How do passive radiators work?
Since purchasing my pair of SDA-2s, I have been wondering about how they achieve such great bass response without a powered sub-woofer. I know that it has something to do with the passive radiator, but I am having a hard time comprehending how. Does anyone here what to take a jab at explaining it to a humble layman, ideally with an obscure metaphor.
-M
-M
Post edited by mhanig on
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Passive radiator (speaker)
From Wikipedia, the free encyclopedia
Passive radiator enclosure with front mounted passive radiator; back or side mounting is also used
A speaker enclosure using a passive radiator usually contains an "active driver" (or main driver), and a passive radiator (also known as a "drone cone"). 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.
This following paragraph is entirely incorrect and was written by someone who does not understand how a passive radiator works: [Contrary to popular belief, the passive radiator will not 'increase' the volume or add to the sound pressure produced by the speaker assembly. Instead it will actually detract from it, much as having a second driver out of phase with the main one. This is because as the main driver cone moves backward, it increases air pressure in the cabinet and moves the passive radiator cone forward (and vice versa) - in the opposite direction to the main driver's cone. This means that audio produced by the active driver - particularly lower frequencies - will be canceled out, reduced in level, and made more difficult to localize, all due to destructive interference of the sound waves. Hence cabinets which include a passive radiator will perform poorly with respect to sound amplitude, compared to a single driver in a cabinet. This effect can be observed by holding the cone of a passive radiator still while playing a tack containing lower frequency notes through the active driver. When the cone is allowed to move, the sound will be less prominent. When it is stopped the same, louder sound that would be produced by a single driver is heard.]
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They work exactly like the (tuned) port in a bass reflex cabinet (Helmholtz resonator) - the mass of the PR is tuned to match the mass of air in the equivalent resonant system.
A bass reflex cabinet and a jug with an open mouth are both Helmholtz resonators and their physics are identical. -
I'll try to keep this simple....pr has no magnet or voice coil....basically work on air pressure of active drivers...hense the word passive.. Designed into systems for low frequency response
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Help me understand this: when the active driver pushes forward, the passive pulls backwards, and vice versa. Doesn't this produce out of phase waves that cancel out?Monitor 5 Series II
RTA 11T -
Help me understand this: when the active driver pushes forward, the passive pulls backwards, and vice versa. Doesn't this produce out of phase waves that cancel out?
Nope......
All you need to know
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So insightful Ivan, lol.
@Nabman , it would seem as simple as that, but you forget that air is compressible, and therefore there will be a delay in reaction between the active driver and the PR. Along with the compression delay, there's also the mass of the PR to take into account, and Newton's 2nd law. -
Remember that the PR serves the same physical purpose as the volume (mass) of air contained by the volume of a port in a Helmholtz resonator
Actually, I believe, that, at the port resonance (and, thus -- again, I believe -- at the PR resonance), the sound wave at the port is in phase with the output of the driver: http://audiojudgement.com/bass-reflex-speaker-design/
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At frequencies on either side of resonance, things get kinda complicated, phase-wise.
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There's magic goin on and I like it when the designers get it right!
I love the sound from Polk audio's Monitor series from the past with all those passive radiator designed speakers.
Most people just listen to music and watch movies. I EXPERIENCE them.
“Anger is an acid that can do more harm to the vessel in which it is stored than to anything on which it is poured.”
--Mark Twain.
“If at first you don't succeed, then skydiving definitely isn't for you.” - Steven Wright -
There's magic goin on and I like it when the designers get it right!
I love the sound from Polk audio's Monitor series from the past with all those passive radiator designed speakers.
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Magic science
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Most people just listen to music and watch movies. I EXPERIENCE them.
“Anger is an acid that can do more harm to the vessel in which it is stored than to anything on which it is poured.”
--Mark Twain.
“If at first you don't succeed, then skydiving definitely isn't for you.” - Steven Wright -
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Fluid coupling... reminds me, I need to change my speaker fluid. Kidding of course. But I have wondered if you could fill with another gas (nitrogen perhaps) and have a perfect seal, how it might improve the PR performance. Or, if you could maintain a slightly positive pressure to reduce delay.Stan
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Magic science
Like little fairy's flying around in there pushing on the PR to make sure they stay in phase. Got it!Gustard X26 Pro DAC
Belles 21A Pre modded with Mundorf Supreme caps
B&K M200 Sonata monoblocks refreshed and upgraded
Polk SDA 1C's modded / 1000Va Dreadnaught
Wireworld Silver Eclipse IC's and speaker cables
Harman Kardon T65C w/Grado Gold. (Don't laugh. It sounds great!)
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more like Maxwell's Demons.
Re: changing speaker fluid... it's getting harder and harder to find places that'll accept it for environmentally responsible recycling and not charge an arm & a leg for it. Me? I just walk way out in the woods and dump it by the decomposing carcass of a 1949 Chevy.
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Of course, one of the advantages of a PR vs. a port is the elimination of sounds (chuffing, etc.) from the port of a loudspeaker. Many people have expressed their displeasure over such unwanted acoustic artifacts.
The proliferation of PR designs in the 1970s seem to have been a response to the high incidence, in the 1960s, of...
wait for it...
Port Noise Complaints.
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^^^^
*rimshot*
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mhardy6647 wrote: »Remember that the PR serves the same physical purpose as the volume (mass) of air contained by the volume of a port in a Helmholtz resonator
Actually, I believe, that, at the port resonance (and, thus -- again, I believe -- at the PR resonance), the sound wave at the port is in phase with the output of the driver: http://audiojudgement.com/bass-reflex-speaker-design/
Passive radiator is no different than a port.
Mass of Radiator and length of port determine tuning frequency,
Diameter of port and diameter of radiator determine the airflow.
Passive radiator, loses deep bass at a faster rate below tuning frequency, compared to a port.
Radiator loses some efficiency compared to port and will be limited by excursion of cone surround.
Port can have chuffing noise IF too small in diameter. -
Passive radiator is no different than a port. .
Not true. Ports are WAY cheaper to manufacture. Diameter and length of a port determine range of frequencies to pass through and it is also affected by cabinet volume.Mass of Radiator determines tuning frequency
Not really. A super light PR will have a broader frequency range. As you add mass you essentially choke out it's ability to reproduce higher frequencies. It also reduces it's efficiencyDiameter of port and diameter of radiator determine the airflow.
Nope. Airflow is determined by the drivers used.Passive radiator, loses deep bass at a faster rate below tuning frequency, compared to a port.
Really? Have you ever heard of Polk Audio? My SDA 1C's dip down to 15 hz and the SRS2's that use the exact same number and size of drivers and a larger and heavier PR has a frequency response that goes down to 12 hz AND are more efficient than the 1C
s. Besides have you EVER heard of a speaker with a port that bests 12 hz and 92 db efficiency?Port can have chuffing noise IF too small in diameter.
Hey! You got one right! Sweet! Congratulations!
Gustard X26 Pro DAC
Belles 21A Pre modded with Mundorf Supreme caps
B&K M200 Sonata monoblocks refreshed and upgraded
Polk SDA 1C's modded / 1000Va Dreadnaught
Wireworld Silver Eclipse IC's and speaker cables
Harman Kardon T65C w/Grado Gold. (Don't laugh. It sounds great!)
There is about a 5% genetic difference between apes and men …but that difference is the difference between throwing your own poo when you are annoyed …and Einstein, Shakespeare and Miss January. by Dr. Sardonicus
