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Author Topic: Microphone placement and standing waves Ken Lackner Phenomenal Film Handler Posts: 1907 From: Atlanta, GA, USA Registered: Sep 2001 posted 09-17-2007 11:28 AM                     It is my understanding that when doing an EQ, microphones should never be placed on a room centerline due to standing waves. I was thinking about this, and there is something I don't understand. The node in a standing wave pattern will only occur in the center at specific frequencies, and there will always be nodes at other points besides the center. There is always a chance that one (or more) of the four mics could be in a "dead" spot (node) for one or more frequencies, no matter where you place them. So why is there so much emphasis on not placing a mic in the center?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 09-17-2007 01:21 PM                     With any good EQ one should walk the room with the PN playing and listen to where the mic(s) are going to be placed...you will be able to hear a point that is clearly not representative of the rest of the room. As you get into exact centers...you will find such places rather easily and merely moving one's mic a few feet either way will allow you to get a better picture of the room. One of the reasons for using more than one mic is to minimize the affect any one set of nodes have on the overall response. Another problem I see with many an EQ is the belief that the mic they are using is acoustically flat. Many a time I've heard people spout off about how their USL mic plexer uses the same mics as THX's R2 (and D2) thus it is the "same thing" but less expensive. Well, no it isn't...yes they are using the same or similar mics but one thing the THX system has over all others I've seen is a means to have all mics properly calibrated on each 1/3rd octave band. When we first got our USL mic plexer we A/Bed it to the THX R2 and the mics are just not inherently flat, especially at the HF and the USL system is load dependent on being flat in the Lf department (we actually sent our Abacus analyzer out to USL so they could measure the roll off). There is a lot at play when tuning a room and one should definately know what it is they are seeing on the analyzer screen before turning knobs (or moving virtual faders) willy-nilly. Walking the room is a good first start to ensure what you see on the analyzer screen has some meaning for the room rather than the one isolated seat. Steve  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 09-17-2007 09:48 PM                        Have you played with the LinearX pcRTA? We had one sitting in the office, so I said, send it over and I will see what it does and compare it to the D2 we are getting in a few days (yay...). It is obviously an "older" product since it is based on a ISA card, but that came in a roughly showbox sized portable chassis, and then you control it through the serial port from your notebook on which the analyzing software runs. Even though it is a Windows 3.11 program, it runs without problems on XP (well, the current version is from 2005) and connects to the serial port through the Keyspan adapter without any problems. I have just set it up and played around with it a little bit, but I haven't had an opportunity yet to test it in the field and compare it against other analyzers. But it does look very thought-through and it has a lot of advanced functions. Plus the mics come with calibration files as well. It would be interesting to know if anyone has actually worked with that.  |  IP: Logged Ken Lackner Phenomenal Film Handler Posts: 1907 From: Atlanta, GA, USA Registered: Sep 2001 posted 09-18-2007 09:12 AM                     I understand why multiple mics are used. And I understand that moving the mic just a foot or two could drastically affect the readings. My question is, why is there so much emphasis placed on specifically avoiding the center, when anomalies could occur just about anywhere in the room?  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-18-2007 09:31 AM                     Michael, I tried out a linear-x unit years ago and found that the R-2 beat it out easily in many aspects including the resopnse of the mics. When I was in St Charles, Il. I had the good fortune to have a friend that worked at Riverbank Labs at my disposal to do some testing of different units. You'd be amazed at the differences in mics and also in the different units claimed filter accuracy... This is where R-2 really blew away several other units. The Ivie PC-40 also had great filter and mic accuracy and this is the main reason I still use my PC-40 unit for some stuff today(dated it is!) The R-2 also has the stuff built into it that helps you verify if a room actually works or not at least to some calculated specs that do seem adaquate and that is most important in our field. I also wouldn't consider purchasing any unit that HAS to operate from a desk top computer these days. I think the linear-x was a good unit in its time and its still a powerful tool but its pretty dated today. P.S. Ken, If your room has lots of audible standing waves then the room shold really be treated properly to minimize them. Mark  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 09-18-2007 05:13 PM                        quote: Ken Lackner My question is, why is there so much emphasis placed on specifically avoiding the center, when anomalies could occur just about anywhere in the room? Because they are more likely to occur on the center axis or other geometrically centered areas. Like Steve said, that's why it is a good idea to take a little walk and listen carefully to the pink noise to see if you hear anything that is noticeably different in certain spots from the general area. Mark - the pcRTA is actually portable, because it has that shoebox sized chassis but yes, it is a bit heavy and clunky, not any more so than the R2 though. But you don't need a multiplexer, as that is done in the unit. You just connect the 4 mics with a cable assembly they call the "snake" to a DB15 which is on the ISA card (which goes in one of the 4 slots seen on the case). Anyway, I think it is rather unlikely that it will see much action in the field since we do have 2 R2s and the D2 just shipped out today, but it still is an interesting piece of equipment.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-19-2007 09:27 AM                     Michael, I think you would find the R-2 all the more useful tool. Also, running the mics all the way down that snake opens the door for many undesriable possibilities to occur. Having the multiplexer nearer the mics and sending a line level signal down the snake is the better option. Mark  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-19-2007 02:16 PM                     There is a AES ANSI standard for filters in RTAs and I believe that that linear X meets it  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-19-2007 06:03 PM                     Gord, While there is an ANSI standard you'd be surprised how many analyzers are off and that while some of the bands are dead accurate others are off. The best units are the R-2, and the PC-40. The PC-40's filters are each individually adjustable. Klark Teknik was the worst of the analyzers that I've owned as far as filter accuracy. I have had the Klark Teknik, IVIE PC-40, Linear-X(that I tried out), my R-2, and several others tested by my friend who was at Riverbank over the years. Sadly, Riverbank has been sold to another group and last I heard was in limbo. Mark  |  IP: Logged

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