Bass reflex port calculator
A ported box is a Helmholtz resonator: the air in the port and the air in the box form a mass on a spring.
L
Hz
cm2
Port length
17.94cm
- Total port area
- 50.27 cm2
- Peak port air velocity
- 20.00 m/s
- Velocity is below the chuffing threshold Above about 17 m/s ports become audible.
- 0
- Port diameter to stay quiet
- 86.8
- Air displaced by the cone
- 500.0 cm3
- Wavelength at the tuning frequency
- 10.719
Show the working
- Box volume = 50
- Tuning frequency = 32
- Port diameter = 80
- Number of ports = 1
- Driver cone area = 500
- Peak cone excursion = 10
- Total port area = 1 * pi * (80/20)^2 = 50.2655
- Port length = 32 > 0 && 50 > 0 ? (23562.5 * 50.2655) / (32^2 * 50) - 0.732 * sqrt(50.2655/1) : 0 = 17.9427
- Air displaced by the cone = 500 * 10/10 = 500
- Peak port air velocity = 50.2655 > 0 ? 2 * pi * 32 * (500 / 50.2655) / 100 : 0 = 20
- Velocity is below the chuffing threshold = 20 < 17 ? 1 : 0 = 0
- port_area_needed = 500 * 2 * pi * 32 / 1700 = 59.1359
- Port diameter to stay quiet = sqrt(4 * 59.1359 / pi) * 10 = 86.7722
- Wavelength at the tuning frequency = 32 > 0 ? 343 / 32 : 0 = 10.7188
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- port length = 23 562.5 A / (f^2 V) - 0.732 sqrt(A), with A in cm2 and V in litres
- the subtracted term is the end correction for a flanged port
A longer port for a lower tuning eventually will not fit in the box, which is the practical limit on how low a ported design can go.
Below the tuning frequency the port unloads the driver and excursion rises sharply. A high-pass filter protects it.