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Natural Science Forum / Physics / Acoustics / May 2008Room resonance in live performance
Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal environment. , so hard wall surfaces, using a pair of speakers on tripod stands so about head height. Fine at low sound level, but sometime into the set they turn the level up. I'm guessing, that at some frequency about 2KHz, the room starts ringing, not microphony/ howl-round as its quite stable in level, whenever that pitch is being emitted. What is this effect called ? Again assuming this is associated with the separation between the wall in front of the speakers reflecting back to the wall behind the speakers, is there a way of calculating what this frequeny would be for a given room dimension and relative speaker position. And would it be possible to make up some device to ping short but high level bursts of various frequencies, in that range, via the PA and speakers , during sound-check, to determine what this frequency is and then ideally knotch-out but more likely attenuate using the graphic on the mixer ? > Well I assume that is what it is. Scenario musician/s in a pub, ie > non-ideal [quoted text clipped - 17 lines] > this frequency is and then ideally knotch-out but more likely attenuate > using the graphic on the mixer ? when it howls it is called feedback and there are devices that put out "pink" noise (band limited white noise) that allow you to set response using a graphic EQ and a audio frequency display. You said 'hard surfaces'. This makes me think 'uneven reverb decay vs frequency'. Yes, it did make me think that. Also el cheapo speakers and cardioid dynamic mikes tend to be peaky in the upper midrange... I think they design em like this to make vocals crisper or something... maybe they dont know how to make em flat so they sell the midrange boost as a 'feature'. I'd fire the marketing guy and hire a real engineer. All that 3.5KHz is just bouncing around the room in a circle going nowhere. Mario Andretti could win a race in a station wagon because of his excellent driving technique. I could make that Sows Ear of a PA system sound like a Silk Purse given a 15 or 30 band graphic eq on the house mix due to my impeccable frequency response judgement and my immeasureable modesty. > You said 'hard surfaces'. This makes me think 'uneven reverb decay vs > frequency'. Yes, it did make me think that. Man, all I thought about are the women that could be in the pub and the beer. > Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal > environment. , so hard wall surfaces, using a pair of speakers on tripod [quoted text clipped - 12 lines] > this frequency is and then ideally knotch-out but more likely attenuate > using the graphic on the mixer ? The effect is called standing waves. It occurs in rooms or volumes with parallel walls or other shapes that reinforce multiple reflections. The frequencies involved are usually much lower than the 2kHz you mentioned. The frequencies are multiples of 1/2 wavelength. So, for a 20 foot space, wave lengths would be 40 ft, 20 ft, 10 ft, 5 ft, 2 1/2 ft etc. This would be frequencies of 27 Hz, 55Hz, 110Hz, 220Hz, 440Hz, etc. Note that these frequencies when above about 200Hz get right in the middle of the mid range frequencies where our ears are most sensitive. Rooms with this defect "color" the sound because of these reflections. Rooms or spaces with good acoustics have few if any parallel surfaces. They have floating "clouds" or panels set at angles and many curved surfaces and sloping ceilings. Look at any good auditorium. They also use a certain amount of acoustic absorption material to reduce reflection. However, too much absorption deadens a room and kills a 'live" sound. There is a balance between too much reverberation (echo) and too dead. The art of acoustics is to get the reflections and reverberation right without standing waves or too much echo. Yes impulses are used to test the acoustics of rooms. A gun shot is sometimes used. An acoustic analyzer takes an timed FFT of the room response as the reflections and reverberations die away. This tells exactly what the faults of a room are, how long the reverberation lasts and what frequencies are emphasized. Corrective action is another matter. >Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal >environment. , so hard wall surfaces, using a pair of speakers on tripod [quoted text clipped - 12 lines] >this frequency is and then ideally knotch-out but more likely attenuate >using the graphic on the mixer ? Reminds me of the time I lined all the walls with 2" rigid fiberglass. (Home Theater) The room was so damped, it was hard to hear people talking to me. D from BC British Columbia Canada >>Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal >>environment. , so hard wall surfaces, using a pair of speakers on tripod [quoted text clipped - 20 lines] >British Columbia >Canada I've been inside an anechoic chamber used to test hearing aids. Scary! You hear your own heart and various gurgling noises ;-) ...Jim Thompson | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: "skypeanalog" | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave > I've been inside an anechoic chamber used to test hearing aids. Scary! > You hear your own heart and various gurgling noises ;-) Eat something before you go in. ;-)  Signaturehttp://improve-usenet.org/index.html Use any search engine other than Google till they stop polluting USENET with porn and junk commercial SPAM If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm On May 4, 9:44 am, Jim Thompson <To-Email-Use-The-Envelope-I...@My-Web- Site.com> wrote: > On Sun, 04 May 2008 09:25:03 -0700, D from BC > [quoted text clipped - 27 lines] > I've been inside an anechoic chamber used to test hearing aids. Scary! > You hear your own heart and various gurgling noises ;-) That's because you where scared and your heart was pounding. :) That creeeeck - creeeeck noise was your knees, or at least it was mine. The grind-pop-pop as I turned my head was by far the most disturbing noise. > ...Jim Thompson > -- [quoted text clipped - 6 lines] > > America: Land of the Free, Because of the Brave > Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal > environment. , so hard wall surfaces, using a pair of speakers on tripod [quoted text clipped - 12 lines] > this frequency is and then ideally knotch-out but more likely attenuate > using the graphic on the mixer ? As the levels go up, ear fatigue starts setting in. This can lead to all sorts of odd non-linear effects including the one you describe. Remember, live sound in a bar/pub setting is more than just the room, the band and the sound system. Pulling back, say with a low energy song can help reset the ears as well as reset the level of the (recently discussed) "cafe effect." A dynamic EQ that pulls down the upper end as the SPL rises may help here. Behringer's DEQ 2496 is one of the lower cost boxes that provides such a function. Being interactive with both the band and the venue, you'd have to experiment a bit to find parameters that work in your setting. [yes, there's a bit of hand waving in my response, thus: YMMV] Later... Ron Capik -- >>Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal >>environment. , so hard wall surfaces, using a pair of speakers on tripod [quoted text clipped - 3 lines] >>not microphony/ howl-round as its quite stable in level, whenever that pitch >>is being emitted. That is a resonance of the sort that requires more description. Ihad one studio owner that complaied of suc. when I measured the RT o that room, there was a very long one only in the 160 Hz 1/3 octave nband. I looked for opposing flat surfacrs, but found very few, and none that could readily explain what we heard. Some of his wall were set to NOT be parallel which confounds the issue. I suppose that one should start with the few flat parallel surfaces we found. >>What is this effect called ? Beats me. "Room resonance" is all that comes to mind. >>Again assuming this is associated with the separation between the wall in >>front of the speakers reflecting back to the wall behind the speakers, is [quoted text clipped - 4 lines] >>this frequency is and then ideally knotch-out but more likely attenuate >>using the graphic on the mixer ? > As the levels go up, ear fatigue starts setting in. This can lead to > all sorts of odd non-linear effects including the one you describe. [quoted text clipped - 3 lines] > (recently discussed) "cafe effect." A dynamic EQ that pulls down the > upper end as the SPL rises may help here. I don't understand the meaning of "puls down the upper end". There are two upper ends; that of the frequwncy range, and athat of the dynamic signal level. Which do you mean? > Behringer's DEQ 2496 > is one of the lower cost boxes that provides such a function. Never heard of it. what does THAT do? > Being interactive with both the band and the venue, you'd have to > experiment a bit to find parameters that work in your setting. > [yes, there's a bit of hand waving in my response, thus: YMMV] Enough with the hand waving! Angelo Campanella |
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