Difference between revisions of "Audio Fundamentals"

Latest revision as of 15:38, 18 February 2022

A guide for audio basics.

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Contents

• 1 OVERVIEW
• 2 SOUND
  • 2.1 Wave Elements
  • 2.2 Timbre
  • 2.3 Sounds in Spaces
• 3 MICROPHONES
  • 3.1 Microphone Basics
  • 3.2 Dynamic and Condenser Mics
  • 3.3 Mic Pickup Patterns
  • 3.4 Noise Avoidance
  • 3.5 Extra Microphone Details
    • 3.5.1 Proximity Effect
    • 3.5.2 Balanced Audio
    • 3.5.3 Hi/Lows
    • 3.5.4 Levels
    • 3.5.5 Filters
• 4 AUDIO
  • 4.1 Audio Concepts
  • 4.2 Audio System Components
• 5 RECORDING
  • 5.1 Space
  • 5.2 Digital Audio
  • 5.3 DAW
  • 5.4 Recording Workflow

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OVERVIEW

This page will help with the basics of audio to help you whether you want to record a song, interview, podcast, or gather sounds for a video. There are many resources to help you learn these skills. See Media Loan's Website for more information on who can check out equipment and how to do it. The Media Loan Catalog shows all equipment available to eligible students.

SOUND

Wave Elements

All sounds that we hear are just air pressure variations hitting our ears at different rates. A speaker or piano radiates these variations of air pressure, while a microphone captures the variations. If you hit a drum or pluck a guitar, it makes sounds because it is moving back and forth repeatedly. When moving out, it produces higher pressure for air particles to squeeze together and then when moving back, the surface creates low pressure for air particles to spread out. This process repeats until the energy put into the drum or guitar has been dissipated and it stops moving. The high and low pressure repeating is a wave.

• Frequency: The rate of the wave is called the frequency. Frequency is measured in how many times a cycle of high to low pressure occurs per second. The cycles per second can be measured in hertz (Hz). If a guitar string vibrates 440 cycles per second, we would call that a frequency of 440Hz. That 440Hz note would be called an A note because of the frequency. We hear frequency as higher or lower pitches. A high pitch bird song will be a much higher frequency like 10,000Hz and a low pitched bass note could be 60Hz. Frequency is pretty synonymous with tone, note and pitch.

• Amplitude: The intensity of the wave is called the amplitude, we hear this as loudness and often describe it as volume. If you pluck the guitar string twice as hard and get the wave to oscillate back and forth twice as far, it has twice the amplitude and therefore we hear it as louder. Another common measurement of intensity is decibels (dB), which is an averaging of amplitudes over time.
• Phase: There is a third main element of a sound wave called phase. It is the time that the wave starts compared to other waves. It can be important to consider because sounds can be "out of phase" where their high and low pressure areas cancel each other out. If two microphone are picking up the same sound from different distance, they may have be offset and cause these kinds of cancelations.

Timbre

Timbre is pronounced "Tamber". Simply put, it is what the sound sounds like. This is what makes a 440Hz note (which is an A note) sound different when sung, played on a guitar or a violin. The timbre is influenced by two things, the partials and the envelope. Timbre can also be called texture, color and tone.

• Partials: These are additional frequencies that occur when you play a single note. Think of a guitar string playing that 440Hz note. That string actually produces many more frequencies in addition to that simple 440Hz frequency. In addition to the whole length of the string vibrating at 440Hz, the string is also vibrating halfway at twice the speed making it 880Hz. In addition, 1/3 of the string vibrates 3 times as fast being 1320Hz, and 1/4 vibrates at 4 times as fast being 1760Hz and so on. This process repeats infinitely and all vibrations occur simultaneously creating a complex wave, that sounds rich. Each additional frequency is increasingly quieter, or in other words, lower amplitudes. We still call the note we played on guitar A because it has a fundamental frequency (the lowest frequency) of 440Hz while it has partials (all frequencies above the lowest) of 880Hz, 1320Hz, 1760Hz and so on. The way that harmonic partials occur in relationship to the fundamental frequency follows a mathematical concept called the harmonic series, which is the foundation for all musical harmony, explaining why notes sound good together. You can tell the difference between instruments because the amplitude of their partials vary.
• Envelope: Envelope is how the amplitude of a sound changes over time. It is how long the sound takes to get to its loudest point and then how long it's takes to return to silence after the cause of the sound has stopped, like when a violinist takes the bow of the string or when the pianist releases the keys. Each instrument has a different envelope that affect its timbre.

