Subwoofer struggling to do excursion on a box

raven colin

raven colin

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May 27, 2023
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I got a 30$ subwoofer just for fun just to do experiments with. It lacks spec's the only information i found about it is it has a 2 inch coil 80 watts rms per coil and its dual 8 ohm and that's it for the spec's. I put on a (estimated) 65L enclosure with a 4 inch diameter port and its about 36cm in length, but when i started playing bass though it, it sounds pretty decent it hits the 20's and 30's decently, but i started to smell burning coil i looked at it as low notes play and it barely moves at all therefore the pole vent barely sucks any air to cool the coil down. This subwoofer does do a good amount of excursion free air it's definitely something to do with the box. what are my options to fix this issue?
 
All vented systems exhibit low excursion around the vent tuning frequency.
In your case, 65l with a 10.1cm port ~36cm long means that's around 29-30Hz, so if those were the kind of frequencies you were testing with then you're seeing entirely normal results.
Try testing with tones around 40-50 Hz and see if excursion is more apparent.
 
David Morison said:
All vented systems exhibit low excursion around the vent tuning frequency.
In your case, 65l with a 10.1cm port ~36cm long means that's around 29-30Hz, so if those were the kind of frequencies you were testing with then you're seeing entirely normal results.
Try testing with tones around 40-50 Hz and see if excursion is more apparent.
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There is more excursion on those tones but i mostly listen to rebassed and decaf song's on this subwoofer and the songs i listen to mostly are rebassed to 26hz - 40hz that's why i went after a 30hz tune. But since it's not doing much excursion i cant play 26hz - 40hz for very long at high volumes because the coil doesn't get enough air and will get fried.
 
Right, in that case you'll be better to get the tuning frequency down to the lower limit (or just below) of the range you'll use it in. That will allow more excursion with your normal program content without risk of excessive excursion from content lower in frequency than your tuning frequency.
If you want to get down to say 25Hz, your 10.1cm port would need lengthened to about 52cm long. As that's likely to be longer than the depth of the cab, you'll need to put a bend in - ordinary plumbing pipe may be a good source of such a bend.
 
David Morison said:
Right, in that case you'll be better to get the tuning frequency down to the lower limit (or just below) of the range you'll use it in. That will allow more excursion with your normal program content without risk of excessive excursion from content lower in frequency than your tuning frequency.
If you want to get down to say 25Hz, your 10.1cm port would need lengthened to about 52cm long. As that's likely to be longer than the depth of the cab, you'll need to put a bend in - ordinary plumbing pipe may be a good source of such a bend.
Click to expand...
I see, i could fit a 52cm port in my box it will only have 1-2cm for breathing room is that alright? And i watched some videos about ports from "Dakoustics" on youtube on that video i watched he said that "bigger port's play lower than their tuning" and i gave it a thought would shortening the port be better or worse in my case?
 
You don't want it as close as 1-2cm from the back wall, that will likely throw the tuning off and possibly cause it to choke up at higher volume. The rough rule of thumb is to keep the end of the port at least one diameter away from any other surface in the box to avoid problems like that.
Shortening the port would raise its tuning frequency. That would raise the frequency at which you have minimum excursion (& cooling), possibly putting it further into the band of frequencies you're using this sub for, so not a good idea.
Also, all ports tend to unload below the tuning frequency - this means they allow much more excursion, without providing any acoustic gain - we definitely don't want this occurring within the operating band of the sub as distortion will rise at the very least, with damage to the driver being a real risk too.
 
David Morison said:
You don't want it as close as 1-2cm from the back wall, that will likely throw the tuning off and possibly cause it to choke up at higher volume. The rough rule of thumb is to keep the end of the port at least one diameter away from any other surface in the box to avoid problems like that.
Shortening the port would raise its tuning frequency. That would raise the frequency at which you have minimum excursion (& cooling), possibly putting it further into the band of frequencies you're using this sub for, so not a good idea.
Also, all ports tend to unload below the tuning frequency - this means they allow much more excursion, without providing any acoustic gain - we definitely don't want this occurring within the operating band of the sub as distortion will rise at the very least, with damage to the driver being a real risk too.
Click to expand...
I remeasured the inside of my box and if there was a 52cm port there, there would be 6.5cm of breathing room. And while i was there i also extended the port as close to as i could get it to 52cm, the best i was able to do was 49cm because my left over 4 inch pipes were flared. Will that do the job? I would extend it to 52cm in the future but as of for now i no longer have any 4 inch pipes left.
 
David Morison said:
Well, if you've been able to put that extension in then it's worth a try as-is, for sure.
That should drop the tuning to around 25.5Hz, so really, close enough to my 52cm estimate from earlier.
Click to expand...
Well it's been 2 days of me using it with that long of a port and i blew the coil due to thermals just like how i guessed. shame that, that woofer didn't even last a month on me but stuff happens i guess.
 
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