This is a work in progress. Please help us by fixing any glaring errors and writing useful and easy to understand definitions.
ACSS - An acronym for "Audio-gd Current Signal System" which is a variation on the CAST (Current Audio Signal Transmission) circuit design developed by electronics manufacturer Krell and other similar concepts. It is a means of coupling individual gain stages (either within a component or between two ACSS-connected components), which minimizes the audible effects of interconnect cables and potential impedance mismatches, as well as the need for negative feedback to lower distortion.
All discrete - Audio equipment that uses only individual transistors, resistors and capacitors and not integrated circuits (for example: Integrated Circuit Operational Amplifiers, or IC Op Amps for short). Believed by some to sound superior to designs that use IC Op Amps. Refer to "Op Amps" below.
Amplifier - an electronic device for increasing the amplitude of electrical signals. Normally it is the amplitude (or level) of the voltage that is amplified. In Head Fi normally used to refer to headphone amplifiers that take a small audio signal from a CD player, sound card or DAC and amplify it to a level high enough for headphones or speakers. Most devices, even those that send out digital data, contain some kind of amplification circuit.
Analog - audio equipment that stores or transmits sound waveforms as a continuous signal with amplititude and frequency, as opposed to digital which records the waveform of an analogue signal as a series of digital bits (i.e. Ones and Zeros) measured at a number of samples per second.
Attenuator - a device or component that lowers the volume of an audio signal. The most common example is the volume control pot of an amp. See also "potentiometer" and "stepped attenuator".
Balanced - A method of using impedance balanced lines, usually in audio recording, so as to permit the use of long cables without interference by way of common mode noise rejection. See also "Balanced amp", "Balanced headphones" and "Single-ended".
Balanced amplifier - An amplifier that has both non-inverting and inverting output circuitry in each channel. A fully balanced stereo (2 channel) device will contain 4 separate analogue amplification circuits: Left and Right Inverting and Non-Inverting amplifing chains. Some balanced-output equipment use a single ended circuit per channel which outputs into a balancing transformer, to create the balanced signal. See also "Balanced Audio". (needs disambiguation)
For a basic, visual guide to balanced amps, see HeadRoom's Balanced Drive Article Series.
For the technical discussion of what a balanced amp is, see "Why Balanced Headphone Amps?"
Balanced audio is a method of interconnecting audio equipment using impedance-balanced lines [The impedance of each conductor plus (+) and minus (-) is the same referenced to chassis ground.] This type of connection is very important in sound recording and production because it allows for the use of long cables while reducing susceptibility to external noise. Balanced connections use three-conductor connectors, usually the XLR or TRS connector. The three conductors are Plus (+) i.e. Non-Inverting, Minus (-) i.e. Inverting , and ground/common/shield. XLR connectors, for instance, are usually used with microphones because of their durable construction, while TRS jack plugs are usually used for mixer inputs and outputs because of their smaller profile. A Non-Inverting signal may be described as: when a positive acoustic pressure wave strikes a microphone diaphragm, the (+) terminal on that microphone capsule will see a positive going waveform, and if carried through the entire audio reproduction chain, that same positive going amplified signal when applied to the (+) terminal of the speaker of headphone driver will cause the diaphragm to move toward the listener, recreating that same positive acoustic pressure wave to be produced.
An Inverting signal may be described as: when a positive acoustic pressure wave becomes negative going waveform in the audio reproduction, usually caused by passing through an inverting amplifier.
Balanced headphones - Headphones that are terminated with a 4-pin XLR or 2x3-pin XLR plugs to use with a balanced amplifier. The term is a misnomer as headphones are balanced (electrically with both a + and - but no ground) by definition, along with their cable, but are typically terminated with a "single-ended" TRS plug where the signal return wires for the left and right channels are joined together.
Bit rate - The average bits-per-second of a music file. Most commonly seen with MP3 files to indicate the relative level of compression, though compressed lossless files have bit-rates relative to their compression as well.
Bit-perfect - Refers to the playback of music from a computer which hasn't been altered in any way by the computer's software, such as the playback program or sound card drivers before reconstruction into an analogue waveform by a DAC.
BNC - A type of locking connector used for digital connections. See also RCA and XLR.
Burn-in: Changes in the sound from a component over time, with use. Manufacturers sometimes suggest, as have owners, that a component sounds better after a few dozen, or even a few hundred hours of being used. Some electrical components are known to change over time with use, so this is plausible, but whether or not headphone transducers are affected by this is disputed. See the measurements done at Innerfidelity.