Sounds in Spaces

We can hear the size and other qualities of a room based on how much reverb, delay and characteristics that become a part of the sound interacting with the space.

• Delay: Sound bounces off walls and floor spaces. This can cause the sound like the initial sound to hit your ears, but as that sound radiates around the room, you will hear it again as it bounces off the wall and hits your ears after the initial instance. That sound can bounce back and forth hearing it multiple times as it gets quieter. Each delay sounds like an instance of the sound. How long it takes for you to hear the delay and how many times it happens will tell you a lot about the room you are in.
• Reverb: This is different from delay because it sounds like a washed out version of the sound source. You will hear it as the sound itself, then a tail of a jumbled up version of itself. This happens when a sound interacts with a space for it to bounce off many versions of it, bounce off of many angles of the room and get to your ear over a duration of time. Each room has its own unique reverb characteristic.
• Frequencies in Space: Different frequencies of a sound will interact with a space differently. Lower frequency will pass through surface or objects, or get caught up in the corners. Higher frequencies on the other hand, bounce off surfaces or get absorbed by it. These qualities plus delay and reverb tell you a lot about a space without having to consciously think about it. To improve the acoustics of a room, you can often use bass traps, and acoustic paneling to break up the simplicity of a flat wall and corners.
• Direction: How do you know what direction a sound is coming from? Is it left, right, behind you? Your ears are spaced apart so you can hear when a sound hits your right ear first then the left slightly after. This allows you to know a sound is, for example a little bit to your right. The shape of your outer ear helps you know if sound is in front of or behind you by filtering out frequencies. Your brain has come to know the characteristics of how frequencies are removed when it goes through the back of your ear and it tells you that the sound is likely behind you. When editing a song or video, you can move the sound to the left or right speaker using what is called panning.

MICROPHONES

Microphone Basics

All Microphones turn acoustic sound into analog signal. Depending on the mic, it may need to be supplied by a battery or "phantom power," which is usually 48 volts of electricity sent to the mic from whatever you are connecting it to, such as a mixer. There is often a button that says "48v" to enable it. You can do a lot with microphones and there is a great variety of them used for different purposes. We have a catalog of Microphone Catalog with information about each mics polar pattern, transducer type, uses and other useful information.

Dynamic and Condenser Mics

The main component is the transducer. The transducer is the part that converts acoustical energy to electrical energy. There are two main types of microphones at Media Loan: dynamic and condenser.

• Dynamic: Mics are more rugged and can handle high sound pressure levels. They are slower to respond to transients. They are the easiest mic to use because they don't require power to use and can be plugged in and ready to go. Most can endure phantom power, but it is always best to turn it off when not using it
• Condenser: Mics are more sensitive and can pick up faster transients. They also require power. Many of the condensers available in General Access can be powered by a battery. Some of the condensers can be powered from phantom power. Phantom power is a power source that is supplied through the mic cable from a mixer or field recorder. Media Loan has mixers available for checkout that can supply phantom power. Zoom H4n and Olympus LS-100 field recorders are also capable of supplying phantom power.
• Other: These are less common types of transducers
  • Ribbon: We don't have these in General Access, but for those in audio programs that get to use them. These are extremely fragile and will break when you use phantom power, so be certain that phantom power is turned off when plugging it in to the input. This is a small strip of magnetized metal ribbon that responds to air pressure variations and affects a large magnet's electrical charge.
  • Contact: Another lesser used transducer type. It is often a small crystal that creates an electrical charge when impacted by pressure variations. Media Loan only has one contact microphone called the hydrophone and is literally the only mic that should be anywhere near water at all.

Mic Pickup Patterns

All microphones respond to sound differently based on direction. This is called the microphone’s pickup or polar pattern. The shape of the pattern tells you how sensitive the mic is to sound approaching from one direction. There are 3 basic patterns:

• Omni-Directional: Mic picks up sound from all directions.

