how a sensor work  especially acoustic sensors ? the basement of sensors?
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In general, the basis of a sensor is to measure some physical property (sound intensity, temperature, light intensity, position, velocity, acceleration, to name a few) and produce an electrical output that represents the amplitude of the property. There are sensors that don't output an electrical signal but indicate amplitude in some other way such as position. An example would be a mechanical speed governor. But the majority of sensors have an electrical output since we are able to do any needed amount of processing of such a signal.\nMicrophones and motion detectors are examples of acoustic sensors. Microphones generate an electric signal that is an analog of the sound it receives; i.e., the signal shows all the vibrations that make up the sound. This can be amplified and used to drive loudspeakers, recording equipment, etc. (If used to detect motion, this would be called "passive" detection.) Acoustic motion sensors use "active" detection. A very high-frequency loudspeaker sends out an ultrasonic sound signal, which results in a particular signal being reflected from nearby surfaces and returned to a microphone. With nothing in the sound field moving, the microphone receives the same sound frequency as is transmitted and a constant amplitude. A moving object reflects some of the sound back to the microphone at a slightly different frequency (due to the Doppler effect) and with slightly different amplitude due to the changing interference pattern created by the motion. Depending on the type of detector, the frequency and/or amplitude change triggers a detection signal, which then can be used to energize an alarm, open a door, turn on a light, etc.
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A transducer is an element that converts some force to an electrical signal. A microphone converts sound waves to a signal. A speaker receives an electrical signal and generates sound.\n\nYou can use a speaker as a microphone, by plugging it into a mic input! So, it really can be both.
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The phone has transducers that converts air vibrations in the voice to electric current variations (microphone)and one that converts electric current variations to air vibrations (speaker/headphone) by using the battery feed from the telephone system.After connecting the transducers by an electric conduction path or a wireless path (carrier modulation/demodulation)you can speak to and listen to the other end.
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a signal to a device that makes a noise
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good website is www.howstuffworks.com.  See:  http://electronics.howstuffworks.com/ultrasound1.htm
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* There are horizontal and vertical rows of electrodes below it that form a grid. These layers with electrodes are charged with a constant alternating current (AC). \n\n    * As the finger touches the electrode grid, the current is interrupted and the interruption is detected by the circuit board. \n\n    * It works by sensing the capacitance. The human finger is an electrical conductor, and when you place your finger on a TouchPad, a tiny capacitance forms between your finger and the metal electrodes in the TouchPad.\n\n    * The TouchPad sensor's sensitive analog electronics measure the amount of capacitance in each of the electrodes. By measuring which electrodes have the most capacitance, the TouchPad can locate your finger to an accuracy of better than 1/1000th of an inch.
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transducer is a general term for a sensor that changes an physical reading (maybe temperature, position, humidity) to an electrical signal (0 to 5 volts for example).MIKE
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Yeah, what Eugene said.  Electrons going from one end to the other get pushed sideways a bit cause they are moving charges in a transverse magnetic field ;  so there are more on the left than the right but they don't like being close to other electrons much and will move back again if you give them the chance, big fat wires give them more of a chance to move back in the direction that you don't want them to go. \n\nNow you tell me why semiconductor hall effect sensors are so much more sensitive than sensors based on thin metal films.
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Sound travels in a wave.  Actually travels best through a solid.
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(m)\n\nA detection device that requires input energy from a source other than that which is being sensed. Note: An example of an active sensor is a photoconductive cell. 2. In surveillance, a detection device that emits energy capable of being detected by itself. Note: An example of an active sensor is a measuring instrument that generates a signal, transmits it to a target, and receives a reflected signal from the target. Information concerning the target is obtained by comparison of the received signal with the transmitted signal. 3. A measuring instrument in the Earth exploration-satellite service or in the space research service by means of which information is obtained by transmission and reception of radio waves.
