Microphone Theory)

PART I (Microphone Theory) Harvey Gerst: Ok, you're on. At 64, maybe the best thing I can do with my life is to pass on what I've learned from great people that taught me when I was starting out. I think that's why Al Schmitt, George Massenburg, Ed Cherney, and some of the other really big guns spend so much time on the net. We all owe the guys that came before us a lot, and this is our way of paying them back. And that's the only thing I have in common with all those guys I just mentioned - we all kinda drank from the same well back in the 50s and 60s. I'll try to cover as much ground as I can, to give everybody a good basic understanding of the different mic designs, advantages and disadvantages of each design, how mic polar patterns are created, advantages and disadvantages of each polar pattern, and finally where each type might be used, along with advantages and disadvantages of each usage. How's that for a course outline? This is the frequency response curve of a Neumann TLM-103. Not very flat, is it? Does that mean it's a bad mic? Before we can answer that we hafta know how to read one of these curves and how to interprete it. Okay, let's start this with some interesting history as a prelude to the whole mic discussion. "Why" will become pretty clear by the third or fourth paragraph: In a way, the history of microphones and sound all started with Alexander Graham Bell, and Western Union. After Bell won the lawsuit with Western Union over the invention of the telephone, his fledgling AT&T company needed somebody to manufacture phones for them. Western Union had created a manufacturing division (Western Electric) to make telegraph keys and telegraph equipment. Bell bought the Western Electric division and they had the exclusive right to manufacture phones for Bell. By 1910, Western Electric had the ambitious task of creating a coast to coast telephone hookup to tie in with the opening of the Panama Canal, but the problem of amplifying a signal over long distances was still unsolved. In 1913, Dr. Harold Arnold (of Western Electric's research group) saw that Dr. Lee DeForest's "Audion vacuum tube" was the possible solution, and they bought the rights to it and began work on a "high vacuum" tube. This indeed solved their long distance problem, and led to another discovery - a "loud-speaking telephone". In 1916, they received a patent for what we now call a "loudspeaker". With the addition of the "high vacuum" amplifying tube, and another little patent for a device called a "condenser mic", they were suddenly in the P.A. business as well. These inventions opened the door for radio, talking movies, and sound systems in general, and with their other patent for a high quality "amplifier" in 1916, they pretty much defined the science of sound. (It would be another 12 years (1928) untill a young Georg Neumann would start his own mic company in Germany. That same year, Western Electric received a patent for a "dynamic mic" design. The designs Western Electric developed for movie speakers would eventually start companies like Altec and JBL making horns and loudspeakers for Western Electric, and eventually those Western Electric designs became the foundation for their own speaker lines. Western Electric created their own Research and Development arm called "Bell Laboratories", which went on to create the transistor and a host of audio related products. It was Western Electric and Bell Laboratories who we must thank for the development and research into microphone design that we enjoy today. Next, we'll look at some of the different types of microphone designs in terms of advantages and disadvantages. How a "dynamic" mic really works will definitely surprise you (hint: it's NOT just a small speaker in reverse). Dynamic Mics By far, the most popular mic on the market today is the dynamic cardioid mic, so that's as good a place as any to start. "How does it work, what exactly is a cardioid, and how and where would you use it" will be our focus today. Let's look inside one and see what we find: Well, it has a cone (like a small speaker), a voice coil (like a small speaker), and it sits in a magnetic gap (like a small speaker), so isn't it just a small speaker in reverse? Yes, and no. The operating principle is the same, but the execution is very different. When's the last time you saw a 3/4" speaker that went down to 30 or 40 Hz? Here's how it's done: The system resonance is chosen for a mid band frequency. By itself, the capsule's response looks something like this: (the black line) just one big resonant peak, with the response falling off rapidly on each side of the peak. Now you can tame that peak by putting in a resonant chamber that's tuned to that peak, which will give you two smaller peaks on either side, like red line: And if you add two more resonant chambers, tuned for each or those peaks, you wind up looking more like the blue line and finaly the green line: but remember, it's still a lot like a bunch of tuned coca cola bottles inside there. Now ya gotta do all of this stuff JUST to get the response usable - never mind about the mic pattern yet! A lot more to come!! Everybody still with me at this point? Any questions? Chris F : 1. So, if I understand your first post, you explain that the frequency response of your basic garden variety dynamic mic is not really a curve, but rather a series of mechanically engineered peaks, right? But we don't necessarily hear it that way, because our ears/brains fill in the sonic spaces the same way our eyes/brains do when we look at a newspaper photo that really consists of a bunch of dots rather than an actual picture. Is that pretty close? 2. Is the reason for that the size of the diaphagm? It would make sense that, in order to truly reproduce a sound in the extreme low register, the diaphragm would need to be as large as the soundwave corresponding to the lowest note on the recording, which would be both incredibly impractical and terribly funny....can you see Roger Daltrey swinging one of those suckers around? So instead of that, the initial response peak is spread out so that it covers more range more evenly. Am I close, or did I get off track by assuming too much? Harvey Gerst : 1. Yes and no. The broad band resonators and filters actually do smooth out those peaks pretty well, but you hafta remember it's all done with mechanical tricks and if you hit it with enough energy in a susceptable frequency range, it will resonate. 2. No, that's also a function of excursion and mic design. Small omnis for example, can get down to 1 Hz fairly flat. A little too much assumption which I'll try to explain in the next installment. Blinddogblues: Harvey, what I would really like to see is some help on mic selection and placement for various instruments. Especially what I consider hard to mic instruments like piano, acoustic guitar, etc... Harvey Gerst: That's coming, but it's important to learn HOW a mic does what it does, so you can understand it's limitations and best uses. Once that's understood, the rest is pretty much, "Well duh, of course!" And you'll be able to say with authority, "No, a large diaphragm mic does not go lower in frequency than a small diaphragm mic", and you'll understand why. And that will lead you to "when should I use this mic or that mic?", except by that time, you'll know the answer yourself, and beautiful women will flock to you at parties, men will whisper in awe as you walk by, and young girls will seek your autograph, all because YOU understand how mics work and how to use them. Seriousturtle: Harvey, i looked at your website today. is it normal to have a zillion microphones? just wondering. you know, if you're throwing any out in the trash, i could take them off your hands.... Harvey Gerst: Actually, no, it's not normal, but since I'm a bottom-feeder, it's cheaper for me to have a lot of low cost mics around for different colors than it is to have a lot of expensive mic pres - for different colors. Most of the mics I bought very cheap, at pawn shops, garage sales, ebay, newspaper ads, etc. I'm still paying on a few of the more expensive ones. At retail, it looks impressive, but I've never paid more than $50 for a Shure SM-57, way under $100 each for the Sennheiser 421s, etc. My only "bought brand new" mics are my Coles, the Neumann TLM-103, the Oktavas, and the Marshalls - everything else was purchased used, for very cheap. And I actually traded some old stuff I had for the Neumann. Seriousturtle : Do you like the sound of the TLM-103 for vocals, which is what i assume you use it for? how much different does it sound than the Ui 87? keep preaching about the response of the mic, cuz i'm interested in how an unflat response is considered good.

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