N See: newton.
NAB (National Association of Broadcasters) A professional trade organization for people working in the radio and television industry.
NACFM (National Association of Church Facility Managers) "A non-profit organization established in 1995 to promote
networking and educational advancement opportunities for Facilities Management Professionals. NACFM provides an informational resource for church and religious facility management professionals by discussion groups, forums, panels, lectures, seminars, or similar programs or activities (such as this web site) designed to fulfill the purposes of the organization." Another great resource for audio contractors, integrators, etc.
NAG (needed acoustic gain) Acoustics. The gain in decibels required by sound reinforcement to achieve an equivalent acoustic level at the farthest listener equal to what the nearest listener would hear without sound reinforcement.
Nagra Recording. Famous Swiss company whose field tape recorders became the standard for all professionals. The name is so associated with portable recorders that today it is becoming a generic label, like Kleenex or band-aid or XLR.
NAH (nearfield acoustic holography) Acoustics. A sound radiation measurement system developed by Professors J. D. Maynard, E. G. Williams, and Y. Lee at the Pennsylvania State University, Department of Physics in 1985. See: "Nearfield acoustic holography: I. Theory of generalized holography and the development of NAH," J. Acoustical Soc. of America, Oct., 1985, Vol. 78, Issue 4, pp. 1395-1413.
nai Musical Instrument. Romanian panpipes, also called Moldavian pan flute.
NAMA (Native American Music Awards) See NAMMYS.
NAMM (National Association of Music Merchants is the original name; today it is officially the International Music Products Association but they didn't change the acronym) A professional trade organization for people working in the music business -- primarily in retailing and manufacturing of music making products. [Popular saying is that NAMM means Not Available; Maybe May.]
NAMMYS The name of the award given yearly by NAMA (Native American Music Awards).
nano- Abbr. n A prefix for one billionth (10-9)
nanoweber Abbr. nWb A unit of magnetic flux equal to one billionth (10-9) of a weber.
Napier, John (1550-1617) Scottish mathematician who invented logarithms and introduced the use of the decimal point in writing numbers. [AHD]
Napier's bones Mathematics. A set of graduated rods used to perform multiplication quickly. Lord Napier is credited with creating them to expedite arithmetical calculations. [Kacirk]
NARAS (National Academy of Recording Arts & Science) See The Recording Academy.
NARM (National Association of Recording Merchandisers) An industry organization made up primarily of music retailers acting as an advocate body for the common interests of merchandisers and distributors of music to industry and public policy makers.
narrow-band filter Term popularized by equalizer pioneer C.P. Boner to describe his patented (tapped toroidal inductor) passive notch filters. Boner's filters were very high Q (around 200) and extremely narrow (5 Hz at the -3 dB points). Boner used 100-150 of these sections in series to reduce feedback modes. Today's usage extends this terminology to include all filters narrower than 1/3-octave. This includes parametrics, notch filter sets, and certain cut-only variable equalizer designs.
National Electrical Code See NEC.
National Music Museum Located in Vermillion, S. Dakota on the Campus of The University of South Dakota, it has more than 13,500 items in its collection (largest in the world) with some 800 on display at any one time.
National Recording Registry Set up by Congress in 2002 to preserve historical recordings. The inaugural list of recognized recordings includes "The Message," by Grandmaster Flash and the Furious Five. (1982)
naturals Musical Instruments. The white note keys on a piano or organ keyboard; so named for the "natural" notes of the C major scale: C,D,E,F,G,A,B).
nay (also ney) Musical Instrument. A Persian end-blown bamboo flute.
N curve (normal curve) Same as Academy curve.
NC (noise criterion) curves A unit of measurement for the ambient or background noise level of occupied indoor spaces, i.e., a measure of its noisiness -- true story; real word. The measured noise spectrum (done in octave bands using an SPL meter) is compared against a series of standard noise criteria (NC) curves to determine the "NC level" of the space. The standard NC curves take into account the equal loudness contours of Fletcher-Munson to accurately reflect the listening experience. Each NC curve is assigned a number (in 5 dB increments) corresponding to the octave band SPL measured over the octave centered at approximately 1500 Hz. A space is then said to have a background noise level of "NC-20," for instance, which would be very quiet, comparable to a quality recording studio. Compare with RC rating.
near-end Telecommunication term referring to your end; the local room, as opposed to the far-end.
near-end crosstalk Crosstalk that is propagated in a disturbed channel in the direction opposite to the direction of propagation of the signal in the disturbing channel. The terminals of the disturbed channel, at which the near-end crosstalk is present, and the energized terminal of the disturbing channel, are usually near each other. [IEEE Std 802.5]
near-field Acoustics. The sound field very close to the sound source, between the source and the far field. Technically, a distance less than one wavelength at the frequency of interest.
near-field focalization (NFF) See: focalization.
near-field monitor A loudspeaker used at a distance of 3-4 feet (1-1½ meters) in recording studios.
