How ports work
The short answer:
Ports deal with all the deep lows and give the woofer(s) an easier ride - tune them right and use a good geometry and there are no downsides. Tune them too high or too low and the tone won't be as good and the cab won't handle as much power. Make them too big and the cab will be huge (this hardly ever happens); make them too small and they'll stop working right as you play louder, killing your bottom end (this is a really common problem). We design, test, adjust, test, etc and make sure they're as good as can be.
This explanation was originally written regarding ports in bass cabs, i.e. cabs used for bass guitar and double bass, for live sound, but this same science and engineering is applicate to ports in all loudspeaker cabinets, from your in-ear headphones to the biggest PA subs.
The better answer:
There are a huge number of misconceptions about ports - here's some of them:
- The ports relieve the pressure in the box and make it easier for the woofers to move
- The ports push air out when the woofers move back and suck air in when the woofers move forwards
- The ports 'fire' sound out in whichever direction they face - so rear ports need a wall behind them to bounce the sound back to you
- Ported cabs sound boomier than sealed cabs
So let's address them one at a time:
1. The ports do not relieve the pressure at all - at frequencies more than an octave above the tuning frequency the ports act just like a solid wall and don't allow any air through. At frequencies within about an octave above the tuning frequency and a third of an octave below the tuning frequency, the port takes energy from the vibrating woofer and uses it to set the plug of air filling the port into vibrating back and forth motion.
This reduces the amount the woofer is vibrating (the amplitude heading towards zero at the tuning frequency) thus hugely increasing the excursion limited power handling. The port is much better than the woofer at vibrating at these low frequencies (the woofer's motion is damped by the mechanical compliance and electrical/magnetic circuit) so at the tuning frequency it produces almost 100% of the sound, with its output diminishing to about 20% of the sound half an octave above the tuning frequency.
2. The only time where the ports are pushing air out when the woofers move back and sucking air in when the woofers are moving forwards is well below the tuning frequency - at this point the port has no useful function and in fact is detrimental as it makes the woofers think that they are not in a box at all but in free air - which causes the excursion to vastly increase but little sound come out as the positive woofer movement is mostly cancelled by the negative port air movement and vice versa.
This is why it is critical to not tune a cab too high, and is one of the reasons why a lot of cabs (particularly small ones) struggle with low notes, especially at the bottom of the E and B (and lower) strings (overly high port tuning increases the mid-bass which is the only sort of bass ('fake' bass) that some cabs can do in quantity).
3. The wavelengths of the sounds coming from a loudspeaker port are huge compared to the size of the port and even our very large ports are small relative to the wavelength's emitted. Sound is a longitudinal wave which travels through the air in the form of pressure changes. When the wavelength is long relative to the size of the source and you're up close to it (the true nearfield) the wave is considered to be a pressure mode (you'll have felt this standing next to a subwoofer stack in a club or putting your head up against you bass cab). As you move further away from the source that pressure mode transforms into a velocity mode. Because the frequencies are long relative to the source size, the source has no ability to control their direction - they are emitted omnidirectionally.
This means that once the pressure wave just outside the port exit has transformed into a velocity wave, the output goes everywhere! Consequently, in the far-field the port output will sound the same whether the port is on the front/side/back/top/bottom/etc of the cab. The output will be different if there is a wall behind the cab but it will differ in the same way regardless of the port position/direction.
4. The reason ported cabs are deemed to have worse transient response than sealed cabs is because their transfer function is different. An 'ideal' ported cab has a 24dB/octave cut-off but an 'ideal' sealed cab has a 12dB/octave cut-off. The steeper cut-off rate causes the transient response to be less accurate - this can be proved mathematically and is true to all filters - the steeper the slope the greater rate of change of the phase shift and thus the greater the group delay.
However, an 'ideal' ported cab isn't actually ideal! The way our 12XN ported cabs are designed means that the LF roll-off is actually much closer to a 12dB/octave transfer function, which results in excellent transient response (in conjunction with the higher efficiency and power handling of a ported vs sealed cab).
The passive radiator - a speaker that think it's a tuned port
You may have seen loudspeakers which have 'passive radiators' or 'drone cones' - this is a woofer with no magnet and no voice coil, in other words just the cone mounted via flexible suspension onto a frame. Looking at one of these is the easiest way to understand ports. Let's say your woofer has a free-air resonant frequency of 35Hz and Q of 0.25. Once we put this woofer into an enclosure the air in the box acts as a spring, raising the resonant frequency of the cab by reducing its suspension compliance. The woofer now has a resonant frequency of 70Hz and Q of 0.5. (Q is how much a device wants to resonate at its resonant frequency: 0.707 is often considered the ideal balance of frequency and transient response, 0.5 is called 'critically damped' and has less bass response but better transient response, 1 is under-damped so gives more bass response but poor transient response). The problem with a resonance of 70Hz and Q of 0.5 is that it will sound thin in the lows.
