Loudspeaker time-alignment usually simply referred to as "time-alignment" or "Time-Align" is a term applied in loudspeaker systems which use multiple drivers (like woofer, mid-range and tweeter) to cover a wide audio range. It involves delaying the sound emanating from one or more drivers (greater than 2-way) to correct the transient response, improve accuracy and, in non-coaxial drivers, improve the directivity or lobe tilting at the crossover frequencies. It employs adjusting the front-to back spacing of the individual drivers so that the sound output is truly simultaneous.
In 1975 Ed Long [1] in cooperation with Ronald J. Wickersham invented the first technique to Time-Align a loudspeaker systems. In 1976 Long presented "A Time-Align Technique for Loudspeakers System Design" [2] at the 54th AES convention demonstrating the use of the Time-Align generator to design improved crossover networks for multi-way loudspeakers systems. This technique relied on subjective evaluation of various square pulses swept through the crossover frequencies. The Time-Align Generator locked the pulse on an oscilloscope so that it could be viewed. The Time-Align technique was employed by Long on the UREI 813 Studio monitor [3] introduced in 1977. Long also manufactured near field studio monitors from the late 1970s through the 90's utilizing the Time-Align technique. In 1977 Long trademarked Time-Align and later trademarked its derivatives, Time-Aligned and Time-Alignment. (Must include the dash). Long Licensed the Time-Align trademark to UREI, Bag End Loudspeakers and others.
This section possibly contains original research .(September 2018) |
Since hi-fi audio requires that the loudspeaker be capable of faithful reproduction of the recorded material, it follows that a loudspeaker that better covers the audio spectrum will have better hi-fi performance. Therefore, most hi-fi loudspeakers employ multiple drivers to cover the audio spectrum satisfactorily.
At the very least, such a loudspeaker may be 2-way employing a woofer (or mid-range/mid-woofer) and a tweeter. Higher end loudspeakers may be 3-way or even 4-way. For the sake of this article and simplicity, a 2-way speaker system will be assumed - consisting of a woofer and a tweeter. Since the woofer covers the lower-end of the audio spectrum and the tweeter covers the upper-end, the dividing point between the two being the crossover frequency, it is of utmost importance that, at the crossover frequency the outputs of both drivers should acoustically sum so as to be seamless, without any peaks or dips, otherwise the loudspeaker is said to colour the sound.
A typical characteristic of a 2-way speaker is that at the crossover frequency, due to the physical distance between the centres of the woofer and tweeter, the sound that emanates from the combination is not omni-directional, but lobed. Within the region of the lobe the sound level, at the crossover frequency is much higher as compared to outside the lobe. Therefore loudspeaker designers try to make the main lobe as fat as possible, by using drivers that are as small as possible in diameter so as to allow closer spacing between them. However, the lowest frequency of interest (bass) puts a lower limit to the woofer's diameter. Therefore, such speakers will always have lobing.
