Chapter 4 notes study guide
Chapter 4 notes study guide EI 300
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This 12 page Study Guide was uploaded by Alex Ellzey on Friday October 7, 2016. The Study Guide belongs to EI 300 at University of Southern Mississippi taught by Jeff Rassier in Fall 2016. Since its upload, it has received 16 views. For similar materials see Intro to Audio Production in Journalism and Mass Communications at University of Southern Mississippi.
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Date Created: 10/07/16
Microphones First piece of equipment in our sound chain Theory of Electromagnetic Induction: o When a piece of conductive metal goes through a magnetic field electricity will be generated o To create an electromagnet is a simple as running current through a coil of wire wrapped around a ferrous metal Three types of microphones: dynamic, condenser & ribbon o Dynamic: Moving coil with magnet Requires no power Generally, more rugged than condenser or ribbon Not as sensitive as condenser Not as flat a frequency response as condenser mics Emphasize mid-range o Condenser: Moving diaphragm only (no coil to move) Generally, flatter frequency response than dynamic mics More sensitive than dynamic mics Requires power: phantom power from pre-amp or a batter Never plug in or un plug a condenser mic with phantom power on Not as rugged as dynamic mics o Ribbon: Uses a corrugated strip of metal (usually aluminum) in a magnetic field Produces a “warm” pleasing sound to the ear Rolls off high frequencies – or does not pick them up well VERY DELICATE Requires no phantom power (power will melt the ribbon) Can be more accurate than dynamic mics, but not as accurate as condenser Directional response: a microphone’s ability to pick up sound from different directions o we measure directional response in a full circle (360 degrees) around the microphone. o When we make a graph of this circle to represent how well the mic picks up sound from all angles, we call this a polar pattern On axis: refers to the direction(s) at which mic is the most sensitive Off axis: refers to the angles the mic is least sensitive to sound Omnidirectional: microphones pick up sound equally from all directions o Omnidirectional mics are pressure mics Meaning: diaphragm reacts the same to sound pressure variations. No matter the directions of the sound source. Rear is enclosed: sound only creates pressure on the front side of diaphragm, regardless of direction source Directional: microphones pick up sound better from some angles while rejecting sound from other angles o All directional mics are pressure gradiant microphones o Sound is allowed to strike the front and back of the diaphragm 2 Differences in pressure created by sound striking both sides of diaphragm will cancel out sound from some directions Bidrectional OR Figure 8: microphones are the simplest form of pressure gradients mics. o If a sound comes directly from a side of a fully exposed diaphragm, the sound waves travel across the front and rear of the diaphragm and effects both sides equally o diaphragm is not moved by sounds from the side o true ribbon mics are bidirectional Cardioid microphones pick up sound in a heart shape towards the front of the mic. o Slots called ports in the side of the mic body allow sound to reach the back of the diaphragm about 90 degrees out of phase from when it reaches the front of the diaphragm o This created destructive interference when sound comes from in back of the mic and constructive interference when sound comes from the front Hyper/super-cardioid o Same types of directional pick up patterns can be created electronically by shifting the phase of dual capsule condenser mics Leave both capsules in phase and you’ve got an omnidirectional pattern Shift the rear capsule out of phase and you have a figure 8 pattern Shift the phase in increments between the two and you get various shapes of cardioid patterns Proximity Effect: when a directional microphone is brought closer to a sound source low-end frequencies are more prominent 3 Microphone Frequency Response: the ability of a microphone to pass frequencies through itself in relation to the original frequencies in the sound wave o if a mic passes all frequencies at the same time, we consider it to have a flat frequency response o most mics tend to emphasize or de-emphasize certain frequencies. On-Axis and Off-Axis Frequency Response: directional mics will have quite different frequency responses with sounds that come from directions off-axis compared to on-axis o On-Axis refers to the direction(s) at which a mic is the most sensitive o Off-Axis refers to the angles the mic is least sensitive to sound Low Frequency Rumble: almost all buildings transmit low frequencies that mics will pick up. The low frequencies often come from heating and air- conditioning system, elevators, nearby traffic or even wind: they literally shake the building. o Sometimes, the very instruments you are recording will cause low frequency vibrations to travel through the room. o Low frequencies can have enough energy to shake a mic stand and cable and can be picked up by a mic. Using a shock mount can significantly reduce the amount of non-acoustic vibration that is transmitted to a mic Some mics will have a low-frequency or bass roll-off switch that will filter out low frequencies NOISE IS ANYTHING YOU DON’T WANT!!!!!!!!!!! Transient Response: how quickly and accurately a microphone can react to a sound o can vary a great deal from one type or model of microphone to