NEC (National Electrical Code) The name for the United States electrical safety standard (NFPA-70).
negative Electronics. An excess of electrons in a conductor or semiconductor.
negative feedback The act of comparing a fraction of the output signal to the input signal at the input to an amplifier in such a way that the amplifier will keep this fraction of the output signal always exactly the same as the input signal. Negative feedback is of prime importance in designing with op amps and audio power amplifiers. As applied to audio amplifiers, negative feedback is first attributed to Bell Labs scientist Harold S. Black, as described in the Bell Labs Technical Review, 1934 and his monumental 87 page U.S. patent 2,102,671 filed in 1932.
negative logic An electronic logic system where the voltage representing one, active, or true has a more negative value than the voltage representing zero, inactive, or false. Also known as negative-true logic, it is normally used in electronic and computing data and communications switching systems for noise immunity reasons. [IEEE Std 1451.2]
negative resistor (aka negistor; also negative impedance) Electronics. A resistor having the opposite characteristics of Ohm's Law, i.e., the current goes down if the voltage goes up and vice versa. Certain devices and circuits display this trait and are used mainly to make negative resistance oscillators.
negistor See negative resistor.
NEMA (National Electrical Manufacturing Association) Founded in 1926 and headquartered in Rosslyn, VA.
neodymium Abbr. Nd Audio Transducers. Popular rare-earth metal used to make superior magnets for loudspeakers and microphones. Neodymium iron boron magnets have a more linear frequency response, are more powerful and smaller, with higher output levels than conventional iron magnets. First used by Electro-Voice in the late '70s. [No, it was not named after Neil Diamond.]
neodymium motors See: bonded magnet motors.
nephew node Networks. A node with one port in standby and all other ports (if any) disabled, disconnected, or suspended. The peer uncle node proxies for the nephew node during bus resets. [IEEE Std 1394b]
network Generally used to mean a multi-computer system (as opposed to a single computer bus-type system) where multiple access is allowed from more than one computer at a time. Characterized by full two-way (duplex) communications between all equipment and computers on the network. See CobraNet for an example.
network glossary See CobraNet glossary for many useful terms.
Neumann, Georg (1891-1976) German inventor, entrepreneur and audio industry pioneer of high-quality microphones.
neume or neum Music. A sign used in the notation of plainsong during the Middle Ages, surviving today in transcriptions of Gregorian chants. [Middle English, series of notes sung on one syllable, from Medieval Latin pneuma, from Greek, breath.] [AHD]
neural spectrogram Hearing. "The idea that the auditory system takes apart the acoustic information in the same way that a spectrogram does. The separation of different frequencies is done by the basilar membrane. Information about the intensity and phase of different frequency components is kept separate in different neural pathways originating at different sites on the membrane. This separation of frequency information is maintained all the way up to the brain. Intensity activity in each neural pathway corresponds to the darkness of the streak on the spectrogram for the corresponding frequency." [Bregman]
neutral Electronics. 1. Having no electrical charge. 2. Common point of a star-connected generator or transformer winding. [IEEE Std 1020]
Neve 1073 Console Module Designed by Rubert Neve in 1970, this mic-preamp with 3-band EQ set the standard for all mic channels to follow.
Neville Thiele Method™ See: NTM
Newman, Robert B. (1917-1983) American engineer specializing in acoustics most remembered for being on of the founders of Bolt, Beranek and Newman in 1948.
New Orleans Jazz & Heritage Festival Beginning in 1970, hit the link to read the fascinating history of this forty-plus-years old festival.
Newport Folk Festival One of America's oldest and most famous folk music festivals held in Newport, R.I., usually in July.
