Applications of Resonance Circuits

Resonant circuits (series or parallel) are used in many applications such as selecting the desired stations in radio and TV receivers. A series resonant circuit is used as voltage amplifier. A parallel resonant circuit is used as current amplifier. A resonant circuit is also used as a filter.
In musical instruments, resonance is used to create sound waves.
Resonance is used in magnetic resonance imaging (MRI) machines to generate images of the inside of the body.
In radio and TV broadcasting, resonance is used in antennas to select specific frequencies and reject others.
In general, any time you see an unexpected peaking in the frequency response of an element or a system, it is normally caused by a resonance condition. If the response has a detrimental effect on the overall operation of the system, a redesign may be in order, or a filter can be added that will block the frequencies that result in the resonance condition. Of course, when you add a filter composed of inductors and/or capacitors, you must be careful that you don't add another unexpected resonance condition. It is a problem that can be properly weighed only by constructing the system and exposing it to the full range of tests.

Stray Resonance

Stray resonance, like stray capacitance and inductance and unexpected resistance levels, can occur in totally unexpected situations and can severely affect the operation of a system. All that is required to produce stray resonance would be, for example, a level of capacitance introduced by parallel wires or copper leads on a printed circuit board, or simply two parallel conductive surfaces with residual charge and inductance levels associated with any conductor or components such as tape recorder heads, transformers, and so on, that provide the elements necessary for a resonance effect.

Graphic and Parametric Equalizers

We have all noticed at one time or another that the music we hear in a concert hall doesn't quite sound the same when we play it at home on our entertainment center. Even after we check the specifications of the speakers and amplifiers and find that both are nearly perfect (and the most expensive we can afford), the sound is still not what it should be.
In general, we are experiencing the effects of the local environmental characteristics on the sound waves. Some typical problems are hard walls or floors (stone, cement) that will make high frequencies sound louder. Curtains and rugs, on the other hand, will absorb high frequencies. The shape of the room and the placement of the speakers and furniture will also affect the sound that reaches our ears. Another criterion is the echo or reflection of sound that will occur in the room.
Concert halls are designed very carefully with their vaulted ceilings and curved walls to allow a certain amount of echo. Even the temperature and humidity characteristics of the surrounding air will affect the quality of the sound. It is certainly impossible, in most cases, to redesign your listening area to match a concert hall, but with the proper use of electronic systems you can develop a response that will have all the qualities that you can expect from a home entertainment center.
For a quality system a number of steps can be taken: characterization and digital delay (surround sound) and proper speaker and amplifier selection and placement. Characterization is a process whereby a thorough sound absorption check of the room is performed and the frequency response determined. A graphic equalizer is then used to make the response "flat" for the full range of frequencies. In other words, the room is made to appear as though all the frequencies receive equal amplification in the listening area.

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