Have you ever wondered why a bass port is typically situated in the rear of a speaker and not on the front? Well, you're not alone. That's exactly the question we'll be addressing in this episode of Ask The Expert.
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In order to answer this question properly, it makes sense to first discuss how a bass port actually works.
Briefly explained; the point where the bass rolls off from the woofer – i.e where the frequencies in the source material are lower than what the woofer is physically capable of reproducing – can gain extra output through the use of a port, which effectively extends the frequency response of the woofer itself, and allows it to produce deeper bass than would otherwise be possible. With this explained, we can safely conclude that a speaker with a bass port is a more efficient speaker.
Of course, the original question was: “Why is the port on the back of the speaker and not on the front?”
With a port, there is always a risk of some port noise. When the sound is being pushed back and forth through the port, depending on the sound pressure, and depending on how loud your speakers are playing, you can sometimes experience excessive noise coming from the port itself. This is, of course, not ideal. We don't want to be able to hear this excess noise from the port, nor do we want to hear the excess midrange frequencies coming out through the port, which can often also be an issue.
So the solution to alleviate these sort of problems is, quite simply, to place the port on the back of the loudspeaker. It seems almost obvious and not very "technical" but, by having the port on the back, those excess undesirable noises are lot less audible, simply because they have a harder time reaching your ears from around the back of the loudspeaker.
Another interesting discussion about bass ports – and one that often leads to the question in itself "why on the back of the speaker? That's where my wall is" – is the common misconception that a rear-firing bass port makes placing the speaker close to a rear wall impossible.
This is, in fact, not the case – as long as the distance between the back of the speaker and the wall is more than the diameter of the port itself, then the performance and efficiency of the port is not negatively impacted at all. This mainly due to the fact that sound is dispersed from a loudspeaker in a somewhat spherical shape, i.e in all directions at once, and not linearly.
in this sense, it doesn't really matter where the port is situated because the sound from the woofer and the port somewhat blend together anyway, due to the very long wavelengths of the particular frequencies that they produce. They blend together and disperse from the loudspeaker like a sphere, therefore making their location – front or rear – irrelevant in terms of sound dispersion.
If, however, you have personally experienced that a front-ported speaker sounds better close to a wall than a rear-ported speaker does, then that's more likely to be a question of sound tuning, meaning that the manufacturer decided to tune the speaker so that it sounds better closer to the wall. It's not actually due to the placement of the port itself.
To summarise; the placement of a port on the back of a speaker helps alleviate unwanted and excessive port noise, but it has little to no bearing on the performance of the loudspeaker, and certainly no bearing on the feasibility of placing the loudspeaker close to a rear wall.