Glossary of Audio and Acoustic Test & Measurement Terms

The standard deviation calculated at each point of the curve. See Statistics chapter of SoundCheck Manual for formula.

A procedure for solving a mathematical problem in a finite number of steps that frequently involves repetition of an operation. Algorithm here refers specifically to a procedure that encodes audio information (so that it can be sent at high speed across data lines) and decodes the transmitted information into audio at the receiving end.

The instantaneous magnitude of an oscillating quantity such as sound pressure. The peak amplitude is the maximum value.

Without echo

Determines which of the selected curves is the closest/furthest from the average of the curves. See Statistics chapter of SoundCheck Manual for formula.

See IEEE 269 and/or B&k Frequency Analysis Text Book definition

Used in a sequence to Jump over steps in a sequence according to the Pass/Fail criteria of a step.

crest factor = absolute peak value / rms value = max[|x|]/rms(x)

A sound-level meter reading with a B-weighting network simulating the human-ear response at a loudness level of 70 phons.

A sound-level meter reading with no weighting network in the circuit, i.e., flat. The reference level is 20μPa.

A logarithmic form of any measured physical quantity and commonly used in the measurement of sound. The decibel provides the possibility of representing a large span of signal levels in a simple manner as opposed to using the basic unit Pascal. The difference between the sound pressure for silence versus a loud sound is a factor of 1,000,000:1 or more, and it is not practical to use these large numbers. Doubling of Sound Pressure = 6 dB Doubling of Sound Power = 3 dB Doubling of Perceived Sound Level = 10 dB (approximately)

dB---the term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1.

The difference between sound pressure level in any given direction in the acoustic far field and the average sound pressure level in that field.

Device Under Test

A contour representing a constant loudness for all audible frequencies. The contour having a sound pressure level of 40 dB at 1,000Hz is arbitrarily defined as the 40-phon contour.

The process of adjusting the frequency response of a device or system to achieve a flat or other desired response.

That part of the sound field in which sound pressure decreases inversely with distance from the source. This corresponds to a reduction of approximately 6 dB in level for each doubling distance.

Unwanted interaction between the output and input of an acoustical system, e.g., between the loudspeaker and the microphone of a system.

Efficient algorithm to calculate the Fourier Transform.

A filter that passes all frequencies above a cutoff frequency.

A filter that passes all frequencies below a certain cutoff frequency.

Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds "seem" reduced in intensity at lower listening levels.

An environment in which a sound wave may propagate in all directions without obstructions or reflections. Anechoic rooms can produce such an environment under controlled conditions.

The number of times per second that the sine wave of sound repeats itself. It can be expressed in cycles per second, or Hertz (Hz). Frequency equals Speed of Sound / Wavelength.

Principle where louder sounds render soft sounds inaudible in nearby frequency bands. This is the principle behind perceptual encoding.

The changes in the sensitivity of a circuit, device, or room with frequency.

Full Scale Deflection

Telco communication that is bi-directional. ISDN is full duplex, so each end of the connection can simultaneously transmit to the other.

The lowest frequency of a note in a complex wave form or chord.

Refers to the transmission of audio via a POTS (Plain Old Telephone) circuit. Frequency response is limited to about 3.5kHz.

To increase in level. The function of a volume control.

Protocols usually implemented in hardware that let one data device tell another that conditions are right (or wrong) for communications. A simple example: a printer telling a computer that it is OK to print.

Also called overtones, these are vibrations at frequencies that are multiples of the fundamentals. Harmonics extend without limit beyond the audible range. They are characterized as even-order and odd-order harmonics. A second-order harmonic is two times the frequency of the fundamental; a third order is three times the fundamental; a fourth order is four times the fundamental; and so forth. Each even-order harmonic second, fourth, sixth, etc.- is one octave or multiples of one octave higher than the fundamental; these even-order overtones are therefore musically related to the fundamental. Odd-order harmonics, on the other hand third, fifth, seventh, and up-create a series of notes that are not related to any octave overtones and therefore may have an unpleasant sound. Audio systems that emphasize odd-order harmonics tend to have a harsh, hard quality.

