I’m an audiophile and home theatre enthusiast. I keep hearing the term “passive radiator” and when I google it everything just goes over my head. Can someone explain like I’m 5?
In: Technology
A/V Equipment generates heat. The heat needs to be removed from the device to prevent it from overheating and damaging itself. Some systems use cooling fans. But cooling fans generate noise, which is not ideal for a device that is supposed to deliver audio or video to a quiet room. A passive radiator is a system which radiates (distributes) the heat away from the device without the use of fans or other active cooling measures.
It’s a speaker that doesn’t have any of the electromagnetic parts. It is placed in the same housing as a normal speaker and is moved by the sound vibrations coming from that speaker through the frame of the box and the air inside it. It’s effectively an additional way to tweak the sound and the electricity-to-sound conversion efficiency beyond just changing the shape of the speaker box. Basically simulates the better bass sounds of a much larger speaker housing.
A radiator here on Earth, surrounded by a fluid such as air or water, transfers the heat via radiation (where hot surfaces just emit Infra-Red) or by convection, where the heat moves from the radiator into the air its in contact with.
A non-passive radiator has a fan that moves the air around so that cooler air moves in and takes the hot air’s place. They work well but can be a source of noise. The movement of the air and the fan moving it make noise.
A passive radiator has enough surface area to put enough of the heat into the air without using a fan. They’re silent. But they have to be LARGE compared to an actively-cooled radiator for the same amount of performance.
The wiki page has a good diagram [here](https://en.m.wikipedia.org/wiki/File:Passive_radiator_enclosure.svg).
It’s a cone without the coil. It doesn’t produce sound on its own, but sitting in a box with an active speaker, it will resonate alongside the main speaker. It does a similar thing to a port/vent, helps the bass escape.
Imagine you want to light up a room, so you put in some light bulbs, which are like speakers in a sound system. A passive radiator is like adding mirrors to light up dark corners of the room that the light bulbs don’t reach, or adding filters that change the color of the light. Passive sound radiators can change where the sound will go, and also resonate with certain sound ranges.
I can’t offer an ELI5 explanation, but it is acoustically equivalent to a port. A port is a device which uses a resonating mass of air in a pipe to augment the sound coming out of the front of the speaker by using the out-of-phase radiation in the rear of the speaker. This resonance constructively combines with the sound in the bass frequencies. Ports tend to make noise if the speaker is playing very loud, low frequencies, and to solve this problem, you need to make the diameter of the port larger, but this then requires that the port be longer. It’s very easy to end up needing a port that is too long and too voluminous to practically fit in the box.
A passive radiator replaces that resonating mass of air in the port with a resonating speaker cone, very similar in construction to any other woofer, but generally having additional weights. These weights lower the frequency at which this cone resonates (the cone is attached to a ‘spring’ in the form of the spider and surround of the passive radiator).
It is relatively easy to make a passive radiator which can output more bass with less extraneous noise than a practically sized port could. It is probably the best way to augment bass in a high-end speaker, with the biggest downside being the cost compared to a plastic or cardboard tube.
High end passive radiators are engineered to perform like woofers, they just lack an electrical motor structure to generate movement on their own, but similar attention is paid to the stiffness of the cone, the excursion and output potential of the radiator, the stiffness of the chassis, and the resonances of all of these components.
One way of making a speaker is to place your driver in a ported box, which basically turns your speaker into a tube with a driver at one end and a hole in the other end.
Depending on how long you make the “tube”, you can change the resonant frequency of the speaker; this is the same as how a trombone gets longer and shorter and this changes the pitch. Changing the length of the tube, and so the resonant frequency of the box can be helpful to, for example, increase the bass response of your speaker.
A passive radiator is a basically a speaker “hack” to do the same thing. In a speaker with a passive radiator, instead of having a hole in the box, we completely seal the box, we then add a springy diaphragm to one part of the box; something that keeps the box air sealed but can still move in and out.
As the pressure inside the box goes up and down from your driver moving back and forth, the pressure will push the passive radiator in and out. By adjusting the properties of the passive radiator, such as it’s mass and how springy the diaphragm is, we can adjust the resonant frequency of our sealed box, which again can be used to change the sound, such as increasing the bass.
Passive radiators are often useful if your trying to keep the size of the box small, but want to boost the bass, as to do this with a ported speaker would need a long “tube” and so would need a big box.
I read “passive redditors” and was also curious, because I’ve only met “aggressive redditors”
It’s a mechanism to use the “backstroke” of a speaker to produce more sound.
In a sealed box or an infinite baffle you’re just isolating the two sides of the speaker to prevent the backwave from canceling what the front of the speaker is doing.
In a ported box, you use a column of air pushed by the backwave to reinforce certain bands of the sound produced by the front.
A passive radiator works similarly to a port except instead of pushing the column of air in the port, you’re pushing an unpowered speaker.
So the passive radiator is passive in that it is not powered by an amplifier, it has no motor structure, but it responds to the pressure created by the backwave of the powered speaker to reinforce a certain range of frequencies.
With ports you tune which frequencies are reinforced by changing the size of the air column, ie the volume of the port.
With a passive radiator you tune it by altering the mass of the radiator, they generally have a stud for attaching weights.
Ok so imagine you have a box that the speaker lives in.
This box is totally sealed. No air in or out
Now install another speaker in the box…but don’t plug it in.
It will move with the pressure changes of the other speaker and produce sound “passively
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