You’ll see many variations for loudspeaker cabinets, including vented (and ported), transmission lines, aperiodic , horn loaded and sealed boxes. This article attempts to describe some of the benefits of ported loudspeaker enclosures, dispelling some myths and explains some of the finer points of design.
Which type chosen by a designer depends largely on the characteristics of the bass driver selected, the space that the speakers are to be played in, the choice of music and the bass characteristics desired. The final consumer of course may select on a smaller number of variables, but usually wants the best and tautest bass performance for the loudspeaker size and room characteristics possible.
For near field monitors, sealed boxes or rear ported boxes are usually selected, whereas larger spaces usually require larger driver area coupling to a larger room volume where larger loudspeaker driver area or travel improves bass delivery and extension.
There are pros and cons to each type of enclosure design. For example, sealed enclosures generally have superior transient low frequency speed but are ultimately limited in how low they can play unless large drive units in very large cabinets are employed. The other unfortunate fact is that unlike several decades ago, there are very few large diameter woofers available which are suitable for use in sealed designs today.
Vented cabinets can be more efficient. This is because the vent or port uses the energy created by the vibrating woofer and translates it into a resonating plug of air producing the lower frequencies much more efficiently. The vent, or port limits the amount that the woofer needs to vibrate. The woofer is less efficient at producing bass because at low frequencies, its motion is both electrically and mechanically damped. The port starts producing some of the sound from around an octave above tuning, and by the tuning frequency all of the bass is produced by the port or vent, with nothing being produced by the woofer directly. The woofer’s travel at this point is minimised to almost nothing at all. This increases power handling and output making it a very efficient way of generating bass and reduced driver distortion if properly tuned. The port stops producing bass output at about 1/3 of an octave below tuning.
Ported enclosures rely on something called the Helmholtz Resonator principle to work. This is something discovered by a German physicist called Hermann Von Helmholtz and related to how two combined systems (a large vessel and a smaller ported cavity) react at a certain resonance which is related to the sizing of the port for any particular cavity and resonance point. It is governed by the equation:
Fh = (ν/2π) x (√(A/(Vo x Le))) x C
Where Fh = tuning frequency
ν = velocity of sound in air (343m/s)
A = Cross sectional area of the port
Vo = static net volume of air in the cabinet
Le = length of port
C = constant determined by the port geometry and speed of sound.
So knowing these points, you can fine tune a port in theory to generate a reinforcement of a chosen frequency using the bass driver to act on the internal cavity air mass which causes a sympathetic resonance within the air mass in the port.
The value of F should not be randomly chosen but is a direct function of the Thiele Small parameters for the chosen driver. In particular the Qts (total electrical and mechanical damping) of the driver needs to be known.
The tuning frequency is then calculated from
Fb = Tr x Fas where Fb = box tuning frequency, Tr = tuning ratio and Fas = Free Air resonance of=driver
The tuning ratio is derived from the Thiele-Small parameter for Qts ( a measure of compliance and control of the drive unit calculated from mechanical suspension compliance and electrical control from the voice coil and magnet).
To get this right though demands careful driver selection and cabinet tuning and such systems are more complex to design and get right than sealed loudspeakers. Group delay (time delay between applied signal and bass being produced) is greater than for a sealed design. A good ported or vented design will have a group delay inside of 15 milliseconds compared with that of a sealed system of about 5 milliseconds. Few designs seem to get this right though with many commercial designs that RFC has tested having GD values of over 25 milliseconds. Anything much over this figure is synonymous with what some perceive as boomy (tuning the cabinet too high can also exacerbate boom issues) or over-hanging bass from some ported designs but correctly designed, you would be hard pushed to tell the difference between a decent vented design and a sealed box. The RFC Raptors for example have very low group delay, sub 15 millisecs.
The other consideration is port or vent design and how this interacts with the listening room. Too narrow a port area will increase turbulence and cause “chuffing” as well as being far less efficient at coupling to the room compared with a larger port area. The difference between getting this right or wrong even if the correct air mass is used can be significant. Rule of thumb is that the port opening should be 30% minimum of the diameter of the bass driver. This is considerably larger than used in many commercial designs, so many pay scant attention to this important consideration. The larger opening increases coupling efficiency to the room of the resonating air mass within the port and therefore increases the efficiency of the transmission of the velocity wave for the low frequency output. It should also be understood that air in the port does not flow through the port, and the drive unit does not see the vent or port as a “leaky” system. Correctly designed, the air resonates, and for frequencies above cabinet tuning, the enclosure behaves like a sealed enclosure as far as the drive unit is concerned.
Tune a cabinet too high and as well as limiting bass (and we’ve all been to hifi shows and auditions where we listen to ported speakers that have artificially lifted bass then cut off suddenly and have nothing below 60Hz!) it can ultimately damage the drive unit. What happens in these circumstances is that below the cabinet tuning, the woofer self resonance is much lower than cabinet tuning, so that the air mass within the port does not resonate at these frequencies but the enclosure effectively behaves like an open baffle. The woofer excursion then increases, so rear movement of the woofer pushes air from the port and vice-versa. This un-damped movement peaks at driver self resonance where it will eventually start bouncing the voice coil off the end stops with too much (ie very little) applied power. The port has no useful function at all below cabinet tuning frequency in such circumstances and will not produce any bass output. All this will do is wreck the drive unit.
