I am making a sub woofer box to go behind the seats in a small isuzu cab over truck. Space is at a premium so I have had to make an odd shaped box to achieve the volume needed. Incidentally I managed to get all the volume needed(25 liters x2) for the two Dayton DCS205-4 8 inch sub woofers I've chosen.
My question is about port proximity to the rear of the driver..... I don't think I have too much choice and it needs to be set up the way it is in the pictures.
What do you guys think about the port entrance so close to the back of the speaker?
My question is about port proximity to the rear of the driver..... I don't think I have too much choice and it needs to be set up the way it is in the pictures.
What do you guys think about the port entrance so close to the back of the speaker?
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The port, being that close to the floor of the enclosure would tune lower than expected.
Eyeballing the port length, I'd suggest that it is likely much too long for your intended usage. The small enclosed space of a vehicle will produce gain starting at a frequency higher than in a room in a home.
Do you have a means of setting a high pass filter?
Regarding the sizing of the port, larger diameter ports have lower velocity, though physical length must be increased. High velocities have high losses. In an ideal world, 11 meters / second or less velocity is preferred, though in practice this is not possible with some drivers. ( their T/S parameters are geared to small box use )
Depending on the music you listen to, i'd suggest a tuning in the mid to high 30's.
25 liters net ( after subtracting any bracing, the driver displacement ( typically only the magnet volume ) and considering the port as a solid projection tuned to 25 hz will require a 50 mm ID port 13.36 inches long ( with the inside unflanged )
30 hz 25 liter NET would require a 50 mm port to be 8.84 inches long
35 hz 25 liter NET would require a 50 mm port to be 6.11 inches long
Small enclosures are more sensitive to port length changes, since the volume ratio of the port to the enclosure size is greater than a much larger enclosure volume, and the net volume changes quickly when port length changes.
If you can attach the port tube in a temporary fashion and use a frequency generator to find the lowest excursion around the expected tuning, this is the best way to go.
diyuser201, thanks for the fantastic reply, posts like yours are the reason i have been reading diyaudio a lot over the last ten years.
Yes the tuning has been tuning lower than expected/calculated in winisd. I ended up using a 65mm internal diameter piece of pipe i found laying around my yard. Currently I am at 340mm and tuned to 30hz. I am using a dats v2 doing impedance sweeps to see where i am tuned to. I am looking for a tune of 32/33 hz as i do listen to plenty of modern bass heavy music in amongst everything else i listen too.
My original post was more specifically about what the effect may be of the port being so close to the speaker though. As you can see in this picture one end or the port is actually right up against the driver basket.
Yes the tuning has been tuning lower than expected/calculated in winisd. I ended up using a 65mm internal diameter piece of pipe i found laying around my yard. Currently I am at 340mm and tuned to 30hz. I am using a dats v2 doing impedance sweeps to see where i am tuned to. I am looking for a tune of 32/33 hz as i do listen to plenty of modern bass heavy music in amongst everything else i listen too.
My original post was more specifically about what the effect may be of the port being so close to the speaker though. As you can see in this picture one end or the port is actually right up against the driver basket.
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I see no issue with the port in it's current location. You will have a low pass filtering effect from the seat back, and frequencies shorter than the tube length ( lower than approximately 1015 hz ) will not make it to the end of the port.
It is possible that you may get some noise from the spider assembly or leads under very high excursions, though this will most likely be quite a bit lower in level than the low frequencies produced by the woofer. In short, I'd not worry about it.
I'd suggest a small amount of polyfill or rockwool in the enclosure, keeping it away from the vent terminus, and away from the cone.
It is possible that you may get some noise from the spider assembly or leads under very high excursions, though this will most likely be quite a bit lower in level than the low frequencies produced by the woofer. In short, I'd not worry about it.
I'd suggest a small amount of polyfill or rockwool in the enclosure, keeping it away from the vent terminus, and away from the cone.
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