Cerwin-Vega Releases Bass Management Information
Cerwin-Vega has released Understanding Bass Management In PA Systems, an eleven page PDF document that many musicians will find informative. The short pamphlet reveals several aspects of sound reinforcement in easy to understand language.
Understanding the Crossover Excerpt
Thus far, you’ve made your best choice of type, size, and cost of a subwoofer. You also have the amplifier and speaker cables sorted out. All that’s left is getting the bass to the amplifier, and the midrange and high frequencies to their appropriate drivers in the system. This is accomplished with a frequency dividing network, commonly called a crossover. A crossover is used to divide the signal—effectively sending separate frequency bands to the appropriate amplifier or driver. At this point, we are still referring to a non-powered subwoofer (an external amp is required).
There are two main types of crossovers used in speaker cabinets today. The first type of crossover is called passive. This crossover uses a series of capacitors and coils to divide the frequencies. This is the crossover type most commonly used in home stereo speakers to send bass to the woofer and highs to the tweeter. It may include a level control for the tweeter as well, usually on the speaker jack plate. This type of crossover can also be used in higher power systems to divide the mid and
high frequencies. Some low powered subwoofers found in music stores may also include a passive crossover, but for high powered subwoofer duty, a passive crossover would be very large, heavy, and inefficient.
Because of the size and weight limitations of passive crossovers, there is another type of crossover called the electronic or active crossover. These crossovers are placed before the power amplifiers in a system where the signal is still at line level. They are matched to the system by the number of bands they divide the signal into. As an example, if you are using a subwoofer with a mid/high cabinet, you would, at least, require a 2-way crossover. Your mid/high cabinet may already have a passive type crossover built in that handles the mid/high dividing duties. You may see a switch on your mid/high cabinet marked “full range” and “bi-amp” that can bypass the internal passive crossover. In the “bi-amp” position, your system would need a 3-way crossover — a channel for low frequency, mid frequency, and high frequency — wired to a separate amplifier channel for each frequency band.
The manufacturer’s recommended settings from your owner’s manual should be followed when setting up your electronic crossover. These settings will include the frequencies to define the crossover point, and may include a slope specification. The slope value determines how much of the frequency range is used for the crossover implementation. A higher value (such as 24dB/octave) means a steep crossover slope over a minimal frequency range. A lower value (such as 12dB/octave) means a wider frequency range is used for the crossover function. These specifications are frequently listed in decibels per octave, usually in multiples of 6 such as 12dB/octave or 18db/octave.
You should also understand there are two main groups of electronic crossovers—known as analog and digital crossovers. The analog crossover is a simple device that, for the most part, divides frequencies only. In addition to frequency division, the digital crossover may offer several other functions such as equalization, limiting, and time delay. These additional features can replace many outboard signal processors in an equipment rack.
The time delay function is especially useful when using folded horn subwoofer cabinets, since the sound in the horn has a much longer path to travel than the sound coming from the mid/high portion of the audio. By delaying the mid/high a few milliseconds, you can get both the bass and the mid/high sound to arrive at your ears more or less simultaneously. To properly define your delay settings, you should refer to the speaker manufacturer’s specifications. If you are uncertain how to use time delay, then you should avoid adding it to your system, as this can cause serious phasing problems.