 |
 |
|||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||
|
|
|
|
MICROPHONES 101by Eric BoyerCONTENTS:SOUND – WHY WE’RE HERESound is a form of kinetic energy which travels through a medium and is later interpreted by the brain after arrival at the ear. In other words, if a tree falls in the woods and there is no one around to hear it, does it still make a sound? Technically no, because sound is only sound once it has been interpreted by the brain. Sound is technically a periodic variation in atmospheric pressure caused by contact with a vibrating body. For our purposes, the medium is air but can also be a liquid or a solid. Sound has many characteristics that we can measure and has some interesting physical properties that you should be familiar with. We’ll discuss some of the basics here and then tackle some more advanced concepts in future additions to this text.Sound Pressure WaveSound travels through the medium in all directions (360??) from its source (the vibrating body) to your ear in the form of a wave of energy. We call this a sound pressure wave because the motion of the vibrating body displaces molecules within the medium (air) and creates pressure as it does so. This pressure can be measured on a barometer as atmospheric pressure and varies as the sound pressure wave moves through the medium. This process is known as compression and rarefaction, compression being an area of greater than normal atmospheric pressure, and rarefaction being an area of lower than normal atmospheric pressure. These pressure variations cause a corresponding vibration of the tympanic membrane (aka, the eardrum) which is the first step in the brain’s interpretation of this form of energy as sound. return to top Characteristics of SoundAlmost every sound we hear, unless it is a pure sine wave (a completely symmetrical manmade tone), contains other components that give audio cues to the brain about the nature of that sound. There are seven primary characteristics that when studied together can give us a pretty good idea of exactly what kind of sound we’re dealing with. Musically, these characteristics determine the timbre of a musical instrument. Timbre is really best thought of as the instrument’s “voice” and is what makes a C played on a violin sound different than the same note played on a piano, guitar or clarinet, for example. These seven characteristics, which can all be measured or quantified in one way or another, though not always in standardized units, are:
return to top FrequencyAs it relates to sound, frequency is a measurement of the rate of vibration (also called oscillation) of the vibrating body that creates the sound pressure wave. Frequency is directly related to pitch – the higher the frequency, the higher the pitch. We measure frequency in units called Hertz (Hz) which measure the number cycles per second of the vibrating body. For example, 440 Hz (concert A) is higher in pitch than 262 Hz (middle C) and both are lower in pitch than 1 kHz (1000 Hz), which is used as a standard test frequency. The generally-accepted range of human hearing is 20 Hz – 20 kHz or twenty cycles per second to twenty-thousand cycles per second. As we age, our ability to hear the higher frequencies declines dramatically. Frequency is also used to measure many other repeating functions such as the duty cycle of AC voltage, computer processing speed and broadcast transmission spectrums.  return to top AmplitudeAmplitude can be thought of as the “size” of a sound pressure wave when measured vertically (the horizontal measurement is called wavelength). Amplitude measures the displacement, or the amount of molecules, pushed through the air during each compression and rarefaction. On a graph, it is the distance from the center line and the peak or trough of a wave – the magnitude of displacement. The more molecules you displace (the more energy you expend), the greater the displacement of the tympanic membrane, thus the louder the sound appears to the brain. Amplitude is measured in decibels (dB) on a logarithmic scale and is probably one of the most misunderstood units of measure in audio. We’ll discuss the dB in further detail in the M.A. and Ph.D. sections of the course. For our purposes now, we need to know that the dB is used to notate the intensity (relative amplitude) of a sound pressure wave as compared to a known reference. Amplitude directly correlates to perceived volume. Greater amplitude = LOUDER!return to top PhasePhase describes the time relationship of the peak amplitudes of two or more sound pressure waves. Say what? Think of it this way: if two sine waves of the same frequency leave the source at two different points in time, they will arrive at the listener’s ear also at different points in time. Yet these two waves are mathematically related and will interact with each other at points where they intersect.The collective results of this interaction are referred to as “phase coherence” and can be measured in degrees. A relationship of 0?? means two or more sound pressure waves are “in phase” and are traveling through the medium at precisely the same point in time. Any relationship other than 0???is considered “out of phase” and can be measured and more importantly, heard – as either of an additive or subtractive nature, depending on the mathematical relationship between the sound pressure waves. A 3???phase difference would be considered “slightly out of phase” whereas 30???would be considered “significantly out of phase.”  