another 4 dynamic mics, due to the mass of the voice coil attached to the diaphragm, are slow to react to acoustic energy. o They are less accurate than a condenser mic with a much lighter weight diaphragm Ribbon mics, typically have better transient response than a dynamic mic; however the weight and stiffness of the ribbon do not allow it to react to sound as accurately as a condenser mic…particularly with high frequency waves, because they have less energy to move the ribbon than lower frequency waves Large vs. Small diaphragm: the lower mass of a small diaphragm will react quicker to sound, return to its original position quicker and can be more accurately moved by high frequency waves that have less energy Output Characteristics of Microphones: 1. Sensitivity: the output of a mic in volts in comparison to the input in SPL 2. Equivalent Noise: noise created by the mic itself (usually rate in dBa). 3. Overload: distortion from a sound too loud for the mic to handle. 4. Impedance: an electrical rating that helps match the output of a mic to the input of a mic preamp. Impedance is measured in Ohms. Low impendance mics are usually better. A low impedance mic should usually be connected to an input with a higher impedance. If a mic is connected to an input with lower impedance a loss of signal strength will result Balanced/unbalanced cables: balanced cables have at least three conductors/wires. o Unbalanced: have just two wires o Balanced: having a third wire in a balanced cable allows cancellation of interference. Twisted pairs of conductors do an even better job of rejecting interference 5 o The cable contains two pairs of wires: one pair for positive and one pair for the neutral conductors Audio cables (interconnects) are the most often neglected link in the sound chain Cheap wire, old frayed wire, cable that is too long, the wrong type of cable or the wrong impedance cable will negatively affect your sound quality o High frequencies are often lost or distorted over poor cables o Noise and interference can enter your sound chain through poor cables (especially mic cables) because the signal voltage is low TIPS FOR AUDIO CABLES 1 Use high quality wire Oxygen free copper Twisted pair of conductors Braided or twisted copper shield that completely envelopes the conductors Internal reinforcement that protects the wire strands from breaking 2 Use high quality connectors Switchcraft Neutrik Amphenol 3 Use high quality solder 4 Use as short a cable as possible 5 Wrap cables WITHOUT TWISTING them so as not to weaken the wire stands 6 Do not allow cables to be walked on or stretched 7 Occasionally clean connectors with a good contact cleaner to remove corrosion Microphone preamps: boost the low output of mics up to more useful level o Preams are gain device. The input of mixers and recording devices usually require a line level input: 6 +4 dBv for balanced lines and typically -10 dBv for unbalanced o A mic preamp will raise the low voltage of mic level to line level YOUR CHOICE OF MICROPHONE PREAMPS CAN BE AS CRITICAL AS YOUR CHOICE OF MICROPHONES o Generally, solid state preamps are more transparent (accurate) and can produce much less noise, distortion and coloring the preamps with tubes in their gain stage However, tube preamps can create very pleasant even-order harmonic distortion that results in warm tones and coloring that solid state preamps cannot produce The quality and color of your sound will be determined in part –large or small—by your mic preamp. o Each model of preamp will have its own characteristics: Some are meant to be deadly accurate while others produce lots of warm fuzzy sound…and there are lots of preamps that fall between these two extremes o Just like microphone selection, the process of choosing a preamp most often comes down to personal taste, and it should involve some experimenting to find the preamp that suits your or your client’s taste Preamps vary a great deal in price. Some may give you a lot of band for your buck, you usually “get what you pay for” o Don’t expect a $90 preamp to perform like a $2,000 unit Microphone placement techniques: how and where you place a mic can be just as if not more important than the choice of the microphone itself 7 Distant miking: placing a mic(s) 3 or more feet from the sound source o picks up more of the full tonal qualities of the instrument than closer placement o picks up more reflected sounds from the room than closer placement Distant miking is a technique often used to record ensembles with stereo pairs of mics o The acoustics of the room will play a large roll with distant miking and the objective is to find the right balance between the direct and reflected sound o Having a good sounding room is critical with distant miking Height can be an important part of distant placement o Floors can be a major source of reflected sound and phase interference. Raising the mics well above the floor can reduce phase problems by increasing the distance from the reflective floor (more distance=less acoustic energy) Noise (both from the room and the self-noise produced by the mics, can become an issue with distant miking o The further you move the mics from the sound source, the greater the signal to noise ratio becomes o Also room noise increases in comparison to the level of your sound source with distance Close miking: placing a mic within 3 feet or less of the sound source o Creates a sense of nearness or presence o Can cover up room noise o Can reduce reflected sounds or other instruments Because the close proximity to the