Q: Love the magazine! You always do such a great job explaining things. I'm curious how "loading" a sub works? And, if it works so well, why doesn't everyone do it?
Thank you in advance,Ryan SippelOtsego, WI
Loading is a term that describes using a speaker (or antenna, light, any number of other emitters) in an application where its surroundings affect how it perfoms. For example, a speaker that's designed and tested in a totally "free field" environment will sound dramatically different when reacting with the acoustic load from another object. If you were playing a subwoofer in a cabinet suspended at least 20' off the ground and away from any walls, you'd hear pretty much just the speaker/cabinet combination.
If you move the cabinet closer to a wall, energy that would spill around the back of the cabinet would now reflect off the wall to hopefully reinforce the bass from the speaker. This is called "half space" and you can try this at home by placing a home stereo speaker against the midpoint of a wall. Notice that as you back the speaker against the wall, the bass increases. Now, if you move the speaker along the wall into a corner, you'll hear another big change as the speaker is now in "quarter space." Drop it into the floor corner where the two walls and the floor meet, and you're in "eighth space." The bass will be stronger, but will also sound different. Put a megaphone-style cone over the front of the speaker and you now have a "horn-loaded" driver. In each case, you prevented stray energy from moving in directions other than where you can appreciate it.
In building a subwoofer cabinet for higher-impact situations, I like to fire the sub into the back of the rear seat. This acts as a load on the speaker, reducing the speaker's tendency to drone uncontrollably and making the bass appear to hit more quickly. The tradeoff is that it takes more amplifier power to do this. Having an obstacle in the way will cause a bit of backpressure that can help to damp the sub's resonance. However, with anything acoustic, you can take things too far and have a sonic mess as a result. The point here is that it isn't always ideal for the sub to play unrestricted in the general direction of the listener as if we're trying to get a natural "free field" character.
Here's where it gets interesting (or frustrating): The second you put a sub cabinet in a car, it's automatically loaded by some amount. In essence, you're sitting with a smaller speaker cabinet inside a larger speaker cabinet. The car will change pressure, causing the speaker to experience acoustic or pressure loading and the character of sound will change as well. Roll down the window and the loading of the speaker changes again! In our 20Hz Advanced Bass Design class, we connect test gear to the students' speaker enclosures and discover all the ways sound can be affected by design changes and surroundings. The whole objective is to get the speaker and cabinet combination to work best with the car to deliver the type of bass you want.
So to summarize the answer, everybody is using loading in some form, but only trained professionals know how to make it work to their advantage. Of course, some are just plain loaded.