A speaker enclosure using a passive radiator (PR) usually contains an "active loudspeaker" (or main driver), and a passive radiator (also known as a "drone cone"). The active loudspeaker is a normal driver, and the passive radiator is of similar construction, but without a voice coil and magnet assembly. It is not attached to a voice coil or wired to an electrical circuit or

power amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...

. Small and Hurlburt have published the results of research into the analysis and design of passive-radiator loudspeaker systems. The passive-radiator principle was identified as being particularly useful in compact systems where vent realization is difficult or impossible, but it can also be applied satisfactorily to larger systems. Logitech Z4 satellite 1

In the same way as a ported loudspeaker, a passive radiator system uses the sound pressure otherwise trapped in the enclosure to excite a resonance that makes it easier for the speaker system to create the deepest pitches (e.g.,

bassline Bassline (also known as a bass line or bass part) is the term used in many styles of music, such as blues, jazz, funk, Dub music, dub and electronic music, electronic, traditional music, traditional, or classical music for the low-pitched Part ( ...

s). The passive radiator resonates at a frequency determined by its mass and the springiness (compliance) of the air in the enclosure. It is tuned to the specific enclosure by varying its mass (e.g., by adding weight to the cone). Internal air pressure produced by movements of the active driver cone moves the passive radiator cone. This resonance simultaneously reduces the amount that the woofer has to move.

Design considerations

Passive radiators are used instead of a reflex port for several reasons. In small-volume enclosures tuned to low frequencies, the length of the required port becomes very large. They are also used to reduce or eliminate the objectionable noises of port turbulence and compressive flow caused by high-velocity airflow in small ports. In addition, ports have pipe resonances that can produce undesirable effects on the frequency response. To a first-order approximation, the passive radiator works identically to a port. Passive radiators are tuned by mass variations (M_mp), changing the way that they interact with the compliance of the air in the box. The weight of the cone of the passive radiator should be approximately equivalent to the mass of the air that would have filled the port which might have been used for that design. If the passive radiator's acoustic mass equals that of the port, and the PR's compliance is negligible, then the frequency response behaviour of these two types of systems will be virtually identical. Although the frequency response of a passive radiator will be similar to that of a ported cabinet, the system low-frequency roll-off will be slightly steeper (5th-order rather than 4th-order), due to a

notch Notch may refer to: * Notch (engineering), an indentation or slit in a material * Nock (arrow), notch in the rearmost end of an arrow * Markus Persson (born 1979), a Swedish game designer known by his online alias "Notch", best known for creatin ...

(dip) in the frequency response caused by the V_ap (compliance or stiffness) of the passive radiator. This notch occurs at the PR's free-air resonant frequency and causes slightly poorer transient response. Despite this, perhaps due to the lack of vent turbulence and vent pipe resonances, some listeners prefer the sound of PRs to reflex ports. PR speakers are only slightly more complex to design and are generally more expensive as compared to standard bass reflex enclosures.

Applications

Passive radiators are used in Bluetooth speakers, home stereo speakers, subwoofer cabinets and

car audio Vehicle audio is equipment installed in a car or other vehicle to provide in-car entertainment and information for the vehicle occupants. Until the 1950s it consisted of a simple AM radio. Additions since then have included FM radio (1952), 8 ...

speaker systems, particularly in cases where there is not enough space for a port or vent system. While most

studio monitor speaker 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 ...

s are either ported bass reflex designs, or closed-back without a vent or passive radiator, Mackie's HR824 and HR624 monitor speakers have a passive radiator installed on the rear of the cabinet. Focal also sells a studio monitor with a passive radiator called the SM9.
Respective examples of a smart speaker and a portable Bluetooth Speaker utilizing passive radiators are Apple's HomePod mini and Ultimate Ears'

UE Boom UE Boom is a portable speaker manufactured by Ultimate Ears, supporting Bluetooth and wired connections. UE Boom has been praised for its industrial design, loudness, battery life, speakerphone capability, and its suitability for outdoor use, but ...

References

{{Reflist , refs= {{cite journal , first=R. H. , last=Small , title=Passive-Radiator Loudspeaker Systems Part 1: Analysis , journal=Journal of the Audio Engineering Society , volume=22 , number=8 , pages=592–601 , year=1974 , url=http://www.aes.org/e-lib/browse.cfm?elib=2739 {{cite journal , first=R. H. , last=Small , title=Passive-Radiator Loudspeaker Systems Part 2: Synthesis , journal=Journal of the Audio Engineering Society , volume=22 , number=9 , pages=683–689 , year=1974 , url=http://www.aes.org/e-lib/browse.cfm?elib=2731 {{cite journal , first=D. H. , last=Hurlburt , title=The Complete Response Function and System Parameters for a Loudspeaker with Passive Radiator , journal=Journal of the Audio Engineering Society , volume=48 , number=3 , pages=147–163 , year=2000 , url=http://www.aes.org/e-lib/browse.cfm?elib=12074 Loudspeaker technology Audio engineering

Design considerations

Applications

See Also

References