spl is a sound pressure level caused by a sonic wave.

you can also confuse it with Pioneer , they Use SPL with their high end speakers/sub for marketing purposes.

often denominated in db, higher the better.

PRE, head unit, in dash car unit, mp3 player all are same.

1 din are standard size ( like in corolla 1974, civic 92), 2 din are double in size (1996 civic and afterwards).

Components have separate tweeters and crossovers, tweeters for high frequency sounds that are sensitive to ears (like a guitar) and bigger drivers are for bass (like drum beats).

Crossover network separate different frequencies (mid low high) optimized for different drivers

they also do multiband processing for audio signal to adjust equalized, compressed, echo.

Components are far superior in terms of sound quality

Example

XS-GTX1620 : Speakers : Xplod� Speakers / Subwoofer : Sony Middle East & Africa

2 way or two driver component in 6 1/2 or 6.5 inch size, there are also 6 3/4 large components (Euro spec).

Coaxials are multielement, upto five ways and in different sizes mot common being 9 inches wide and 6 inches in height, often placed in rear of the car for fill in sound.

one popular choice of pakwheelers and a good 5 way multielement (coaxial) is this.

http://www.pioneer.com.sg/storefront...&CatId=2,27,39

6 X 9 have more bass but less clarity compare to round ones due to their design limitation.