• Uni-Directional: These are mics that pick up best in one direction. The uni-directional category is broken down into cardioid, super-Cardioid, and hyper-cardioid subcategories:
  • Cardioid: Sound is picked up in a heart shaped pattern in front of the mic. This allows you to focus on one sound while excluding other sounds. Also all directional mics are omni directional with low frequencies. This means that a cardioid mic may pick up low frequencies outside of its pick up pattern.
  • Super-Cardioid: Mics have a narrower pattern in the front but they also have a small lobe in the back of the pattern.
  • Hyper-Cardioid: Used with shotgun mics. They have a very narrow pattern in the front. They have a rear lobe like the super-cardioid, but they also have lobes on the side of the mic. They are used to focus in a specific direction. You need to pay attention to the side lobes. If you are standing next to a noisy camera with a shotgun mic the side lobes might pick up the camera along with the intended sounds

• Bi-Directional: This pattern is less common than the others. Sound is picked up from either side of the mic. There are no bi-directional mics in Media Loan’s General Access. Most ribbon mics are bi-directional, and some fancy large diaphragm condensers have variable pickup patterns including bi-directional.

Noise Avoidance

There are a lot of different bad noises to consider when using microphones.

• Location: You want to be in a quiet room, or a space that has environmental noise that you want. If you are in a room with a loud fan, fridge buzz, etc, it will make a noisy recording. If you want location sounds, you could add them in later, but if you know you want to have bird chirping, or loud coffeeshop chatter in the back of interview, these noises can work to your advantage.
• Placement: You want the mic to be close enough to your source that it is picking up the person or instrument more than anything else, but not so close that it starts to get too "boomy" where the low frequencies get picked up more than others. This boomy-ness from close mic'ing is because of what is called proximity effect.
• Mic Bumps: Handling the mic or bumping the mic stand will make a noise that can be hard to effectively remove later. It is best to avoid these noises. Some mics are built to endure handling noise better, like the Shure SM58. For field recording, you could use a shock mount like the boom pole or pistol grip to minimize handling noise for shotgun microphones.
• Pops: You can use a pop filter or a windscreen to help reduce some bad sounds that might get recorded on a microphone. When people make "B" or "P" sounds they generate a big burst of air that will hit the microphone and not sound very good. These are called "plosives." Using a pop filter helps break up that burst of air and you are only left with good natural sounding vocals. These pop filters can also help reduce the sharp "S" sounds that can get unnaturally picked up by microphones, called "sibilance".
• Feedback: This is a really scary thing to deal with in audio, but you only need to worry about it if you have speakers and microphones in the same space and the speaker is amplifying what the microphone is picking up. It is called feedback because the mic picks up itself on the speakers, which then amplifies itself and it feeds back until it makes an extremely loud whining sound. The easiest thing to do to avoid this is have your mic in a different space than the speakers, but if you need them to be together, you could be sure the mics are pointed away from the speakers, so they wont pick them up. Do mic tests to make sure that with your placement, it won't feedback.

Extra Microphone Details

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Proximity Effect

When most uni-directional microphones are placed close to a sound source the bass frequencies increase in level. This is called the proximity effect. Sometimes this is desired. People may speak into a mic that is close up to get a deeper sound out of their voice. Sometimes you may not want this sound. If you are recording some one who moves back and forth from the mic too much the bass sound of their voice may not be even. Some cardioid microphones are designed to eliminate the proximity effect. The EV RE-15, RE-16 and RE-18 are designed this way. They have a row of slots down the side of the mic. If they are covered up there will be the proximity effect. So be careful how you hold the mic.

Balanced Audio

Audio signals can be carried on two types of cables; balanced and unbalanced. A balanced cable use two wires for the audio signal and one for the shield. The 3 pin XLR connector of a mic cable is the most common example of a balanced cable. An unbalanced cable has one wire for the audio signal and one wire for the shield. Balanced signals are less likely to pick up extra noise and can be run for hundred’s of feet if needed. Unbalanced signals are more likely to pick up noise but they are okay to use for short distances. Most of the mics at Media Loan use balanced XLR connectors. A few of the mics have an unbalanced cable with a mini or 1/4” connector. Most of our recording equipment uses unbalanced mini or 1/4” inputs.

