How Ports Work: A Barefaced Exploration

(A tidied up transcript of this video: https://www.youtube.com/watch?v=4Y851S9_M9I )

By Alex from Barefaced

Hello, internet. It’s me again, Alex from Barefaced, and it is week three. I'm going to continue explaining things that we do and why we do such things.

Last week, I was talking about the first prototype guitar cab. Now, to explain how things progressed, I think you need to know how ports work—also known as bass reflex devices or Helmholtz resonators, if we're being fancy.

What Is a Port?

Well, what it is, is a hole—get some light, can you see this one here in a cab? What makes it different from just a generic hole is that a properly designed port has been tuned to resonate at a certain frequency.

Question Number One: What Does Resonance Mean?

Resonance is a property of any physical thing that, when excited—be that a bass string, a drum skin, a cymbal, or indeed a floor if you jump on it and it keeps wobbling for a moment after, or a car suspension—there’s a good example: a classic American car chase moment in a car with knackered dampers when the car comes to a halt and it just bounces. It’s got a natural resonant frequency.

Obviously, our voices have natural resonant frequencies, and we can adjust where they go up and down by messing with things internally that control air passages and vibrating chords, as they're known in the vocal cords.

Tuning a Port

This port is tuned. Now, you can make a hole in a box, and it will inherently have a tuning frequency.

The first example of Helmholtz resonators being used in music that I know of is in things like—well—guitar sound holes, violin f-holes, things like that. They all have a resonant frequency. How they work is the air in that space has a mass, and the air is bouncing on the air behind it that’s acting as a spring. So it's literally a mass bouncing on a spring. You’ve got your mass, you’ve got your spring, and the pair bounce together and have a natural frequency they want to move at.

Now, obviously when you play a string on a guitar, the frequency that it's moving at depends on the note you play. But of course, we must not forget that when you play a note, the note isn't one frequency. It's made of a huge stack of frequencies—starting with the fundamental, and then, if it’s a tuned instrument like a guitar, they are a stack of integer multiples: 50 Hz, 100 Hz, 150 Hz, 200 Hz, 250 Hz, 300 Hz, and so on.

You also get percussives, some of which can be subharmonics—so below—and also above. You know: your scratches, your squeaks, your noises, the thump when you push a string and don’t actually let it move at its own speed. So you’re controlling a sort of lower frequency energy.

But you play that note, and the string produces a whole swathe of sounds from all those harmonics and non-harmonic frequencies. The sound then comes off the string, the energy goes into the body, and the sound hole acts as a Helmholtz resonator to improve the efficiency and output of these sounds at low frequencies.

The same happens with a speaker and a port in a box. That port will resonate around its resonant frequency. It will not only work at its resonant frequency—its tuning frequency—but it will work around that region. Once you get above it, it stops moving—far enough above it—and once you get a certain amount below it, it starts moving the wrong way.

Demonstration Time

I thought I would demonstrate this using these excellent props that I have. This here magazine / critical tool, an excellent old issue of Bass Player, entitled "Get Great Time". actually. (It's not just bassists—all musicians need great time. It's not the role of the bassist or the drummer to keep time; you all should be feeling it. Feeling it.)

So this is going to be our speaker cone—our lightweight bit of papery cardboardy stuff.

And this vintage book here—The Physics of Music, which a friend’s father gave me, originally published in 1944 (this is a 1964 edition, so it’s 14 years older than me)—is going to represent the mass of air in the port.

You may wonder why the mass of air in the port is a thicker, heavier thing than the cone. Well, it kind of is, in the sense of music—in the sense of how the physics of this is working.

The cone has a higher resonant frequency than a correctly tuned port, or is certainly operating at higher frequencies.

So, here is our speaker. Now let’s imagine, just for the sake of it—I’m not going to put them in a box or anything—you’re going to have to imagine the box. And it doesn’t even matter if the port’s at the back of the box or at the top, bottom, side, or front. It doesn’t matter. The frequencies we’re dealing with are so long that their output from the port is omnidirectional.

