Audiopulse

Sealed Systems

Sealed systems have several advantages, they are typically the smallest type of system you can build, easiest, have the least group delay (change of phase with respect to frequency), naturally protect the driver at lower frequencies, have a tame 2nd order roll-off characteristic, and they are the least temperamental when it comes it linear response. While all this sounds lovely, they also have one big shortcoming: They are the least efficient system, especially for low frequencies, so they can quite easily have the most non-linear distortion because the driver(s) has to rely exclusively on their own linear displacement to produce SPL for the entire spectrum. Compared to un-EQ’ed sealed systems, 4th order (vented) systems can have upwards of 6 to 9dB more SPL at tuning (usually below 40Hz) which translates in a 2 to 3 fold advantage. At higher frequencies, sealed sysetms and ported systems with the same driver and power produce similar SPL levels.

Sealed systems are often considered ideal if your purpose is “SQ” (Sound quality). This is probably the wrong reason to do a sealed system. Sealed systems should be used if space is a consideration or if the driver you’re using has a high Qts (above 0.5) then it probably should only be used in a sealed box to prevent unwanted ringing at Fb. Low Qts drivers don’t often benefit from being used in a sealed box and can quite literally be anemic in the low end response because of the high electromotive damping factor. EQ boosts are necessary to bring up the low end response if the driver’s Qts is too low. Higher Qts drivers tend to balance nicely in a sealed box and can have great low end response without Eq in the right volume. If the drivers Qts is high, larger volumes are needed otherwise ringing could occur, while low Qts drivers generally need a only small sealed box, but will still require EQ. If your goal is a very small sealed box, you’ll likely still need to use a low Qts driver and then EQ it.

4th Order Ported Systems

Ported, or rather 4th order vented systems, are commonly stereotyped to have poorer sound quality then that of sealed systems. While this is far more myth than fact, it is true, that improperly ported systems are can devastate the frequency response, and while the driver may be producing very low distortion, the response curve can sound atrocious and affect the sound quality of the system. The fact is, ported systems are much more efficient than sealed systems and therefore can product more SPL with lower distortion especially in the lower frequencies. For full range bass, blind A/B tests suggest that ported systems (especially when passive radiators are used) are preferred to sealed systems and this is attributed to their dynamic headroom or rather ability to play louder with less distortion.

Not all mfr recommend enclosure specs are idea either. If you’re very serious about making your 4th order system linear, than proper modeling using accurate TS parameters will give you very good insight as to what you can expect from your system. We recommend that the ported box be made slightly larger than what you model unless the program accounts for non-linear thermal compression where BL lowers as a function of heat. Also remember that increasing the volume decreases tuning, so adjust for that if you need to by changing the port dimensions.

Port tuning is a function of the driver, the port dimensions and the box volume. Yes, we said driver! However the driver’s affect on the tuning is very slight and we can omit that safely and simply make tuning a function of purely the box and port dimensions. Longer ports and/or larger boxes decrease tuning, while the smaller boxes or shorter ports increase tuning. It is important that the port area is suffice the driver. High BL drivers with large cones and lots of power behind them needs lots of port area, while a lesser driver may not need quite as much. It can be difficult to fit ports in certain types of boxes with certain types of drivers at least if you’re attempting to maintain linear response so again, modeling software is critical. Multiple ports work the same as one port. It is simply a construction convenience; however, it is ideal to have the least amount of friction in a port so heavy flaring on both ends is ideal as well as using one port over more than one port to decrease the surface area of the port walls.

In a 4th order vented system, the driver does indeed roll off at 24dB per octave after tuning as the driver and port resonator’s energy cancel each other out as the system unloads. At tuning the port produces nearly all of the SPL of the system for a very short frequency range. As frequencies go higher, the active driver does more and more of the job. Because the system is vented, there is no air pressure to protect the driver as the port unloads. Also, unlike sealed systems, the driver’s excursion exponentially increases below tuning, so unless the driver is filtered, then mechanical damage can quite easily occur with surprisingly little power. This filter is often called a subsonic filter and should be set at or just below the tuning frequency so that the active drivers physically excursion reaches two maximum peaks, one before and one just after tuning. At tuning (Fc) the active driver will nearly stand still and the port does all the work. This is also the point where the system pulls the most current and power so if the amplifier is not up to the job, clipping is usually heard at Fc first.

4th Order Passive Radiator Systems

If you did not read “ported systems”, go ahead and read that then come back here. Passive systems are very similar to ported systems. They are a 4th order system, have a steep roll off below tuning, gain lots of low end sensitivity from the resonator and require approximately the same internal air volume. The difference is, ports are literally replaced with passive radiators. Now there is not easy conversion, but there are several key differences that could make PR systems ideal. PR systems don’t outright suffer from port turbulence or port resonance. These two distortion mechanisms can compromise sound quality in a ported system especially if you can hear the port! You’ll not likely be able to hear a passive radiator unless its suspension is compressing at Fc.

Passive radiators should ideally be used in pairs of 2 with a single active but this is not always necessary. Because they are resonated by the active driver and have a lot more mass and a lower resonate frequency they tend to have more momentum and move further which is why pairs are often used to maintain linearity and decrease potential compression. Audiopulse VMP’s are high excursion passive radiators and can be used on a 1 to 1 ratio with an active driver. Using one active with one passive or two passive one side of the baffle has a distinct advantage if the subwoofer is coupled or bolted to the vehicle. The subwoofer can convert the momentum of the passive to car and behave like an LFE bass shaker. If the passive are mass balanced then the subwoofer will not behave this way, either way the caustic energy would be relatively the same. The Audiopulse VMP also has another distinct advantage: It can be returned without rebuilding the box my simply loading or unloading hte mass washers. Adding mass to a passive radiator is analogous to increasing the length of a port. Adjusting tuning for maximum SPL or maximum low end extension is now possible.

Other Systems

The 4th order bandpass boxes are not a system we recommend for car audio. They have limited SPL and very limited bandwidth. In practice a well executed 4th order bandpass box will only be as loud as a sealed box, but with a smaller working frequency range especially in the higher frequencies. A better approach is to attempt a 6th order system where both the front and rear chambers are either vented or use passive radiators. You can then vent between those chambers to create an 8th order system, but these more elaborate types of systems are very complex and may not offer practical benefits over a typical 4th order vented box for full range bass. Horned systems or transmission line systems can also be used to increase efficiency and SPL. But these boxes tend to be very large and also difficult to design.