How about an iPhone-ProTrack SLM

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Alesis finally got around to making shields for securing an iPhone or iPod touch to the ProTrack stereo iPod recording device. This means that you can insert your iPhone (any model) or iPod touch (2G or later), plug in a phantom-powered measurement microphone, run SoundMeter, and turn the unit upside down for a handheld (albeit somewhat large) sound level meter. Of course, if you use SignalScope Pro, you’ll also have access to 1/3-octave and narrowband spectral analysis.

One downside is that the headphone output is completely blocked for the iPod touch. The shield itself blocks the headphone output of the iPhone, although a drill could solve that problem fairly easily.