I am working on a couple of new ported subwoofer designs. I continue to struggle with the same issue I have always come up against for high-excursion drivers in ported designs. This issue is the balance of port size, port air velocity, and port resonance. I have been on forums and have read articles for years, concluding that there are many conflicting opinions.
I see many comments about the maximum acceptable port velocity being < 10 m/s, < 26 m/s, and < 34m/s. In my opinion, < 10m/s is unworkable. This limit imposes physical requirements on the design that result in something nobody wants in their living room. This limit also often results in port resonance at low frequencies. Many have said they worry more about port chuffing/compression and do not worry much about port resonance. I’m not sure I agree. The same high-power, high-excursion conditions that generate high air velocities also generates harmonics. If port resonance occurs below 2x crossover frequency, the largest harmonics will be allowed to sing.
I am currently running a subwoofer that generates port velocities in the mid-upper 30’s m/s at about 22Hz at max excursion. I can tell you the chuffing is audible when driven with a 22Hz sine wave input. This effect, however, is never really observed during normal listening. Max excursion happens well above the driver’s thermal power limit, so the chuffing is never sustained and is likely more a short-duration transient signal issue and may be masked by the actual signal which is quite loud. In current designs, I consider these air velocities a little high, and I shoot for something below 30 m/s.
I am looking to use a slot port about 1.5-inch x 14.75-inch. It will have about 0.5-inch round over on each end to take the sharp edges off (I realize more would be better, but I’m trying not to complicate the build). To get 20Hz tuning, I end up with 1st port resonance around 180Hz and max air velocity of about 30 m/s (this is at 44mm p-p excursion). This includes a high-pass filter to limit excursion below the port frequency. It seems to me this will have to be a fair compromise.
Am I out of my mind? Are people here really building < 10m/s ports?
I see many comments about the maximum acceptable port velocity being < 10 m/s, < 26 m/s, and < 34m/s. In my opinion, < 10m/s is unworkable. This limit imposes physical requirements on the design that result in something nobody wants in their living room. This limit also often results in port resonance at low frequencies. Many have said they worry more about port chuffing/compression and do not worry much about port resonance. I’m not sure I agree. The same high-power, high-excursion conditions that generate high air velocities also generates harmonics. If port resonance occurs below 2x crossover frequency, the largest harmonics will be allowed to sing.
I am currently running a subwoofer that generates port velocities in the mid-upper 30’s m/s at about 22Hz at max excursion. I can tell you the chuffing is audible when driven with a 22Hz sine wave input. This effect, however, is never really observed during normal listening. Max excursion happens well above the driver’s thermal power limit, so the chuffing is never sustained and is likely more a short-duration transient signal issue and may be masked by the actual signal which is quite loud. In current designs, I consider these air velocities a little high, and I shoot for something below 30 m/s.
I am looking to use a slot port about 1.5-inch x 14.75-inch. It will have about 0.5-inch round over on each end to take the sharp edges off (I realize more would be better, but I’m trying not to complicate the build). To get 20Hz tuning, I end up with 1st port resonance around 180Hz and max air velocity of about 30 m/s (this is at 44mm p-p excursion). This includes a high-pass filter to limit excursion below the port frequency. It seems to me this will have to be a fair compromise.
Am I out of my mind? Are people here really building < 10m/s ports?
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