Newport Jazz Festival Founded in 1954, it was the premier jazz festival in America, held annually in Newport, R.I., usually in August.
newton Abbr. N. The International System unit of force. It is equal to the force required to accelerate a mass of one kilogram one meter per second per second.
Newton, Sir Isaac (1642-1727) English mathematician and scientist who invented differential calculus and formulated the theory of universal gravitation, a theory about the nature of light, and three laws of motion. The sight of a falling apple supposedly inspired his treatise on gravitation, presented in Principia Mathematica (1687). [AHD]
NEXT (near-end crosstalk) Category wiring. Interference between signals on twisted pair cable caused by damage (usually a loosening of the tight twist required for high speed transmission) occurring close to the connector.
ney See: nay.
NFF (near-field focalization) See: focalization.
ngoni Musical Instrument. A West African stringed instrument.
NIC (negative impedance converter) Op Amps. A circuit that acts like a negative load, i.e., it adds rather than consumes energy--magic, really.
NIC (network interface card) Ethernet. The PC expansion board that connects a device to a LAN, usually Ethernet-based.
NIC (noise isolation class) Noise Measurements. "A method for rating a partition's ability to block airborne noise transfer." [From link]
nickelodeon 1. An early movie theater charging an admission price of five cents. 2. A player piano. 3. A jukebox. [AHD]
NIH (not invented here) Popular abbreviation found in technology land, used to described a prima donna corporate attitude that everything they do must be original. Stems from an unhealthy attitude that the only solution worthy is their solution, thus rejecting ideas and inventions not theirs. Makes for very unproductive and unhappy engineering teams.
NIME (New Interfaces for Musical Expression) Music. International organization for the development of new musical interface design.
Nipper The famous "His Master's Voice" RCA dog. A bull terrier and fox terrier mix born in 1884 and named for his propensity to nip at peoples' legs.
NIST (National Institute of Standards and Technology) Government organization who produces (among vastly other things) Special Publication 811: Guide for the Use of the International System of Units (SI), the authority on this subject.
Nixie tube Vacuum Tubes. A numerical read-out cold-cathode tube first developed by Haydu Brothers Laboratories in 1955, later purchased by Burroughs Corporation who registered the name. The name Nixie was derived by Burroughs from "NIX I", an abbreviation of "Numeric Indicator eXperimental No. 1." Hit the link for photos.
No also Noh Theater. The classical drama of Japan, with music and dance performed in a highly stylized manner by elaborately dressed performers on an almost bare stage. [AHD] Similar but older than Kabuki.
node Acoustics. A point of minimum amplitude in a one-dimensional standing wave field. A nodal line is a line of minimum amplitude in a two-dimensional filed, and a nodal surface is a surface of minimum amplitude in a three-dimensional field. [Morfey]
noise General. Sound or a sound that is loud, unpleasant, unexpected, or undesired. Physics. A disturbance, especially a random and persistent disturbance, that obscures or reduces the clarity of a signal. Computer Science. Irrelevant or meaningless data. From Latin meaning nausea, discomfort and seasickness. [AHD]
noise cancelling headphones Special headphones incorporating a microphone built into the headset that samples the ambient sound and adds it back out-of-phase to the headphone signal. This method actively cancels or nulls out background noise -- works best with low frequencies. See Sennheiser NoiseGard® for an interesting demo.
noise cancelling microphone A special dynamic microphone designed so both sides of the diaphragm are exposed to the sound field. Close direct sound strikes primarily one side of the diaphragm causing it to move while sounds from far away tend to be canceled because they strike the diaphragm from all sides with no net force.
noise color People working in pro audio
know the terms white noise and pink noise, but few recognize
the terms "azure noise" or "red noise," but they are real terms. Noise
that is not white is called colored noise and will have more energy
at some frequencies than others, analogous to colored light.
White noise and pink noise are well defined and known; much less so are the others.
White noise is so named because it is analogous to white light in that it contains all audible frequencies distributed uniformly throughout the spectrum. Passing white light through a prism (a form of filtering) breaks it down into a range of colors. Examination shows that red light is characterized by the longer wavelengths of light, i.e., the lower frequency region. Similarly, "pink noise" has higher energy in the low frequencies, hence the somewhat tongue-in-cheek term.
The Federal Standard 1037C Telecommunications: Glossary of Telecommunication Terms defines four noise colors (white, pink, blue & black) and is considered the official source. No official standard could be found for the others.
