Audio Theory

Basic Audio Theory

This section covers basic Audio Theory that all guitarists and bassists should thoroughly understand before moving on to the next sections on this webpage. Some of the subjects covered in audio technical theory will be on – Pickups, Connectors, Cables, Balanced and Unbalanced,  Polarity, Line Levels, Impedance, Speaker Types, and Wattage (just to name a few)…

 

1. Guitars and Bass Guitars – Basic Technical Info

Basically the most technical information about your instruments you will need to know (at this point) is about pickups, volume / tone controls, and cable jacks.

Pickups – A device (electromagnet) located under the strings on an electric guitar that converts the acoustic sound (string vibration) into an electrical signal so the signal can be processed through an amplifier. Active Electronic pickups require a source of power – so many Acoustic Electric guitars use batteries for their active pickups(see next picture).

audio theory

Volume / Tone (EQ) Controls – These are the knobs used to control the volume and tone on your guitar or amplifier, effect pedals, etc. They come in different shapes (flat, round, etc) and resistances (all depends on what your guitar manufacturer designed), some can be 1 meg (1 million ohms), 250k (250,000 ohms), just to name a few sizes (there are many different sizes). The technical name is “Potentiometer” (Pot).  These use “Active” pickups which require a source of power (9-Volt, AA Batteries).

audio theory 

The Cable Jack – Is basically the output of your guitar (the place where you plug your guitar cable into). Cable jacks can eventually wear out, and can cause the audio to cut in and out (the technical word for a signal that is occasionally cutting in and out is “Intermittent” – this term can also be used for when a problem comes and goes, only happens once in a great while, etc). “Intermittent” is a good technical term to know and understand.

audio theory 

2. Basic Audio Theory – Balanced and Unbalanced Connections (the Basics).

In Sound Reinforcement systems (PA systems) balanced connections are “preferred” to unbalanced connections because they are far less susceptible to interference (noise).  Most high end equipment will come with balanced inputs and outputs (either XLR or TRS or a mixture). Unbalanced connections may work in certain systems, but it is always best practice to use balanced connections (try to avoid unbalanced connections).

It would be very beneficial to print out the Handout for Balanced / Unbalanced connections – it shows a diagram that will help understand the theory and concepts better…

Balanced / Unbalanced Audio Connections Handout – CLICK HERE!

Balanced Connections use two conductors (two signal carriers) and a shield wire. Each conductor will carry the same signal potential, but at different polarity (signals are reversed – one signal is positive, and one signal is negative). Any equal energy (noise) will be at the same potential on the conductors and will be rejected. A Balanced input amplifies only the “difference” between the two signals, and will reject any part of a signal (noise) that is the “same” in each of the conductors.

audio theory

audio theory

Note: A stereo patch cable with TRS connectors (pictured left) at both ends is a good example of a cable used with Balanced connections. It is important to understand that the cable used will have 2 conductors (red / black or clear / black) and a shield wire (see example to the right). Cutting off the shield wire can cause unwanted interference.

Unbalanced Connections – Unbalanced connections utilize two conductors – one conductor is carrying the signal and the other conductor will be at ground potential. Noise energy on the shield wire will be grounded (go to ground and bypass the output, but noise energy on the signal conductor will flow through and be amplified with the audio signal.

audio theory

 audio theory

Note: A guitar cable with 1/4″ (mono phone connectors – pictured left) at both ends is a good example of a cable used for Unbalanced connections. The cable used will have 2 conductors – a red or clear wire, and a shield wire (see example to right). RCA cables (CD players) are another type of cable used with unbalanced connections.


3. Sound Levels –  Line Level, Microphone / Guitar Level, and Amplifier levels…

Sound Levels:  All audio signals transmit sound information that have a certain amount of “strength” associated with the signal inputs and outputs. Guitar / Mic levels are generally very weak, CD player output levels are fairly strong, and Audio Amplifiers have high power outputs.  This section of audio theory will describe the sound levels that you should be familiar with (the theory behind “levels” is much more technical than what we will detail on this page). We will cover Line level, Guitar / Microphone  levels, and Amplifier levels below – all important aspects of audio theory.

Line level is a term used to designate the strength of an audio signal used to transmit analog sound information between audio components such as iPods, CD, MP3, and DVD players, mixing consoles (small or large), and audio amplifier inputs. Line level signals are always “amplified” (usually through a pre-amp, etc) to ensure a good strong signal.