Hi/Lows

To convert from a balanced to an unbalanced signal you need a transformer and Media Loan calls its adaptors with transformers hilows. You need a hi-low to plug a mic with an XLR connector into a device with a mini input. This keeps the audio balanced from the mic to the hi-low. There is only a short section of unbalanced signal from the hi-low to the recorder. Media Loan has 1/4” and mini hi-lows. Make sure that the you get the right type for your needs. Also, all of the mini hi-lows look like a stereo connector but they are really mono. If you use a hi-low with a stereo device like a mini-disc recorder or palmcorder it may only record to one of the 2 tracks and you will only hear it in one side of the headphones.

Levels

A microphone generates a very low level signal. It always needs to be amplified before we can use it. A mic level signal can range from -60 to -20db. The level of signal from devices such as vcrs and cassette decks is -10db. Some recorders have a special mic input which amplifies the mic signal to line level. If you plugged a CD player output into a mic input it would sound distorted. Some recorders such as the 4-track cassette recorders have inputs that can take a mic or line signal. But there is a mic preamp control on the recorder. It’s normally called the trim control. You need to adjust this to set levels with a microphone.

Filters

Some of the microphones have high pass filters. A filter removes specific frequencies from the audio signal. A high pass filter removes the low frequencies and is also called a bass roll off or low cut filter. A low pass filter removes the higher frequencies and is also called a high cut. Normally, you should wait until the recording process is complete to perform drastic equalization, but sometimes during recording the low frequencies will be unwanted noise like wind or the microphone shaking. When this is the case, it can improve the recording to cut out the lower frequencies. The Sennheiser 421 has a 5 position bass roll off switch. The EV RE-15 and RE-16 have a two position switch. Media Loan also has several XLR barrels that operate as high or low pass filters between the microphone and the recorder.

AUDIO

Audio Concepts

• Signal Flow: Sound waves in the air can be turned into an electrical signal called an analog signal using a microphone, combined with other signals using a mixer, then sent back out as a new physical sound waves using a speaker. No matter the size or complexity of the system, it is important to think of it in terms of signal flow. What order is the signal flowing through different cables and gear, and what is happening to the audio at each step? This line of thinking will help with quick adjustments and troubleshooting issues.
• Gain Staging: An important concept to know is mic vs line level signal. Microphones output a very weak signal out of their cables, so whatever it is connected to needs to use a "preamplifier" to amplify that signal to a level so that it can withstand going through circuits and being recorded. A preamp brings a mic level up to line level. In this process you face the biggest decision of gain staging (for more info on this read the gain staging section below). You need to choose if your signal is mic or line level as it goes into the preamp, which may be in a mixer or audio interface.
• Stereo: We have two ears, so we can simulate sounds in space by using stereo audio. Instead of one "channel" of audio, there are two different ones. One is the left channel, and the other is the right channel. These will go to each left and right speaker or headphone and ultimately the left and right ear. Most audio tools can use "panning" to control what goes to the left or right speaker, but most of the time it is a combination of both or the same. Most songs and movies will be in stereo and have some aspect that can be heard, for example, the background vocals might be spread out between the left and right channels, or a car driving by will be heard going from left to right. You can also record in stereo by using two microphones instead of one. You could then take the audio from the first mic and put it in one speaker, then the audio from the other mic in to the other speaker. Many audio field recorders like the H4n have built in stereo mics because they are a popular mic technique for field recording.

Audio System Components

There are categories used to identify audio gear, but many items combine elements from multiple categories. For example, a mixer may also act as an interface. It is important to be aware of these categories and that they may be combined depending on the gear.

• Microphone: See MICROPHONES section above.

