Question from a noob about using RTA for a PA - Home Theater Forum and Systems -

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post #1 of 3 Old 04-25-13, 11:05 PM Thread Starter
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Question from a noob about using RTA for a PA

Hello all. This is my first post on this forum and I thank members in advance for their patience. I am wanting to use the RTA specifically to do an initial set up for our PA system when playing local clubs. I have built a hybrid very portable system consisting of

Two Fishman SA220 powered line arrays running from 125hz up
Two Small subs I built each using (2) 12" woofers in an isobaric configuration with the Fishmans on poles on top. Each has a 500 RMS plate amplifier on the 18" overall cube
Two JBL JRX 112 2 way monitors as side fills, and two more of the JRX 112s as stage monitors. The monitors are powered by (2) Crown 500 / channel amps
16 Channel Mackie Analog Board
Rane electronic crossover for the Fishmans and woofers
Monitors are all full range
I also have a DBX 15 band stereo EQ for the mains and one for the monitors

We play old school blues and jazz at very modest levels and are striving for a very clean sound. The system actually sounds really good but I would like to be able to set up a basic tune to a room and then compensate by ear.

Is this something that this software will allow me to do? I have a DBX calibration mic, Radio Shack SPL meter and test CD with various types of Pink Noise to include individual tracks with 1/3 octave noise available. I have the Cd ripped to IPOD and can also just use the DVD drive in my laptop.

The system is run stereo and I am planning on essentially doing four set up tests, main left, main right, monitor left, monitor right, then play all four as we will for live music. The only live instruments that really play through the system are my upright coming from a DI and the kick drum.

Is the approach I am taking valid? Any suggestions would be greatly appreciated as I am just a crusty old mechanical engineer that is using the golden years to pursue a really fun hobby. We have been very well received by live crowds, I would just like to try and maximize our sound and cut a bit of time out of the set up.

Thanks in advance

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post #2 of 3 Old 04-26-13, 11:15 AM
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Re: Question from a noob about using RTA for a PA

First, welcome to the Forum, Shikzachaser!

To start, I’d ditch the idea of running the system in stereo. Stereo is for home audio systems, where the listener sits in the sweet spot. The great majority of the audience in a performance environment will not be sitting in the sweet spot. Take it from me, it really stinks when they put the guitar solo in the speaker on the opposite side of the room from where you’re sitting.

Aside from that, it’ just not practical from an equipment standpoint. Ordinarily you could get by with a single stereo equalizer for the main speakers and monitor mix, one-channel for each. Running the system in stereo means you now need an extra equalizer. Not to mention a stereo crossover instead of a mono unit, a stereo effects processor, etc.

The traditional way to EQ a PA system is to run pink noise through both speakers. There’s no good reason to pink noise the monitor speakers, as they are typically equalized to eliminate feedback, not to optimize sound quality.

REW can provide the pink noise signal, so no need for an external signal source. REW works best with a mic with a custom calibration file. The calibration file compensates for any frequency response inaccuracies the mic might have, so you have for all practical purposes a perfectly flat measurement mic. Without the calibration, with any measurements you take, you don’t know where the speaker’s response ends and the mic’s begins. So, your measurements will only be as good as the response of your dbx mic.

However, that ultimately might not matter much, as with the equipment you have you won’t be able to do much with whatever REW shows anyway. A 15-band equalizer is really just a sophisticated tone control. In order to properly address any response issues you need at least a 1/3-octave equalizer or even better, a parametric equalizer.

Ultimately, there’s only so much you can achieve towards the goal of “a very clean sound” with the system itself. You can have an awesome system, but still have muddy sound if the individual mics and instruments are not equalized properly, both in the house system and any stage amplifiers. You can see more about that here.


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post #3 of 3 Old 04-26-13, 02:53 PM
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Re: Question from a noob about using RTA for a PA

I have a somewhat different viewpoint.

In 20 years of doing live sound on smallish systems it's be stereo 95% of the time. You won't get true imaging the the audiophile sense but you'll get many advantages, like being able to pan a guitar amp (that's too loud and beaming down one side of the venue) to the side that's not getting enough. Many keyboards sound better in stereo even for people not in the sweet spot. Effects like reverb are pretty much all stereo unless you are using a spring unit (and even then...). You can always simply pan things center if you don't want stereo, but with a mono system you're stuck with mono. With a stereo system you have built in redundancy and diagnostic tools.

Monitors with flat response sound louder and clearer (than those with uneven response, at the same measured SPL) and are less prone to feedback right off the bat. The flatter they are the less feedback control you'll have to do to deal with microphone frequency response and room issues, at least at reasonable stage volume. Higher stage volume destroys that balance and you have to resort to aggressive eq to make them "cut", but that leads to lowered intelligibility, requests for more volume, more aggressive eq and so on in a cycle of increasing levels and worsening sound. I get the feeling your stage volume will be reasonable.

I do agree on the eq. Graphic eq is useful for dealing with things as they happen but even 1/3 octave isn't that precise. Your 2/3 octave eqs are rather blunt instruments. A digital parametric with presets for each type of speaker (based on your measurements) would give you a more solid starting point, and the graphics would be handy to compensate for variations in venue acoustics.

You mentioned "RTA". You don't want to use RTA to set up the eq for your speakers. You want to do a transfer function that takes time into account, which RTA doesn't do. RTA is helpful during a show to indicate the exact frequency of feedback.
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