Round and Slot Ports

Bob Brines

Member
2003-01-31 10:11 pm
I did some modeling of a project I am working on, and an item came up that I thought might be of general interest. This is a MLTL type cabinet in two flavors. One is with a slot port at the bottom of the cabinet, the other with a round port near the bottom. The driver is positioned to completely remove the 5th harmonic, which also reduces but does not eliminate the 3rd harmonic. This is done for cosmetic reasons. Placing the driver at ~1/4 down from the top puts the driver near ear level, while putting the driver at ~1/2 down from the top to eliminate the 3rd harmonic is well below ear level.

Whether a slot port or a round port is used, the frequency response plot is essentially identical up to 500Hz. If this cabinet were to be used as the bass for a FAST system, the choice of ports becomes purely cosmetic. However, if the cabinet were to be used for a single-driver system, there are further considerations.

Note the group of high-order harmonic that begins at 500 Hz. Even with adequate stuffing to smooth out the bass response, these harmonics are high enough to color the mid-range. These harmonics appear in the same range as the baffle hump, which exacerbates the the problem, particularly for those who refuse to use EQ.

The position of a round port is important. If the port was at the bottom of the pipe, the response would look essentially identical to the slot port. However, moving the port up the pipe moves a null across the harmonics. Therefore, the port can be placed wherever needed for the smoothest output. This can be verified by measurements of the actual cabinet. Unfortunately, the construction of the cabinet will never match the assumptions of the model. The only way to get exact results is to build a test mule that will allow the port to be moved for testing.

In conclusion, if the design is for a bass speaker, the port position is irrelevant. If the cabinet will be used full-range, then the best results will be had from a properly positioned round port. As an aside, the best results are obtained by using the smallest port consistent will the expected power output. Small short ports produce lower volume harmonics than large long ports.

But then, if the port is placed on the back of the cabinet, the upper harmonics will be dispersed as they reflect off of the wall and this whole discussion is moot. Theory is theory, practice is practice.

Bob
 

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A height off the floor of 1/2 the radius makes the area at the perimeter of the port equal to the area of the port. From there it is a quadratic expansion to the perimeter of the speaker. So for a 4" port, the distance off the floor would be 1" for equal areas.

How it actually works in the real world, I don't know beyond the pair of CHR70.2 computer speakers which are sitting on my desk. I am very happy with them, and they image quite well.