Passively radiating
Join Our Community Subscribe to Paul's PostsYesterday we covered the basics of ports in speaker boxes that would otherwise be called sealed boxes were it not for the hole and associated tube punched into the speaker box to extend the bass.
Port calculations used to be a black art but today the science is well known and there are many online port calculators that take the complicated mathematics and make it easy to implement for designers. By far, the port is the most common means of extended bass response in a cabinet.
The second most common bass extender for loudspeakers is called a passive radiator. The first passive radiator I ever saw was in a Polk loudspeaker. Polks and several other speakers of the day all used passive radiators for extended bass response. Many still use this technique today, Thiel, Definitive and Golden Ear come to mind.
A passive radiator is actually nothing more than a fancy port, but if I were designing a loudspeaker for sale and was willing to accept a hole in my speaker box to extend the bass (which I would not), then a passive radiator or a folded transmission line would definitely be on the top of my list relative to a port.
A passive radiator is a woofer without a motor. The motor of a loudspeaker driver is made up of a large magnet and a voice coil. The voice coil is an electro magnet – meaning if you put electricity into it you create a magnetic field – and when you power the voice coil with music from your power amplifier, it pushes away from and is drawn towards the large permanent magnet on the back of the driver. The woofer cone is what is moving back and forth – and the only part of the woofer you actually see – and it is this back and forth movement of the woofer cone that pressurizes the air in the room to get sound.
A passive radiator is the cone without the voice coil and magnet – covering a big hole in the cabinet. The radiator is placed in the same box as the woofer itself is and, when the woofer starts to move back and forth to make bass, the rear pressure of that woofer pushes the passive radiator in and out to get increased sound.
One might think that if the passive radiator is being driven by the rear of the main woofer that it would be out of phase and thus cancel, instead of boost but as mentioned yesterday, it’s actually one full cycle out of time and thus it reinforces. The radiators do their most where the woofer actually does the least – right at what’s known as box resonance – the point where the woofer is making the least amount of bass.
By controlling the mass or weight of the passive radiator’s moving surface, the designer can control how this works and the bass can be pretty effective. Thiels use this technique to great advantage as well.
Tomorrow the folded transmission line.
Paul I’m having a problem with your terminology. A port is an opening. Loudspeaker enclosure descriptions generally refer to how the bass driver problem is handled. The problem; without some way to prevent it, a woofer’s rear radiation which is out of phase with the front radiation will find its way around the driver to cancel it out. The lower the frequency the greater this effect. Ported enclosures can’t be sealed, it’s a contradiction. What I think you are referring to is the midrange and tweeter rear radiation. If they’re in the same box and not protected from the high pressure the woofer creates internally, their performance will be compromised. That’s why most midrange and tweeter drivers have a built in rear enclosure to protect their backs. If not they need to be in a sealed sub enclosure or separate enclosure.
The passive radiator must be in a sealed enclosure. If there were an opening, pressure from the rear of the driven woofer would not move the passive cone, the opening would allow air from the back of the driven woofer to escape and coupling between the two drivers would be poor. But even in an acoustic suspension woofer/enclosure there must be a small opening if only a pin hole to allow changes in external air pressure to equalize the internal pressure or the speaker would become a manometer altering the neutral point of the driver.
Well, obviously if the passive radiator was out of phase there’d be bass cancellation – which actually the opposite is true. The radiator is in the same enclosure as the woofer but it is tuned such that it work just like a port – where there is a rear phase reversal placing it out of time by one cycle.
Actually it is out of time by the speed of sound divided by the path length difference which is the sum of the distance between the passive radiator and the front wall plus the distance from the front wall to the active woofer. Typically this is about 5 or 6 milliseconds. The reflection off the front wall inverts the phase without delaying it in time just the way it does with a rear port or an open baffle design. The difference is that the passive radiator more linearly loads the back pressure of the active woofer than a port. That is, it is less frequency dependent. The overall system can have a lower Q.
Oops, its the path length difference (distance) divided by the speed of sound. Must have been thinking upside down :-)
Paul I have a question: if I was building a sealed cabinet for a guitar amp and wanted to use a complete speaker as a passive radiator would it work or would the magnetic field created by the motor reduce the effectiveness?
As long as the terminals of the driver were disconnected it shouldn’t matter at all.
thank you for the response
Paul,
When was the passive radiator introduced, and by whom? Was it JBL with their S8R (R for radiator)? That was a great sounding speaker, but the bass did not sound as precise as it did with their big horn-loaded speakers. Is less precise and clean bass a tradeoff one gets with passive radiators?
Some of the cleanest, most effortless, best bass I ever heard (but without the terrific bass extension you get with subwoofers) was from the professional 4 woofer horn loaded cabinets Ampex had JBL make for the 70mm film process Todd-AO. There were 5 of them behind the screen (5 independent channels + one surround) in some of the theaters United Artists and Magna equipped for Todd-AO). Current movie bass sounds muddier — but stronger on the very bottom — by comparison.