• Cable: For most uses, cables are straightforward, you find the cable with a plug that connects to the socket you want to connect. The most common are XLR, 1/4", 1/8" (mini 3.5mm aux), and RCA. If you don't know what connector you need, you can look up your equipment make and model to find a manual with connector information. USB and other computer cables can carry digital audio signal if your gear is designed to utilize it. For advanced systems, you may want to learn the difference between balanced and unbalanced signal.
• Interface: In audio, an interface refers to an audio device which connects an analog system to a digital system. They are used to take multiple analog signals (voltage) from a mixer or microphone, and convert it to digital signals (1's and 0's). It is also used to direct digital audio from your computer out into your analog system. You can use USB to connect the interface to the computer to record the digital audio. Other gear can function as an audio interface because some have the ability to connect analog inputs to digital outputs that then go into the computer to be recorded. Media Loan has two different models of dedicated audio interfaces available. There is also a microphone that acts as an interface called "Blue Yeti". There are 2 kinds of analog mixers, and 2 field recorders that all have the ability to act as an interface.
• Mixer: A mixer is a type of equipment used to take in multiple audio signals, "mix" them, and send them to another part of the audio system like speakers, or a device to be recorded. Mixes also include different effects, such as phase inversion, panning, EQ, and bus/aux insert groups.
• Recorder: Anything that stores audio. A field recorder, like the "LS-100", is the most popular kind of recorder in Media Loan. But if you were to use an interface and record on a computer, that computer would be a recorder. Media Loan also has tape recorders.
• Amp: An amp, or amplifier, takes an analog signal and makes increases the gain, which you can think of as volume. Guitar players call the speaker they connect their guitar to an amp, but it is actually both a speaker and an amp in one box. Most amps are just an amp with no speaker, and some devices are just speakers and they need to be connected. If a speaker has a fan in it, that is probably because it is cooling an amp. Most gear that receive signal from microphones has a preamplifier, which is a kind of amp that makes a mic level turn in to line level (discussed in the above gain staging section). Most amps, that aren't specifically preamplifiers, convert line level to speaker level so it can have enough gain to be heard on a speaker.
  • ML doesn't have any amps without speakers, but we do have guitar amps (with speakers)
• Speaker: There are many different uses for speakers, and each may have their own different type of speaker. Of course there are headphones and guitar amps. For events the bigs ones facing an audience are often called mains, or house. If you have any facing the performers so they can hear themselves they are called stage monitors. Speakers used in recording studios are called monitors, because you are monitoring the mix.
• Support: This term isn't an audio industry term, but rather how Media Loan categorizes items that hold audio gear. These kinds of gear can be overlooked, but are very helpful to getting clean audio. Media Loan has mic stands, speaker stands, pistol grips, boom poles, gorilla pods, goosenecks, and stereo mount adapters.

RECORDING

Space

The key decision in finding the right space is how you can best eliminate noise. You want to avoid fans, road noise, other people walking around, wind and any other unintended sound. It is hard to remove these kinds of sounds from a signal, it might be possible though if you use noise reduction tools, but don't expect it to. At this point in time, most recording is digital: your signal gets sent to a computer and turned into a file. Unless you put a tape in your device, it is most likely digital.

• Studio: any room used for recording could be called a studio. Professional studios have a control room to separate the recording engineer from the performers. Evergreen has 4 General Access recording studios, and 5 recording studios with control rooms, but you need to be in an audio class to use them. You could record in any quiet room and get good results, you just have to consider not only noise, but how reverberant the room is. When you make a sound, can you hear that sound bounce around the room? This is usually caused by flat parallel surface. You can put up curtains, or other acoustic treatment techniques. There is no perfect audio system, you need to design it based on your needs, look at the Audio Systems Components above for ideas on where to get started.
• Field: Field recording is anything that isn't in a studio, usually outside. People often use devices called field recorders which is like an all in one audio recording device. It functions as the mic, interface, computer, and speakers. Although most phones have these abilities too, a field recorder can capture much higher quality sound and has the ability to connect with professional microphones. The microphones that come on most field recorders like the LS-100 work great; they are in a stereo XY configuration, so you can get nice wide stereo sound or a whole field. If you want record a specific sound like an interview or a bird, you may want something more directional like a shotgun mic, like from the Sennheiser combo kit. Shotguns are often paired with windscreens and boom poles for getting sound for video. You could use a lavalier mic, which is a tiny mic that clips onto a person's shirt for high quality close mic'ing. For recording only one sound and rejecting all other sounds, Media Loan's best options are the parabolic mic kits, often used for nature recording and sports. You should always have headphones to monitor your sound. You never know when its going to rain, so it's a good idea to carry your gear in a dry bag, available at Media Loan. To reduce the noise of recording, it is good to use shock mounts for microphones like a boom pole, or a pistol grip.

