than a 1db variance from 28hz to 42hz, then it picks up to peak at about 49hz and barely rolls off from there. But my port was tuned to 28ish what gives? I'm glad you asked. Take for example at 29hz we are just about 10db louder than with the port hole just being open. That's a huge improvement in low end extension. So as you can see the port was able to greatly improve low end extension but the box itself was wanting to still peak higher though. Not as high but still higher. This shows that port tuning doesn't always determine where a box is going to be loudest. (keep in mind this a daily box with relatively low port area.. I'm sure a burp box would peak much closer to port tuning). But it will definitely help the response of your enclosure. And a box that is about a max difference of 4db from 29hz all the way to 80hz where my tone stopped is pretty alright in my book. IMO I would still like it lower lol but I would probably need a larger box to do this.
Now the gold line. This is basically a 20 cube box sealed. I don't have much to say about this but these three tests where all at the same volume/power. And you can see it's a good 3-5db down at most frequencies. If I had the TSPs for my HDDs I would throw this in Winisd or BBP6 and see how the different graphs compare but I don't really want to pull a sub out now and I'm not even sure my woofer tester is working right atm.
Keep in mind none of these tests are low level testing either. this is much closer to high power as most of these were 50 to 100 watts depending on the frequency.
So to recap.. I guess lower tuned ports ftw. High tuning is good for spl. And sealed just sucks... lol no but funny.
Now here are in car measurements. I put the mic on the center console pointed towards the windshield. I chose this position to best reflect the vehicle response as a whole. Not close to any walls for loading to occur etc..
The red line is with the driver door open. Not much change though. The take away from here is that even though my box was fairly flat the cabin still made the spl start rolling off at 60hz with a over a 48db per octave slope. The low end was a little higher than with just the box so that tells us the cabin itself is helping pick up the low end. Whether that is because the cabin or how the port is being affected by the vehicle I don't know. But I do know the overall response has a less steep rolloff down to 20hz.
And when I took SPL measurements at full tilt from 30hz to 55hz there was less than a 3db variance at full power. As you can see from the in car response that pretty much holds true. I can post those up too if you guys want. Those are less controlled though as holding voltage was different at every burp due to pulling more power and not being able to keep my belt from slipping.
Sadly I can't make a response curve of the cabin gain right now with these results as they would be skewed from different volumes. (not just output power but how close the mic is to the source also changes the measured SPL). But you can infer from this picture based on the relative dbs on the curves what the cabin is doing to the wave.
Let's see.... hmm. All testing was done with certain controls. For example I tested the output voltage of my soundcard from 20hz to 80hz to know that it was a flat output. The mic already came with a calibration file but I can't really test it so I just have to rely that it is accurate.
I hope this will enlighten what effect different ports have and in/out of vehicle testing.
Oh yeah and ToolMaker FTW:
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4 15s + Flat Wall + 10k = Loud