The following list of noise colors is loosely based on a rainbow-prism light analogy, where a prism creates a rainbow effect by separating white light passed through it into a visible spectrum labeled red, orange, yellow, green, blue, indigo, and violet from lowest to highest frequencies. Also shown is the approximate slope of the power density spectrum relative to white noise used as the reference:
red noise also called brown noise or Brownian noise after Robert Brown: -6 dB/oct decreasing density (most amount of low frequency energy or power; used in oceanography; power proportional to 1/frequency-squared); popcorn noise.
grey noise: A random pink noise within the audible frequency range subjected to inverted A-weighting loudness curve per IEC 61672. It gives the listener the perception that it is equally loud at all frequencies.
blue (or azure) noise: +3 dB/oct increasing noise density (power proportional to frequency).
purple (or violet) noise: +6 dB/oct increasing noise density (power proportional to frequency-squared; most amount of high frequency energy or power).
black noise: silence (zero power density with a few random spikes allowed).
Other noise colors exist for specialized fields like video/photographic/image processing, communications, mathematical chaos theory, etc., but are not found in pro audio circles.
noise criterion (NC) curves See NC curves.
noise dose or noise exposure Limits of noise exposure published by OSHA (US Department of Labor, Occupational Safety and Health Administration). It is a expressed as A-weighted SPL (dBA) term. Here are some common examples:
90 dBA: 8 hours max
95 dBA: 4 hours max
100 dBA: 2 hours max
105 dBA: 1 hour max
110 dBA: 30 minutes max
115 dBA: 15 minutes max
noise figure The ratio between the Johnson noise (or thermal noise) of the equivalent input resistance of a circuit and its measured noise, expressed in decibels. It is the ratio of the output noise to the input noise, so the answer is always positive, with a theoretically noise-free device having a noise figure of 0 dB.
noise gate An expander with a fixed "infinite" downward expansion ratio. Used extensively for controlling unwanted noise, such as preventing "open" microphones and "hot" instrument pick-ups from introducing extraneous sounds into the system. When the incoming audio signal drops below the user set-point (the threshold point) the expander prevents any further output by reducing the gain to "zero." The actual gain reduction is typically on the order of -80 dB, thus once audio falls below the threshold, effectively the output level becomes the residual noise of the gate. Common terminology refers to the gate "opening" and "closing." Another popular application uses noise gates to enhance musical instrument sounds, especially percussion instruments. Judicious setting of a noise gate's attack (turn-on) and release (turn-off) times adds "punch," or "tightens" the percussive sound, making it more pronounced. A noise gate is to an expander as a limiter is to a compressor. See the RaneNotes Dynamics Processors and Signal Processing Fundamentals.
noise map Serato DJ. The Serato control vinyl and control CDs each contain a proprietary Noise Map Control Tone that allows Scratch Live to track the motion of the record, simulating the same movement with digital audio. The Serato noise map was co-invented in 2002 by Serato cofounders, Steve West and AJ Bertenshaw ("AJ"). An important distinction is that Serato's noise map is not time code, or anything like time code. It is a unique and proprietary method of tracking the control record's motion based on the mathematical concept of a maximum-length pseudo random bit sequence, which guarantees uniqueness for the shorted possible section. It is a continuously varying signal rather than a sequence of discrete consecutive location labels as is the case with time code schemes. A noise map makes it is more difficult to determine any exact location since a form of fuzzy pattern matching must be used. However, this increased complexity is more than compensated for by the resultant lower latency, the lack of any need to synchronize with lengthy code words within the signal, and better robustness to noise. In AJ's words: "The idea is to get a sequence of bits in which you can find your position uniquely in the shortest number of bits, without having to synchronize. The problem with time code is that you have to know where each code ends and the next begins. With the noise map you don't have the problem because the code is continuous."
noise measurement filters See weighting filters.
noise reduction Abbr. NR Recording. A signal processing function used to reduce the amount of background noise. See expander.
noise reduction coefficient Acoustics. A single-number rating of the sound absorption properties of a material; it is the arithmetic mean of the sound absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest multiple of 0.05. [Harris]
noise shaping A technique used in oversampling low-bit converters and other quantizers to shift (shape) the frequency range of quantizing error (noise and distortion). The output of a quantizer is fed back through a filter, and summed with its input signal. Dither is sometimes used in the process. Oversampling A/D converters shift much of it out of the audio range completely. In this case, the in-band noise is decreased, which allows low-bit converters (such as delta-sigma) to equal or out-perform high-bit converters (those greater than 16 bits). When oversampling is not involved, the noise still appears to decrease by 12 dB or more because it is redistributed into less audible frequency areas. Further digital processing usually reverses the benefits of this kind of noise shaping. See the RaneNote Digital Dharma of Audio A/D Converters.
noise temperature The standard reference temperature for noise measurements is 290°K.
noise weighting See: weighting filters.