A healthy young person generally can hear frequencies from approximately 20 Hz to 20000 Hz, and sound pressure levels from 0 dB to 130 dB or more (threshold of pain). The smallest perceptible change is 1 dB.

The human ear is less sensitive at low frequencies than in the midrange. Turn your volume knob down and notice how the bass seems to "disappear". To hear low bass requires an adequate SPL level. To hear 25Hz requires a much higher SPL level than to hear 250Hz.

The unit of frequency, abbreviated Hz. The same as cycles per second.

See filter, high pass.

The opposition to the flow of electric or acoustic energy measured in ohms.

A very short, transient, electric or acoustic signal.

Sound pressure versus time measurement showing how a device or room responds to an impulse.

Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase."

Under far field/free field conditions, sound intensity varies inversely with the square of the distance from the source. In pure spherical divergence of sound from a point source in free space, the sound pressure level decreases 6 dB for each doubling of the distance.

The subjective judgment of intensity of a sound by humans. Loudness depends upon the sound pressure and frequency of the stimulus. Loudness was defined by Fletcher and Munson (1933) as a physiological description of the magnitude of an auditory sensation. The definition of loudness was later refined as a definition of the attribute of auditory sensation corresponding most closely to the physical measurement of sound intensity, but is not always accurate. Loudness is a subjective quantity and all measurement techniques are based on assumptions and interpretation.

The process by which the threshold of audibility for a sound is raised by the presence of another (masking) sound. A masking noise is one that is intense enough to render inaudible or unintelligible another sound that is also present.

The maximum value at each point of the curves being compared.

The average value at each point of the curves being compared

An acoustical-to-electrical transducer by which sound waves in air are converted to electrical signals.

The minimum value at each point of the curves being compared.

Locations close to the sound source between the source and the far field. The near field is typically characterized by large sound pressure level variations with small changes in measurement position from the source. This is a physical region in space where the inverse square law does not apply.

Traditionally, noise has been defined as unwanted, undesired, or unpleasant sound. This makes noise a subjective term. Sounds that may be unwanted and undesired by some may be wanted and desirable by others. Noise is sound, as defined in this document: a pressure variation, etc. In order to keep terms used in soundscape management as non-subjective as possible, sounds should be classified as either appropriate or inappropriate, rather than as "noise." or "sound." The appropriateness of any sound in a given area of a park will depend on a variety of factors, including the management objectives of that area.

Noise Free Interval (natural sounds only)

The length of the continuous period of time during which only natural sounds are audible. Though little research has been conducted to relate how this measure correlates with visitor judgments or with common experiences in park settings, it should provide a reasonable measure of the existence and availability of periods with only natural sounds. It is also a metric that requires no acoustics knowledge to be meaningful. Over the coming years of soundscape data collection, the NPS will acquire such data and develop an understanding of how this metric can best be used to aid in assessing and managing park soundscapes.

An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. For each octave lower in frequency that a speaker tries to reproduce, the speaker needs to move four times as much air!

The segment of the frequency spectrum separated by an octave.

Frequency ranges in which the upper limit of each band is twice the lower limit. Octave bands are identified by their geometric mean frequency, or center frequency.

Frequency ranges where each octave is divided into one-third octaves with the upper frequency limit being 2* (1.26) times the lower frequency. Identified by the geometric mean frequency of each band.

LPK[nd] ----ten times the common logarithm of the square of the ratio of the largest absolute value of the instantaneous sound pressure in a stated frequency band during a specified time interval to the reference sound pressure of 20 micro pascals.

Percent Time Above Natural Ambient

The amount of time that sound levels from human-caused sound(s) are greater than sound levels of natural ambient sounds in a given area. This measure is not specific to the hearing ability of a given animal, but a measure of when and how long human-caused sound levels exceed natural ambient sound levels.

The amount of time that various sound sources are audible to animals, including humans, with normal hearing (hearing ability varies among animals). A sound may be above natural ambient sound pressure levels, but still not audible to some animals. This information is essential for measuring and monitoring human-caused noise in national parks. These data can be collected by either a trained observer (attended logging) or by making high-quality digital recordings (for later playback). Percent Time Audible is useful because it is a measure that is understandable without any acoustics knowledge. It is a measure that can be specific to a given animal, and it is a metric that correlates well with park visitor judgments of annoyance and with visitor reports of interference from certain noise sources with natural quiet and the sounds of nature.