Digital Audio

Microphones create electrical signals that are analog signals. These analog signals are usually converted into digital signals, by being "sampled" by an Audio Interface. Digital signals are usually much easier to work with. They can be sent from one device to another, and be recorded and manipulated using digital audio tools.

• Sample: A sample is the smallest unit to represent sound, it represents a single position of a speaker or microphone. You can think of it as the comparative air pressure associated with a single instance of the sound. It is like the frame rate of a movie. Movies usually have about 30 frames per second, whereas audio usually has 44,100 samples per second (generally written as 44.1kHz). As a side-note, modern music refers to small sections of sound as a sample which is a different use of the word, because an actual "sample" is just a single slice of audio that in itself doesn't sound like anything. All the samples put together in order make a waveform.
• Sample Rate: The sample rate is how many sample there are per second like the 44.1kHz is a common sample rate. Thats 44,100 samples per second. Analog signal doesn't have samples, it is just a continuous stream of varying electrical level with infinite temporal resolution. You are losing sound information by converting the signal from an analog signal to digital signal, but most people can't hear the difference, which is why most audio work is done digitally.
• Bit-Depth: A samples resolution is defined by it's bit-depth which is how many 1's and 0's are required to describe each sample. A higher bit-depth means each individual sample has more possible positions it could be in.
• File Type: Once recorded, these 1's and 0's are stored as a file. There are many file types, MP3 may be the most recognizable, but it is better to use higher quality ones like .wav or .flac if possible. CD's and DVD's are made possible by the ability to have 1's and 0's being represented as either a tiny reflective (1) or non reflective (0) surface on the plastic. Tapes and records on the other hand are analog and have infinite detail between what would be reduced to a sample.
• Artifacts: All of these medium and tools have "audio artifacts" which are errors that indicate the medium being used. You can hear the blockiness of low quality youtube videos, you can hear the tape pitch warble on a run down tape deck, a vinyl record player might get stuck and loop a couple times. all of these problems are good to be aware of an can be used to trick the user into thinking that they are listening to one medium rather than another.

DAW

Digital Audio Workstation, or DAW for short, is versatile tool for recording and editing your audio. There are many different kinds, but they all rely on the same principle of sound design, so if you learn one, you can learn the others. They have a mixer view or some kind usually at the bottom to represent the same kinds of controls you would have on a physical mixer. There is also a track view, where you get to see the waveforms recorded, from left to right, in each track stacked vertically. Most DAWs have the ability to use midi which might be in a midi track, mixed in with all the audio channels.

• Recording: You can either bring in recorded file from a field recording or record directly from an interface. The interface will have different channels that you will want to connect to the channels of the virtual mixer (usually at the bottom). This is done with I/0 or input/output. Find these labels on a channel and select the input that you want to record. Sometimes, interfaces aren't recognized instantly so you may need to go into the DAW or the computer preferences to enable the interface as your audio device.
• MIDI: Midi is a very powerful tool in DAWs. It is the representation of musical note information like pitch, velocity (volume) and timing. It in itself doesn't make sound, but it is very useful because you can tell virtual instruments to make sound based on the notes in midi data. It also can connect to many kinds of audio gear using a MIDI cable, or nowadays a USB cable, to carry MIDI signals.
• Mixing: On a computer, audio mixing is similar to if not interchangeable with the concept of audio editing. You are adjusting gain (volume) of each channel, adding effects, and adding "automations" to adjust those qualities over time. You can sequence midi, or chop up and move around audio layering

Recording Workflow

1. Test out your equipment and space before your performer gets there. Make sure you are getting clean signal with no noise.
2. With your musician, run a mic check and set your gain (described in the above gain staging section).
3. Make sure that the performer can hear what they need to; themselves, what is being recorded, the pre-recorded sounds, whatever they need.
4. Record the take, check it, continue.
5. Save your files in at least 2 locations for safety. There are audio files that you want to save. If you are using a DAW there is what is called a project file where you do your mixing and effect in a timeline. It is just a simple file that notes what audio files and when on the timeline in addition to any effects you might use. You will want to save this and the audio files together. Many times the program will make a folder to hold all these files, but it is good to check what and where your files were saved to before closing out of the program to make sure you have everything you need.