Click here to purchase the entire book in PDF format. Shelving Filter The nice thing about high pass and low pass filters is that you can reduce (or eliminate) things you don't want (like low-frequency noise from air conditioners, for example.) But, what if you want to boost all your low frequencies instead of cutting all your high's? This is when a shelving filter comes in handy. The response curve of shelving filters most closely resemble their high- and low-pass filter counterparts with a minor difference. As their name suggests, the curve of these filters level out at a specified frequency called the stop frequency. In addition, there is a second defining frequency called the turnover frequency which is the frequency at which the response is 3 dB above or below 0 dB. This is illustrated in Figure 6.17. Figure 6.17: The frequency responses of a low-shelving filter with a turnover frequency of 100 kHz. The blue curve shows a maximum boost of 12 dB and the black curve shows a maximum cut of 12 dB. The transition ratio is sort of analogous to the order of the filter and is calculated using the turnover and stop frequencies as shown below. $$R_{T} = \frac{f_{stop}}{f_{turnover}}$$ (7.7) where $R_{T}$ is the transition ratio. The closer the transition ratio is to 1, the greater the slope of the transition in gain from the unaffected to the affected frequency ranges. This is because if $R_{T} = 1$ then $f_{stop} = f_{turnover}$. These filters are available as high- and low-frequency shelving units, boosting high and low frequencies respectively. In addition, they typically have a symmetrical response. If the transition ratio is less than 1, then the filter is a low shelving filter. If the transition ratio is greater than 1, then the filter is a high shelving filter. The disadvantage of these components lies in their potential to boost frequencies above and below the audible audio range causing at the least wasted amplifier power and at the worst, loudspeaker damage. For example, if you use a high shelf filter with a stop frequency of 10 kHz to increase the level of the high end by 12 dB to brighten things up a bit, you will probably also wind up boosting signals above your hearing range. In a typical case, this may cause some unpredictable signals from your tweeter due to increased intermodulation distortion of signals you can't even hear. To reduce these unwanted effects, super sonic and subsonic signals can be attenuated using a low pass or high pass filter respectively outside the audio band. Using a peaking filter at the appropriate frequency instead of a filter with a shelving response can avoid the problem altogether. The most common application of this equalizer is the tone controls on home sound systems. These bass and treble controls generally have a maximum slope of 6 dB per octave and reciprocal characteristics. They are also frequently seen on equalizer modules on small mixing consoles.