NOM (number of open mics) An acronym believed first created in 1967, or 1968, by Bill Snow after he retired from Bell Labs and went to work at Altec Lansing Research. It's use was popularized by Dan Dugan, the father of the automatic microphone mixer and Altec Lansing, the manufacturer of his first design. In Dan's original design, the automatic mic mixer, like human operators, turned the gain down on unused mic channels and turned the gain up on active channels, all the while ensuring that the overall level remained roughly constant. As a rough approximation, each doubling of the number of open mics (NOM) cuts the gain by 3 dB, i.e., as more mics are opened up the mic mixer reduces overall gain. If not, as mics open and close, the reverberation and ambient noise fluctuates unacceptably. NOM attenuation techniques work to provide the gain, stability, and low noise qualities of a single open mic with the benefits of multiple mics. [Historical Note: This concept was first written about by C.P. Boner & R.E. Boner, in their paper "The Gain of a Sound System" April 1969, reproduced in Sound Reinforcement: An Anthology (Audio Engineering Society, NY, 1978.]
nominal This word has several definitions but the one of importance to pro audio is its engineering sense meaning: insignificantly small; trifling: a nominal amount. It can also mean "according to plan or design." [AHD]
NOMM (number of open mics & mixers) A term created by Rane Corporation extending the concept of NOM (above) to include multiple mixers as well as microphones. As used by Rane, it is NOM-like in that feedback stability is maintained, however, since large systems have mics across multiple mixers, Rane includes these mixers in the NOMM calculation. For example, in audio conferencing, when the chairman is speaking and someone else quickly answers "yes," coughs, or drops a pen, most mic mixers running in NOM mode are annoying because they reduce the level of the chairman mic just because someone else made a noise. The Rane NOMM approach avoids this annoyance by keeping the chairman's mic at the same gain while still allowing the interruption to be heard, yet at a reduced gain from its full gated level.
nomogram 1. A graph consisting of three coplanar curves, each graduated for a different variable so that a straight line cutting all three curves intersects the related values of each variable. 2. A chart representing numerical relationships. [AHD]
non-acoustic voice sensor A device that can detect a person's voice without the person actually speaking out loud. See TERC.
nonlinear Electronics. The amplitude of the output is not linearly proportional to the input. If a true sine wave were transmitted through a nonlinear device, its shape would be changed. [IEEE] Contrast with linear.
nonlinear distortion Introduction of frequency components not present in the original signal. Contrast with linear distortion.
nonvolatile Refers to a memory device that does not lose its data when power is removed from the system.
norator Electronics. A two-terminal circuit element with a voltage-current (V-I) characteristic curve consisting of all points on the V-I plane (except zero/zero because that is a nullator). The device does not define either the voltage or the current. Used to model complex circuits for analysis. For example the output of an ideal op amp can be modeled with a norator, i.e., its output voltage and current can assume any value. Paired with a nullator creates a nullor.
normal distribution See: Gaussian distribution.
normalize Recording. The process of linearly increasing all digital samples by the same amount so that the largest original sample reaches a given level. This is done in order to create a maximum signal while maintaining the S/N ratio, and with no clipping when the level selected is 0 dBFS. Analogous to an analog audio system's volume control. See: 10 Myths about Normalization for an interesting discussion of this subject.
normalling jacks See patchbay.
normative Standards. A mandatory set of instructions or references. [IEEE]
Norteño Music. A genre of Mexican music.