Capacitive Reactance - see Impedance and Inductive Reactance
Caps - short form for Capacitors. These store energy inside a device temporarily, which can have a number of functions such as: storing energy in an amplifier power supply, DC coupling, tone controls and filtering. Where that includes the music signal passing through, it is popular to replace the capacitors in a device with higher grade or "boutique" brands to improve the sound.
Circum-aural - Headphones that sit around your ears, as opposed to Supra-aural.
Closed headphones - Closed-backed headphones. Have a wood, metal or plastic cup covering the outside of the drivers.
Coloration - The effect of a device on the music signal. As opposed to neutral.
Cups - the cups on the outer sides of the drivers on closed headphones, sometimes made of wood, metal or plastic. Where a headphone is open, it has a grill.
Current - the flow of electrons is essentially what "current" means in an electrical context, and is measured in units denominated in Amperes (or "Amps"), named after André-Marie Ampère.
Customs - Custom fit In Ear Monitors (IEMs). Impressions are taken of a persons ear which are then sent to the factory to be made into a mould which goes around the IEM drivers. This results in better isolation, sound and comfort than regular IEMs with "universal" tips. Custom fit IEM's are also known as CIEM's.
DAC - Digital to Analogue Converter. Converts digital music data from a CD, computer or portable music player (PMP) back into an analogue waveform. CD and DVD players, computers and PMP's have them built in. They can be bought in a separate box for when you want to bypass your devices internal one because you want a higher quality one. DAC's and headphone amps are often bundled into one package.
Damping factor - With headphones (as with loudspeakers) “damping factor” describes a particular amplifier’s ability to control undesirable movement of the driver element (principally with dynamic headphones, less so with electrostatic or planar designs) near the resonant frequency of the headphone. It principally can be noticed in the “tightness” and "definition" of low-frequency music, particularly transients, as the amplifier effectively “puts the brakes on” the driver to stop excessive movement. Damping factor will vary with different combinations of amplifiers and headphones, as each presents a different load at different frequencies to the other component.
DAP - Digital Audio Player, ie: An iPod or similar.
DBT - Double-blind Test. A test where both the tester and subject don't know what is being tested, so as to eliminate bias. Discussion of it is forbidden except in the Sound Science forum due to it causing too many pointless arguments. While people call for them to validate people's claims of hearing the differences between components, just conducting a DBT in itself is not necessarily valid by peer-review grade scientific standards. Regardless, they can be useful for one's own benefit.
Decibel - Just as the word "calorie" is actually a measure of energy (replaced in modern physics by the term "joule") which most people only associate with food, "decibel" is actually a mathematical term used in many different scientific contexts. It can refer to gain within an electrical circuit, or ratios between signal and noise, but most commonly on Head-Fi it is used to express sound pressure levels (SPL) . . . the word decibel is very often abbreviated as "dB".
DHT - Directly Heated Triode. Generally made in reference to a preamplifier or amplifier designed to use directly heated triode (DHT) tubes (ie. 45, 2A3 or 300B tubes).
Digital - signals that describe music in a binary encoded format that represent a music signal by the volumes of an analog waveform at a certain frequency of samples per second. Unlike analog it can be stored, transmitted, and reproduced repeatedly with no degradation.
Distortion - Refers to a change in a signal away from its original form. The amount of distortion in audio equipment is measured as Total Harmonic Distortion (THD) or Total Harmonic Distortion plus Noise (THD+N) or Intermodulation Distortion (IM) in Decibels. The effects of distortion in audio equipment are many and varied and the relevance of degrees of distortion and types on what we are able to detect audibly is the subject of much debate.
DIY - Do It Yourself. Many amplifier, DAC and other designs are available to build yourself, some with only a minimum of skill required. Usually cheaper than buying the same thing built commercially. See also the DIY Forum.
DIYmod - A DIY version of the iMod.
Dual Mono - Equipment where the left and right channels are completely separate amplifiers, including the power circuits. In most amps, the power circuitry is shared by both channels.
Dynamic headphones - Regular headphones that use a voice coil driver around a magnet.
Electrostatic headphones - "Stats" for short. Use an electrically conductive membrane clamped a short distance between two metal grills. They require special balanced amplifiers that output very high voltages, but can produce a very accurate sound with a lot of detail. The membranes are basically capacitive plates which move based on the varying voltage (i.e. the audio signal) applied to them. Most commonly manufacturered by Stax in Japan, but other well known models were made by Koss and Sennheiser in the past. Most of the discussion of these can be found in the High End Audio forum, specifically the Stax Thread (New).
EQ - "Equalization". Using software or hardware to adjust the relative volume of frequencies in audio. Bass boost or treble boost are two common examples.