A typical 2-way speaker uses a woofer and a tweeter as mentioned earlier. Usually the tweeter is much smaller and thinner than a woofer. What this means is that the radiating surfaces (at the voice-coil or the dome/dust-cap depending on design, also known as the "acoustic centre") of the drivers are not in the same plane - the tweeter's radiating surface is usually much forward of the woofer when both are mounted to the same flat panel. Even though this physical offset may be of the order of 20-40mm, at typical crossover frequencies (≥ 1 kHz), this offset is enough to cause tilting of the main lobe. The image below shows this:
The woofer is the larger of the two, located below the tweeter. This is the most commonly used 2-way configuration. As can be seen the tweeter is much thinner and its acoustic centre is ahead of the woofer's acoustic centre. Due to this, at the crossover frequency since both drivers are producing the same frequency, the sound waves from the tweeter arrive at the listening position P earlier than those from the woofer. Due to this, at P there is non-ideal summing of the waves (they might reinforce or cancel out). Thus the main lobe points away from P towards a position P' (which for this particular speaker is lower than P). [4]
For most purposes the tilted lobe poses no problems and indeed many speaker systems do not use time alignment. However, there is a type of crossover called the LR4 or LR2 crossover, which has certain unique features that make time-alignment worthwhile for speakers that use it. This particular crossover has the property that at the crossover frequency the electrical summing is flat (i.e., there is no peak or dip) and the signals being sent to the woofer and tweeter are always in phase (180° out of phase in the LR2 case, which is corrected by simply inverting the tweeter's signal). When used with a loudspeaker that is time-aligned, the loudspeaker's main lobe now points exactly forward (i.e., straight) and does not have a 3dB peak in response. This makes the LR2 or LR4 crossovers ideal for audio as compared to the Butterworth type. Even without a LR crossover, it is worthwhile to have the main lobe point forward so that the speakers will illuminate the listening position evenly, resulting in better overall system performance (such as imaging or audibility). [5]
In this technique (usually) the tweeter's signal is phase-shifted (usually a delay since tweeter is ahead of the woofer). This phase shift introduces an equivalent time shift in the tweeter's sound wave such that it corrects the time difference between the woofer and tweeter due to the physical offset. With a variable phase-shift filter, it becomes very easy to time align almost any speaker without having to physically alter anything. This method is also much easier and convenient over physically time-aligning the drivers. [6] However, acoustic measurement is almost always required in this case, since the phase adjustment cannot be done by hearing alone.
In this technique the drivers are physically offset such that their acoustic centres lie in the same physical plane. This technique is used when no other means of time alignment are available or meant to be used. It simplifies setup for the end user in that they do not need any special electronics to align the drivers. However, this technique requires that the exact depths of the acoustic centres be known at design time, so that the physical offset may be introduced in the front panel of the speaker where the drivers mount. [6]
A common way to do this is so that the front-panel has a "step" (as shown in the above image) where the tweeter mounts at some distance behind the woofer. This step can cause more errors in summing than the time delay between the drivers due to the diffraction of the tweeter's sound waves around the step. [7] Sloping and rounding the edges of the step helps in reducing diffraction, but it cannot be eliminated completely. Also, the more gradual the slope, greater is the vertical separation between the drivers, which in turn again causes thinning of the lobe (i.e., increase in vertical directivity) at the crossover frequency.
Another way to introduce physical time-alignment without having to physically shift the tweeter backwards is to tilt the speaker itself upwards (or have the front-panel sloping instead of vertical). This method will cause the physical on-axis plane itself to be tilted upwards - so it virtually brings the physical plane in line with the required on-axis plane. However, now the listening position is off-axis relative to either driver at all frequencies. [7] This is the simplest of all methods (especially tilting the speaker itself upwards) in that it can be done for any speaker and lends itself more easily to setting up the speakers by trial-and-error.
A loudspeaker is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A speaker system, also often simply referred to as a speaker or loudspeaker, comprises one or more such speaker drivers, an enclosure, and electrical connections possibly including a crossover network. The speaker driver can be viewed as a linear motor attached to a diaphragm which couples that motor's movement to motion of air, that is, sound. An audio signal, typically from a microphone, recording, or radio broadcast, is amplified electronically to a power level capable of driving that motor in order to reproduce the sound corresponding to the original unamplified electronic signal. This is thus the opposite function to the microphone; indeed the dynamic speaker driver, by far the most common type, is a linear motor in the same basic configuration as the dynamic microphone which uses such a motor in reverse, as a generator.
Audio crossovers are a type of electronic filter circuitry that splits an audio signal into two or more frequency ranges, so that the signals can be sent to loudspeaker drivers that are designed to operate within different frequency ranges. The crossover filters can be either active or passive. They are often described as two-way or three-way, which indicate, respectively, that the crossover splits a given signal into two frequency ranges or three frequency ranges. Crossovers are used in loudspeaker cabinets, power amplifiers in consumer electronics and pro audio and musical instrument amplifier products. For the latter two markets, crossovers are used in bass amplifiers, keyboard amplifiers, bass and keyboard speaker enclosures and sound reinforcement system equipment.
