What is an audio filter? Is filtering the same as equalization? What role does filtering have with regards to audio fidelity? Filters play a role in the successful conversion of analog/digital signals, speakers/crossovers, and record productions. It’s time to dive into an investigation of filter types, their capabilities, and quality.
We all know what filters do. A coffee filter allows the brewed liquid to pass through the ground coffee beans into the cup positioned below. A fuel filter stops impurities from getting into your carburetor and causing your engine to run rough. Audio filters work the same way except that the stuff being filtered is audio frequencies. The range of frequencies extends from the low end through middle register sounds to the high frequencies and beyond. Audio engineers use filters to remove or attenuate undesirable frequency components.
Imagine you’re doing a recording session in a studio or location that is located close to a construction site or a subway. The rumbling equipment or trains generate low frequencies that can vibrate structures, walls, and even the nearby ground causing low frequencies to get into the room where you’re recording. As hard as studio designers try to physically isolate live recording rooms from the outside world, sometimes intrusive frequencies get past the architectural barriers.
Recording engineers do their own isolating with “shock mounts” for microphones. These are elaborate cages that suspend the microphone in the middle of the cage using elastic bands. Low frequencies can’t get to the microphone. If somehow they do, the engineer can switch on a HPF or High Pass Filter to remove annoying low frequencies. Many studio quality microphones have these filters built in to the housing of the mic. The HPF allows the high frequencies to pass through but stops the low frequencies…say below around 75 Hz.
If that’s not enough to get control of the subway noise, the console strips come equipped with filters of their own. There are High Pass Filters, Low Pass Filters, Bandpass Filters, Notch Filters, and Bandcut/Band Reject Filters. Equalizers are also available to craft the timbral characteristics of a sound but they are active devices, which can both attenuate and amplify specific frequency areas. Filters are capable of removing specific frequency ranges. They cannot boost anything.
A filter has the following parameters. There is the corner/cutoff frequency, which is the frequency where the amplitude is attenuated by -3 dB. If the engineer wants to remove the sound of a whining HVAC air handler, they would run the signal through a Low Pass Filter (LPF) and then adjust the Corner /Cutoff Frequency at the point where the whining is tolerable but the meat of the sound is still in tact. The Corner Frequency is adjustable on most designs.
The steepness or slope of the attenuation is called the “order” of the filter. We have first, second, and third order slopes corresponding to 6 dB per octave, 12 dB per octave, and 18 dB per octave. The higher the order the steeper the slope. A third order filter is extremely aggressive in removing frequencies near the Corner Frequency.
Filters can be built to operate on analog and digital signals.
Part II tomorrow.