Line Level Notes:

a) The voltage associated with a line level signal is around 1 volt.

b) 1 volt is equal to +4dBu (Line out usually means +4dBu).

c) Never plug a line level source into a microphone input (the only way to get this to work is to use an attenuator.

audio theory

 

Guitar / Bass Guitar / Microphone levels are the terms used to designate the strength of an  audio signal used between audio components such as Guitars, Microphones, and Bass Guitars. Guitar / Mic level signals are extremely weak (there are no amplifiers inside the microphones), and have to be “amplified” (usually through a pre-amp, etc) to ensure a good strong signal. Active pickups in guitars require a power source (usually a battery) to bring this lower level up to a stronger level (line level). Typical Line / Mic switch on the back of a small mixer (shown).

Guitar / Mic Level Notes:

a) The voltage associated with Mic / Guitar level signals is around 1 milli-volt.

b) Guitar / Mic levels are about -60dBu (can be between -30dBu and -80dBu).

c) Microphone preamplifiers come with a gain control to provide (add) between 30 and 80dBu of gain (the gain control should be enough to get the mic level up to line level). The picture shows one example of this…

audio theory

Amplifier level is a term used to designate the strength of an audio signal used to transmit analog sound information between an audio amplifier and speakers.  Amplifiers will usually receive a “line level” signal at its input – this line level signal will then be “amplified” through the circuits of the amplifier to a speaker (or speakers).


Amplifier Level Notes:

a) The voltage associated with amplifier levels is usually measured in Wattage (Watts, W).

b) Amplifier levels are dependant on the size of the amplifier (can be 1 Watt – 3000 Watts or more).

c) Amplifier levels are also dependant on the “load” they see (speaker size – Ex: 4 ohm, 8 ohm speakers).

 

It is important to note that the power output levels associated with audio power amplifiers can be very high – use caution when working with power amplifiers. 

Weak Signals:  If you send a poor input signal (weak signal) from a mixing board to a power amplifier – it can affect an amplifiers output (there will less signal going into the amp, so the amp will be less efficient).  Weak signals from mixing boards can also cause noise.  Why? Because to get more output out of the amplifier, you will have to turn it up higher than normal (due to the weak signal).  You are not only turning up the weak signal, you are also turning up the noise associated with the signal. 

audio theory

Attenuators are devices that “attenuate” (decrease) signal strength. Let’s say that you have a microphone mixer with only mic level inputs, but you would like to connect an CD player to it. An attenuator can be used to decrease the line level signal and transform it down to microphone signal strength.

audio theory

 audio theory

Impedance – We’ll cover some basic audio theory on impedance here…

Impedance (Z) can be defined as “resistance” – an opposition to the flow of electrical current that is representing the audio signals, and is measure in Ohms. The theory behind impedance can get very technical so we will not go into great detail in this booklet (just enough to help you understand the basics). Although impedance is sometimes only associated with speakers (which is incorrect), impedance can be measured at equipment Outputs (Source) and Inputs (Loads).

Output Impedance:  (also called “Source Impedance”) Is the impedance (resistance) that is Inherent” in the signal source. Output Impedances include: Microphones, Mixers, Signal Processors, Electric Guitars, Amplifiers, etc.

Note: Due to the nature of magnetic pickups – electric guitars and basses usually have high output impedances, but many manufacturers are now designing pickups with lower output impedances.

Input Impedance:  (also called “Load Impedance”) Is the impedance (resistance) that is “seen” by the signal source – Input Impedances include: Speakers, Mixers, Signal Processors, Amplifiers, etc.

Impedance Matching:  Is best described as the “appropriate” connection of Output and Input sources (when the output impedance of the source device is acceptable to the input impedance of another device). See examples:

It is important to note that in Sound Systems that “Inputs” are always “High” Impedances, and “Outputs” are always “Low” Impedances.

Example using an Electric Guitar and Guitar Amp…

audio theory

 

Example using a typical PA system set up…

audio theory

 

 Conclusion

This Audio Theory menu page is just a brief overview of some important audio theory basics. Understanding basic audio theory will help you maintain a good quality sound from your equipment (which is pretty much the goal of all musicians – clean audio).