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Guide for Goobers--PA Basics and Glossary for Newbies

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  • #16
    Brand and model aside, how is it that a less-tuned system responds worse to the feedback killer than a well-tuned system?

    Feedback always happens for the same reason. That reason is you cross the unity threshold point (where the sound of the speaker is as loud into the mic as the sound of your voice into the mic). So maybe the question is how and where did I do that.

    Let's try a simplified explanation ...

    Let's consider one mic and one speaker. There is a distance from your mouth to the mic and a different distance from the speaker to the mic. So when the sound from your mouth and the sound from the speaker arrive at two different times it sets up a comb filter. The top of the teeth of the comb are in phase and the gaps between the teeth are out of phase. You can only get feedback on the top of a tooth in the comb and cannot get feedback in the gaps. When you change these distances you will change the number and the width of these teeth.

    Your sound system is not perfectly flat ... it has peaks and dips ... especially off axis from the speaker and because of the pattern of the mic. Now consider that ragged frequency response. As you turn up the system when one of the peaks in your system crosses the unity threshold right on top of one of the tops of a comb tooth ... presto ... you get feedback.

    A "more tuned system" simply minimizes the sound level of the speaker from reaching the mic. The first thing here is to aim the speaker away from the mic as best you can while still pointing it in the direction needed. Now there are two tunings you need to make. The first is tuning the speaker so that the sound reaching the audience sounds pleasing. The second tuning is cutting those frequencies that present themselves as peaks to the mic. These may or may not be the same thing. In fact they can be exactly opposite.

    Feedback (if systems had no distortion harmonic or otherwise) would be a single frequency and would be close to a sine wave. So specific that you could remove that single frequency. The human ear can't identify such a narrow bit of info so your brain just fills it in (integrates it). If fact your ear/brain combination (on average) can't hear the number of points that would be 1/3rd of an octave at frequencies below about 400Hz and about 1/6th of an octave above that. There is of course a first frequency that crosses the threshold and then a second and a third and so on.You need to grab those single frequencies only without grabbing anything else.

    The standard practice is to throw a 1/3rd oct graphic at this ... but a 1/3rd octave graphic has filters that are about an octave wide ... which you can hear and will miss. They are called 1/3rd oct graphics because the filters are SPACED a third of an octave apart.

    So what you really want to do is tune your speaker for listening with a 1/3rd oct GEQ for the listening part and use a very narrow parametric EQ for the feedback part.
    Don Boomer

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    • #17
      Limiters for speaker protection "limit" the amount of voltage an amplifier can deliver to a speaker. This should go a long way to protecting the speaker from thermal burnout (you still need to protect against excursion limits with HP filters). It should be said for the record that it doesn't matter what the waveform looks like ... only the amount of power contained in it ... square wave, sine wave or anything in between. Amp clipping itself does not burn speakers and underpowering does not burn speakers. Too much voltage for too long will burn the voice coil or separate it from the former itself. Protecting your amp from clipping has very little to do with anything except your amp (and most amps have clip limiters built in)

      Let's start with the basics and add exceptions as we go along

      Set the limiter for about a 20:1 ratio or higher. In the real world there is almost no practical difference between 20:1 and infinity:1 (because you have almost nothing to drive above threshold. If you set the threshold at "0" you'd have to drive 20 dB more to get the output to rise 1 dB). Anything less than 20:1 doesn't give you much protection. The attack and release will depend of the averaging style of your limiter and the frequencies involved. You probably want the attack on the order of 15ms - 20ms for woofers and maybe 5ms for tweeters (not written in stone). The release is generally set to be 10 times the attack time.

      Now comes the tricky part ... how do you set the threshold?

      Now you need to know how much voltage your speaker will handle. It will be called the "continuous", "average", or "rms" rating. If it is called "program" then cut that number in half and if "peak" divide by 4 (generally). Understanding the "real" number is a bit difficult ... so setting a limiter like this depends on how good this number really is. Adjust (down) as necessary for your own comfort.

      It is helpful to know by which method the manufacturer has used to make this rating. If done by the AES method then it considers the minimum impedance. Speakers have a "nominal" rated impedance e.g. 4 ohms or 8 ohms but the true impedance varies with frequency. It would be typical for an 8 ohm speaker to vary between a low of maybe 6 ohms (at low frequencies) to a high of 20 or 40 ohms at high frequencies. So your amp is delivering varying amounts of "wattage" at different frequencies. You may have to subtract a little bit of power handling ability.

      Convert the speaker rating to Volts;

      sq root (speaker rating * impedance)

      so a 500W 8 ohm speaker looks like this ...

      square root (500*8) = 63.24 volts

      (just type "square root (500*8) into google and it will do the math for you)

      At this point you can simply drive signal through your limiter to your amp and bring down the threshold until you reduce the output of the amp to 63Vac measured on an AC voltmeter (assuming you have proper gain settings)
      Don Boomer

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      • #18

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        • #19
          These curves show the equal loundess of a set of frequancyies. The 3+ Khz area is most interesting.

