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So, I know nothing about sound.(Reference: Well, I know where the on button is on my board and the gain knob... a couple weekends ago I was running a followspot for an event at my school. Early on in the show I heard the most god awful feedback I'd ever heard. Strike that. I ever felt. The theatre was shaking. At first I dismissed it as a passing train, but quickly came to my senses, thanks to the audience's groans. I nearly laughed afterwards, for my mistaking the tremors for a train. So this event made me quite curious, and for my edification, I'd like to know: Is this within the "normal" range of feedback? Any thoughts / comments greatly appreciated.
None. In a properly configured, eq, compressed, limited, system... feedback really should not be an issue. Also, you have to have a board op that somewhat knows what they are doing... it kinda helps.
It can be in the normal range of feedback, it usually occurs at lower or higher frequencies. But in your case, if it was long and loud enough for the building to shake, then it is time to fire the old sound board op, and hire a new one.
I did a show in one venue where they boosted the snot out of the subs (until I fixed it with my DriveRack). The subs were flown above the apron of the stage, a good five feet behind the open mics. To make matters worse, the console didn't have any HPFs on it. If one of them started to feedback, you'd feel it before you heard it.
As you are in a highschool setting... yes it is completely with "normal" that's the cool thing about Highschool, you get a chance to learn from your mistakes. Typically you tend to get most feedback in the higher frequencies, but sub-woofer range feed back isn't impossible.
That is an unusual problem, But getting to know the quirks of your system takes time.

when i first started, I used to get horrible feedback all the time, It occured when the levels on the mics onstage we set to low levels. This was a perplexing problem. I eventually figured out that the feedback was caused by the clearcom system.

So It is possible that it isn't a mic level issue, but rather something that you wouldn't expect.
IF the feedback went on that long, chances are you have damage to the speakers. Usually that happens is that there is a mic in the wrong place with an on and off switch that gets turned on, or a condenser mic that needs phantom power, and initially the phantom power is off, and then turned back on.

I have certainly seen where the clearcom can feed back on itself but not sure how it would feed back into the system unless the clearcom feed was back thru the system.

I have a good feedback story. I was running sound for the wedding of some very good friends (I'm actually how the two first met). This took place about 4 years ago, well into the digital music age. So the father of the groom had this really crappy mono recording of his church's organ playing here comes the bride on an old WELL USED cassette. He handed it to me the morning of the wedding and was so excited about how amazing it was going to sound for the wedding. I popped it in the high quality Denon cassette deck and the tape immediately disintegrated. I had three hours to wedding start time and we had no here comes the bride to play. I grabbed the denon deck, the dead cassette, and headed home. I used my best splicing skills to patch together the tape. Played it into my P.C. and then did my best to cut paste and edit together what sounded like a pretty good version of the song, removing the verse where the tape shredded, and burned it to CD. Back to the church just in time for the service to start and I had saved the day... or so I thought.

Since I was no longer using the cassette deck for play back, I grabbed a new blank tape and threw it in the deck to make an audio copy of the service. All my processional music on CD ended, the pastor was about to talk and I hit record on the Cassette... The pastor said, "Ladies and..." and was blown away by the deepest bone rattling feedback I've ever heard. I immediately pulled the pastor's mic down... the feedback kept going... I pulled all the sliders down including the master... it began to rattle my teeth. I fortunately had the training and presence of mind to ask myself, "What was the last thing I did to screw this all up?" I pressed stop on the cassette deck and the feed back ended. The church busts out in laughter as the pastor appeared rather flustered and confused and started over again. No one's ever heard a sound like that and had no idea that feedback could possibly be that low. So they don't immediately think I screwed up (which is nice). Afterward, the father of the bride actually thanked me for screwing up. He said he was so nervous he felt like he was about to pass out, but the brief diversion and moment of laughter helped him through. "It's all part of the service sir".

So, how do you get feedback out of a cassette deck with all the sliders off? Well the deck had an extra set of heads for live monitoring of the recording as it happens. The board was set up so that aux send A/B was output level to the cassette deck for recording. Aux C/D was output level to the monitor system. The aux send to the monitor system had been accidentally left on as I had set monitor levels earlier in the morning before the stupid tape broke. I created a feedback loop that went from the cassette playback heads to the board, out through aux send C/D on the channels used for cassette playback to the monitors, back in through the pastor's microphone and that channel's aux send A/B on the board out to the cassette deck to record.

Don't try this at school it's bad for your gear.
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So, any advice for learning about sound + theatre. Is there really much to it? Aside from the overly complicated stuff I'm not likely to touch?

Heh, you should know better than to ask that in a "sound" forum. I've been doing it for 30 years and I can't recall a single day I didn't learn something new. Get over the idea that there is anything overly complicated that you won't ever touch. That goes against human nature. Ever see anyone walk up cold to a wall and touch it to see if the paint was wet. Probably not. Now put up a "Wet Paint" sign and see how many people have to touch it now that its a "don't touch' thing.

If you are really interested, the best money you will ever spend is buying the Yamaha Sound Reinforcement Handbook.

You can't read a book and presto you are a sound engineer, but if you work at understanding what is in that book your trial vs error ratio will be greatly improved.

Good sound seems to be variable parts science, mechanics, art, superstition, habit, snake oil and voodoo. If you don't believe it ask 25 sound guys how to mike a grand piano. You will get 25 different well supported arguments why their way is best. Truth is, there is no one right answer for every piano, and every environment. You learn what works for you and build on it.
That story about the tape deck feeding back reminds me of the event I was doing with a DJ who kept complaining he couldnt hear enough low ends as he was scratching his records, so we added a SB1000 to his monitor stack (The SB1000 is a dual 18inch 1400W Subwoofer). It was rattling him so badly that the needles on his record decks were bouncing causing low frequency feedback... but he loved the sound of the subtle feedback and we ran with it. Sometimes you just know where your feedback is going to come from, other times you dont!

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