Overporting: definition & effects?

[Mos Fetish]

I understand overporting to be when the "optimum" or indicated length of the port is exceeded in order to extend the low end of the box.

Correct?

Symptons of overporting? Low power handling. Too much bass. Soft & flabby bass. Excessive cone movement.

Correct?

Just trying to clarify. Any thoughts appreciated.

Thanks in advance.

[kelticwizard]

If you exceed the optimum length of your port, it means you are tuning lower than optimum. Depending on how much extra length you are adding, you should have a fairly thin bass, unless you want to give it a special boost in the lower octave or so. You will extend the bass downward, because reflex boxes really don't work too well when driven below Fs, IF you give it that extra boost. Otherwise, your bass will sound thin.

[Mos Fetish]

Hmm, I'm obviously confused :-)

So a "fat" sounding bottom end is more likely to be caused by a higher tuning that creates a hump in the bass response?

[kelticwizard]

Yes, absolutely.

The "hump" will be a bit flabby, but then if you are building a PA or party type speakers, often that sound corresponds to what you want the bass guitar to sound like.

Tune your speakers too high, and you get a higher F3 but a hump in the bass. Depending on how big the hump is, some listeners might prefer that sound.

[Paul W]

You'll also find this referred to as "EBS" or Extended Bass Shelf.

[Ron E]

Overporting is a vague, misleading term. In all honesty you can almost never have enough port, although you can definitely have too little. "Mistuning" is a more accurate word.

EBS is much more than just a "big port", and it does not cause flabby bass unless the driver cannot handle the lower tuning and is distorting..

[Svante]

I am not sure of the proper english terminology, in Swedish, the term for what you describe is "sub-tuning" (in direct translation).

It is not very well defined what the tuning is below, however. In some cases it is when it is below a maximally flat design. In other cases it is when the port is tuned lower than the free air resonance of the driver.

Anyway, lowering the port resonance makes the response slope towards lower frequencies, but also pushes the cutoff knee towars lower frequencies. In other words, the bass gets deeper but weaker.

"Sub-tuning" may be a way to compensate for the room gain, which typically introduces a boost of low frequencies (below ~100 Hz). Too many loudspeakers today are IMO designed to play in free space, even though they are used in rooms.

[kelticwizard]

Mos:

Below is a graph of the same driver-ACI SV12, in two enclosures.

The red is a flat response, in a box 8.2 cu ft and tuned to 18 Hz.

The blue is in a box half that size-4.1 cu ft, and tuned to half an octave higher-25 Hz.

This setup has been suggested by David Weems in his book Designing, Building, Testing Loudspeakers. He recommended for a hobbyist-not so much someone after super hifi, but a good, serviceable way to put a woofer in a smaller box. You get a 2.3 dB hump in the half-size box.

The sound isn't bad, and some people like that slightly fat bass sound. I think this illustrates the principle.

[Svante]

Originally posted by kelticwizard
The blue is in a box half that size-4.1 cu ft, and tuned to half an octave higher-25 Hz.

Hmm, isn't this the oppsite of what poster #1 suggested? Here the tuning is slightly higher than the "optimal".

[elambert]

Here is a better idea of what is being discussed. The driver used in this sim is a Dayton RS Series 15" HO Sub. FS on this driver is 21hz.

White Line: Max Flat Response, 3.55 ft^3 tuned to 22hz
Yellow Line: EBS -3db, 7.1 ft^3 tuned to 16.35 hz
Magenta Line: EBS -6db, 8.3 ft^3 tuned to 12.76 hz

As you can see the bass in the EBS alignments start rolling off earlier however extend the bass lower at the expense of spl higher up. So that would give you your "weaker" bass because the EBS -3 alignment is a full 3db down at 30hz where the flat response alignment is just starting to drop off at 30hz. However, at 17hz where the EBS alignment starts to roll of it's shelf the max flat is almost 8.5db down.

AS in all things speaker related it is a compromise. Extend the bass lower, or keep the power up for a sharper rolloff higher.