Absorbs energy because sound can enter its porous surface to be dissipated by being reflected off the material's fibers. In this mechanism, sound energy is converted into a minute amount of heat within the absorbing material.
Acoustic Power Level (PWL)
The total sound energy radiated by a source per unit time. The unit of measure is the acoustic Watt. Also known as sound power.
The amplitude of the carrier voltage is caused to vary directly with the modulating voltage.
Amplitude-Frequency Response (Magnitude Response)
The variation of gain, loss, amplification, or attenuation a function of frequency.
At extreme volumes, when the voice coil smacks into the back of the pole plate or the spider hits the top plate, it creates a very unpleasant sound.
An electric circuit element composed of two metallic plates separated by a dielectric, used to store electric charge. Note: Capacitance also reacts to AC. While an inductor becomes more reactive at high frequencies, the capacitor becomes more reactive at low frequencies.
By definition, the output device(s) in a Class A amplifier is always conducting or “on” and provides output current during the complete signal cycle (over a 360-degree interval). Because the device is always turned on, the theoretical maximum efficiency is only 12.5% for a resistively biased single-ended output stage, to 25% for a current source biased output stage, to 50% for a “push-pull” output stage. Also, maximum efficiency is only approached at maximum output; at low output the efficiency is much worse.
In Class B operation, the output device only conducts for 180 degrees of the signal cycle, so it is only on or conducting half of the time. To make a useful audio amplifier two Class B devices must be used in a “push-pull” configuration to generate output over the full 360 degree waveform cycle. The amplifier’s output devices are connected to positive and negative voltage supplies and are driven alternately to “push” current and “pull” current from the loudspeaker. A small forward bias on the output devices must be adjusted precisely to turn on and cutoff exactly at the zero crossing of the waveform. Too little bias and the amplifier will conduct for less than 180 degrees and slip into class-C operation, resulting in offensive “crossover distortion”. Too much bias and the amplifier will conduct more than 180 degrees, dissipate more power and also increase distortion. The higher efficiency of the Class B amplifier is due to the fact that when one output device is conducting the other is cutoff and therefore no power dissipated by the cutoff transistor. Theoretical maximum efficiency is about 78.5%.
In between class A and class B operation is class AB. The output devices are biased to that the output devices are conducting for more than 180 degrees of the signal cycle but less than 360 degrees. Some amplifiers are heavily overbiased to give a few or several Watts of class A operation at low output power. When the amplifier output is increased, it slips into class B operation, where only one of the output devices is conducting and the other is cutoff. The theoretical maximum efficiency falls in between class A and class B (50 -78.5%). Actually, most consumer grade class B amplifiers are actually marketed and described as class AB amplifier to avoid the negative connotations of class B amplifiers that persists from the early days of transistor amplifiers where thermal drift and immature transistor technologies produced many amplifiers that exhibited very audible crossover distortion.
The occurrence of short term overload in an amplifier. (Geek version: A flat-topped deformation as viewed on and oscilloscope of the waveform on the output of an amplifier.) There are two ways an amplifier can clip: the desired output voltage exceeds the amplifiers power supply voltage (aka “the rails”) or the amplifiers power supply cannot deliver the amount of current needed to maintain the desired voltage on the output.
Comb filtering is the spatially perceived constructive and destructive interference between two like sounds, one being delayed relative to the other, usually due to a physical separation of two speakers or drivers operating that are driven in the same frequency band.
The crossover is a circuit that divides, shapes and allocates the high and low frequencies to different drivers. A crossover is used when a speaker system has more than one driver. It keeps the drivers operating in their most accurate frequency range.
Current is movement of electrical charge. It is assumed that electrons (which have a negative electrical charge) travel from a negative to a positive potential. Engineers however use the convention that current flows from positive to negative – a convention that predates the discovery of protons and electrons. In semiconductor physics, the concept of “holes” moving through a semiconductor can also describe current flow. (Holes are places that an electron can fill or vacate.) Regardless of how good the electronics are, there is always some random movement of electrons that cause electrical noise. Current is measured in Amperes, or amps for short. Note: current flow is analogous to water moving through a hose.
Decibel notation is a convenient way to express very large differences in numbers, but also corresponds closely with our perception of sound. A figure in decibels can describe the magnitude of the difference in power, voltage or sound pressure, etc. If two powers PI and P2 differ, their difference can be expressed in decibels as dB= 10 log (P I/P2). It is meaningless to state absolute values of power in decibels. The decibel represents a ratio, and therefore a reference level must also be stated if absolute values are required. For example "a power of20 dB" is meaningless, but "20 dB relative to 1 Watt" means 100 watts. In most cases the reference level is implicitly understood. A good example would be sound pressure level where the reference level (0 dB SPL) is always understood to be the threshold of hearing. Note: Fundamentally, the Bel (after Alexander Graham Bell) is defined as the logarithm to the base 10 of a power ratio. However, the Decibel is a more convenient unit to use, and is defined as 10 times the logarithm to the base 10 of a power ratio (I Bel = 10 decibels).
Diffraction is the bending of sound waves around an object and the re-radiation of energy through an opening. Diffraction makes it possible to hear sounds around comers and behind walls, or through a small opening in an obstruction.
Diffusion is a scattering of sound. A diffuser converts a sound traveling in one direction into many lower-amplitude sounds moving in many directions. Note: An irregularly shaped surface tends to diffuse sound, provided that the irregular structures are similar in size to the maximum wavelength of the sound striking them.
Usually presented graphically, it is a function of the radiation of the transmitted or incident sound wave in a specified plane and at a specified frequency.
