The woofer in a ported speaker box can cause air masses in the port and within the speaker box to pulse in common resonance with the woofer. This is why a port in a speaker box is tuned. The tuning is to the specified resonant frequency. The speaker cone's forward movement is matched to the air mass in the port. The two masses vibrate in phase with each other. This causes the cone of the driver to be virtually motionless, and the audible sound comes mainly from the port.
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Things you need
- Thiele-Small parameters
- Port configuration equations
Determine the resonant frequency of the loudspeaker driver. Resonant frequency is denoted as "fs." It is one of the primary Thiele-Small parameters. These parameters, also know as T/S parameters, are provided with every good-quality component loudspeaker. They can also be found in the Thiele-Small online database.
Use the following equation to tune a round port to a desired frequency:
LV = [(1.463 x 10^7 x R^2) / (Fs^2 x Vb)] - 1.463 x R
Fs = the desired tuning frequency of the enclosure in hertz (Hz).
Lv = the length of the round port in inches.
R = the inside radius of the port tube.
Vb = the internal volume of the enclosure in cubic inches. Convert cubic feet to cubic inches by multiplying by 1728 (1728 = 12 times 12 times 12).
Calculate square vents by using the formula below. It provides a value to R for area in the formula in Step 1 for round ports:
R = square root of (A / Pi).
A = the area of a square vent or port (height x width).
Pi equals approximately 3.141592. (You may remember Pi from high school or junior high school math.)
Apply the derived information for round or square vents/ports to derive a precise port length for the resonant frequency to tune the speaker box. A port length of approximately 5 inches is a good guideline for resonant frequencies ranging from 20 to 80 Hz.
Tips and warnings
- The principle involved in tuning a ported speaker box is akin to blowing across the top of a glass bottle to produce an audible low tone.
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