Phase is the measure of progression of a periodic wave. Phase identifies the position at any instant which a periodic wave occupies in its cycle. It can also be described as the time relationship between two signals.

The time or angular difference between two signals.

The loudness level in phons of any sound is defined as being numerically equal to the dBSPL of a 1000 Hz tone that is judged by the average observer to be equally loud.

Noise with a continuous frequency spectrum and with equal power per constant percentage bandwidth. For example, equal power is any one-third octave band.

A subjective term for the perceived frequency of a tone.

The positive or negative direction of an electrical, acoustical, or magnetic force. Two identical signals in opposite polarity are 180 degrees apart at all frequencies. Polarity is not frequency dependent.

Calculates the square root of the sum of the squares of each Y value in a spectrum. See Power Sum in Post-processing chapter of SoundCheck manual.

A measure of process performance. The relationship of the +/- 6 value to the user specified limits.

As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.

A tone with no harmonics. All energy is concentrated at a single frequency.

A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform spectral distribution of energy.

For large surfaces compared to the wavelength of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equaling the angle of reflection.

The bending of sound waves traveling through layered media with different sound velocities.

The quality of electrical or acoustical circuits that results in dissipation of energy through heat.

A natural periodicity, or the reinforcement associated with this periodicity.

Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate. A stretched piano string, when plucked, will vibrate for a while at a certain fundamental frequency. Plucked again, it will again vibrate at that same frequency. This is its natural or resonant frequency. While this is the basis of musical instruments, it is undesirable in music-reproducing instruments like audio equipment.

The sound in an enclosed or partially enclosed space that has been reflected repeatedly or continuously from the boundaries.

The persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped; by extension, in some contexts, the sound that so persists.

A room so designed that the reverberant sound field closely approximates a diffuse sound field, both in the steady state when the sound source is on, and during the decay after the source of sound has stopped.

The tailing off of a sound in an enclosure because of multiple reflections from the boundaries.

Square root of the average of the squares of the signal. Measures the power of a signal.

electronic specification for serial data connections between digital terminal equipment (DTE) and data communications equipment. (DCE) Signal is unbalanced. May be either synchronous or asynchronous. Can reside on multiple connector sizes, but most commonly appear on DB9/DB25 connectors. 50' transmission limit.

The rate at which an analog signal is sampled, or digitized. For instance, when digitizing audio for a CD, the audio is captured at a sample rate of 44.1KHz, or 44,100 times per second, creating a very close, but not perfect, digital representation of the analog waveform.

The range or distance between the noise floor (the noise level of the equipment itself) and the test signal or program material.

A periodic wave related to simple harmonic motion.

The unit of measurement for subjective loudness.

A wave motion in air, water, or other media. It is the rapid oscillatory compressional changes in a medium that propagate to distant points. It is characterized by changes in density, pressure, motion, and temperature as well as other physical properties. Not all rapid changes in the medium are sound (e.g. wind distortion on a microphone diaphragm).

The reduction of the intensity of sound as it travels from the source to a receiving location. Sound absorption is often involved as, for instance, in a lined duct. Spherical spreading and scattering are other attenuation mechanisms.

J-energy added to an elastic medium by the presence of sound, consisting of potential energy in the form of deviations from static pressure and of kinetic energy in the form of particle velocity.

The capacity of a structure to prevent sound from reaching a receiving location. Sound energy is not necessarily absorbed; impedance mismatch, or reflection back toward the source, is often the principal mechanism.

W/m2 the quotient obtained when the average rate of energy flow in a specified direction and sense is divided by the area, perpendicular to that direction, through or toward which it flows. The intensity at a point is the limit of that quotient as the area that includes the point approaches zero.

The degree of acoustical separation between two locations, especially adjacent rooms.

Of airborne sound, a sound pressure level obtained using a signal to which a standard frequency-weighting has been applied.

The weighted sound pressure level obtained by frequency weighting, generally A- or Cweighted. The weighting used must be clearly stated: For L Aeq, "A" denotes that A-weighting was used, and "eq" indicates that an equivalent level has been calculated. Hence, L Aeq is the A-weighted, energy-equivalent sound level.

