Illusion Audio Nd-15
www.illusionaudio.com
A unique design and an incredibly shallow mounting
depth make Illusion Audio's Nd-15 a sub to consider.
by Brian Smith
The Nd-15 is a rather unique 15-inch woofer that features a carbon fiber cone and a rubber surround. It's uniqueness lies in the fact that the woofer's motor structure is mounted on the front of the driver rather than the rear. This allows a dramatically shallow mounting depth of about 4 inches. The Nd-15 was delivered for testing in a sealed enclosure of about
2.5 cubic feet.

Subjective
Inverted motor structure or not, one of my favorite combinations in a car is a 15-inch woofer in a sealed box. However, not all 15's are created the same. Most would probably be more at home in a stadium rig than in a car. Big heavy cones are great if you have a large space to fill with low frequency energy, but in a car, all that extra mass tends to cause mush. The better sounding 15-inch car systems tend to have light cones and a narrow impedance peak that occurs relatively high in the frequency band. (We'll look at the impedance curve in the objective section, but you won't find many materials that are lighter for a given strength than carbon fiber.) The only weakness that we encountered during our listening tests was a tendency to compress just a bit on very high level sub-bass passages. Otherwise, the Nd-15 sounded great, and was both transiently and tonally accurate. All that and a 4-inch mounting depth? Sweet.

Objective
Impedance measurements on the Nd-15 reveal a maximum of 83 ohms and a minimum of 4.3 ohms over its usable range. These values occur at 56 Hz and 20 Hz, respectively. Average impedance measured 15.1 ohms. Out-of-car measurements show a smooth roll-off below system resonance. In-car measurements show a 1-watt sensitivity of 101 dB at 10 Hz and very smooth, extended response.

The 300-watt trace (top) shows a maximum SPL of 123.3 dB at 11.5 Hz and about 3 dB of power compression. It's a fact that all speakers will exhibit power compression at some output level.

*Note: Ever since cone speakers have been marketed, it has been normal practice to cover the voice coil with a minimum of at least a felt cloth. This seems to have been forsaken in this design (perhaps for aesthetic reasons), so a word to the wise -- watch how you handle this puppy during the install. Be cautious about chips, especially metal ones, and, above all, be sure to cover this thing with a fine mesh grill cloth to prevent foreign particles from lodging in the voice coil.
Price & Contact: $699; 602-966-3455; www.illusionaudio.com