What is a filter in the context of musical instruments?

Also posted at www.WildRockStudios.com

Musical instrument makers have been using filters to shape the sound of their instruments since, I suppose, the first musical instrument. For example, the evolution of early bowed string instruments to the modern violin reflects a corresponding evolution of filter design. And the evolution continues!

Well-studied and frequently-cited filters critical to a violin are:

• Types of wood and thickness
• Physical shape and dimensionsType of glue used
• The varnish
• Topology of the arching of the top and back
• The “graduation” (varying thickness) of the violin top
• The bass bar
• The sound post
• The bridge
• The tailpiece
• Manner of attaching the neck
• Length of the strings
• Shape and overall aperture size of the sound holes
• Composition of the strings (e.g. gut, steel, aluminum, titanium and various allows, etc.)

Every variable listed above, and many more, interact in ways that are still not fully understood, even by the most well-informed experts on musical instrument acoustics, affecting how a violin body filters the signal induced at the bridge by the vibrating strings. This is one of the reasons that the violin and its relative instruments are still closely-studied instrument in doctoral dissertations and masters’ theses.

Today, violinists and fiddlers fine-tune the sound of their instruments by varying the type of strings, the thickness and height of the bridge, the placement of the sound post, the type of tailpiece and even the “tail gut” that attaches the tailpiece to its hitching point (called the tail button). On a modern violin or fiddle, the tail gut could be made of actual “gut” (twisted sheep intestine), stainless steel, nylon or braided Kevlar cord. The matter of strings is even more complex. Each brand and sub-brand of string, apart from the string’s performance on an actual instrument, is well known for its tension, responsiveness to the bow, its “darkness”, “brightness”, “warmth”, “focus” and so forth.

Once the sound from a violin or any other musical instrument is generated, there is yet more filtering performed by a microphone, the sound reinforcement system, the room acoustics, and the physical structures that comprise the hearing process.

So, anything through which a signal passes (in our case, a music signal), whatever its form, can be regarded as a filter.

Examples:

• Cross-over circuits in speaker cabinets
• Treble, mid-range and bass boost circuits on a pre-amp
• Graphic equalizers
• Effects boxes (often called “stomp boxes”), commonly used by guitarists

It has been said "When you think about it, everything is a filter."

Filters can be physical or electronic. Electronic filters can be analog, making use of various capacitors, oscillators and so forth. They can also be digital, which operate on sound that has been represented as a series of numbers. The filtering is a computation that takes one sequence of numbers (the input signal) and produces a new sequence of numbers (the filtered output signal). A digital filter is fundamentally the implementation of a software algorithm.

It is noteworthy that many digital filters, such as a digital “octave splitter” (generates an output an octave or more lower than the input), first convert an incoming analog signal to a digital signal, does its processing, and then converts the signal back to an analog signal that is output to either an amplifier or another effects box.

High-Pass and Low-Pass Filters

This is often a point of confusion.

• A low-pass filter rejects frequencies above a certain point.
• A high-pass rejects frequencies below a certain point.