One common misconception about phase problems is that they can be “fixed” with the “phase switch” on a recording console or microphone preamplifier. That switch, correctly called a “polarity switch” can’t really “fix” anything. All it can do is reverse the poles of the microphone preamplifier circuit. Without getting too far ahead of ourselves, the polarity switch makes the negative pole of the circuit positive and the positive pole negative. So it technically does change the phase of a circuit because switching polarity actually means that the circuit is 180???out of phase. But the important point is to understand that if you have a phase problem to begin with (the result of poor mic placement), reversing the polarity only means you have the same phase problem but now it’s backward! return to top WHY MICROPHONES? THE WONDERFUL WORLD OF TRANSDUCERSSince the invention of reproduced audio, sound engineers have needed a means to get sound from the physical environment into the storage or broadcast medium. Whether radio or television broadcast, or professional or home recording studio, or even sports arena turned concert venue, all sound must be transformed into a form of energy that can be broadcast to a wide audience or stored for later retrieval.At its most basic level, a microphone is a transducer. A what? A transducer is any device that converts one form of energy into another, one of which is electrical. There are many different kinds of transducers – electrochemical (batteries, fuel cells), electromechanical (electric motors, wind generators), electromagnetic (cathode ray tubes, analog tape heads), photoelectric (LEDs, solar cells), and one particular kind of transducer called electroacoustic. As the name implies, an electroacoustic transducer converts sound into electrical energy or vice-versa. And as you can guess, a microphone is an electroacoustic transducer. Some other examples include loudspeakers and headphones, phono cartridges, disc cutting lathes, and guitar pickups. All of these devices have one common function – to convert acoustic energy (our sound pressure wave) into electrical energy or go the other way as in the case of loudspeakers and convert electrical energy into acoustic energy.  Lucky for us, it’s not that simple. Let’s add a critical component to our definition – the word useful. The art of transforming energy from one useful form to another is a complex and elaborate task which requires meticulous precision and rigorous engineering, one which becomes even more imposing when we add the subjective nature of musicality. The microphone designer must not only create a device that conforms to all manner of standards established by various organizations around the world, but she must also design a useful transducer that sounds good! We could fill libraries (and we have) debating that very topic. In sound engineering, the transducer is often thought of as the weak link. It is indeed the transducer that adds more noise, distortion and other artifacts to the signal than any other component in the entire audio path. And when you think of how many transducers are involved in recording and reproducing music (see our list above), it becomes clear that each link in the chain becomes that much more important. As we continue you will discover that it is actually you, the engineer, that is the single most important link in that chain – specifically your selection and application of the tools of the trade. There are hundreds of kinds of microphones in use today in every imaginable field. They come in all shapes and sizes and like anything, from automobiles to screwdrivers; each kind of microphone is best suited to a specific task. If you take only one thing from this program, make it this: the art of recording starts with selecting the right tool for the job - really, the essence of this craft is microphone selection and microphone placement – an art form in itself. Microphone manufacturers that restore those expensive handmade vintage classics often do so because they recognized the value of those beautiful microphones – not their monetary value (which just keeps rising), but the intangible value that they bring to the party. Sure, they are merely transducers, just like every other microphone out there – and also like all transducers, they have the potential to be the weak link in the chain! All they can do is turn sound into voltage. But - and here is the essence of this study - it is how they turned that sound into voltage that made them the legendary tools that they were and still are to this day. Today’s top-of-the-line microphone products have been built on the foundations laid by those venerable vintage microphones, but have often incorporated the advantages of superior modern electronic components and high-tech manufacturing equipment. A select few modern microphone manufacturers have even made significant improvements in visual design, fit and finish, fusing elegant attractive designs with the best that modern technology has to offer. In consideration of their contribution to this course material and dedication to microphone education, Blue Microphones deserves special recognition of their efforts to provide recordists with superior performing microphones. Blue Microphones design their products to accomplish a variety of tasks and to deliver a unique and musically-pleasing sound whether live on stage, sitting in the live room of a multimillion dollar studio, or tucked away in your own personal creative space. Like you, they are creative artists who recognize the importance of reliable easy-to-use tools that deliver professional results with a minimum of effort and equipment. Each Blue microphone has its own unique sound, but there are three specific characteristics they have built into every microphone that are universally important to the performance of a microphone:
return to top MICROPHONE PRINCIPLESMicrophones have been around since the late 1860s as engineers worked on a practical solution for a new communication device that would change history – the telephone. Though there are countless microphone designs in existence, modern microphones are divided into two broad categories: dynamic microphones and condenser microphones. Dynamic microphones generate their own output voltage whereas condenser microphones require an external power source.Another type of microphone is the ribbon, also technically a dynamic, but often categorized separately because its design is so vastly different from that of a standard dynamic transducer. There is great debate among sound engineers as to just what makes a “good” microphone. We like the idea that the “best” microphone is really the one that best accomplishes the task at hand. But how do we know what that even means? There is actually a formula that you should memorize which helps identify “best” sound: Ears + Years = Engineers (E+Y=Es). In other words, there is no simple answer to the question “what is the best vocal mic?” or “what is the best guitar mic?” To answer that question, the engineer must really rely on his knowledge and experience along with the elusive and subjective definition of exactly what sounds “good.” If you’ve ever been in a professional recording studio, you probably noticed that they didn’t have just a few mics casually strewn about – they had several different microphones, each carefully placed with respect to the sound source and to the other microphones in the room. And they probably had hundreds more in the mic locker – each awaiting selection by an engineer seeking a certain result and using her “ears and years” to achieve it. The purpose of this discussion is to introduce you to the principles that guide microphone design. It is important to understand these principles because they will help you make the kinds of choices that will ultimately answer the question posed above. We will discuss these principles in even greater detail in future additions to the course. return to top The Moving Coil DynamicThe dynamic is a very simple microphone which is sometimes also called a moving coil microphone. Here’s how it works: a coil of wire (called the “voice coil”) is attached to a very thin mylar disc (called the membrane) and precisely oriented in the field of a permanent magnet. As air molecules displaced by a sound pressure wave make contact with the membrane, they displace the membrane in a manner proportionate to the frequency and amplitude of the sound pressure wave, causing the voice coil to move through the magnetic field. The result is an AC output voltage proportionate to the frequency and amplitude of the sound pressure wave. This process is called electromagnetic induction, sometimes called the Faraday Principle for Michael Faraday, the British scientist who, in 1831, discovered the phenomenon which gave rise to a host of devices we now take for granted like electric motors, generators, phono cartridges and yes, moving coil dynamic microphones.Due to their simplicity, dynamics tend to be rugged and inexpensive and have a higher maximum SPL rating (sound pressure level – how much amplitude they can handle without blowing up!) than condensers. They are often found in environments where durability is a plus – on stage at live concerts, remote broadcast facilities, etc.. But many dynamics also find their way into recording studios right next to their more expensive electrostatic counterparts – capacitor or condenser mics. return to top The CondenserCondenser microphones operate on the principle of electrostatic induction rather than the electromagnetic induction employed by simpler and less expensive dynamics. The primary component in a condenser mic is the capsule. Similar to the dynamic, the consdenser also employs a thin mylar disc which responds to excited air molecules created by a sound pressure wave. Unlike dynamics, however, there is no voice coil. Rather, the condenser employs two diaphragms (called “plates”), one fixed and one moveable. Both are coated with a conductive material, usually gold mixed with other precious metals to increase conductivity. The two plates form a capacitor, an electronic component capable of storing an electrical charge. A capacitor is said to have a value – a measurement of how much voltage it can store (measured in units called farads, also named for Faraday). The value is determined by a number of factors, one of which is distance between the plates. So when the moveable plate is displaced by the sound pressure wave, the distance between the fixed and moveable plate varies thus constantly changing the value of the capacitor and causing an output voltage proportional to the value of the capacitor. Decreased distance between the plates means increased capacitance, thus increased voltage at the output. You will sometimes hear condenser microphones referred to as capacitor microphones for this reason. (condenser is an archaic name for a capacitor). Additionally, in order for electrostatic induction to work, we must apply an external polarizing voltage source. Some condensers require batteries, but the majority require an external power supply known as phantom power – which is a 48 volt DC voltage supplied by the microphone