Quarter Wave Theory for Building Walls

The 1/4 Wave or ABCD Theory was a big fad in SPL for quite some time, especially in walled subwoofer systems. With this article I hope to explain what it is, how to calculate it and you can draw your own conclussion from there.

In a wall if the port opening is located ¼ of the tuned frequency’s wave length away from the windshield/microphone you gain constructive interference. This occurs because the originating wave (from the port) and the reflected wave (from the windshield) will "mix" with each other at the peaks & troughs of their wave form. This effectively boosts the amplitude (hence volume) of the wave. Thats the theory.

Aside from helping to boost the SPL measured at the microphone, you'll also get a condition in which woofers will handle far more power that they are rated to handle, provided the burp is very short. Ever seen a shattered cone in an SPL car and the owner blames it on "unloading", this is what they mean. That the load on the cone (caused by the pressure waves) wasnt great enough to hold them together, a good SPL system exploits the waves load to increase the power handling of the system.

If the system is run at a frequency other than tuned frequency, the proximity of woofers and ports to the microphone is more critical because destructive interference between woofers and the port is possible. With destructive interference, the waves mix and the peaks are lowered as they dont co-incide.

OK so all this sounds great ! How can I give it a try ? Determine the distance (in feet) from the baffle to the windshield. Multiply that measurement by 4. Divide 1132 (which is the speed of sound at feet per second at sea level) by the result. Thats the 1/4th wavelength frequency that corresponds to the distance between the baffle and the windshield.
In other words, that’s the frequency which you should be tuning your system to.