The lowest amplitude measurable by sound monitoring equipment. Most commercially available sound level meters and microphones can detect sound levels down to about 15 to 20 dBA; however, there are microphones capable of measuring sound levels below 0 dBA.

Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.

W---in a specified frequency band, the rate at which acoustic energy is radiated from a source. In general, the rate of flow of sound energy, whether from a source, through an area, or into an absorber.

Fluctuations in air pressure caused by the presence of sound waves. Sound pressure is the instantaneous difference between the actual pressure produced by a sound wave and the average barometric pressure at a given point in space. Not all pressure fluctuations detected by a microphone are sound (e.g., wind over the microphone). Sound pressure is measured in Pascals (Pa), Newtons per square meter, which is the metric equivalent of pounds per square inch.

Sound Pressure Level (LP or SPL)

The logarithmic form of sound pressure. In air, 20 times the logarithm (to the base 10) of the ratio of the actual sound pressure to a reference sound pressure (which is 20 micropascals, and by convention has been selected to be equal to the assumed threshold of human hearing). It is also expressed by attachment of the word decibel to the number. A 10 dB increase in SPL represents a perceived doubling in loudness sensation and a 3dB increase is typically a "just noticeable difference" to an average listener.

Sound waves can be thought of like the waves in water. Frequency determines the length of the waves; amplitude or volume determines the height of the waves. At 20Hz, the wavelength is 56 feet long! These long waves give bass its penetrating ability, (why you can hear car boomers blocks away).

 the distribution of the energy of a signal versus frequency.

Spectrum (Frequency Spectrum)

The amplitude of sound at various frequencies. It is given by a set of numbers that describe the amplitude at each frequency or band of frequencies.

An instrument for measuring, and usually recording, the spectrum of a signal.

A measure of sound clarity that indicates the ease of understanding speech. It is a complex function of psycho acoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment.

The plus/minus sigma values evaluated on each point of the curves. See Statistics chapter of SoundCheck Manual for formula.

Short Term Fourier Transform

The quality of a sound that distinguishes it from other sounds of the same pitch and volume. The distinctive tone of an instrument or a singing voice.

The response speed of the detector in a sound level meter. For Slow response, the response speed is 1 second. Slow time weighting is frequently used in environmental sound measurements. Fast response time is 1/8 second (0.125). This is less frequently used, but will detect changes in sound levels more rapidly. Fast and Slow time weightings were developed, in part, to slow needle movement (called a "decay" factor) in analog meters so investigators could read and record sound levels. New digital sound level meters, while changing numbers rapidly on the screen, store sound level data in memory for later analysis, thus, the ability to read numbers on the screen is less important. Hence, the most accurate "weighting" is none. Generally, 1-second Leq data are appropriate; however, when measuring sudden onset sound events such as sonic booms, more frequent data (many readings per second) may be appropriate.

A short signal used in acoustical measurements to make possible differentiating desired signals from spurious reflections.

Refers to a device adding harmonics that were not in the original signal. For example: a device that is fed a 20Hz sine wave that is also putting out 40Hz, 80Hz, etc. Not usually a factor in most modern electronics, but still a significant design problem in loudspeakers.

The ability of a component to respond quickly and accurately to transients. Transient response affects reproduction of the attack and decay characteristics of a sound.

Instantaneous changes in dynamics, producing steep wave fronts.

Sounds or a frequency higher than 20,000 Hz.

The unit of electrical or acoustical power. 1 watt = 1 joule per second

The unit of power used to rate the output of audio amplifiers. For a wattage number to have meaning the distortion level and impedance must also be specified.

A particular type of disturbance that travels through a medium by virtue of the elastic properties of that medium.

Wavelength is the distance a wave travels in the time it takes to complete one cycle. A wavelength can be measured between successive peaks or between any two corresponding points on the cycle. Wavelength (ft) = Speed of Sound (ft) / Frequency (Hz). (speed of sound at sea level is 331.4 meters/second or 1087.42 feet/second).

Adjustment of the unweighted frequency response to account for a given human psycho acoustic

Noise with a continuous frequency spectrum and with equal power per unit bandwidth. For example, equal power in any band of 100-Hz width.