Phase plug

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Diagram of a compression driver. The phase plug is shown in dark purple. Phase plug and compression driver.gif
Diagram of a compression driver. The phase plug is shown in dark purple.

In a loudspeaker, a phase plug, phasing plug or acoustical transformer is a mechanical interface between a speaker driver and the audience. The phase plug extends high frequency response because it guides waves outward toward the listener rather than allowing them to interact destructively near the driver. [1]

Contents

Phase plugs are commonly found in high-powered horn loudspeakers used in professional audio, in the mid- and high-frequency bandpasses, positioned between the compression driver diaphragm and the acoustic horn. They may also be present in front of woofer cones in some loudspeaker designs. In each case they serve to equalize sound wave path lengths from the driver to the listener, to prevent cancellations and frequency response problems. The phase plug can be considered a further narrowing of the horn throat, becoming an extension of the horn to the surface of the diaphragm. [2]

History

An electromechanical driver of the sort later used in loudspeakers was invented by German industrialist Werner von Siemens in 1877 but no practical amplification existed to create the loudspeaker until 1921. [3] Various loudspeaker designs were produced in the 1920s, including General Electric engineers Chester W. Rice and Edward W. Kellogg mating an acoustic horn to the speaker driver in 1925. [4] In 1926, Bell System engineers Albert L. Thuras and Edward C. Wente modified the horn loudspeaker by inserting the first phase plug between the driver and the horn. [5] This phase plug directed sound waves into the horn throat from the center of the diaphragm and from a ring around the perimeter of the diaphragm, by way of center hole and annular slot, for the purpose of improving "the transmission characteristics" of the loudspeaker "at the upper portion of the sound frequency range." [6] Based on their joint research, the two engineers were awarded consecutive U.S. patents: Thuras filed a patent for a novel electrodynamic diaphragm design, and Wente filed a patent for the first phase plug. [6] [7] The principles laid out by Thuras and Wente have influenced every subsequent phase plug design. [8]

Compression drivers

Two types of dome-type phase plug: one with radial slits and one with concentric ring slits, also called annular or circumferential Slot vs circumferential phase plug.svg
Two types of dome-type phase plug: one with radial slits and one with concentric ring slits, also called annular or circumferential

In horn loudspeakers, the phase plug serves to carry sound waves out from all areas of the compression driver diaphragm through the compression chamber to the horn throat such that each pulse of sound reaches the throat as one coherent wave front. [9] With a successful implementation, high-frequency performance is extended higher. [10]

The phase plug is a complex and expensive element of the compression driver. [5] Its manufacture requires fine tolerances. Phase plugs are machined in metals such as aluminum, or cast in hard plastic or Bakelite. [10] Meyer Sound Laboratories chose a lightweight plastic because of its resistance to temperature and humidity. [11]

Many variations exist in phase plug design, but two types have evolved to match two major diaphragm types: dome and ring.

Dome-based diaphragms are similar to the 1920s Thuras/Wente patents, and are still in common use today. Phase plugs that interface with dome-type diaphragms include a wide variety: designs with radial slots, designs with concentric annular ring slots, and hybrid designs with a combination of annular and radial slots. Altec engineer Clifford A. Henricksen reported on the differences between radial and "circumferential" types of phase plugs at Audio Engineering Society conventions in 1976 and 1978. [12] [13] The radial design is easier to produce, but it does not differentiate between sound waves from the perimeter of the diaphragm and sound waves from the center. At high frequencies, the diaphragm does not act as a perfect piston; instead, it displays rippling, modal properties related to its stiffness and density. Because of the speed of wave propagation through the diaphragm material, the center of the diaphragm moves slightly later than the perimeter. Radial slots in the phase plug do not correct for this small time difference, which affects the highest frequencies. Concentric circular slots may be able to correct for the diaphragm's rippling behavior but the positioning of the slots is critical. Circular slots may allow resonances to build up between the diaphragm and the phase plug—resonances which cause wave cancellations and a corresponding reduction in frequency response at the resonance frequency. [5]

The less common ring diaphragm is a later development intended to minimize the problems related to wave propagation through the diaphragm material. This design requires a radically different shape of phase plug, but radial slots and concentric rings may still play a part. [5]

The combined area of the phase plug slots is typically about one-eighth to one-tenth of the area of the diaphragm. This gives a pressure-to-volume velocity change ratio in the range of 8:1 to 10:1, which serves to match the impedance of the diaphragm to the horn throat. [8] [14] A larger slot area admits more sound wave energy but also reflects more energy backward onto the diaphragm. A smaller slot area traps more wave energy between the phase plug and the diaphragm. In researching the diaphragm/phase plug interface, David Gunness found that only half the wave energy, at best, travels directly from the diaphragm through the phase plug slots and out to the listener. The other half (or more) causes cancellations within the space between the diaphragm and the phase plug, or causes temporal anomalies (time smear) upon leaving the phase plug later than the direct sound. To minimize the problem, Gunness modeled the behavior mathematically and used digital signal processing to apply a polarity-reversed version of the undesired wave behavior to the original audio signal. [15]