preamplifier. We’ll discuss phantom power in greater detail in the M.A. and Ph.D. sections of the course.The diaphragms that form the plates of a condenser microphone capsule tend to be extremely thin and of much lower mass than those found in dynamics. Thus condensers are able to reproduce a range of details often not heard with dynamics. It can be said that condensers have a much higher sensitivity than dynamics – meaning that they can “hear” better than their electrostatic counterparts. Of course the flip side of this is that they are a lot less rugged than the thicker diaphragms found in dynamics. For this reason, condensers are generally found in recording studio environments where they are for more likely to be treated with care rather than thrown into a road case and carted off to the next gig. Generally engineers believe that condensers have a smoother sound or are of a more “hi-fi” nature while dynamics are a bit more “rough around the edges” yet more durable. It would seem that these characteristics would dictate application, which is true to a certain extent, but if we remember our rule that “the best microphone is the one that best accomplishes the task at hand” we may consider other factors in our microphone selection process. return to top Frequency ResponseThis is a general term used to describe the capacity of an audio device to reproduce a specific range of frequencies. Most devices are rated at “20 Hz – 20 kHz” - ironically, the range of human hearing. Ok, actually it’s not that ironic – audio circuit designers engineer to this specification, of course! Unfortunately this doesn’t tell us much about the way a microphone sounds. Telling us the frequency response of an audio device is “twenty to twenty” is a lot like telling us a car you’re looking at “includes tires!”We need a little more information so engineers devised the frequency response curve which graphically illustrates the way a microphone responds to certain frequencies. It can be thought of as a study of the relative amplitude of specific frequencies or bands (groups of frequencies) that would be found at the output of a microphone given a specific input. It is best to think of a frequency response curve as a rough guide. What the graph is really telling you is “this is how the microphone responds in a laboratory with a specific set of frequencies at a specific amplitude on-axis to the transducer.” The graph can be helpful in determining the general sonic characteristic of the mic (does it have more midrange? or is the bottom more emphasized? does it roll off high frequencies?) but it tells us little about how that microphone will sound in front of a guitar cabinet or under the lid of a grand piano and nothing about how the mic will interact with the room, the objects in it, the performer’s technique, etc.. To learn that information, we must use an extremely sophisticated and sensitive set of audio measurement devices. Fortunately, we were born with them – our ears! (E+Y=Es).  return to top Directional Response, Polar PatternsAnother characteristic we may find useful is a graphic representation of a microphone’s directional response. This is another study of relative amplitude, but this time we are looking at the effect of the angle of incidence on the microphone’s output. We call these studies polar patterns because of the pickup pattern they form around the microphone’s center (its pole).Microphones can be broadly classified in on of two ways: directional or omnidirectional. If we draw an imaginary perpendicular line down the center of the transducer, this line is the microphone’s axis. A sound pressure wave that reaches the transducer exactly aligned with that imaginary line (0o angle of incidence) is on-axis where a sound that reaches the transducer from any other angle is considered off-axis. From studying the polar response graph of a transducer we learn that directional microphones “hear” differently than omnidirectional microphones. An omni responds equally to any pressure variation on a 360o axis - it “hears” everything around it equally. A directional microphone is just the opposite – it “hears” differently from different angles. As we will learn from studying the polar response graph, a directional microphone’s sensitivity to certain frequencies is variable proportionate to angle of incidence. You will also note that directional sensitivity corresponds to frequency, higher frequencies being more directional than lower frequencies. There are three general polar patterns found on modern microphones: omni (omnidirectional), figure-of-8 (bidirectional) and cardioid (unidirectional). The cardioid pattern is further divided into wider and narrower versions sometimes called supercardioid and hypercardioid. Some condenser microphones are capable of producing multiple polar patterns. These are called, interestingly enough, multipattern mics! These microphones feature a switch that allows the engineer to change the polar pattern to suit her needs. They generally offer omni, figure-of-8 and cardioid, while some will add hyper- and supercardioid patterns. The Blue Kiwi and Cactus actually offer the engineer nine different patterns to suit a variety of requirements. In the example below of a directional cardioid capsule (Blue’s B6), compare 125 Hz (the light red line) to 16 kHz (the black line). Generally speaking, off-axis sounds tend to be more “colored” than those closer to on-axis.   This completes our introduction to Microphones. Please check in with us for future additions to this course material return to top |