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ACTIVE OR PASSIVE CROSSOVER

A crossover network for a subwoofer has the primary job of filtering out high frequency program that is above the intended operating range of the sub. This application is known as a low pass filter, also sometimes referred to as a high cut filter. As a matter of either convenience or system integration the low pass filter is sometimes combined with a second filter that removes low frequencies from the high range speakers. The name for that filter is- wait for it- a high pass filter (or low cut). Whatever bandpass we are shaping, there are two approaches to the application of these concepts. The most common approach is the passive network, which in various configurations is by far the most common for loudspeaker systems in general. A passive network refers to the use of high power handling capacity components AFTER the amplifier and before the speaker load itself. While there are some advantages as far as flexibility and cost, there are some significant downsides as well. The insertion loss that results from the inductive components lowers system efficiency, and passive filter circuits can also affect phase response and damping.

An active network derives its name from the placement of the filter BEFORE the amplifier; the network is designed to operate at line level. The formulas that are used to calculate the crossover are similar in theory to the passive designs, but the voltage and current involved are obviously much lower. There is consequently much less of the associated distortion and loss that we have described for the passive network, plus a few other significant advantages. Active crossovers send the filtered output to the amplifier responsible for each particular frequency range. Although there are cost, space, and complexity issues with using multiple amps, the advantages are improved headroom, lower distortion, and much greater control.

SUBWOOFER DRIVER SELECTION

The selection of the actual subwoofer driver must by necessity be considered throughout the process of choosing the actual type of enclosure. Each enclosure type can demand that a driver of a particular specification is used in order to provide the performance that is expected. In general, sealed box designs tend to expect a driver to have a relatively high Qts, low Fs, very good xmax, and relatively soft suspension. Vented boxes will usually expect the driver to have a somewhat lower Qts, and remember that Fs will need to also be fairly low since you cannot usually tune the box below that frequency. Horn loaded designs do not need a driver to have a low resonance because the length of the path of expansion, and the mouth area of the horn, determine low frequency cutoff.

A useful formula to determine the suitability of a particular driver for a particular box is the Equivalent Bandwidth Product (EBP), also known as the Equivalent Bandwidth Ratio (EBR). It is determined by taking the driver free air resonance (Fs) and dividing it by the electrical Q (Qes). If the resulting number is around 50 or less, you are looking at a driver that is best suited to a sealed type box. If the EBP is between 50 and around 100 or more, the speaker will work best in a vented enclosure. Horn loaded designs will perform best with an EBP of 150 or even higher.

Look closely at xmax values and any indicators of excursion limits for the driver that you are evaluating. The lowest musical octave that the subwoofer has to reproduce will cause far more stress than any higher frequencies. Linear long excursion capabilities combined with good cone control will provide high output low distortion sub bass. Most modern subwoofers will run out of mechanical power handling before they exceed their thermal power handling capacity.

Some users prefer to mount their subwoofer in a "down-firing" orientation, although this particular configuration is not that common for automobile applications. The cone mass of the driver must be taken into consideration, as the tendency for the cone to settle downward can move the voice coil out of the gap and affect xmax and excursion capabilities. The formula that should be used to determine if a subwoofer is acceptable for this kind of arrangement is:

As long as the amount of "sag" does not exceed 5% of xmax, the driver will work just fine in a horizontal configuration.

SINGLE OR DUAL VOICE COILS

A conventional woofer has a single voice coil that moves in the magnetic field and provides the motive force for the cone. A dual voice coil has a second coil wound concentrically with the first coil, and has its own separate terminations. The primary advantage of this arrangement is the ability of the dual voice coil subwoofer to accept both left and right amplifier outputs in a single driver. This can be of obvious benefit in compact, low cost applications since bass is non-directional anyway. An alternative feature of the dual coil speaker is that choices can now be made to most effectively connect the subwoofer to the amplifier. If for example the dual coils are rated at 4 ohms each, you can decide whether to connect them in series (8 ohms) or parallel (2 ohms). This leads to even greater flexibility because the connected voice coil pairs can be connected to additional voice coil pairs to achieve a specific target impedance.

 
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