instrument, the mic receives more acoustic energy in the form of direct sound when close miking o Other sounds will be somewhat diminished 8 o Less leakage from other instruments will enter a close mic 3:1 distance rule: to lessen leakage and phase problems, for each unit (feet/inches) that a mic is placed from its sound source it should be spaced 3 X that distance from the nearest mic o ex: if you mic a guitar at a distance of 2 feet, the instrument and the mic next to it should be 6 feet away. Accent miking: a medium distance technique that can bring out (accent) an instrument in an ensemble without the “in your face” sound of a close miking o this allows the accented instrument to sound more natural in comparison to the distant mics Ambient Miking: intended to pick up more of the reflected sound of the room than the direct sound of the instruments o can be used to capture the natural reverb of the room o can be used to pick up the audience o can be used to create a large aural space Stereo microphone techniques: use of two or more mics to create a stereo image will often give depth and spatial placement to an instrument or overall recording o One of the most popular specialized mic techniques o SPACED PAIR (A/B) technique used two cardioid or omni directional mics spaced 3-10 feet apart from each other panned in left/right configuration to capture the stereo image of an ensemble or instrument Effective stereo separation is very wide The distance between the two mics is dependent on the physical size of the sound source o Ex: if two mics are placed 10 feet apart to record an acoustic guitar; the guitar will appear in the center of the stereo image 9 Most likely too much spacing for such a small sound source A closer, narrower mic placement should be used in this situation The X-Y techniques uses two cardioid mics of the same type and manufacture with the two mic capsules placed either as close as possible (coincident) or within 12 inches of each other (near-coincident) and facing each other at an angle ranging from 90-135 degrees, depending on size of the sound source and the particular sound desired o The pair is placed with the center of the two mics facing directly at the sound source and panned left and right o Due to the small distance there is no phase problems M-S or Mid side stereo technique involves a cardioid mic element and a bi-directional mic element o Usually housed in a single case, mounted in a coincident arrangement o The cardioid (mid) faces directly at the source and picks up primarily on-axis sound o Bi-directional (side) faces left and right and picks up off-axis sound o Two signals are combined via the M-S matrix to give a variable controlled stereo image Decca Tree: setup includes 3 omnidirectional microphones in an upside down “T” pattern. o left and right mics are placed 6 feet apart, the third is placed 3 feet out and centered in the front o to mix the side mics are planned hard left and right, the output of the middle mic is then sent to both left & right channels o Originally used in orchestral situation, fitted on a tall boom and suspended high up in the air, roughly above the conductor 10 Three separate stands were sometimes used to set individual heights o Great sounding stereo miking techniques often used in large orchestral or choir performances Also be used as a room miking technique for recordings When used for room miking drums its wide stereo image captures the nuances of bigger environments better than most other techniques In smaller rooms, Decca tree does not work as well BLUMLIEN: the pair consists of an array of two matched mics of bi-directional (figure 8) pickup pattern, positioned 90 degrees from each other o The mic capsules are placed as close to each other as physically possible, generally with one centered directly above the other Recording direct or using a direct input box: o A DI BOX allows you to patch an instrument (particularly guitar and bases with pickups) directly into your mixer or recording device No mic is necessary with a DI box The instrument is plugged into the DI box and another cable connects the box to the recording device Lowers the voltage of the line level output coming from the instrument and allows you to input the signal at mic level into your recording system o DI box will give you a cleaner and closer sound than mixing an amplifier: none of the noise from the head and amp enters the sound chain 11 However none of the gritty sounds and colors produced by the amp are passed either & often you want those effects too Another great option with a DI track is to re- amp the track TIPS FOR USING A DI BOX WITH GUITARS: o Turn the volume knob on the instrument all the way down: this will reduce the amount of hum produced by the guitar’s pickup o Turn the treble tone control up: this will reduce pickup noise if you need to boost the high frequencies with an EQ during post-production Passive or active DI o PASSIVE DI’S: are simple transformers that just reduce the impedance of the signal from guitar pickup from around 50K Ohms to a mic level around 100-200 Ohms o ACTIVE DI’S have a built in pre-amp and can be set to line or mic level output can provide phantom power Mic placement GUIDELINES: o Experimentation and critical listening are keys to good mic placement…patience is also a virtue o There is no “one best place” to always mic an instrument o Finding that “sweet spot” takes time and talent and guidelines are intended to give you a good starting point for the process 12
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