The difference, in decibels, between the overload level and the minimum acceptable signal level in a system or transducer. Note: The minimum acceptable signal level of a system or transducer is ordinarily fixed by one or more of the following: noise level, low-Ievel distortion, or interference.
Efficiency is the percentage of electrical power converted by the loudspeaker into acoustical power. Note: it is incorrect to use the term efficiency in place of sensitivity.
The time rate of repetition of a periodic phenomenon. The frequency is the reciprocal of the period. The unit of measure is cycles per second, or Hertz.
A process of changing the frequency of a tone (called a carrier tone). The difference frequency relative to the carrier tone varies directly with the information signal (or modulating voltage).
Frequency components which are an integral multiple (1,2,3,4,etc., times) of the fundamental frequency.
A standard unit of frequency that equals one cycle per second.
Parameter used by speaker engineers to characterize how much current the speaker will draw from an amplifier. Low impedance means correspondingly higher current. Impedance is the opposition to current flow in an AC circuit, specified in ohms. Impedance is to an AC signal what resistance is to DC. Impedance differs from resistance in that impedance implies that the load is not a simple resistance, but a combination of resistance, inductive reactance, and capacitive reactance.
A circuit element, i.e., a coil of wire, in which electromagnetic induction generates electromotive force. Note: This induced voltage opposes the applied voltage, reducing current flow (in other words, a “frequency-dependent resistor". The higher the alternating current frequency, the greater the induced voltage and the greater the opposition to the applied voltage.
This is defined as the sound pressure level of a 1000Hz tone that sounds as loud as the sound in question. The unit of measure is the phon. Note: Extensive measurements have been made to determine the loudness of pure tones and narrow bands of noise as a function of frequency and sound pressure level. The resulting equal loudness contours are known as the Fletcher-Munson curves.
To regulate some parameter of a high-frequency carrier wave by means of the lower-frequency information signal.
The interval between two frequencies having a ratio of 2: 1.
The amount of current flow is determined by two things: the resistor's resistance value and the supply voltage. More resistance will decrease current flow. Conversely, a higher voltage will increase current flow. The relationship is expressed by Ohm 's Law, which states that: V=IxR Note: If we know any two values, we can calculate the third using Ohm 's Law.
Circuits provide multiple paths through which current can flow. Parallel circuits differ from series circuits in that the voltage across each branch of a parallel circuit is the same as the applied voltage.
Phase as applied to loudspeakers describes a time difference in the incidence or arrival of a sound wave. Phase can also describe how reactive a load is compared to the load's resistance. A phase difference (measured in degrees and known as “phase angle”) is introduced because capacitive reactance causes voltages to lag slightly behind current flow in time. In an inductor, the opposite occurs, current lags voltage.
Power is an expression of the rate at which energy is transferred, or the rate at which work is done by the voltage and current. The unit of measure is the Watt. Power is the product of voltage and current. To calculate the power produced by an audio waveform, you need to know voltage and current, voltage and load impedance, or current and load impedance. To calculate power you need the RMS values: take the square root of the arithmetic average of a set of squared instantaneous values of the voltage and/or current; aka: Root Mean Square or RMS. Despite being widely used, there is no such thing as RMS power; “Watts RMS” is a misnomer. When you multiply Volts RMS and Current RMS you just get Watts, not “Watts RMS”.
The variation of sound power as a function of frequency.
Hard surfaces which do not allow sound to penetrate the surface; nearly all energy is reflected back toward the source. Sound is reflected at the same angle at which it struck; i.e., "the angle of incidence equals the angle of reflection." (Think billiards.)
Resistance is the opposition to current flow. It reduces the number of electrons flowing through the conductor. The higher the resistance, the less current flow in the conductor (given the same voltage). The unit of electrical resistance is the ohm. Note: Resistance is analogous to a kink in a water hose when water is moving through it.
1) The persistence of sound in an enclosed space, as a result of multiple reflections after the sound source has stopped. 2) The sound that persists in an enclosed space, as a result of repeated reflections or scattering, after the source of the sound has stopped.
Sensitivity is expressed as the sound pressure level with 1-Watt input measured 1 meter away. Note: Technically, sensitivity should be expressed with a constant drive signal of about 2.83 V; which corresponds to 1 Watt into an 8 ohm load.
In a series circuit, there is only one path through which current can flow. In a series circuit, the current remains the same while the voltage across each circuit component is different.
Simple Point Source
A source that radiates sound uniformly in all directions under free-field conditions.
The non-uniform distribution of current flow in a cable. The overall effect is current tends to flow near the surface of the conductor. Because the current is confined to a smaller cross section of the conductor, the apparent resistance of the conductor Increases.
Sound is produced by variations in air pressure around the average steady-state barometric pressure, caused by the physical movements of objects and their surface-contact in the air. The alternating variations of pressure at a sound source cause sound waves to radiate out from the source in a similar manner as waves are caused in water by a dropped stone. The waves travel with a certain velocity of propagation, which depends on the medium through which they are traveling (air, water, metal, etc.), and they transmit energy at a certain rate (expressed in watts). Sound pressure levels are typically stated in terms of average pressure variations (RMS) about the barometric level.
The average rate of sound energy transmitted in the specified direction through a unit area.
Sound Pressure Level (SPL)
The sound pressure level of a sound, in decibels, 20 times the logarithm to the base 10 of the ratio of the measured effective sound pressure of this sound to a reference effective sound pressure. When sound pressure levels (SPL) are stated, they are usually quoted in decibels above the average threshold of hearing level.
Speed of Sound (C)
The speed of sound in air is approximately 1132 ft/sec.
A periodic wave having a fixed distribution in space which is the result of interference of progressive waves of the same frequency and kind. Characterized by the existence of maxima and minima amplitudes that are fixed in space.