Woofers

Horn-loaded woofer showing a phase plug in black Horn-loaded woofer with phase plug.jpg
Horn-loaded woofer showing a phase plug in black

Phase plugs may be placed in front of woofer cones, especially in horn-loaded loudspeaker designs. In the same fashion as compression driver phase plugs, the intent is to minimize higher-frequency wave interference near the driver. In this case, "high frequency" is relative to the intended bandpass; for example, a 12-inch (300 mm) cone woofer might be expected to reproduce 550 Hz energy near the top of its intended range, however, the wavelength of 550 Hz is approximately twice the diameter of the woofer, so wave energy at that frequency traveling laterally from one side to the other will be out of phase and will cancel. With a phase plug in the center, such lateral wave energy bounces off of the obstruction and is reflected outward toward the listener. Phase plugs for woofer cones are typically solid plugs positioned over the woofer's central dust cap, or in the center of the woofer, replacing the dust cap. [16] [17]

Related Research Articles

Loudspeaker electroacoustic transducer that converts an electrical audio signal into a corresponding sound

A loudspeaker is an electroacoustic transducer; a device which converts an electrical audio signal into a corresponding sound. The most widely used type of speaker in the 2010s is the dynamic speaker, invented in 1924 by Edward W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an alternating current electrical audio signal is applied to its voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move rapidly back and forth due to Faraday's law of induction, which causes a diaphragm attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound. The sound source must be amplified or strengthened with an audio power amplifier before the signal is sent to the speaker.

Tweeter loudspeaker for high audio frequencies

A tweeter or treble speaker is a special type of loudspeaker that is designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz. Specialty tweeters can 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).

Mid-range speaker

A mid-range speaker is a loudspeaker driver that reproduces sound in the frequency range from 250 to 2000 Hz. It is also known as a squawker.

Woofer loudspeaker driver designed to produce low frequency sounds

A woofer or bass speaker is a technical term for loudspeaker driver designed to produce low frequency sounds, typically from 40 Hz up to 500 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.

Electrostatic loudspeaker Sound playback device

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.

Horn loudspeaker type of loudspeaker used in handheld megaphones and stationary speaker installations for public address

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.

Klipsch Audio Technologies is an American loudspeaker company based in Indianapolis, Indiana. Founded in Hope, Arkansas, in 1946 as 'Klipsch and Associates' by Paul W. Klipsch, the company produces loudspeaker drivers and enclosures, as well as complete loudspeakers for high end, high fidelity sound systems, public address applications, and personal computers.

JBL American audio hardware company

JBL is an American company that manufactures audio equipment, including loudspeakers and headphones. There are two independent divisions within the company; JBL Consumer produces audio equipment for the consumer home market, while the JBL Professional produces professional equipment for the studio, installed sound, tour sound, portable sound, and cinema markets. JBL is owned by Harman International Industries, a subsidiary of South Korean company Samsung Electronics.

Full-range speaker

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.

Loudspeaker enclosure Acoustical component

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" 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" or even 21" speakers in huge enclosures which are designed for use in stadium concert sound reinforcement systems for rock music concerts.

The Isobaric loudspeaker configuration was first introduced by Harry F. Olson in the early 1950s, and refers to systems in which two or more identical woofers operate simultaneously, with a common body of enclosed air adjoining one side of each diaphragm. In practical applications, they are most often used to improve low-end frequency response without increasing cabinet size, though at the expense of cost and weight.

The Air Motion Transformer (AMT) is a type of electroacoustic transducer. Invented by noted physicist and scientist Dr. Oskar Heil (1908–1994), it operates on a different transduction principle from other loudspeaker designs, such as moving coil, planar magnetic or electrostatically driven loudspeakers, and should not be confused with planar or true ribbon loudspeakers. In contrast to a planar ribbon loudspeaker, the diaphragm of the AMT is of pleated shape similar to a bellows. The AMT moves air laterally in a perpendicular motion using a metal-etched folded sheet made of polyethylene terephthalate (PET) film. The circuit path embossed on the PET membrane, acts as the voice coil unit. The diaphragm is then housed between 4 stacks of steel pole-plate pieces positioned at 45° within a high-intensity, quadratic, opposing magnetic field. The air motion transformer with its sheet film equally exposed at 180° behaves as a dipole speaker, exciting front and rear sonic waves simultaneously.

Acoustic suspension

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. Acoustic suspension was invented in 1954 by Edgar Villchur, and brought to commercial production by Villchur and Henry Kloss with the founding of Acoustic Research in Cambridge, Massachusetts Speaker cabinets with acoustic suspension can provide well controlled bass response, especially in comparison with an equivalently-sized speaker enclosure that has a bass reflex port or vent. The bass vent boosts low-end output, but at the tradeoff of introducing phase delay and accuracy problems. Sealed boxes are generally less efficient than a reflex cabinet, so a sealed box speaker cabinet will need more electrical power to deliver the same amount of acoustic bass output.