A tweeter or treble speaker is a special type of loudspeaker that is designed to produce high audio frequencies, typically deliver high frequencies up to 100 kHz. The name is derived from the high pitched sounds made by some birds (tweets), especially in contrast to the low woofs made by many dogs, after which low-frequency drivers are named (woofers).
A mid-range speaker is a loudspeaker driver that reproduces sound in the frequency range from 250 to 2000 Hz.
A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 20 Hz up to 80 Hz. The name is from the onomatopoeic English word for a dog's bark, "woof". The most common design for a woofer is the electrodynamic driver, which typically uses a stiff paper cone, driven by a voice coil surrounded by a magnetic field.
An electrostatic loudspeaker (ESL) is a loudspeaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field.
A horn loudspeaker is a loudspeaker or loudspeaker element which uses an acoustic horn to increase the overall efficiency of the driving element(s). A common form (right) consists of a compression driver which produces sound waves with a small metal diaphragm vibrated by an electromagnet, attached to a horn, a flaring duct to conduct the sound waves to the open air. Another type is a woofer driver mounted in a loudspeaker enclosure which is divided by internal partitions to form a zigzag flaring duct which functions as a horn; this type is called a folded horn speaker. The horn serves to improve the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material and the less-dense air. The result is greater acoustic output power from a given driver.
Rudolph Thomas Bozak (1910–1982) was an audio electronics and acoustics designer and engineer in the field of sound reproduction. His parents were Bohemian Czech immigrants; Rudy was born in Uniontown, Pennsylvania. Bozak studied at Milwaukee School of Engineering; in 1981, the school awarded him an honorary doctorate in engineering. Bozak married Lillian Gilleski; the two had three daughters: Lillian, Mary and Barbara.
A full-range loudspeaker drive unit is defined as a driver which reproduces as much of the audible frequency range as possible, within the limitations imposed by the physical constraints of a specific design. The frequency range of these drivers is maximized through the use of a whizzer cone and other means. Most single driver systems, such as those in radios, or small computer speaker designs, cannot reproduce all of the audible frequencies or the entire audible audio range.
A loudspeaker enclosure or loudspeaker cabinet is an enclosure in which speaker drivers and associated electronic hardware, such as crossover circuits and, in some cases, power amplifiers, are mounted. Enclosures may range in design from simple, homemade DIY rectangular particleboard boxes to very complex, expensive computer-designed hi-fi cabinets that incorporate composite materials, internal baffles, horns, bass reflex ports and acoustic insulation. Loudspeaker enclosures range in size from small "bookshelf" speaker cabinets with 4-inch (10 cm) woofers and small tweeters designed for listening to music with a hi-fi system in a private home to huge, heavy subwoofer enclosures with multiple 18-inch (46 cm) or even 21-inch (53 cm) speakers in huge enclosures which are designed for use in stadium concert sound reinforcement systems for rock music concerts.
Studio monitors are loudspeakers in speaker enclosures specifically designed for professional audio production applications, such as recording studios, filmmaking, television studios, radio studios and project or home studios, where accurate audio reproduction is crucial. Among audio engineers, the term monitor implies that the speaker is designed to produce relatively flat (linear) phase and frequency responses. In other words, it exhibits minimal emphasis or de-emphasis of particular frequencies, the loudspeaker gives an accurate reproduction of the tonal qualities of the source audio, and there will be no relative phase shift of particular frequencies—meaning no distortion in sound-stage perspective for stereo recordings. Beyond stereo sound-stage requirements, a linear phase response helps impulse response remain true to source without encountering "smearing". An unqualified reference to a monitor often refers to a near-field design. This is a speaker small enough to sit on a stand or desk in proximity to the listener, so that most of the sound that the listener hears is coming directly from the speaker, rather than reflecting off walls and ceilings. Monitor speakers may include more than one type of driver or, for monitoring low-frequency sounds, such as bass drum, additional subwoofer cabinets may be used.
