Passive radiator (speaker)
Encyclopedia
A speaker enclosure
using a passive
radiator usually contains an "active driver
" (or main driver), and a "passive cone" (or drone or passive radiator). The active driver is a regular driver, and the passive is typically the same or similar, but without a voice coil
and magnet assembly. It is only a suspended cone, not attached to a voice coil or electrical circuit. The passive radiator usually has some means to adjust its mass
, thereby allowing the speaker designer to change the box tuning. Internal air pressure produced by movements of the active driver cone moves the passive radiator cone as well.
Passive radiators are used instead of a reflex port for much the same reasons—to tune the small volume and driver for better low frequency performance. Especially in situations in which a port would be inconveniently sized (usually too long for practical box configurations). They are also used to eliminate port turbulence and reduce motion compression caused by high velocity airflow in small ports (especially small diameter ones). Passive radiators are tuned by mass variations (Mmp), changing the way their compliance interacts with motion 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. Passive radiators do not behave exactly as do (more or less) equivalent bass-reflex
designs in that they cause a notch in system frequency response at the PR's free air resonant frequency; this causes a steeper roll-off
below the system's tuned frequency Fb, and poorer transient response. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers, however, are more complex to design and likely to be more expensive as compared to standard reflex enclosures.
The frequency response of a passive radiator will be similar to that of a ported cabinet, with two exceptions. The system low frequency roll-off in a passive radiator design will be slightly steeper, and will have a notch
(dip) in frequency response
due to the Vas (compliance, or stiffness of the speaker cone) of the passive radiator. The goal in designing a passive radiator is to adjust the tuning so that this notch is below audible levels.
Loudspeaker enclosure
A loudspeaker enclosure is a purpose-engineered cabinet in which speaker drivers and associated electronic hardware, such as crossover circuits and amplifiers, are mounted...
using a passive
Passivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
radiator usually contains an "active driver
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
" (or main driver), and a "passive cone" (or drone or passive radiator). The active driver is a regular driver, and the passive is typically the same or similar, but without a voice coil
Voice coil
A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
and magnet assembly. It is only a suspended cone, not attached to a voice coil or electrical circuit. The passive radiator usually has some means to adjust its mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
, thereby allowing the speaker designer to change the box tuning. Internal air pressure produced by movements of the active driver cone moves the passive radiator cone as well.
Passive radiators are used instead of a reflex port for much the same reasons—to tune the small volume and driver for better low frequency performance. Especially in situations in which a port would be inconveniently sized (usually too long for practical box configurations). They are also used to eliminate port turbulence and reduce motion compression caused by high velocity airflow in small ports (especially small diameter ones). Passive radiators are tuned by mass variations (Mmp), changing the way their compliance interacts with motion 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. Passive radiators do not behave exactly as do (more or less) equivalent bass-reflex
Bass reflex
A Bass reflex system is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.A reflex port is the distinctive feature of a...
designs in that they cause a notch in system frequency response at the PR's free air resonant frequency; this causes a steeper roll-off
Roll-off
Roll-off is a term commonly used to describe the steepness of a transmission function with frequency, particularly in electrical network analysis, and most especially in connection with filter circuits in the transition between a passband and a stopband...
below the system's tuned frequency Fb, and poorer transient response. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers, however, are more complex to design and likely to be more expensive as compared to standard reflex enclosures.
The frequency response of a passive radiator will be similar to that of a ported cabinet, with two exceptions. The system low frequency roll-off in a passive radiator design will be slightly steeper, and will have a notch
Band-stop filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter...
(dip) in frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
due to the Vas (compliance, or stiffness of the speaker cone) of the passive radiator. The goal in designing a passive radiator is to adjust the tuning so that this notch is below audible levels.