How a passive radiator helps a simple cab perform better
If we take then a passive radiator with a free-air resonant frequency of 20Hz and Q of 0.3 and put it into the same box, its resonant frequency shifts to 40Hz and Q of 0.6. What then happens is at 100Hz the woofer is producing all the output but as you head down towards 80Hz the passive radiator starts to scavenge energy from the woofer and re-emit it itself. By 40Hz the passive radiator is taking most of the energy from the woofer and re-emitting it as sound. If the woofer was just on its own, although it would be taking in approximately the same about of electrical energy it would be losing most of that energy as heat and not creating a useful amount of sound down low.
Adding the passive radiator massively improves the conversion of electrical energy to sound energy at very low frequencies because it is a better low frequency resonant system than the woofer alone. The way the energy is transferred from woofer to passive radiator is that the passive radiator pushes back against the air in the enclosure, creating high pressure against the woofer around the PR's resonant frequency. This high pressure improves the transfer of energy into the air (this is what makes compression drivers and horns more efficient than normal dome tweeters), so more sound energy comes out of the woofer into the inside of the enclosure and then into and back out of the PR into the outside world.
Ports are not just a hole or tube that lets air in and out
Hopefully by thinking of a port as being like a passive radiator we can get away from the misconceptions of it blowing air out or sucking air in or of acting like a leak which reduces the back pressure in the box. As we've explained above, it actually increases the back pressure in the box at certain frequencies thus improving the efficiency of the system!
A subwoofer that's made of air?
So how does a port differ to a passive radiator? Instead of being a paper/cardboard/plastic/aluminium/etc membrane vibrating back and forth it is instead a mass of air doing the vibrating. In a perfect port in a simplified world this mass of air never changes - it is always the same lump of air which vibrates back and forth and thus creates vibrations in the outside air that you can hear. If you look at passive radiator design information you'll find that PRs should have approximately double the volume displacement of the woofers it is working with - so you'd pair a 10" woofer with two 10" PRs of equal excursion or a 12" woofer with a 15" PR of 30% greater excursion. This may seem a little strange - why would the PR that's helping out in the lows need to be able to move more air than the woofer which is driving it?!
Volume displacement requirements
Well the answer is that to produce a given SPL at 50Hz takes twice as much excursion as at 100Hz - this is obvious if you think about it because the woofer moves in one direction for twice as long at 50Hz so it travels twice as far outwards before it starts coming back in and then moves twice as far inwards before it starts going back out and so on. Our woofer reaches its peak excursion about 2/3 of an octave above the tuning frequency of our PR. At that point the PR is moving about 40% of the air and the woofer 60% - to put it another way, without the PR helping out the woofer would need to move about 65% more air.
Down at the tuning frequency the PR is moving about 90% of the air and the woofer 10%. So in the case of our hypothetical cab the PR doing 50% of the work at 56Hz, increasing to moving 90% of the air at 40Hz and then dropping back down to sharing half of the air movement requirements at 33Hz. This 33-56Hz region requires twice as much air be moved as the 66-112Hz region where the woofer is doing the lion's share of the work because it is an octave below. Hence the PR must have double the volume displacement of the woofer or the full ability of the woofer will not be able to be utilised.
Below the tuning frequency...
The key difference between a port and a PR is that at very low frequencies a port just acts a like a huge hole in the cab - basically it turns it into an open baffle design where the woofer back wave starts to cancel the front wave. A PR can obviously never act like a hole because it is a solid membrane - instead it acts like it is in free air itself and thus sucks energy out of the woofer at its free-air resonant frequency and produces out of phase output with the main woofer causing a distinct notch in response. When we see this plotted on a response graph this notch makes the roll-off of a PR appear steeper than with a port but it isn't really a change in transfer function so much as an additional function. Head lower in response and the port and PR response curve tend to converge and continue on the same slope together.
Why ports need to be big - not just little after-thought tubes stuck in the cab
Getting back to the ports, this large volume displacement requirement certainly doesn't go away. We don't want our port to be blowing air in and out, we want the air in the port to resonate in a controlled fashion and thus set the air in the room vibrating, creating extra bottom end. The larger the area of our port, the better it can do that because the shorter the distance the port air needs to oscillate back and forth. This is why our ports are big - so they work right!