Bi-amping and tri-amping is the practice of using two or three audio amplifiers respectively to amplify different audio frequency ranges, with the amplified signals being routed to different speaker drivers, such as woofers, subwoofers and tweeters. With bi-amping and tri-amping, an audio crossover is used to divide a sound signal into different frequency ranges, each of which is then separately amplified and routed to separate speaker drivers. In Powered speakers using bi-amping, multiple speaker drivers are in the same speaker enclosure. In some bi-amp set-ups, the drivers are in separate speaker enclosures, such as with home stereos that contain two speakers and a separate subwoofer.
Acoustic suspension is a method of loudspeaker cabinet design and utilisation that uses one or more loudspeaker drivers mounted in a sealed box or cabinet. Acoustic suspension systems reduce bass distortion that can be caused by stiff motor suspensions in conventional loudspeakers.
An electrodynamic speaker driver, often called simply a speaker driver when the type is implicit, is an individual transducer that converts an electrical audio signal to sound waves. While the term is sometimes used interchangeably with the term speaker (loudspeaker), it is usually applied to specialized transducers which reproduce only a portion of the audible frequency range. For high fidelity reproduction of sound, multiple loudspeakers are often mounted in the same enclosure, each reproducing a different part of the audible frequency range. In this case the individual speakers are referred to as drivers and the entire unit is called a loudspeaker. Drivers made for reproducing high audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers, and those for low frequencies are called woofers, while those for very low bass range are subwoofers. Less common types of drivers are supertweeters and rotary woofers.
KEF is a British company specialising in the design and production of a range of high-end audio products, including HiFi speakers, subwoofers, architecture speakers, wireless speakers, and headphones. It was founded in Maidstone, Kent in 1961 by a BBC engineer Raymond Cooke OBE (1925–1995).
A coaxial loudspeaker is a loudspeaker system in which the individual driver units radiate sound from the same point or axis. Two general types exist: one is a compact design using two or three speaker drivers, usually in car audio, and the other is a two-way high-power design for professional audio, also known as single-source or dual-concentric loudspeakers. The design is favored for its compactness and behavior as an audio point source.
The midwoofer-tweeter-midwoofer loudspeaker configuration was a design arrangement from the late 1960s that suffered from serious lobing issues that prevented its popularity until it was perfected by Joseph D'Appolito as a way of correcting the inherent lobe tilting of a typical mid-tweeter (MT) configuration, at the crossover frequency, unless time-aligned. In the MTM arrangement the loudspeaker uses three drivers: Two mid-range for the low frequencies and a tweeter for the higher frequencies, with the tweeter being placed between the mid-range drivers. D'Appolito initially configured his design using a 3rd order crossover, D'Appolito has since amended this original recommendation in favor of 4th order topology. However, this does not impart any significant effect on the MTM design's unique characteristics.
Edward M. Long was an audio engineer known for introducing the first near-field studio monitors and the first Time-Aligned loudspeaker crossover.
Acoustic lobing refers to the radiation pattern of a combination of two or more loudspeaker drivers at a certain frequency, as seen looking at the speaker from its side. In most multi-way speakers, it is at the crossover frequency that the effects of lobing are of greatest concern, since this determines how well the speaker preserves the tonality of the original recorded content.
Veritone Minimum Phase Speakers, or VMPS, was a loudspeaker manufacturer founded in 1977 by speaker designer Brian Cheney. Many VMPS speakers received favorable reviews from audio critics, such as the RM40, which was awarded Best of CES in the High-End Audio category in 2002. VMPS was in operation for over 35 years, from January 1977 to December 2012, when it closed soon after the death of company owner Brian Cheney on December 7, 2012.