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Chapter 5(A): Microphones This Is Where It All Begins Really – the Microphone Emulates the Human Ear and Responds to the Pressure Variations of the Sound Waves

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Chapter 5(A): Microphones This Is Where It All Begins Really – the Microphone Emulates the Human Ear and Responds to the Pressure Variations of the Sound Waves Chapter 5(a): Microphones This is where it all begins really – the microphone emulates the human ear and responds to the pressure variations of the sound waves. Microphones range from extremely inexpensive “throw-away” units, to precise studio instruments costing tens of thousands of dollars. They all sound slightly different, and if you ask any three audio professionals what they think of a particular microphone, you will probably get at least 5 different answers. In recent years, the quality of even inexpensive microphones has dramatically improved, and there are some real bargains out there. There are even quality microphones available which plug directly into a computer USB slot. But if you hope to be able to make a wise selection among the plethora of options, you need to know some very basic information. So, once again, we begin with some definitions. Transducer: Any device that transfers the energy of one system to another. These may be the same or different types of systems (i.e. motion to electricity, electric signal to sound, sound pressure to electricity etc.). Microphone: A transducer that converts sound pressure to an electrical signal1. The first broad classification of microphones is as to the manner in which this conversion takes place, and consists of Dynamic and Condenser (or Capacitor) microphone types. Dynamic Microphone: One in which the electrical signal is produced by the movement of a conductor through a magnetic field. There are two major types of dynamic microphones, moving coil, and ribbon. Moving Coil microphone: A coil of wire wrapped around a tube is attached to the rear of the microphone diaphragm (the disc that is moved by the force of the sound pressure), and suspended in a magnetic field. Movement of the diaphragm moves the coil, cutting the lines of magnetic force and inducing a flow of electrons in the wire of the coil. Strength of the resulting signal depends on the strength of the magnet and the number of turns of the wire in the coil, as well as the amount of movement allowed by the diaphragm suspension. Figure 1: Moving Coil Microphone Elements Ribbon microphone: (also dynamic) Instead of having a moving coil of wire, ribbon mics have a corrugated strip of metal - actually foil as it is extremely thin - that is suspended in a magnetic field. As metal is a conductor, the movement of the foil induces a voltage as it cuts across the magnetic lines of force. 1 So – do you abbreviate it as “mic”, “mike” or “mic.”? Again, 5 soundies in a room, 6 opinions likely. I like to use “mic” since it’s not a “mikerophone”, it’s a “microphone”. I usually skip the period because it keeps the darn word processor from capitalizing the next word and I’m basically lazy. Basic Sound Engineering PP271: Chapter 5a 64 Figure 2: Ribbon Mic Elements This produces a voltage that is relatively small and a very small current as the resistance of the foil is very, very small. Ribbon microphones always have a matching transformer that presents appropriate impedance2 to the system (though these are still "low impedance" devices) and provides a voltage more similar to that produced by other microphones. While modern ribbon microphones can be quite robust, it is important to remember that many vintage ribbon mics are extremely delicate, and should be protected against shocks and overloads. Since even modern versions tend to be quite expensive, it probably makes good sense to be in the habit of treating them gently. Condenser (or Capacitor) Microphone: These are microphones in which the diaphragm is one part of a capacitor3 (once called a condenser). Movement of the diaphragm causes the distance between the plates to change which varies the capacitance of the device, causing a corresponding change in voltage if a voltage is being applied. Figure 3: Condenser Mic Elements 2 More detail later – for now: Impedance is the complex sum of Resistance and Reactance – everything that “impedes”, or gets in the way of the flow of electrons in a circuit. 