          The Fletcher-Munson
          http://en.wikipedia.org/wiki/Fletcher-Munson_curves

          And the newer (1956) Robinson /Dadson version of the same : though it may not be more accurate.
          http://en.wikipedia.org/wiki/Robinson-Dadson_curves
          -token canadian

          Lest we forget: double-blind tests make audiophiles look twice as stupid. CRAIG V 2007

          Just for fun, what do you think would happen if you decided to take a nap in the fast lane of a freeway? agedhorse -2008

          Funny, I'll bet I have a good one sitting on my shelf. agedhorse -2008

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          • #20
            I see that Soundcraft UK have posted the full Soundcraft Guide To Mixing onto YouTube. Previously this set of 18 videos was only available on DVD.

            Check it out here: http://www.youtube.com/view_play_list?p=4CF0FA9E71C443FE

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            • #21
              Here's a link to practice on training your ears on how to ring out your monitor system for offending feedback frequency don't worry it's free and also is virus free.

              http://sourceforge.net/projects/sft/

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              • #22

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                • #23
                  If the board and the amps clip at the same time, and your board has the pro standard of +26dBu output capability, but the input and output of the DRPA (and 260 and many other DSP's) clip at +20dBu, then you will be clipping the DSP 6dB before clipping everything else. This is probably one of the most common user errors with DSP in general. Internally, most DSP's operate at around +6dBu before internal clipping, so there's input and output scaling that occurs inside the DSP boxes.

                  For proper system operation, set the DRPA's limiters off, set the DRPA to unity gain (0dB in, and out's to average around 0dB... some may be a few db + and the rest a few dB - ) then do the same thing you did before w/ pink noise but set the output level of your console to +12dBu. Turn your amps up to the point of clipping or limiting and THEN set your limiters on DSP so that the amps just no longer clip. Now, when you drive the system hard, the DSP's limiters and drive electronics will have 8dB of headroom on which to work. The term is called complance, and is the differential on which the limiter's gain reduction algotithems can operate while the rest of the DSP's I/O remains in linear operation. The penalty is a little noise.

                  With the limiters, it's a fine line between adequate steepness and overshoot versus audible artifacts, but if you are using this primarily as protection, it's not a big deal.

                  This is good general DSP information, and understanding the interanal AND exteral gain structure of all devices in the system are a big part of troubleshooting performance issues and maximizing the full potential of any DSP product.
                  -------------------------------------------------------------------------------------------------------------------------------------------------------------------
                  Former product development engineer: Genz Benz, a KMC Music/FMIC/JAM Industries Company, continuing factory level product support and service for Genz Benz

                  Currently product development engineer: Mesa Boogie

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                  • #24
                    Here some free room analyzing software tools to help tune your PA.
                    You will have to register but it free to register to.

                    http://www.hometheatershack.com/roomeq/

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                    • #25
                      EQing techniques and how to by Dave Ratt sound man for the RHCP and many more pro touring bands.

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                      • #26
                        Finally buying an equipment trailer. I live in cold, cold Michigan.

                        Wondering what of my PA equipment I can leave locked in the trailer in sub-zero temps between gigs.
                        Equipment - We're talkin: Yamaha Club series non-powered speakers/monitors. Yamaha mixer. Yamaha amp/mixer. TC M-One effects unit. Pedals, direct boxes. Lots of cords and cables.

                        We get a lot of temp changes in Michigan, freezing/thawing/freezing.

                        What is your experience with leaving stuff in your trailer/van?

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                        • #27
                          What is your experience with leaving stuff in your trailer?

                          It can get really wet from the trailer sweating on the inside.

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                          • #28
                            I've never done that.

                            I have left all my gear in an unheated garage all winter (in NJ) with no ill effects.
                            Unless you count being out of tune.

                            I usually did warm up the garage with a space heater for an hour or so before practicing though.

                            Other than condensation issues, I'd let everything adjust to room temperature for 30-45 minutes before playing.
                            "If you don't feel like dancing when you play, neither will anyone else." Bassgeek

                            Musicman StingRay 5 Fretless H Piezo / G&L L2500 MIA / Reverend Rumblefish 5XL
                            Reverend Rumblefish 5L FS PM
                            Epiphone Les Paul
                            Mesa Boogie 400+ (backup amp)
                            Hughes & Kettner Fortress / QSC PLX 1804 / Bergantino NV412
                            Eden WT800B / Peavey 215
                            Markbass Little Mark II (backup amp)
                            Eden Nemesis ENX-260 / Mesa Boogie PH112 / Mesa Boogie PH112

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                            • #29
                              I leave mine in my trailer. but the only thing i take out is my mixer, that comes in the house when i am done with my jobs ( each job). but as far as my other gear i just leave it in there and it's is fine. I have a cargo trailer , I think a 5 x 8 cargo trailer and my gear is always fine. at the end of winter i always get my gear out and test it out to make sure it made it good through the winter months and it always works.

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                              • #30
                                In my experience, summer tends to be worse than winter because of the higher humidity. Neither is too bad though. The only thing I've really noticed an effect on are steel ball corners and latches on cases, and it's not too serious.

                                In the winter, let your stuff adjust to the indoor temperature when you bring it in for the show and let the condensation evaporate, and you should be fine.

                                Desiccant packets inside cases and racks would help with the humidity issues, if that's an option for you.

                                I don't typically leave guitars, especially acoustics, in the trailer in summer or winter. I know people who do, but I don't. You definitely need to let those acclimate to the indoor temperature when you finally bring them inside.
                                B.

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