Filter
滤波器/Lv4 Bo1 Qi4
n/a
CHEARS: True
EARS: True
CMT: False
EARS2 Encyclopedia: True
CHEARS:
源自《音乐声学与心理声学》（第三版）、霍华德、安格斯、人民邮电出版社、2011。
1.7.1 (Page 57 and 58)
EARS:
A device used to affect certain parts of the spectrum of a sound, by causing the attenuation of certain frequency bands, while allowing other bands to pass unattenuated. Some common types of filters are:
- high-pass filters (which attenuate low frequencies below the cut-off frequency);
- low-pass filters (which attenuate high frequencies above the cut-off frequency);
- band-pass filters (which combine both high-pass and low-pass functions);
- band-reject filters (which perform the opposite function of the band-pass type);
- octave, half-octave, third-octave, tenth-octave filters (which pass a controllable amount of the spectrum in each band);
- shelving filters (which boost or attenuate all frequencies above or below the shelf point);
- resonant or formant filters (with variable centre frequency and Q).
The Q of a filter is a measure of its resonance and is defined as the ratio of centre frequency to the bandwidth. The narrower the bandwidth, the higher the Q, and the more the filter will 'ring' or go into oscillation when stimulated by a signal with energy near the centre frequency. A constant Q filter varies its bandwidth as a function of the centre frequency, always keeping the ratio between them the same.
A group of such filters may be interconnected to form a filter bank. (Reduced from Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)
With Comb-Filtering, a sound is passed through a group of filters, generally distributed equidistantly. The mode of sound processing changes the spectral content of sounds around the selected frequencies, possibly boosting them to the extent of generating resonance, when the width of the filtered areas become very thin. Sinusoidal frequencies then start to oscillate in response to the feedback created in those frequencies. (Source - Rodolfo Caesar (1992). The Composition of Electroacoustic Music. PhD Thesis, University of East Anglia.)
Filters have also been used as musical 'instruments' in live performance, most significantly in a number of works by Karlheinz Stockhausen, including Mikrophonie I (1964), Prozession (1967), and Kurzwellen (1968). The filters were used to transform the sounds of a large tam-tam picked up by microphones moved by performers over the surface. In the latter two works the filters were also applied to the sound from a contact microphone attached to a viola. (Source - Richard Dobson (1992). A Dictionary of Electronic and Computer Music Technology. Oxford University Press.)
EARS 2:
A filter is a manipulation tool which can alter the relative balance of frequencies within a sound.
What is a filter?
A filter changes the frequency makeup of a sound by making parts of it weaker.
Filters allow you to focus on parts of a sound that are of interest to you, or to take away parts that you don’t like, such as noise or ‘hiss’.
As we learnt in Pitch and Soundscape Music, sounds are often made up of many different frequency parts.
By viewing the Spectrogram of a sound it is easy to see the different frequency parts which make up that sound.
Using filters we can change the balance of these frequency parts, removing some to make others stand out.
Filters in Action
Below are a some sounds which have been filtered.
Can you hear the difference between the sounds before and after filtering?
What has changed?
Filter Properties
Filters have two main controls:
Frequency: the frequency at which the filter operates.
Quality: the width and shape of the filter (narrow or wide / gentle or steep).
By altering these controls we can change the output sound, as demonstrated in the video:
Filter Types
There are several types of filter, and each creates a different type of effect.
Each type is explained below, with the same original sound being transformed by the different filters.
Original Sound - Keys And Door
This sound contains many different parts, the high jangle of they keys and the low thud of the door closing. Listen and compare it with the following filtered versions of the same sound.
Low-pass filter
Allows all frequencies lower than its cut off point to pass (for example, if the cut-off frequency is set at 500 Hertz (Hz), all of the frequencies over 500 Hz will disappear). Does not allow any higher frequencies to pass. Best used to remove unwanted high sounds, or to highlight only the low frequency portions of a sound.
Low-pass Filter
You can hear only low frequencies, all of the key jangles have been removed.
Composition Tip
Low-pass filters can be useful when we want to highlight only the bass frequencies of a sound.
High-pass filter
Allows all frequencies higher than its cut off point to pass. Does not allow any lower frequencies to pass. Best used to remove unwanted bass sounds, or to highlight only the high frequency portions of a sound.
High-pass Filter
All low parts of the sound have been removed. Leaving only the jangling of the keys. We no longer hear the low thump of the door closing.
Composition Tip
High-pass filters can be very useful for removing low background noise, like the rumble of traffic, from recordings.
Band-pass Filter
Allow one band of sound to pass through. Best used to highlight frequencies of sound in a specific area. The Quality of the filter will affect its width.
Band-pass Filter
Only a narrow band of sound is allowed through the filter. We lose both the high and low frequency parts of the sound.
Composition Tip
Being able to focus on one specific are of the sound allows us to highlight a specific aspect that might be unique.
Band-reject Filter
This filter will not let a certain band of sound to pass through. Best used to remove sounds at a specific frequency. The Quality of the filter will affect its width.
Band-reject Filter
In this example only a narrow band of sound is removed. We can see this clearly in the spectrogram and we hear that this middle part of the sound is missing.
Composition Tip
The band reject filter can be useful for removing hum or noises that fall within the mid frequency range of a recording.
Fact
The bass and treble controls on your stereo, or car radio, are filters.
When to use filters?
Filters allow us to transform the frequency makeup of sounds. They can therefore be very useful to us.
We can use filters to balance sounds, removing very loud portions of the frequency range, or boosting areas of interest which are quiet.
Combined with Automation filters can be very useful in creating Gesture and a sense of movement.
We can use a Filter Sweep to draw people’s attention to different portions of sound and to cary the listener on a journey into sounds.
Activity
Load up a session in Compose With Sounds and begin to explore filters yourself.
Try to make use filters in a composition.
Encyclopedia:
A filter changes the frequency makeup of a sound by making parts of it weaker. Filters allow you to focus on parts of a sound that are of interest to you, or to take away parts that you don't like, such as noise or 'hiss'
(Some filters, also known as Equalisers, can make parts of the sound stronger).
Controls
Filters have two main controls:
•Frequency: the frequency at which the filter operates.
•Quality: the width and shape of the filter (narrow or wide / gentle or steep).
Filter Types
There are several types of filters:
Low-pass filter – Allows all frequencies lower than its cut off point to pass (for example, if the cut-off frequency is set at 500 Hertz (Hz), all of the frequencies over 500 Hz will disappear). Does not allow any higher frequencies to pass. Best used to remove unwanted high sounds, or to highlight only the low frequency portions of a sound.
•High-pass filter – Allows all frequencies higher than its cut off point to pass. Does not allow any lower frequencies to pass. Best used to remove unwanted bass sounds, or to highlight only the high frequency portions of a sound.
•Band-pass Filter
– Allow one band of sound to pass through. Best used to highlight frequencies of sound in a specific area. The Quality of the filter will affect its width.
•Band-reject Filter
– Will not let a certain band of sound to pass through. Best used to remove sounds at a specific frequency. The Quality of the filter will affect its width.
Fact
The bass and treble controls on your stereo are filters.
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