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How can I use a RTA?


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Hi guys, bought a Audio Source RTA-One used and did not get instructions on how to operate it. I was voted to be the sound man in the band so I bought the unit to help aid me in setting up the EQ. Any advice please on how to set the EQ up using the RTA as aid? It's a hand held unit that ranges from 31.5hz to 16khz. It has 10rows of lights that bounce from -10 db to 10 db. It came with a pink noise generator. i know you run the pink noise through the PA but that's as much as i know. Your talking to a new rookie sound man excited about the craft of sound mixing so I'd really appreciate any help. I know this is the best sound forum with the best technicians on the net and appreciate your imput, thanks!
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Zorro, did it come with a mic? If not, you need to use a high quality omni-directional. The white/pink noise generator in the unit is to be run thru the PA (mono). The unit will output a frequency (20-20k) thru your "mains" which will be picked up by the mic and displayed on the unit as the "room mode EQ". Simply look at the units EQ output graph and adjust your mains EQ to get the units EQ display flat as possible. (This is how the one I used to use worked. -YMMV) warning: when you have the RTA's display reading flat, after adjusting your EQ out to the mains, it will probably be too "flat" for chest thumping rock and roll, rave, house, acid, disco, etc.(really good for jazz, though) so further adjusting may/will be needed, it should be used to get you in the "ballpark", not as the final setting. btw: I dumped all that and now I just bring a CD player and a few CD's of the type of music that will be played, and tweak the PA by ear. Yea, it isn't audiophile flat, but it works.
In two days, it won't matter.
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Once you have that pink noise running through the PA, you need to have a microphone suitable for measurement purposes to send the sound of the PA to the RTA. A mic like the Earthwerks TC30K would probably do the job. (Flat frequency response from 20Hz to 20kHz is what you want when choosing a microphone to help you EQ a PA.) From there, remember to attenuate frequencies rather than boost them, and be aware that you can't fix phase problems with EQ. (You can make them worse, however.) -Danny

Grace, Peace, V, and Hz,

 

Danny

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Great advice! It has a built in mic. I really appreciate the responses. I feel more confident now I got good advice. My ears are good (I'm a musician) but I lack the confidence in setting the EQ up by ear. Plus I'd like to make a good impression with my first sound job that will be coming up soon. This is a great forum!!
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I'm not an expert, but I have done this several times. It is more important to tame feedback and maximize system gain before feedback than to shape the sound of the system. Basically, the room has a certain frequency response that will usually be less than flat. Your PA system also has a response curve that is usually not flat. When you combine the two systems (the room and the PA system) you end up with some frequencies (usually corresponding to so called "room modes" or resonant frequencies) that feedback at much lower overall gain settings than others. By attenuating these frequencies, you can raise the overall gain of the system. The Real Time Analyser allows you to view these frequencies graphically so that you can easily identify which frequencies need to be trimmed. You can do this (coarsely) without an RTA if you have a good ear. Just have someone do a "check one two" into an open mic while you slowly raise the system gain. The room mode frequencies should begin to ring or feedback (hopefully) one at a time. If you have a good ear, you can cut these frequencies with your graphic EQ. This practice is called "ringing out the system". This is usually self limiting after you have found 4 or 5 resonant frequecies; going much beyond that is difficult. The RTA allows you to more precise control. It is important to use pink noise, not white noise as the sound generator. White noise has equal power at all frequencies, but our ears do not hear that way. Pink noise is weighted to approximate the frequency response of human hearing. You may also find when using the RTA, some notable dips in the frequency response. These may result from a crossover that isn't matched to your speakers, or it may be due to your speakers poor frequency response, but most likely it is the result of acoustic phase cancellation. You can not fix phase cancellation with an EQ. The best you can do to control phase cancellation is to direct your speakers to provide as much direct sound to the audience area with as little overlap as possible, and try to keep the speakers away from corners, obstructions, walls etc. Phase cancelation is usually most pronouced in the low-mid and bass frequencies. You can often detect phase cancelation by walking around the room with program material that you are very familiar with playing. Listen for bass drop outs at various points in the room. Once you have killed the feedback beast, you you should find that you have a system that is reasonable stable (does not easily feedback) across the audio spectrum. Now you can apply some subtle sound shaping with the EQ. Note - once you have equalized the system, you may have to restore you previous gain structure by adjusting the input gain of your EQ. You don't want to apply so much attenuation with your EQ that you have to drive your board into clipping to get the level you want. If you have a bypass switch on your EQ you can easily deterime the how much gain you have to add to restore unity gain. One last note - in correcting resonant frequencies, it is often better to make small cuts to adjacent bands than to make a deep cut on any one filter. As the filter bandwidths in most graphic EQs overlap, this can be effective. When you make deep filter cuts, you introduce more phase distortion.
Our country is not the only thing to which we owe our allegiance. It is also owed to justice and to humanity. Patriotism consists not in waving the flag, but in striving that our country shall be righteous as well as strong: James Bryce
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