Balancing area, length and volume
Unfortunately the larger the area of your port, the longer it needs to be to maintain the tuning frequency. This has two effects - firstly it makes the whole cab bigger, and secondly it can cause the port to be longer than the cab's depth or width and thus require a bend or kink (we can't practically use the height of the cab in a bass guitar cab because an upfiring port would be liable to have beer spilt into it and a downfiring port will vary in performance depending on the floor). These issues are why passive radiators are common in home cinema subwoofers - their very low tuning would require an incredibly long port, especially in the small enclosures they use. Putting a bend in a port tends to introduce unpredictable losses which reduce the port output and also lower the power level at which air noises due to turbulence (chuffing) will be heard.
The larger a cab, assuming a fixed port area, the shorter the port will be for the same tuning frequency. If we're running into port length/volume issues we investigate making the internal volume of the cab bigger and then seeing out that works out with are target response curve, sensitivity, port function, max SPL etc.
Optimisation for different purposes
Loudspeaker design is proven to be as much an art as a science when it comes to details like port optimisation for its specific purpose. Although there are formulae that can be used to give theoretically "ideal" tuning frequencies, enclosure volumes and port dimensions, these do not take into account the real world usage of the loudspeaker. They would have been very useful before we could easily make accurate computer models for preliminary design or before accurate measurement equipment was accessible - thankfully those days are long behind us and that is how we've managed to optimise our ported Barefaced cabs (the 12XN range) quite so well.
Impedance curve power compression
Not only do larger ports operating at lower velocity couple more effectively with the room, they also maintain the correct impedance curve seen by the amplifier. At the tuning frequency the impedance is at minimum and the amplifier delivers maximum power but when an undersized port exhibits overly high air velocities the impedance around the tuning frequency increases, so not only is the efficiency of the port dropping because of turbulence and frictional losses but also the amplifier is delivering less power.
One could say this is like trying to drive a car across a hot high altitude sandy desert (Atacama?) - you have less power from the engine because the air is less dense (high temperature and altitude = less oxygen to burn the fuel) and you can't put the power down effectively because the wheels are spinning the power away in the sand. If Barefaced made a car for that it would have turbocharging, 4WD, diff. locks and knobbly tyres!
Midrange and treble leakage and resonance
Many people forget that a loudspeaker driver is a two-sided device. The cone vibrates to generate sound and that wide bandwidth sound comes off both the front and the rear of the cone. This means that not only is our speaker generating low frequencies to drive the reflex port, it's also throwing mids and highs backwards into the cab. Some of this energy will inevitably escape through the port, so it's key that it doesn't interfere negatively with the midrange and treble coming from the front of the cone. There are various solutions to this, but it really comes down to three things:
1. Do not position the port so you can see the rear of the cone through it as sound most likes to travel in straight lines (and the shorter the path, the louder the sound remains).
2. Ensure the port geometry does not cause resonances at any leakage frequencies as this will not only accentuate them it will also spoil the time domain response through midrange overhang.
3. Design the inside of the cab to effectively damp mid and high frequencies, thus attenuating the loudness of any port broadband leakage.
Our 12XN ported cabs are particularly well designed with regards to these details, with a notable point being that the port dividers are positioned at unequal distances so that any standing waves in the port will have unequal resonant frequencies - this minimises the chance of there being significant detrimental midrange output from the ports. We go a step further than merely ensuring these resonant frequencies are different - we've also tried to ensure that the harmonics are different too, only coinciding when you're a long way up into the harmonic series.
Ports you can feel
Our ports are up the side of the front of our cabs because that is where you FEEL them best when you're standing up close playing bass. Once you're out in the far-field then it doesn't make any difference but there is definitely a physical benefit when you're on stage near your cab.
We don't use strongly flared ports because although that would allow us to reduce the port area and shrink the total cab size, when a flared port is pushed into turbulence the effective port length drastically shortens, thus raising the tuning frequency which causes a hump in response and more output in the mid-bass, and worse unloading and risk over over-excursion at low frequencies. This causes a negative spiral of even higher air velocities and more turbulence, distortion and compression. Heavily flared ports are great in more controlled environments but we do not see high SPL live sound as the right place for them.
Thermal power compression
Another key feature of our ports is their ability to help cool the cab. By running vertically up the cab they provide a path for the warm air to flow out of the upper sections and cool air to be drawn in through the lower sections. The power ratings you see on bass cabs are thermal ratings taken from the driver manufacturers' ratings, which are done in free air. Is a plywood boxed lined with damping material (which is always a good thermal insulator) the same as free air? Definitely not!
By making many of our cabs as tall and slim as possible we encourage cooling convection currents within the cab and then our vertical ports do the rest to get that warm air out. This doesn't just help with the ultimate thermal power handling before driver destruction, this also reduces thermal power compression by keeping the voice coils cooler. Hot voice coils exhibit higher resistance which reduces the power your amplifier can deliver - this is what causes bassists to have to turn up their amp as a long gig progresses, and not only is this bad for tone it also increases the risk of premature driver death.