Norton's Theorem In simple terms, states that a complex linear circuit (i.e., no exponents or roots in its defining equations) can be replaced by a single current source and a parallel resistor. See All About Circuit's Norton's Theorem for detailed explanation. [After Edward L. Norton.]
notch filter A special type of cut-only equalizer used to attenuate (only, no boosting provisions exist) a narrow band of frequencies. Three controls: frequency, bandwidth and depth, determine the notch. Simplified units provide only a frequency control, with bandwidth and depth fixed internally. Used most often in acoustic feedback control to eliminate a small band of frequencies where the system wants to howl (feedback).
note value See: music note value.
Novachord Musical Instruments. The name for Hammond's first commercially available synthesizer released in 1938. See The Novachord Restoration Project for pictures and text.
noy Psychoacoustics. A subjective unit of noisiness. For example, a sound of 2 noys is twice as noisy as a sound of 1 noy and half as noisy as a sound of 4 noys. [BBC's J.R. comes through again - thanks!]
NPO 1. Ceramic capacitors Temperature coefficient designator meaning negative-positive-zero, i.e., the capacitance drifts negative and positive averaging zero. A marking meaning stable with temperature. 2. Medicine Abbreviation for nil per os, or nothing by mouth
NR See: noise reduction.
NRW+E (Nashville Recording Workshop and Expo) Yearly Nashville event dedicated to "Helping Creative People Make Better Recordings."
NRZ (non-return to zero) A system of binary code where one's are usually represented by a positive voltage and zeroes by a negative number, i.e., neither is ever zero volts.
NSCA (National Systems Contractors Association) "Founded in 1980 as the National Sound Contractors Association, the NSCA underwent a name change in 1994 to better reflect the diversification found within the hi-tech industry of electronic systems. Rather than focusing solely on the installation of audio systems, today's innovative member companies of the NSCA expanded into other fields, including audio, video, intercom/paging, telecommunications, security/access control, and many others." [from NSCA website]
NSP (native signal processing) Intel-designed method of using a powerful microprocessor (like their Pentium CPU) for signal processing functions normally done by separate DSP chips. Not finding many backers.
NSSP (National Standards Systems Network) A Web-based service launched by ANSI, along with government and industry partners. A full search & sales service provides for locating and buying virtually any standard. More than 100,000 global standards are available. Over 25 standards groups provide technical specs for this database, including ISO. The EIA endorsed the project.
NTMTM Crossover Filter (Neville Thiele MethodTM Crossover Filter) Trademarked term for the patented (U.S. Patent 6,854,005) technology developed by Neville Thiele for Whise Acoustics in Australia. Two choices were offered, a 4th-order with rolloff slopes of 36 dB/octave and an 8th-order with 52 dB/octave slopes. The published curves resemble 4th- and 8th-order cascaded elliptic filters. Rod Elliot of Elliot Sound Products explores the details in his paper: NTM™ Crossovers.
n-type semiconductor An extrinsic semiconductor in which the conduction electron concentration exceeds the mobile hole concentration. [IEEE]
null Mathematics. Of or relating to a set having no members or to zero magnitude. Instrumentation. A reading of zero. [AHD]
nullator Electronics. A two-terminal circuit element with a voltage-current (V-I) characteristic curve consisting of only the origin point on the V-I plane, i.e., it always has a value of 0-0. Used to model complex circuits for analysis. For example the input of an ideal op amp can be modeled with a nullator; it has zero input current and acts like a virtual short between its inputs, i.e., zero volts between the inputs, thus an input voltage-current characteristic of 0-0. Paired with a norator creates a nullor.
null modem cable Special wiring of an RS-232 cable such that a computer can talk to another computer without a modem (thus "null" modem). As a minimum, a null modem cable reverses pins 2 and 3 on a standard RS-232 cable -- but other pins may also need changing and shorting together.
nullor Electronics. A four-terminal circuit element consisting of a norator and a nullator; together they model an ideal op amp, i.e., one whose input voltage and current equal zero and whose output voltage and current can be any value.
numerological nonsense John Allen Paulos' wonderful page of rationality helping to fight off the increasing numbers of irrational parents/teachers/politicians/scientists/engineers/audiophiles/(your favorite here).
Nusselt number See Grashof.
nWb See: nanoweber.
nyckelharpa Musical Instrument. A Swedish keyed stringed instrument.
Nyquist frequency The highest frequency that may be accurately sampled. The Nyquist frequency is one-half the sampling frequency. For example, the theoretical Nyquist frequency of a CD system is 22.05 kHz. See the RaneNote Digital Dharma of Audio A/D Converters.