What additional value (in terms of sound quality, dB and other features) does the double DIN provide, apart from its larger size?
The DIN standard is a radio chassis that's 7" wide and 2" tall. A true double-DIN is simply two DIN units stacked; or a single unit that's the same size as two DIN units stacked. Double-DIN head units became common as CD players became more popular; many vehicle manufacturers (especially of Asian cars) created a DIN CD player for use with a DIN cassette/radio unit.
The main advantage to an after-market double-DIN is that it will look better in the dash of a vehicle that uses a stock double-DIN unit. The alternative is a single-DIN, with a DIN-size pocket to fill the extra space. Some manufacturers use the extra size to fit a cassette and CD player into the same unit. Also, a larger head unit can have larger controls and a larger display, making it easier to use. The best in-dash mobile video and navigation units are double-DIN, because they don't require a flip-out mechanism for the viewscreen.
There's not necessarily a sound quality advantage in a double-DIN head unit, but there may be an advantage in reliability, because manufacturers don't have to make design compromises because of size issues.
A disadvantage to double-DIN is that they rarely are available with a detachable face; so they're a tempting theft target.
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