The iPhone’s headphone jack is recessed into the iPhone’s body, so third-party headphones with larger miniplugs won’t work with this recessed jack.
The solution is a headphone adaptor that plugs into this recessed jack and then lets you use your headphones of choice. A number of vendors, including Belkin, Griffin Technology, and Shure have announced such adaptors, and we’ll be taking a look at each. The first one we’ve had in our eager hands is Belkin’s £8 Headphone Adapter for iPhone.
The smallest and simplest of the announced adaptors, Belkin’s is just over two inches long and covered in thick rubber. Plug it into your iPhone’s headphone jack, and you get a standard headphone minijack into which any 1/8-inch stereo miniplug - your run-of-the-mill headphone connector - can be inserted. Every set of headphones I tested worked fine with the Adapter, and the plug end is thin enough to fit in the headphone-jack hole of every iPhone case we’ve received so far.
The Adapter is fairly rigid, which means you need some clearance at the top of the iPhone for the Adapter and the plug from your headphones. (You can bend the Adapter in the middle, and it bends much more easily after you “break it in” a bit, but you should still assume that you’ll need an inch or so of clearance.) The Adapter’s headphone jack also fits tightly with most third-party headphone plugs; when you pull on the headphone plug, you’ll likely unplug the adaptor instead - which is likely what you want to do anyway.
To answer the obvious question being asked by the audiophiles out there: the Belkin Adapter does affect sound quality, but only slightly. I tested the Adapter with V-Moda’s Vibe Duo - an iPhone-focused (read: includes an inline microphone) version of the company’s excellent Vibe. The Duo provides sound comparable to the Vibe - which is to say, much better than Apple’s stock earbuds - and uses a miniplug small enough to fit the iPhone’s headphone jack, so I was able to use the Duo both with and without the Belkin Adapter.
When switching between a direct connection and the Adapter, there was a slight difference in overall sound quality. However, the difference was noticeably only in immediate, direct comparison and was small enough that only the most golden-eared and picky audiophiles (such as myself) will notice - and even fewer will care.