Frequency filtering

What is the frequency?

 The frequency it’s the number of vibrations in the air that one sound creates in one second. We use the Hertz to quantify the frequency. The human ear can detect the sounds that are compressed between 20 to 20 kHz (20.000 Hertz).

Depending of the frequency of one sound it can be high, mid or low:

  • High. The high frequency moves between 2000 to 16k Hz. When the frequency of a sound it’s higher than 20.000 Hz we call it ultrasound, only some animals can hear ultrasounds. An example of a high sound it’s an electric guitar playing a solo, or a whistle of a referee in a football match.Let’s see an example of sound with a high frequency

  • Mid. The Mid frequency it’s compressed between 300 to 2000 Hz. In this frequency we can found the most common sounds.
  • Low. The low frequency it’s compressed between 20 to 300 Hz and sounds more heavy and dense than a high frequency sound. When the frequency of a sound it’s under the 20 Hz we call it infrasound and, like the ultrasounds, it only can be listened by some animals. One perfect example of a low frequency sound it’s the bass of a song.Let’s see an example of sound with a low frequency.

Frequency filtering

There are some digital filters to change and modify the frequency of one sound, but first it’s important to understand what a digital filter is. A digital filter is a system that depending of the in and out signals applies a mathematical process in marked segment of one sound. This process can change the frequency of the sound and also so many different aspects.

Some of the most important digital filters to modify the frequency of one sound are the low-pass filter, the high-pass filter, the band rejection filter and the band pass filter. As we can deduce for the names of the filters some of them works in opposite ways. We can use each filter for different situations or for different intentions, because one filter can change the entire sound.

We can use the digital filters for many reasons. For example, if we want to simulate some place like the inside of a car o something like that we can use a Low-pass filter. Some others filter works perfectly to correct the sound of a live performance. As we can se there are so many digital filters with so many uses.

Low-pass filter

The low-pass allows the entrance of signals with a frequency lower than the cut off reference frequency and attenuates the signals with a frequency higher than the same cut off reference. We can decide the exactly position of the cut off if we want more precision or we can use a standard cut off in our digital edition platform. This filter is just the opposite of the high-pass filter because low-pass filter passes low signals and high-pass allows high signals.

This filter it’s perfect for so many different applications. A low-pass filter works perfect to create fake ambient for movies or to amplify the bass of a song.

High-pass filter

The high-pass filter it’s the opposite of the low-pass filter. It means that the high-pass filter passes the high frequency signals but reduces the amplitude of the low frequency signals. As the low-pass filter, the high-pass has a cut off frequency to mark the frequency limit to pass.

The high-pass filter has a lot of different applications. We can use it to remove unwanted sounds with a low frequency like a lot of noises (cars, footsteps, etc.). Also this filter works as a part of an audio crossover.

Video example of the application of the low-pass filter and the high-pass filter

Band-pass filter

The band pass filter it’s kind of a mix between low-pass filter and high-pass filter. In this filter we mark two different cut off frequency references. The frequency inside the two cut off marks passes and the frequency outside the cut off range are attenuate.

The main applications of this filter are the audio equalization and the noise reduction (because band pass filter can isolate a specific frequency range).

Band-rejection filter

This filter is the opposite of the band pass filter. The band rejection filter passes most of the frequencies unaltered, but attenuates those frequencies compressed by the two cut off frequency references.  This kind of filter can be used to isolate a certain sound with a lot of noise.

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Online external references

  1. http://www.webopedia.com/TERM/A/audio_frequency.html
  2. http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Low-pass_filter.html
  3. http://www.electronics-tutorials.ws/filter/filter_3.html
  4. http://www.ese.upenn.edu/detkin/courses/ee206/miniproject2/bandpass.html
  5. http://www.allaboutcircuits.com/vol_2/chpt_8/5.html
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