You might think, “Oh no, this port’s at the back, so it’s going to be firing sound backwards, and if there isn’t a wall behind it I won’t get that sound.” Doesn’t matter. The sound from that goes in all directions. It doesn’t care that there’s an enclosure in the way in front because that enclosure is really small compared to these massive wavelengths that are dozens of feet long.

Frequency Interaction

So, speaker moves. At high frequencies (I’m not going to move fast enough to do it, so you’ll have to imagine speeding me up), speaker is moving.

At these higher frequencies, this port—the air is just a solid block. It’s just not moving at all.

Then we get to near the tuning frequency, and what will happen is the port starts moving with the speaker. Because the speaker is moving and it’s making the port move, the port actually helps out. The speaker no longer has to move as much because it’s transferring its energy to the port. So the speaker moves less and the port moves more. But they are moving in synchrony.

Above the port tuning frequency, speaker does all the work. Then, we approach the port tuning frequency, and the speaker moves less, and the port moves more.

Then, once we start dropping below the port tuning frequency—not right at it, but as we go below it—they move out of phase.

In phase means moving together. They don’t have to move the same amount, but they have to be moving the same direction and change direction at exactly the same time. That’s 100% in phase—zero degrees out of phase.

Then the phase can shift. Reverse phase—180 degrees—is when that moves that way and this moves that way. So, ba ba.

Once that happens, that means we’re a significant amount below the tuning frequency of the port. The speaker pushes air out as the port sucks air in, and vice versa.

That means, instead of the speaker pressurizing the room and then depressurizing it—which is how we're making low frequencies, squish–suck, squish–suck—this squishes, this one sucks. That means they cancel out.

So below the port tuning frequency—significantly below—you get no acoustic output of those low frequencies. You will get sound, but it will all be higher frequency stuff. You’ll get distortion components that are generated. This might be trying to generate 20 Hz sound, but it’s not going to manage to generate 20 Hz sound. It will probably generate a whole load of distortion harmonics at 40 and 60 and 80, which will then muddy up your sound in strange ways.

Summary

That is basically how ports work. So, if we look back at this: speaker goes in here, moves in and out. Air here moves in and out. The critical thing to understand is this thing here—it’s not a leak.

You don’t make a hole in it to make it easier for the speaker to move. Because if it just made it easier for the speaker to move, all that would happen is the speaker wouldn’t be able to generate these low frequencies because it wouldn’t be able to do this pressurizing and depressurizing of the room.

It is really about the fact that above the port tuning frequency—significantly above—the air mass is like a solid lump that completely blocks the output from the speaker. It doesn’t let it out.

If you put a port directly behind the speaker so it’s in the line of fire, you get leakage. You will get leakage of mid-range and treble frequencies, but you won’t get the lows leaking out—and it’s the lows leaking out that would be a problem causing the capsulation.

So yes, mids and highs can sneak through. But the low frequencies are blocked.

Then, as we get down towards the port tuning frequency, the port is doing its thing. It’s resonating. The speaker’s moving as well. They’re working together to pressurize the room. Once we get below the port tuning frequency—significantly below—the phase has gone from 0° out of phase (100% in phase) and it’s got more and more out of phase until they’re fully 180° out of phase.

That’s when we stay in this state of low frequencies from speaker and port completely cancelling each other.

Wrapping Up

Hopefully, that gives you a vague understanding of what’s going on with a bass reflex port.

Let me know if that made any sense to you. If it didn’t, I can do more explanations, more examples, maybe make some things that show you things resonating. But I think it was a good idea to just chuck this out there, see what you make of it. Ask me questions about what you’ve seen here. Ask me lots of questions about ports, and I will do a follow-up next week or the week after or something like that.

Thank you. I’m signing off now before this gets too long.

Oh—and also, I’ve got a new microphone attached, so let me know how the sound is. I think I’ve set the colour temperature correctly on the camera for the big light we’ve got up there, so again—let me know what you think.

Thank you. I’ve been Alex from Barefaced, and this has been some knowledge for you.

Goodbye.