How to Set Up a PA System with Free or Cheap iPhone Apps

I have a 3G iPhone now because of the toolkit I could have in my hand when working on AV projects. For live sound gigs, it is very small and convenient to use and I now prefer it over my cumbersome laptop and hand held SPL meters. 

Don't get me wrong, I will always use my probe-mic ANSI IEC Type II SPL meter for paid consultations along with my Radio Shack analog meter for details, but when I set up a sound system, I usually try to identify gross fluctuations rather than fine details, so the iPhone app seems to give me my feedback problems and my 'boxy' frequencies to correct.

Method for ringing out feedback with RTA Lite:

1) With amplifiers turned up and balanced properly (unity gain or thereabouts), turn up the master volume fader of the mixer to unity (or '0dB' or about 75% up, depending on the mixer), turn up the mic channel fader to maximum setting (unity or +5dB, depending on how you handle your gain structure), then increase the mic channel trim pot slowly until mild feedback occurs (a slight ringing only!!).

2) Cup the bulb of the mic carefully to induce more feedback, but don't allow spikes! With the RTA Lite app, see which frequency is reacting the most and cut by about 5dB. 

3) For Low frequency resonance, be sure the high-pass filter for the mic channel is disengaged and say some low-toned words directly into the mic to see what is reacting ans cut those frequencies a bit just enough to stop the 'trail' of resonance. For 'boxy' sounds, say words like 'walk' and 'dog' to identify where to cut and what bandwidth of frequencies.

For estimating maximum SPL and headroom with Studio Six SPL Meter:

1) For live bands - set the meter to 100dB on the dial and sing very loudly (or shout, if you can't sing). A very loud singer should be able to sing just over 100dB, A-weighted. The bass drum (or loudest low frequency transient sound) should be between 106-110dB, C-weighted. Anything louder than that and you're on you're own.

2) For speeches - set the meter to 60dB and speak into the main microphone at a distance of about two feet from the mic, to simulate the worst case scenario. You should be reaching conversational level throughout the room (between 55-65 dB, A-weighted). Anything quieter than that and people will complain (get the speaker to talk closer to the mic).

3) For media soundtrack playback - set the meter to 70dB, A-weighted and set the playback levels to about 60-65dB for dialogue. Switch to C-weighted and adjust low frequency EQ so it runs at about 65-70dB (depending on the soundtrack mix). Business presentation media will be as compressed as a TV commercial (around 5dB of dynamic range) and feature films will be as follows - 60dB for A-weighted dialogue and up 80dB C-weighted or more for the loudest sound effects.

The results you get will be fairly accurate and you should get great results for less than five dollars of iPhone apps.

One last note - do you need pink noise to ring out a sound system? The answer is: maybe, maybe not. Pink noise will help to EQ a system to a 'flatter' response, but flat doesn't always mean it will be good-sounding. In fact, trust a reference song or pre-recorded speech when it comes to tuning for naturalness and clarity.

I'd want to tune a large concert system or a movie theatre with pink noise, but not necessarily a night club or conference room. I mainly aim for four areas of correction in this order: upper middle frequency feedback, boxy vocal sounds, low frequency resonance trails and proper SPL delivery, and the two apps I use help me do just that.

Buck Moore

B4UShowit.com