The number of applications for the iPhone that could be useful for people doing recording is growing faster than any one of us can keep up with, but a recent find for me is the 99-cent SPL Meter from Studio Six Digital. It's a simple SPL meter with A and C-weighting, slow and fast response settings, a range wheel exactly like you'd find on a Radio Shack SPL meter, and a classic VU-styled meter with a digital display which reads out to a tenth of a dB. By hitting the info button (i), you can then enter a calibration mode on the back, allowing you to calibrate both the internal mic and an external mic. In calibration mode, you see the numerical dB level reading and a trim number with + and -for adjustment. Presumably, one would use a known, accurate meter in order to calibrate the app. (There is no way to account for frequency response when calibrating.) It's worth mentioning that if you have a first generation iPhone, the internal mic works at a lower sample rate than any external mic, including the included headset mic. The app lets you know that this is the case with a pop-up window and recommends an external mic. However, for doing relative level readings (such as looking for room modes), the internal mic on my first generation iPhone worked fine. 3G users won't see this pop-up menu. In use, the app works just like a standard SPL meter. I now pop it open while mixing, just to get my bearings, certainly during mastering, and was even able to use it while doing a room measurement using low-frequency test-tones. There are more sophisticated SPL meters out there for the iPhone, but this was the only one I saw for a buck. This is a no-brainer investment if you've got an iPhone and make records. By the way, my typing adds over 10 dBA to the ambient room noise in my studio. ($0.99 iPhone App Store download;

Tape Op is a bi-monthly magazine devoted to the art of record making.

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