How does a cd produce sound ? It's Plastic.?

Answer 1

The surface is covered in a reflective coating, usually aluminum but sometimes silver or gold.

Parts of the surface reflect light well, others don't reflect it so well. The different regions (reflective versus non-reflective) are microscopic in size.

As the CD spins, these regions pass in front of a laser which reflects or not off the surface, and the light when it is reflected is picked up by a photocell which converts it to logic pulses.

These logic pulses represent numbers, which in turn represent a digitized form of the music, very much like a .WAV file on your computer. If you have ever used .WAV file editting software like Cool Edit or Goldwave, you probably have a pretty good idea of how different numbers can be strung together to form a sound on the computer -- a CD player is doing exactly the same thing. It is basically reading a great big .WAV file off the surface of the disk (just like your computer reads one from the hard drive) and plays it out.

The website linked by Mr-Know-It-All just above me is a very good explanation -- the part I think you will be most interested is this sub-page of it:
http://electronics.howstuffworks.com/cd5.htm

Here is a good picture showing how the "pits" (non-reflective parts of the disc) look:
http://referate.mezdata.de/sj2003/cd_thomas-ley/res-mdv.ch/pits_qf.gif

When a laser spot shines on the surface, the spot is about twice as wide as the track of pits. If the spot is shining on a flat part of the surface, it is all reflected back and the photo-sensor "sees" the light. But if the spot is shining on one of the pits, some of the light is in the pit and some is not. The pit is a very precise depth (exactly one quarter of the wavelength of the light used, or about nine one-millionths of an inch deep) and due to a funny property of laser light, the part of the light reflected from the "surface" is canceled out by the part of the light reflected from the "pit" so the photo-sensor does not see any light. The website shows this as light being deflected away from the sensor by the "bumps" -- this is not exactly how it happens (it is based on the "destructive interference" of laser light which I just descibed) but it is close enough and probably a lot easier to understand.

Here is more information on how the discs themselves are manufactured:
http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm

Answer 2

Cd Produce

Answer 3

During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. A CD has a single spiral track of data, circling from the inside of the disc to the outside. Along the track is a series of bumps. These bumps can be represented as 1s and 0s much like that of the 1s and 0s used in binary computer code. As the CD spins, a laser follows the circular track and shines over the bumps. Depending on whether a bump is present, the laser registers either a 1 or 0. The series of 1s and 0s are translated into digital audio data that is then played through your speakers. Hope this helps.
-Dan

Answer 4

The CD, unlike a record, uses no mechanical contact to vibrate something and make sound. Instead, the CD has data encoded on it which represents the sound. Specifically, the CD has a number for the loudness of the sound for each 1/44,100th of a second of the music. The player reads these numbers, and then produces a voltage that changes every 1/44,100th of a second to correspond to the number. Your earphone or speaker then moves to the deflection corresponding to the voltage.

For a low note, the numbers change relatively slowly as time passes, so the speaker moves slowly, producing bass. For a high note the voltage changes more rapidly, and the speaker follows, producing treble. Because the CD is a linear system, it can also handle combinations of tones and produce them both at once.

The CD is just a carrier for the numbers that corresponds to the
sound. The numbers could just as well be on a flash memory chip (as in a mp3 player), on a DVD (as in a movie with music), or on your computer hard drive (as when you copy a track from a CD). The player converts the numbers to a varying voltage, which is fed to your speaker to make sound!

Hope this helps.
VK

Answer 5

CD audio is recorded in a digital form--as a series of numerical pressure measurements. This digital recording is a very accurate representation of the air pressure fluctuations associated with the original sounds that arrived at the microphones. During playback, these air pressure measurements are read from the CD and the original air pressure fluctuations are recreated by the speakers. While there are imperfections in the whole process of measuring air pressure fluctuations and recreating those fluctuations, the CD itself doesn't introduce any imperfections--the information read from the CD during playback is absolutely identical to the information that was recorded on the CD at the manufacturer's plant.

The same isn't true of analog recording on a cassette tape. Cassette audio is recorded in an analog form--as magnetizations of the tape surface that are proportional to the air pressure fluctuations associated with the original sounds. During playback, these magnetizations of the tape are analyzed and used to recreate the sounds. But the tape itself introduces imperfections in the reproduced sound. The information read from the tape during playback isn't quite the same as the information that was recorded on the tape at the manufacturer's plant. The tape isn't perfect and the sound that's reproduced by a tape player isn't quite the sound that was originally recorded.

A CD player uses a laser beam to determine the lengths of a series of ridges inside a compact disc. Infrared light from a solid-state laser is sent through several lenses, a polarizing beam splitter, and a special polarizing device called a quarter-wave plate. It's then focused through the clear plastic surface of the compact disc and onto the shiny aluminum layer inside the disc. Some of this light is reflected back through the player's optical system so that it passes through the quarter-wave plate a second time before encountering the polarizing beam splitter. The two trips through the quarter-wave plate switches the light's polarization from horizontal to vertical (or vice versa) so that instead of returning all the way to the laser, the light turns 90° at the polarizing beam splitter and is directed onto an array of photodiodes. These photodiodes measure the amount and spatial distribution of the reflected light. From this reflected light, the CD player can determine whether the laser beam is hitting a ridge or a valley on the disc's aluminum layer. It can also determine how well focused or aligned the laser beam is with the aluminum layer and its ridges. The player carefully adjusts the laser beam to follow the ridges as the disc turns and it measures how long each ridge is. The music is digitally encoded in the ridge lengths so that by measuring those lengths, the player obtains the information it needs to reproduce the music.
563. In a CD player, how is the digital optical signal transformed into an electrical signal?

The ridges and flat regions on a compact disc's aluminum layer determine how laser light is reflected from that layer. As the disc turns and the player's laser scans across ridges and flat regions, the intensity of the reflected light fluctuates up and down. This reflected light is directed onto an array of silicon photodiodes that provide both the signals needed to keep the laser focused tightly on the aluminum layer and the signal that the player uses to recreate sound. The sound is encoded in the lengths of the ridges. A computer monitors the amount of light returning from the disc to determine how long each ridge is and how much spacing there is between it and the next ridge. The computer uses this information to obtain a series of 16 bit binary numbers for each of the two sound channels that are represented by an audio CD. A digital-to-analog converter uses these 16 bit numbers to produce currents that are eventually amplified and used to produce sound.

Answer 6

Read http://electronics.howstuffworks.com/cd.htm. The fact that it's plastic is not relevant.

Answer 7

Here is a real good explanation of how cd's work:

http://electronics.howstuffworks.com/cd1.htm

Answer 8

http://electronics.howstuffworks.com/cd.htm
http://electronics.howstuffworks.com/cd3.htm

Answer 9

Theres ink in the CD that gets burned like a record and the laser picks up these different size bumps as music.