Nothing makes people get on the dance floor and spaz out more than the ‘drop’. The drop is a musical moment we have all heard before, and is normally associated with nonsensical mutterings like “get low when the whistle blow” or “turn down for what”.
Much like drops from a rollercoaster, these musical drops are only effective if they are from a height. In music, these moments of height are created by building musical tension, usually in sections called ‘build-ups’. One very common type of build-up is one that uses a sound that is constantly rising in pitch, also known as a ‘riser’. There is one crucial system that makes risers possible: filters.
Simply put, filters are a type of circuit used to attenuate the frequency content of a signal. There are two basic types of filters: low-pass filters and high-pass filters. Low-pass filters leave low frequencies untouched while attenuating higher frequencies, and high-pass filters do the opposite. So how are these filters made?
Filters can be built in many different ways, but in this article, we will only be looking at RC filters: filters that use only resistors and capacitors. To make sure everyone is on the same page, a capacitor is a device that stores charge in an electrical field. It is diagrammatically represented as two parallel plates. That is all we will need to know for now.
Below is a diagram of an RC low-pass filter.
Let’s get to analysing it! To simulate a high frequency, let’s say that the switch is turned on and off rapidly (many times per second, and hence, a ‘high’ frequency). The switch would never be closed for long enough to allow input signal, Vin, to reach the capacitor. And hence, the voltage measured by the voltmeter, Vout, would never increase much beyond 0.
Conversely, if the switch was turned on and off very slowly to simulate a low frequency, the signal provided at Vin would reach the capacitor, and after long enough, the capacitor would charge fully. Once it is fully charged, the voltage measured at Vout would be equal to the voltage provided at Vin.
I had defined ‘low-pass filters’ earlier: at low frequencies, the output signal is the same as the input signal, or Vin = Vout meaning that low-frequency signals are left unattenuated. High-frequency signals, however, are attenuated or ‘filtered’ out. The frequency at which the high frequencies are filtered out is called the “cut-off frequency”.
The variable resistor in this circuit allows us to vary the cut-off frequency. The resistance of the resistor in the circuit and the cut-off frequency are inversely related: a higher resistance means a lower cut-off frequency. This fact makes filter circuits especially useful.
Let us take the example used before of a riser that allows us to create ‘build-ups’ in music. The cut-off frequency would be constantly increasing, allowing more and more high frequencies to pass through, and we perceive this as rising pitch, creating tension in the music.
Low-pass filters are also particularly useful in electric guitars. Electric guitars have 2 knobs: the volume knob and the tone knob. Many people don’t actually know what the tone knob does, often playing around with it cluelessly. Well, the tone knob actually acts as a low-pass filter with changing cut-off frequency. At 0, the knob allows only very low frequencies to pass, and we often hear a sound we describe as ‘dull’. But as we turn the knob, we find that the filter opens up, allowing more and more high frequencies to pass, making the sound ‘brighter’.
Although we will not discuss high-pass filters in quite as much detail, they are actually quite simple to understand once you have understood how low-pass filters work. They are just the opposite of low-pass filters. All you have to do is switch the positions of the variable resistor and capacitor in the circuit above, and voila, you have an RC high-pass filter.