FLAC - Free Lossless Audio Codec (FLAC) is a "lossless" audio compression format which means audio is compressed without any loss in quality.
FOTM - Flavour of the Month. Headphones or other gear that is having a surge of popularity in the forums, usually as a result of people new to Head-Fi buying it and raving about the improvement it makes over what they had before.
FOTY - Flavour of the year. Audio gear that is still popular even after many months.
FTFY - "Fixed This For You". A usually amusing quote of a post with alterations.
Fun - A term coined by Currawong referring to a tonal balance in a piece of audio gear that has a boosted bass (mid-bass) and treble that is most appealing on first or casual listening but isn't intended to be tonally neutral. Fun gear is characterized by a U-to-V shaped signature, with good bass impact, energetic (but manageable) highs, and mids that - while recessed - are typically not veiled.
Gain - The factor by which the audio signal is increased by an amplifier. Normally in audio circuits it is the voltage that is being amplified. Gain is normally expressed in dBs. The factor by which the audio signal is increased is normally an expression of Volts out/Volts in. For example: a gain factor of 1 Volt in, 10 Volts out = a gain of 10 volts/Volt = 20 dBs of gain.
Grill - The grill protecting the outer side of headphone drivers where the headphone is open or semi-open.
High impedance (headphones) - headphones having an impedance greater than approx. 300 ohms. Typically the impedance will not be higher than 2,000 ohms.
HDAM - An acronym for "Hyper Dynamic Amplification Module," originally developed by audio manufacturer Marantz, where a group of discrete components (transistors, capacitors, resistors, etc.) are used instead of the typically less-expensive "Op Amp", which incorporates the roles these components play in a single integrated circuit chip. On Head-Fi, most commonly used in reference to the HDAMs made by Audio-gd and Burson. As with discrete components vs. Op Amps described elsewhere on this wiki, some listeners claim noticeable sonic advantages to this approach.
House sound - Different products from a manufacturer will usually sound very similar or almost the same. This similar tuning is called the manufacturer or designer's "House sound".
HRTF - Head Related Transfer Function. Roughly speaking, how a person's ears respond to different frequencies and positions of sound. Related to the different frequency responses of people's ears.
IC - Interconnect cable. Two are used in regular analogue connections, one for the left channel and one for right. They usually come terminated with RCA or XLR plugs, depending on the devices being connected.
IEM - In Ear Monitor. Headphones that go inside your ear canals.
iMod - A modification of the 4th and 5th generation iPods by Red Wine Audio to improve the quality of the line out by connecting it directly to the output of the DA chip. Later generation iPods are not used as RWA felt they were sonically inferior after Apple switched to using a Cirrus DAC from the previous Wolfson. See also DIYmod.
Impedance - electrical resistance to the flow of current in an AC circuit (for example, an audio circuit). The higher the impedance of the headphone, the less current will flow thru it. Impedance can vary with frequency in most circuits, including audio circuits. Impedance is comprised of the basic elements of all electrical circuits: inductance (abbreviated by "L"), capacitance (abbreviated by "C") and resistance ("R").
Additional reading: http://www.head-fi.org/a/headphone-impedance
Impedance mis-match - Where efficiency and damping factor are lessened and frequency response may be altered due to incompatible source and load impedances. Normally in audio circuits source impedance (i.e. output impedance) should be relativity low, load impedance (i.e. headphone impedance) should be relatively high. As a rule of thumb, headphone impedance should be 8 or moretimes higher than output impedance. True impedance matching has more relevance in RF circuits and digital transmission circuits.
Intermodulation distortion - "It's when a transducer (microphone, headphone, speaker) mangles once-distinct frequencies by letting them interfere with one another, ie, modulate one another. It's bad because it's by definition a reduction in clarity. In a transducer one craves simplicity. Simple movements, not ultracomplex chaotic flapping and rippling." (Taken from this post.)
Jitter - (Excerpted from Wikipedia) "Roughly speaking, jitter is a significant, and usually undesired, factor in the design of almost all communications links. In conversion between digital and analog signals, the sampling frequency is normally assumed to be constant. Samples should be converted at regular intervals. If there is jitter present on the clock signal to the analog-to-digital converter or a digital-to-analog converter then the instantaneous signal error introduced at that time will produce a form of digital distortion of varying degrees of perceived audibility." On Head-Fi, the term is very over-used to refer to electrical and other issues with digital to analogue conversion that aren't related to jitter.
LOD - Line Out Dock. A dock or cable that connects an iPod or other DAP using the proprietary connector on the bottom to an amp or other device for playback bypassing the internal volume control.
