Capstone Project
For my capstone project, I built a custom guitar speaker cabinet to test how different speakers and cab configurations affect guitar tone.
The cabinet was built out of 3/4 inch birch plywood to be 14.5 inches x 18.5inches x 30 inches with 2x12 speakers. I added the ability to remove the middle 6.5 inches off the back of the cabinet to allow for an open back configuration. Additionally, I added a hole, 2 inches in diameter, on the front of the baffle to allow for a port. There are three pipes and a block that can be in the porthole to allow for different low end frequencies can be emphasized, or you can block it off.
Building the cab was a tedious learning process. After setting the baffle 1.5 inches back from the front of the cab and adding all the sides, the inside dimensions had been decreased to 12.25 X 16.5 x 28.5. This brought the inside volume to 5760.5625 cubic inches. This is relevant to the math of the port holes.
The Math
To calculate the port length we need 3 things: the cabinet's volume, the diameter of the port hole, and the frequency we are tuning. In this case we have a volume of 5760.5625 cubic inches, a port hole diameter of 2 inches, and we are going to solve for 3 different tuning frequencies using this equation:
L =(2.35625 x 10^4 x D^2)/(V x F^2) - (k x D)
L = Length of Port
D = Diameter of port
F = Frequency
V = Volume of cab
k = End correction
The end correction value is determined by the flanges on the ends of the port hole. In this case there is one end flanged - the one on the outside of the baffle. In the case of a one side flange and one side free, the end correction is .732.
For an F of 52 L =
2.35625 x 10^4 x 2^2)/(V x 52^2) - (.732 x 2)
= 7.564 in
For an F of 62 L =
2.35625 x 10^4 x 2^2)/(V x 62^2) - (.732 x 2)
= 5.133 Inches
For an F of 84 L =
2.35625 x 10^4 x 2^2)/(V x 84^2) - (.732 x 2)
= 2.13 Inches
For the purposes of the project, I rounded the L lengths to 7 1/2 inches, 5 1/8 inches, and 2 1/8 inches.
Results and Remarks
This project was aimed at highlighting the importance of the speaker and cabinet in the guitar tone. In the video, when the speaker model changes, the difference is most obvious. The port holes of the cab affect the low end response mostly in a range that is not useful for standard guitar tone, which obscures their impact. The port holes definitely add some low end chunk, but not in a range that is emphasized by the speaker and instrument. The dimensions of the cab certainly differentiate it from other 2 x 12 cabs in that it is a bigger cabinet for a rounder low end that does lend itself to the range of the instrument. It is chunkier than my ADA 2 x 12 cab that is much smaller. Builders in the past have often prioritized factors beyond acoustical engineering to provide easily accessible, affordable cabinets and combos for many to enjoy. One of these such builders was Leo Fender when he was developing his combos in the 60s. These amps go for up to $10,000 on the used market now. I think the main thing I learned was that the best way to change the sound of a cabinet is to put different speakers in it. If you feel like your cabinet is too woofy, you can compliment that with a particularly thin sounding speaker. Port holes are not particularly useful for a guitar cabinet, but can be very helpful for a lower instrument like bass. You see this reflected in what is available in the market. There are far more ported bass cabs. The open back and closed back certainly made a difference in the sound, although not as dramatically as I thought it would. Removing the back of a cab not designed to be open back could be an interesting and nuanced tool.