The Benefits of a Sealed Cabinet, and the Subs That Make ours Possible

One of the first questions we often get asked when people see our loudspeakers for the first time is “where’s the port?” Some are immediately excited when we tell them ours is a sealed design having experienced the fast transient response and more accurate low end that usually typifies a good sealed design, others immediately put off, having associated sealed designs with bass light, low output, high distortion monitors of yesteryear. There are, to be certain, costs and benefits to choosing a sealed deployment, not least of which is the demand that a sealed enclosure places on the performance of its low frequency drivers. But to understand why we’ve chosen a sealed enclosure, it’s perhaps best to start with why most monitor designs employ ports, transmission lines or passive radiators, how these cabinets acoustically work, the differences between these approaches as well as the similarities, and the more or less singular goal of every one of these alignments.

Let’s start with the venerable port. Whether it’s a tube or a slot, flared or straight, a port is another term for a Helmholtz resonator. The essential way it works in a loudspeaker is by tuning this resonant pipe or slot to a specific frequency, which helps increase the low frequency output of the loudspeaker as a whole by taking advantage of the pressure differential at this frequency to sympathetically produce additional pitched output along with the pistonic output of the driver itself. In other words, the port essentially takes some of the load off the driver as far as the volume of air its diaphragm alone must displace in order to reproduce a certain sound pressure level at a given frequency. The lower the frequency, and the higher the sound pressure level, the harder it becomes for any driver to behave linearly, as the cone itself must travel further and further, eventually reaching a point to where the suspension can no longer behave linearly, causing distortion to rise dramatically and eventually even physical damage to the driver. A port helps lighten this load, potentially doubling the amount of low frequency output a given driver can produce.

There are, however, significant problems with ports. Firstly, a port’s pressure differential has precisely the opposite effect below its tuning frequency. Output falls off dramatically, and cone excursion skyrockets – essentially the port now determines the low frequency limit of the loudspeaker as a whole. The electrical and mechanical performance of the driver itself also more or less mandates a certain size for both the ported box and the port itself in order to maintain linear response up until the roll off point (this is a bit of an over-simplification as there are multiple port alignments with different response compromises, but maximally flat magnitude response is the most common choice). For a given tuning, one can always trade a narrower opening for a shorter port, but this comes with its own problems, as the narrower the opening of the port, the higher the air velocity inside the port for a given output level. High air velocity creates a myriad of negative side effects, such as port compression or turbulence (the audible “chuff” that occurs when the air escaping the port must decelerate too rapidly to normalize to the acoustic impedance of the room). An adequately sized port for adequately low speed (a general rule of thumb is no more than 5% of the speed of sound in air) often necessitates a practically infeasible port length.

Passive radiators and transmission lines represent potential solutions to some of these problems. A classical quarter wave folded transmission line is essentially a damped, stuffed maze inside of the loudspeaker itself equivalent to ¼ the wavelength of the tuning frequency. By maintaining a standing column of air within this quarter wave, turbulence and distortion is cut dramatically. The physical space required internally is significant however, and the geometry must be far more precise that a traditional port. Another potential alternative is the passive radiator – essentially a dummy driver with no motor structure (the magnet portion) that allows the internal backpressure that the active driver creates to move it sympathetically with the driver’s output. In this case, the tuning is determined by the mass of the diaphragm itself, with small washers usually added to adjust the tuning. Passive radiators represent a compelling alternative to a port. An equivalent tuning can be created in a smaller area, and some find the nature of the way it reproduces transients relative to a port more appealing. However, the mechanical excursion of the radiator itself is still limited in the same way that the active driver is, so as a rule of thumb the effective surface area/suspension linearity must be 50% greater than the active driver to limit distortion.

All three of these solutions, however, suffer from one immutable flaw – the output of the transmission line, port or radiator is out of phase with output of the driver itself, as it is acoustically reactive to the driver’s output, and the phase relationship varies by frequency, making it next to impossible to align the phase output of the two geometrically. To make matters worse, because the output is acoustically tied to the phase curve of the driver itself, any digital correction applied to the active driver will move the passive element commensurately. For this reason above any other, we chose a sealed cabinet – simply put, this it what allows our DSP correction algorithm to perfect phase in the low frequencies and not just the mids and highs, which contributes greatly to the speed and accuracy of the low end in our monitors.

The downside to this choice, of course, is that our subwoofers need to work much harder for the same amount of output. Which is why we spent a huge amount of time and resources on the custom SEAS designed subwoofer in our loudspeakers. Derived from their Extreme series subwoofer transducers, which were originally designed for the Linkwitz Orion open baffle design, arguably an even more demanding application for a driver, our 8” subs are capable of a massive 28mm of linear peak to peak excursion, and an astonishing 56mm before damage. 2” voice coils, a massive tall boy surround, titanium formers and an exceptionally symmetrical, linear spider allow our subs to take the kind of power and reach the kind of depths that would be considered reasonable for most 12” transducers, while still retaining pistonic linearity all the way to 1.2khz (nearly 3 octaves past its crossover point). In no uncertain terms, with a larger cabinet and a massive port or radiator, we could have gotten even more low frequency output from these monsters. But even in our sealed enclosures, a single one can reproduce a 40hz sine wave at 103db continuous before reaching the end its linear excursion, and because our phase linearity is so accurate, a true 6db center sum is possible in a good room for the pair. And the accuracy and speed we receive in exchange is well worth it.   

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