Compression driver type of loudspeaker

A compression driver is a small specialized diaphragm loudspeaker which generates the sound in a horn loudspeaker. It is attached to an acoustic horn, a widening duct which serves to radiate the sound efficiently into the air. It works in a "compression" mode; the area of the loudspeaker diaphragm is significantly larger than the throat aperture of the horn so that it provides high sound pressures. Horn-loaded compression drivers can achieve very high efficiencies, around 10 times the efficiency of direct-radiating cone loudspeakers. They are used as midrange and tweeter drivers in high power sound reinforcement loudspeakers, and in reflex or folded-horn loudspeakers in megaphones and public address systems.

Electrodynamic speaker driver specialized loudspeaker that reproduces a portion of the audible frequency range

A speaker driver is an individual loudspeaker transducer that converts an electrical audio signal to sound waves. While the term is sometimes used interchangeably with the term loudspeaker (speaker), 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.

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.

Altec Lansing Duplex Line of loudspeakers

DUPLEX was the trade name given by Altec Lansing to its line of coaxial loudspeakers, beginning with the first model 601 in 1943. However, the name was most commonly associated with the subsequent model 604 which was a seminal loudspeaker that became a milestone in loudspeaker development. Well over a dozen different models carried the Duplex name over a near 50-year period. The vast majority consisted of a high frequency (HF) compression driver mounted to the back of a large diameter paper cone low frequency (LF) driver. However, there were also a few models with small diameter LF cones and direct radiator tweeters.

David Gunness Loudspeaker engineer

David W. "Dave" Gunness is an American audio engineer, electrical engineer and inventor. He is known for his work on loudspeaker design, especially high-output professional horn loudspeakers for public address, studio, theater, nightclub, concert and touring uses.

Clifford A. Henricksen American inventor, American musician

Cliff Henricksen is a musician, inventor and audio technologist. He is self-taught as a musician with a graduate degree in mechanical engineering at Massachusetts Institute of Technology (MIT). Throughout his career Cliff has found innovative ways to apply engineering basics to electro acoustics and to audio technology as it applies to music and in particular to live music performance. He has invented and engineered a wide variety of technologies and products well known in the world of professional audio. Today he balances work in audio and work as a performing musician.

Veritone Minimum Phase Speakers American loudspeaker manufacturing company

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.

References

  1. ↑ "Phase plug". Pro Audio Reference. AES . Retrieved 2017-12-17.
  2. ↑ Davis, Don; Patronis, Eugene (2006). Sound System Engineering (3 ed.). Taylor & Francis US. pp. 284–285. ISBN   0240808304.
  3. ↑ "History and Types of Loudspeakers". Edison Tech Center. Retrieved February 15, 2013.
  4. ↑ Holmes, Thom (2006). The Routledge Guide To Music Technology. CRC Press. p. 179. ISBN   0415973244.
  5. 1 2 3 4 Graham, Phil (November 2012). "Speaking of Speakers: Understanding Compression Drivers: Phase Plugs". Front of House. Las Vegas: Timeless Communications.
  6. 1 2 U.S. Patent 1,707,545 "Acoustic Device". Edward C. Wente, assigned to Bell Telephone Laboratories. Applied for on August 4, 1926. Patent awarded on April 2, 1929.
  7. ↑ U.S. Patent 1,707,544 "Electrodynamic Device". Albert L. Thuras, assigned to Bell Telephone Laboratories. Applied for on August 4, 1926. Patent awarded on April 2, 1929.
  8. 1 2 Eargle, John (2003). Loudspeaker Handbook (2 ed.). Springer. pp. 173–179. ISBN   1402075847.
  9. ↑ Nathan, Julian (1998). Back-To-Basics Audio. Newnes. p. 120. ISBN   0750699671.
  10. 1 2 Ballou, Glen (2012). Electroacoustic Devices: Microphones and Loudspeakers. CRC Press. pp. 8–10. ISBN   113612117X.
  11. ↑ "How to Better the Best: The Development of Meyer Sound's High Drivers". Meyer Sound. Archived from the original on February 16, 2013. Retrieved February 16, 2013.
  12. ↑ Henricksen, Clifford A. (October 1976). "Phase Plug Modelling and Analysis: Circumferential Versus Radial Types". AES E-Library. Audio Engineering Society. Retrieved February 16, 2013.
  13. ↑ Henricksen, Clifford A. (February 1978). "Phase Plug Modelling and Analysis: Radial Versus Circumferential Types". AES E-Library. Audio Engineering Society. Retrieved February 16, 2013.
  14. ↑ Eargle, John; Foreman, Chris (2002). JBL Audio Engineering for Sound Reinforcement. Hal Leonard. pp. 125–126. ISBN   1617743631.
  15. ↑ Gunness, David W. (October 2005). "Improving Loudspeaker Transient Response with Digital Signal Processing" (PDF). Convention Paper. Audio Engineering Society. Archived from the original (PDF) on May 12, 2012. Retrieved February 16, 2013. Hosted by EAW.com
  16. ↑ Stark, Scott Hunter (1996). Live Sound Reinforcement: A Comprehensive Guide to P.A. and Music Reinforcement Systems Technology (2 ed.). Hal Leonard. p. 149. ISBN   0918371074.
  17. ↑ "Phase Plug Technology". Preference Audio. OEM Systems. 2010. Archived from the original on April 14, 2003. Retrieved February 16, 2013.
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