Paul McGowan writes:
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.