Passively radiating

Yesterday 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 McGowan – PS Audio, Intl

Ports ahoy

I am in the beginning of a new series that will give my readers an overview of starting fresh with a new sound room: the steps I am taking to build the room, the decisions being made and why.

I mentioned that the Maggies I am leaving and the Infinity IRS I am adding to the sound room are both dipole loudspeakers and a number of you asked me to explain in a little more detail what dipoles are, why I like them and what their issues are.  I will delve into that subject briefly for you but first let’s cover all the speaker types broadly.

The vast majority of loudspeakers are what we call monopoles; meaning they have one radiating surface area, or pole.  A dipole has two – one out the front and one out the rear – each out of phase with the other.  Another loudspeaker type we might have seen is a Bipole, which is a sealed box in-phase version of a dipole – where the front and rear of the Bipole radiate in phase with each other creating a type of spherical wave front.

Monopole sealed box systems, those that cover the lion’s share of all loudspeakers, are for the most part not really sealed because in the bass regions many have a port, passive radiator or tuned transmission line and then there’s sometimes a rear facing tweeter as well – but let’s start with the bass port.

Ports, passive radiators and transmission lines are holes in sealed boxes that let a calculated amount of bass out of the box, relieving some of the internal pressure and generally increasing the low bass level in the room.  I am not a fan of holes in sealed boxes, no matter how well done they are done, but let’s understand the basic types and then move on.

The most common “hole in the box” is called a port.  Ports usually have a port tube associated with them, which is nothing fancier than a PVC pipe used in plumbing.  The tube helps tune the port to the right bass frequency and, if it’s fancy, it may have a smooth flare at its output to reduce “chuffing” noises (sometimes referred to as farting noises).  These ports can be found in every price point from insanely expensive Wilson Audio speakers to the lowest cost speakers on the planet.  The port generally gives better, lower bass response in the room.  What comes out of the port is bass energy from the rear of the woofer – thus that energy is out of phase with the front of the woofer.

Ports are used a great deal because they really work and, in the hands of an excellent speaker designer, can be very effective.  If you want to know why I don’t like ports, put your ear up to a port while the speaker is working and give it a listen.  True enough that chuffing and puffing sound from the port ends up integrating into the room nicely, but you gotta wonder how anything that sounds that a-musical could ever be a good thing.

Tomorrow, passive radiators.


Paul McGowan – PS Audio, Intl.