Lossless - Refers to music file compression that does not remove data to compress the file. The compression can be reversed to reproduce the original file as it was, much like zipping a text file. Eg: FLAC, ALAC.
Lossy - Refers to music file compression methods that remove the least audible sounds from music files to compress them. The sound removal cannot be reversed. Eg: MP3, AAC, Ogg
Low impedance (headphones) - Headphones whose nominal impedance falls below 100 ohms. (see Impedance)
MP3 - MPEG Layer 3 file.
Neutral - does not tonally alter the signal.
Neutral headphone - an individual matter of perception, as every individual has different ear shapes that reflect and filter incident sound differently, and differently between extreme near field vs. free field. Usually refers to headphones that have, based on averages of how the human ears perceive sound (HRTF, Head Related, Transfer Function), a frequency response closest to that of flat for the average person.
NOS - New Old Stock (Tubes).
NOS - Non-OverSampling (DAC). Prior to reproduction as an analog signal, digital audio has samples added at a multiple of the existing sample rate, usually 2x, 4x or 8x to prevent aliasing and distortion in the higher frequencies. However some people don't like the resulting sound of standard over-sampling DACs, feeling they sound unnatural, so some manufacturers make them without this feature, at the cost of higher distortion.
OCCC - Ohno Continuous Cast Copper - wire made by a method such that single crystals are many meters long, as opposed to very short in conventional copper. Some people believe it produces better quality cables resulting in better sound.
OFC - Oxygen Free Copper. Cable wire designed to have minimal impurities.
Ohm - Unit of measurement for electrical resistance or impedance. Also written as Ω. See "Resistance" and "Impedance".
Op Amp - short for "Operational Amplifier", an amplification circuit usually condensed to fit inside a small integrated circuit (IC) chip. Some devices have removable Op Amps which can be switched with other types to achieve a different "sound". Op Amps must be used with feedback in an audio circuit as Op Amps used without feedback have far too much gain to be used in Audio circuits.
Open headphones - Open-backed headphones, where air and sound is free to move around the back of the drivers due to the presense of a grill as opposed to closed cups on closed headphones.
Orthodynamic headphones - "Orthos" for short. A form of dynamic headphones that use a special membrane with embedded voice coil sandwiched between large magnets. Until recently, only one commonly manfactured ortho was in production, the Fostex T-50RP with many vintage Yamaha and other models around. However, with the production of new headphones by Audeze and Hifiman, there has been a major resurgence in interest in this type of headphone. They don't require special amplification as electrostats do, but can reproduce music with a more accurate sound and a lot of detail compared to most dynamic headphones.
Pads - The earpads on headphones.
PCDP - Portable CD Player
Power - For electrical purposes (as with amplifiers driving headphones, for Head-Fi readers) it is basically voltage times current, measured in Watts. 1 Watt = 1 Joule/second.
Potentiometer - Commonly referred to as a "pot" for short. The thing you turn or move to change the volume on an amplifier. A simple way to think of this is a variable resistor with a knob. Turn the knob clockwise and the resistance goes down (volume goes up), turn counter-clockwise and the resistance goes up (volume goes down).
Pre-amp - A pre-amp is an amplifier that prepares a small voltage signal at “line level”, that is, a level of voltage suitable for transmitting over interconnect cables for further amplification. In traditional hi-fi systems, amplifiers were often integrated, that is, they had inputs for small signals from various sources which they amplified, then passed to power amplifier circuitry to drive speakers. In high-end systems, this pre-amplification is separated from power amplification. As line-level signals are close to those required for most conventional headphones, many headphone amplifiers double as pre-amplifiers and many pre-amps can drive headphones. A pre-amp can also be “passive”, containing only an attenuator (a volume control) and no amplification circuitry. A number of DACs also act as pre-amps, having attenuation on the output and extensive amplification circuitry.
RCA - A type of coaxial connector used for un-balanced analogue connections. The center pin is used for the signal and the outer sleeve is connected to the ground. Often used for digital signals in place of BNC connectors for simplicity.
Resistance - Resistance is the physical property of elements (such as copper) to resist the flow of electrons (which is what electricity essentially consists of, in over-simplfication for purposes of brevity) and is measured in units of "Ohms", named after Georg Ohm.
Rig - A complete set-up of gear, from source to amp and headphones including cables, tweaks and anything else.
Sensitivity - In headphones, the amount of sound output, measured in decibels, the headphones will output at 1mW (milliWatt).
Single-ended - A general term to refer to all non-balanced audio equipment where the signal returns for both left and right are connected to the ground. This is the most common type of design for regular hi-fi gear.