It is fairly common though to come across small enclosures tuned slightly too high to achieve raised bass output at around 60Hz. This is deliberately done in some cases to give the loudspeaker the aural appearance of having impressive bass output for size, but invariably the bass cuts off very rapidly and there is nothing much below 60Hz.
It is for the considerations above that correctly designed ports should not really be messed with as the well meaning enthusiast could end up damaging their loudspeakers. Best advice if bass output is too high is first to move the speakers off wall boundaries, then to either seal the ports completely transforming the boxes to a sealed enclosure although for the driver parameters, this is likely to result in much reduced bass due to the T-S parameter of such drivers not being ideally suited to sealed designs. The other method is to introduce a resistive plug of dense foam or similar of about an inch thick into the port. This modulates the speaker’s impedance characteristics, reducing and smoothing bass output as it creates a semi-aperiodic loading. Part-stuffing the ports or using bunches of straws as was popularised some years ago is NOT recommended.
Designing a ported speaker enclosure is not generally recommended as your first foray into loudspeaker building as a DIY project if it’s something you’ve never done before. It is far more difficult to get right than a sealed enclosure or even a transmission line design. There are so many variables from achieving the proper tuning, port noise, group delay, bass depth, and driver/system loading. Get the port length wrong or the diameter wrong and as well as sounding awful, you’ll not achieve the end result desired and could even damage your bass units and amplifier.
Another misconception regarding ported designs is that they and sealed boxes always behave in an “ideal” fashion, and mathematically it can be proven that a ported design rolls off “ideally” at 24db/octave compared with that of a sealed enclosure which rolls off at closer to 12dB/Octave. Two effects of the steeper slope are phase shift, in the case of the ideal port, of 180 degrees. The effect of 180 degree phase shift is the greater group delay mentioned earlier in this article. However, not all ported speakers behave in this mathematically ideal manner (and nor do we wish them to) and they can be made to perform much closer to the sealed 12dB/octave by virtue of careful driver selection and enclosure and vent design, hence the earlier comment that a good ported enclosure can result in sub 15 millisecs group delay. This is little understood, but it is an important point because it is perfectly possible to have most of the benefits of a sealed enclosure with the bass efficiency of a ported enclosure.
So you now see that not only must a driver be selected for sensitivity, impedance load, off axis response at chosen crossover frequency, but also on where its own resonant frequency point lies. A drivers’ free air resonance coincides with peak excursion for minimum power required (ie driver electrical impedance is at a peak). As a driver approaches resonance, the amount of current required to make it oscillate is reduced but the driver is not operating in a controlled manner due to this being a resonant frequency and we do not want to tune our cabinet at this point. A cabinet is best tuned above driver resonance for this reason although some designs to work well with extended bass shelf tuning. EBS tuning doesn’t work well with all drivers and its use should be considered only where you can achieve adequate well controlled bass output below tuning without increasing driver distortion or risking damage to the drive unit. In general tuning too far above or below driver resonance will create problems and reduce efficiency.
Re-examining port area, there are several reasons why, once the correct tuning is established, for the port area and the perimeter to area ratio of the port to be scrutinised. Too small an area causes port turbulence which reduced efficiency and increases turbulence noise. Turbulence is caused by the velocity of the resonating plug of air exceeding about 7m/s. At and above this figure a notch in response is also produced. One needs to consider an ideal port as having a plug of air that doesn’t leave the port, but vibrates within the port area. For the resonance to load the woofer correctly, some movement to pressurise the cabinet is required, so that plug does move back and forth but critically, it is the same plug of air within the port at all times, ie the woofer’s rear movement is not displacing air from the port or sucking air back in on the forward motion (unless the cabinet is tuned too low or too high).
It has already been suggested that by increasing port area for any given tuning point, turbulence is reduced and efficiency is increased. This is true but another factor used to modify port behaviour is to shape the port such that the perimeter to area ratio is increased, such as with our RFC-Canterbury loudspeaker design designed for Tannoy’s venerable 315hPD drive unit. By adopting a carefully chosen tuning point and shaping the ports as elongated slots, the perimeter to area ratio is increased over a circular port, and this increases boundary friction at the vent edges. The effect is to further modify driver impedance such that extra control is exercised over the driver for a wider tuning range and to lower distortion.
So there you have it. This scratches the surface of what is quite a complex design subject but hopefully also dispels some common misconceptions about vented loudspeakers and hopefully demonstrates why considered and thorough design can in fact deliver a very satisfactory and efficient means of bass production. It is for this reason that so many so-called “high end” loudspeakers use some form of vented tuning. It is true to say that most woofer units available off the shelf are optimised for vented use although many can also be used open baffle and some are still suited to sealed enclosure use, where their Qts combined with correct box design results in an optimal system Qtc figure of around 0.71.