A mechanical filter is a signal processing filter usually used in place of an electronic filter at radio frequencies to allow a range of signal frequencies to pass, but to block others. The filter acts on mechanical vibrations. Transducers at the input and output of the filter convert the electrical signal into, and then back from, these mechanical vibrations. The mechanical elements obey mathematical functions which are identical to their corresponding electrical elements. Electrical theory has developed a large library of mathematical forms that produce useful filter frequency responses for use in the design of mechanical filters. Steel and nickel–iron alloys are common materials for mechanical filter components. Resonators in the filter made from these materials need to be machined to precisely adjust their resonance frequency prior to final assembly. The high "quality factor", Q, that mechanical resonators can attain, far higher than that of an all-electrical LC circuit, made possible the construction of mechanical filters with excellent selectivity. Good selectivity, being important in radio receivers, made such filters highly attractive. Contemporary researchers are working on microelectromechanical filters, the mechanical devices corresponding to electronic integrated circuits.

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_______________________________ Today's selected anniversaries:

1905:

In a sign of support for the December Uprising in Moscow, the Council of Workers' Deputies of Kiev decided to stage a mass uprising, establishing the Shuliavka Republic in the city. http://en.wikipedia.org/wiki/Shuliavka_Republic

1946:

The United Nations General Assembly created UNICEF, originally to help provide emergency food and health care to children in countries that had been devastated by World War II. http://en.wikipedia.org/wiki/UNICEF

1962:

Arthur Lucas, convicted of murder, was the last person to be executed in Canada. http://en.wikipedia.org/wiki/Capital_punishment_in_Canada

1981:

About 900 civilians in El Salvador were killed in the El Mozote massacre. http://en.wikipedia.org/wiki/El_Mozote_massacre

2008:

American stock broker Bernard Madoff was arrested and charged with securities fraud in a $50 billion Ponzi scheme, the largest such in history. http://en.wikipedia.org/wiki/Bernard_Madoff

_____________________________ Wiktionary's word of the day:

undownable (adj): 1. Undeniably important; describing that which cannot be played down or ignored. 2. Invincible; describing that which cannot be brought down or overcome http://en.wiktionary.org/wiki/undownable

___________________________ Wikiquote quote of the day:

If only there were evil people somewhere insidiously committing evil deeds, and it were necessary only to separate them from the rest of us and destroy them. But the line dividing good and evil cuts through the heart of every human being. And who is willing to destroy a piece of his own heart? --Aleksandr Solzhenitsyn http://en.wikiquote.org/wiki/Aleksandr_Solzhenitsyn