SPLnFFT is a noise meter (or sound meter) with extra features such as FFT viewer and frequency meter and test signal generator. Which makes an all-rounder product at a very affordable price.
It is "universal" which means it is "HD" on an iPad and it supports native resolution of "retina" displays.

Here are extracts from reviews:
"our favorite sound measurement app" SafetyAwakenings.com
"SPLnFFT is comprehensive, easy to use" TapeOp.com

You absolutely need a microphone to use this App, either internal (iPhone/iPad) or external (iPod 2nd gen).

Top digital display is for measurement of sound power in 'slow' mode (averaged on 1s). Central analog display is for 'fast' mode (averaged on 1/8th s). But it can display 'slow' mode as well on request: just click on 'fast' label.
Peak and average values are displayed below and are holded until you touch the 'reset' label.

A decent noise meter proposes some weightings. Here you can select A,B or C types of weightings (search "A-weighting" on wikipedia for more information).
Bottom view is a real time FFT. You will notice that predominant frequency and next one are highlighted. Their frequencies and relative power are displayed on the right. If your input signal is a pure tone then this App will lock on its frequency and display a very accurate result.
Green line is the result of a real time FFT done with 1024 points. Purple line is the averaged FFT (exponential filtering done on power of each frequency). Blue line is a copy of what the purple line was when the user froze the display the last time.
A test tone at 1kHz or a white noise or a pink noise can be generated and sent to right and/or left outputs (select the type you want in BEEP tab). This FFT view - more precisely an upsized version of it - can be exported as a picture in your own photo library, using the camera button (when display is frozen only). This picture can be exported in a mail as well, or to social networks (when available and configured).
The display can be flipped upside-down to ease reading: just rotate your iPhone while holding it vertically.

Very sensitive: for example it can detect a CRT TV (*) at a distance of few meters ! A dedicated flag is raised to warn you in such situation.
(*): CRT = not Plasma nor LCD

As for ALL sound meters (thus including this one and all others at any price), calibration is suggested if you want to reach extreme precision. This is enabled in this App thanks to a slider to apply some compensation and to three non volatile memories (choice to be made in MIKE tab). How to ? For calibration, you need and external reference, either a calibrated sound meter, or a calibrated noise source.
Note as well that the tiny embedded loudspeaker cannot reproduce extreme frequencies. Thus if you want to hear a real white or pink noise, you will need to use a quality headphone or HiFi system.

This App was designed seriously for my own needs. I am a signal processing expert and you will find other audio applications from myself in AppStore.

***** Any comment, any feedback ? Click on 'support' in iTunes: there is a blog for this. You can even contact me directly by email *****