3 A Capacitor consists of two conductors separated by a non-conductor such that an accumulation of electrons on one conductor drives electrons out of the other via the electrostatic field between them. Electron flow “through” the capacitor is thus dependent on the distance (and material) between the conductors. Basic Sound Engineering PP271: Chapter 5a 65 This signal is quite small, so is applied to a preamplifier to bring it up to the level of a mic signal. Condenser mics must have both power supplies and preamplifiers. Power supply: can be separate plug-in or battery supply inserted in the mic cable, a simple chamber in the mic for a battery, or Phantom Power - wherein the operating voltage is supplied by the mixer board. More later on phantom power, but briefly the voltage is applied to BOTH signal lines and returned by the shield. Better boards with phantom power allow you to switch it on or off for each line, some offer a range of voltages, as condenser mic supply voltages range from 1.5v to over 50 volts. Electret Capacitor Microphones: are condenser microphones in which the diaphragm is polarized permanently at manufacture eliminating the largest drain on the power supply. This allows smaller batteries to be used, as the pre-amp requires a much smaller EMF4 to operate. Acoustical Properties: Receiving end: Our brain has an editing facility that allows us to tune in some sounds and tune out others. It is relatively easy to concentrate on one sound out of a mix when we are present in the room. Rooms like a classroom are actually fairly noisy, but we learn to tune out un-wanted sounds such as hum from florescent lights, ventilator noise, noise from the hall and parking lot, etc. Of course, this assumes there is something that we want to hear in the room that the brain can focus on. One of the keys to this facility is a perception of direction of sounds. We perceive horizontal direction (excluding visual cues) largely by evaluating the difference in arrival time of different sounds at each ear. Thus if a sound arrives at one ear earlier than the other, we perceive its direction to be to the side of the "early ear". Microphones in general do not have this ability - a microphone will "hear" all sounds of equal level equally well. The demonstration of this is simple, put a microphone in a "quiet" room and record for a while. On playback, we hear all kinds of noises that are present in the room, and "heard" by the mic, that we didn't hear during the recording session. We hear them on playback because they are coming from a source that we do not automatically tune out - the speaker - so all the cues our mind uses to pick and choose are defeated. This has great import to us as sound engineers, and different types of microphones have been designed to overcome this and other problems related to recording only what we want.5 Microphone Physical Design. The three basic microphone types for directional sensitivity are: Omni-directional, Bi-Directional, and Uni-directional. Directional sensitivity is a function of the physical design of the microphone (except in the case of multiple diaphragm microphones - more later). Omni-directional microphones: also called pressure microphones as they respond to variations in air pressure without regard to direction. They are characterized by only one path for sound to the diaphragm, and only one side of the diaphragm is open to sound. They are essentially a simple sealed container with a diaphragm as one end. If you look at figure 3 on 4 EMF: Electro Motive Force – a sexier term for voltage. It describes what voltage really is in a circuit – it is the “pressure” that causes electrons to flow. The actual flow of electrons is referred to as “current”. 5 It’s also part of why we never think a recording of our voice really sounds like us. The brain filters out and “equalizes” the sound it receives, and in the case of our own voices it also adjusts for the fact that the ears cannot possibly be in the direct path of sounds coming from the mouth, and that there are other paths for the sound of our voice to arrive at our hearing mechanism. It has no accurate reference to do this though, since it has no input from any device which is in the direct path. Basic Sound Engineering PP271: Chapter 5a 66 page 65 above, you can see that it represents such a pressure microphone. Any pressure variation, regardless of what direction it may come from, will cause movement of the diaphragm. If the microphone body is physically large, the case of the mic may obstruct sounds arriving from off-axis, so they may not be perfectly Omni-directional. Bi-Directional microphones: are equally sensitive to sounds on 0 and 180 degrees (front and back), and insensitive to sounds on 90 and 270 degrees (left and right). Also called pressure gradient mic, as it responds to the relative difference of pressure between front and back. Figure 4: Bi Directional Microphone Elements (top view) Physically the bi-directional mic is simply a diaphragm suspended "vertically" so that sound can reach it from any direction. Sounds from the front strike the diaphragm and cause it to move, but by the time that the same "wave" arrives at the rear of the diaphragm it has shifted in phase and helps to "suck" the back side thus reinforcing the movement of the diaphragm.
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