Slew rate - Slew rate is a measure of the "speed" of an electronic gain circuit (in an amplifier or preamplifier), particularly important for the accurate reproduction of transient musical details in digital audio with wide dynamic range, which may come and go very quickly.
Solid state (SS) - audio equipment that uses transistors and OPAMPs for amplification as opposed to tubes.
SOTA - State of the Art
Source - The first device that sends out an analog signal, such as a CD player, sound card, iPod or DAC. See also transport.
S/PDIF - The name stands for Sony/Philips Digital Interconnect Format (more commonly known as Sony Philips Digital Interface).
'Stats - Electrostatic headphones such as Stax.
Stax Mafia - An old joke about some forum members who insisted electrostats were superior to other types of headphones.
Stepped Attenuator - Higher end potentiometer that uses a separate resistor for each level of volume. Has a shorter signal path and in most cases it results in a cleaner sound than a conventional pot.
Straight wire with gain - The ideal high-end amplifier is said to behave as if it were merely a piece of wire amplifying the sound without any alteration at all (eg: Transparent). Some amplifiers are described as being like this.
Supra-aural - Headphones that sit on your ears, as opposed to Circum-aural.
Synergy - The interaction or cooperation of two or more audio components in an audio system, which, when combined, produce a combined effect greater than the sum of their separate effects : e.g.. the synergy between DAC and Headphone Amp, or Headphone Amp and Headphones. This is partly related to subjective impressions as well as electrical considerations such as input and output impedances between components and how they perform when connected.
T-amp - The Sonic Impact T-amp. Using a Tripath Class-D chip, this 8 watt battery-powered speaker amp sold for around US$20-$40 and was considered as good-sounding as amps costing hundreds of times its price. It can now be found as the Dayton Audio DTA-1 though many other similar designs using the Tripath chips exist.
THD - Total Harmonic Distortion. A measurement of the degree to which a piece of equipment distorts the signal.
THD+N - Total Harmonic Distortion plus Noise. Another measurement of the amount equipment distorts the signal.
Tips - The silicon, rubber or foam tips for in-ear monitors (IEMs).
TOTL - Top Of The Line
Transparent - Does not alter the signal in any way. While essentially impossible, the ideal of high-end audio gear is that it reproduces a music signal with as little alteration as possible.
Transport - The device at the very beginning of the audio chain that holds the music data. Could be a computer, CD transport (with or without DAC), iPod or whatever. Sometimes confused with the source for this reason.
TRS - Tip Ring Sleeve connector. The most common connector used on headphones. Comes in two sizes: 3.5mm (1/8") and 6.3mm (1/4"). The smaller size is the standard on portable gear whereas the larger is standard on home audio equipment.
Tube - Vacuum tube, used for amplification prior to the invention of the transistor. As a great many of a great many types were manufactured to a high quality, they are popular to use in amplifier (or amplification circuit) designs due to their pleasant harmonics as well, sometimes, as coloration of the music.
Tube amp - an amplifier that uses tubes in one or another section of the design for amplification of the sound or power rectification. Common designs are OTL (Output Transformer-Less), transformer coupled, hybrid (using solid state circuits as well as tubes) and others. Contrary to popular belief, they do not necessary produce a "warm" sound.
Tube rolling - The swapping of tubes in an amp to change or improve the sound.
Voltage - Voltage is the electromotive force or pressure that "pushes" large numbers of electrons - which essentially are electricity-in-motion - through a "load" resistance or impedance (consisting of "LCR" characteristics), and is measured in units named after Alessandro Volta. See "Current" above in this same wiki.Voltage is the electromotive force or pressure that "pushes" the electrons down the wire, measured in uni
Watt - Basically voltage times current. It is an expression of the work done. It is also an expression of rate of energy usage. See "Power" and "Voltage" elsewhere on the wiki. In an audio amplification context, it is sometimes amended with "R.M.S." - an abbreviation for "Root Mean Square."
Windows Media Audio (WMA) Lossless - A proprietary lossless audio data compression technology developed by Microsoft which competes with FLAC and Apple Lossless Audio Codec (ALAC).
World Class - Usually used in magazine reviews to suggest that something is of a high enough grade that it can compete with similar gear from any country.
XLR - A connector with three, four or possibly more pins used primarily in audio for balanced connections. With 3-pin plugs and sockets, one pin carries the in phase signal (pin 2 or 3), one the return signal out of phase (pin 3 or 2), and the third (pin 1) is the ground. On headphones, the ground pin isn't used. The 4-pin XLR pinout, when used on headphones, is, respectively L+ L- R+ R-.