Bass Reflex – Ported Cabinets
OK, this is the last post on speaker cabinets for a while, I promise! Completing the mini-series that started by talking about Bass Amp Set Ups and covered open back designs and closed back designs. The main type of cabinet design in use today is a ported enclosure. If you have a cab or a Combo it will almost definitely be of this type.
The ported cab (or vented or reflex design) is basically a closed back cabinet with a very special hole in it. Time for a quick sketch:
I say “hole”, but the port is actually a tube or a long slot (something like a tunnel). This clever design tweak enables the cabinet to produce significantly more low-frequency bass than it would normally for its size. Choosing the exact size and length of the port is critical to getting the desired effect.
Have you ever blown across the top of a bottle? The sound you get is from the air inside of the bottle resonating. Remember that in a sealed cabinet, the bass response starts to fall off at the lower frequencies. Now, imagine a big bottle, tuned to boost those lower frequencies. That is pretty much how a reflex port works. It uses the resonance from the reflex port to artificially extend the bass response of the cabinet. Very neat trick.
There are, as ever, some trade offs. The transient response of a reflex designed cab will not be as good as a sealed box. It may not sound as punchy, giving a slower attack, and some notes may ring slightly. That downside is usually more than offset by the extended bass response and smaller cabinet size, which is why even slap bassists are happy with these sorts of cabinets.
If you want the full lowdown on how a reflex port is designed, knock yourself out here. It is reasonably tricky, and very easy to get wrong. A badly designed reflex port can end up “breathing”, making the sound of the air being drawn into the port become audible, or sounding boomy. That only happens with a bad design, and I haven’t ever heard a commercial speaker cabinet suffering from breathing – although I’ve encountered some slightly boomy ones.
The ports can be in the front or the back of the cabinet, and there can be just one or multiple ports. For most commercial bass cabinets, the design choice is usually a slot or a couple of reflex ports (plastic tubes) in the front of the cab.
Hopefully it goes without saying that blocking the reflex port (with any kind of obstruction!) will stop it working effectively. So , if you didn’t know before, now know what those holes in your cab are for!
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