the Midnight Monitor Oil
by Eddie Cilletti, continued from page 1
MARGIN OF SAFETY
upon a time, I was fast-winding an analog tape past what should have been
the end of any recorded material. A sudden screech and then an eerie silence
jolted me back to consciousness too late to pull my finger away from the
heads. A 1-amp fast-blow fuse did not react in time to protect all of
my beloved Radio Shack Minimus 7s—four in a passive-surround configuration.
At least they were fused. Fig. 2 shows the distorted bobbin after the
voice coil overheated. The two surviving tweeters, which were made in
Japan, didn’t sound anything like the two new replacements from Malaysia.
Yeah, I know these are cheap consumer-grade speakers. My “real” monitors
are by Dynaudio Acoustics, modelBM15A.
All wire has resistance
(stated in ohms per foot and referenced to room temperature) that increases
when hot and decreases when cold. This applies to voice-coil windings
as well, so when you continuously blast those boom boxes, heat makes the
coil less effective at delivering transients. If you listen at loud levels
for an extended period of time, the monitors will sound dull and floppy
from fatigue. Your ears get tired, too.
There is one, quite
remarkable, “solution,” called Ferrofluid (www.ferrofluidics.com), a magnetic
material suspended in a viscous liquid designed to fill the magnetic air
gap in which the voice coil is suspended. The fluid provides three benefits:
greater transfer of magnetic energy, “hydraulic” damping of the resonant
diaphragm and the transfer of heat from the coil to the magnet assembly.
It’s not hard to see, and hear, that Ferrofluid makes a driver more efficient,
less colored and better equipped to deliver transients even when driven
sample pack included fluids for tweeters through subwoofers, including
one optimized for midrange and high-frequency compression drivers. I was
willing to sacrifice a Minimus 7 or two to see how much magic I could
conjure out of them.
Fig. 3 shows the powerfully focused magnet assembly attracting the fluid from its container. It took some experimenting to achieve an optimum, repeatable mic placement.
Fig. 4a shows a 500Hz square wave through an unmodified tweeter, captured with an AKG C-300 with an omni capsule and a Great River transformerless mic preamp. Fig. 4b shows the same tweeter’s response after Ferrofluid treatment. Notice that the amount of overshoot decreases with treatment.
That’s all the room
I have for this month. Look for Part Two in the next issue. Ciao!