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How Stuff Works, my favorite site for things like this question.\n\nhttp://home.howstuffworks.com/metal-detector.htm
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The quartz is a crystal which deforms when an electrical current is introduced. This is the opposite of how a piezo element microphone works; when sound strikes the element ( a crystal similar to quartz) the element deforms and creates an electrical impulse analogous to the vibration caused by the sound.\n\nWhen a steady current is applied to the crystal with the correct feedback circuit, the crystal oscilates at a set frequency. This is in fact how your computer keeps time as well! The entire system (whatever its processor speed is) is governed by a crystal utilizing this exact method.\n\nThe oscilations are then fractioned into larger units of time (since the crystal may oscilate at many millions of vibrations per second) and then coupled to the movement by some type of mechanical transducer.\n\nVery accurate time keeping is created this way.
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Acoustics deals with the creation, control and destruction of sound energy which consists of vibrations of a wide range of wave lenghts through air and other materials.  It is important in designing fine concert halls that they be well balanced between dead and too much resonation or echo.  The shape of the auditorium and the capability of walls, seats and the  audience (simulated by sand bags?) are tested for sound properties.  Special baffles may be installed to absorb excess sound.  Because sound can reflect from surfaces and focus or disperse, dead and live spots must be avoided.\n\nOne of my physics professors was an accoustics hobyist and build an anechoic chamber in the basement of his house.  The walls, etc. were coated with long sharp angles to absorb all sound for his experiments.  He said that he could tell when his son borrowed the sound chamber to play his rock-and-roll guitar because the entire house shook but he couldn't hear a thing.
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Some pressure transducers are a piezo electric crystal and the pressure forms a voltage across the crystal. Works for sound as well.
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Piezoelectric crystals will deform when a voltage is applied to them. If the voltage is an alternating one, they will deform one way, then the other way,. If the voltage is alternating rapidly, the crystal will flex rapidly. If the alternating signal is in the audio range, the crystal will emit sound. (Some very small, lightweight headphones work this way.)\n\nThe opposite is true also. When the crystal is mechanically deformed (vibrated), it will generate a tiny voltage. So they can also be used as microphones. Some piezoelectric crystals are actually ceramic materials. \n\nThey will respond at ultrasonic frequencies, and so are used as transducers-- sources and sensors.\n\nThe way the crystal is made inside is that there are layer upon layer of the same kind of molecule lined up, all in the same direction. These molecules are usually polarized, with a free electron on one end and none at the other end.\n\nSo if a thin film of metal is electroplated on both sides of the crystal (and you have to cut the crystal carefully, to make sure you have the molecules lined up right) you can solder wires to those metal platis and use them as electrodes. When a voltage is applied to one side, the molecules want to rotate in place (without stepping out of formation) as far as they can. The electrons repel one another, and this is what causes the deformation.\n\nWhen the polarity of the voltage reverses, the molecules want to rotate the opposite direction (again, without steping out of formation.) The electrons are repelling on the opposite side.\n\nIf too high a voltage is applied, the molecules break formation, and the crystal cracks. The voltage causes a little spark like an electric arc, that ruins the device.\n\nIn addition to the man-made ceramic materials, quartz, tourmaline and Rochelle Salts have these properties. Radio hams use quartz crystals to keep their transmitters on the right frequencies.\n\nIf you are intersted in things of this type, I suggest taking electronics courses. It will be a challenge, but you seem to have an interest, and I know you will enjoy someday being able to build your own ultrasonic devices, like rangefinders and echo-locators. Good luck!
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Some cases have optical sensors which sense the open door.\nThey are located on the opposite side of the doors.
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I assume you are talking about auto. windsheild wipers?\n\nI don't have specifics on brands of sensors, but they usually work using hydroscopic discs.\nthese discs are made from material that expands when wet. They are connected to a normally open electrical switch. When it rains, the discs expand, closing the switch and turning on the windshield wipers. As the discs dry out, the switch opens again, turning them off.