Passive Radiator

[M6 Driver]

So help me understand prior to the arrival. (cross fingers)

I understand that a rear ported speaker needs to be X inchs from the wall.

So what about a passive radiator does it need follow the same principle?

[Ray Garrison]

Simply put - the distance from the wall will change the sound. Experiment and find what sounds best to you.

To go a bit longer - there are two (at least) different effects here relating to the closeness to the wall. If you're talking about a matter of inches or less, you get into a low pass filtering effect. As you go up in frequency, the angle of dispersion of soundwaves decreases. With a large driver (say a 10 or 15 inch passive radiator) the angle begins to narrow at a fairly low frequency. This means that if you place a big passive radiator close to a wall (say and inch or two away) the lowest frequencies, which are emitted in pretty much an omnidirectional way, won't be severely restricted by the close wall. However, as the the frequency increases, less and less of the sound is radiated "sideways" out of the opening between the speaker and wall, and more and more is "beamed" into the wall and then reflected out. (VMPS uses this to reduce the amount of high frequency distortion emitted by their passive radiators by "slot loading" them.) Whether this means that the frequency response of the passive radiator begins to drop as the frequency goes up or not depends on the wall. If the speaker is sitting in front of a leather covered, upholstered, tapestry covered library wall, then the response will drop percipitously as the frequency goes up (uh, keeping in mind that we're talking solely about the passive radiator here.) If the speaker is sitting in front of a steel plate, it won't change very much at all, due to this effect...

The other effect is sound reflecting off the wall reinforcing or cancelling the direct output. This comes into play when we're talking about longer distances. For example, if the passive radiator extends up to, say, 100 Hz with some output, that's a wavelength of about 132 inches. If the speaker is about 5 feet (uh, 66 inches) from the wall, the total distance from direct radiator to wall and back is 132 inches. The direct and reflected sound are exactly in phase. Soundwaves will reinforce each other. You'll get a nasty response peak at 100 Hz. Drums will sound boomy, male voices will sound very "chesty". If the speaker was placed 33 inches from the wall, the total distance from radiator to wall and back to radiator is 66 inches. This is exactly one half wave. The direct sound and reflected sound would be exactly out of phase (180 degrees out of phase) and would cancel. You'd get a big null at 100 Hz. No output. Kick drums would sound really weak. Move the speaker to 16 inches from the wall, and the round trip is 32 inches. A quarter wave. Now you get neither perfect cancellation or reinforcement. You'll actually add a pretty hefty amount of 2nd order harmonic distortion to the primary sound. Things will sound (guessing here?) warmer and mellower, with a "golden overtone"...

Anywho, the only real way to find out what sounds best is to realize that speaker to wall distance *DOES* make a big difference, and experiment. A lot. With patience. Inches can make a big difference.