Velodyne ULD-12 Owner's Manual page 8

lowfrequeneies.Even at i watt drive levels, man.y loudspeakers haveS to 10%THDat 40Hz. With the 50 watt
drive levels needed to produce realistic bass intensity,these distortion figures rise to astonishing levels. Second,
the distortion products produced byanonlinearwoofer aremuch easier to hear than is thefundamental.fre-
queney. Consider a speaker driven at 30 Hz with 5% Srdharmonic distortion. If the 30 Hz tone is 90 dB SPL
(Sound Pressure Level), equating to aboutSwatts intOa.typicaf woofer,the Srd harmonic; at 90 HZ,would be
down 26dB,.or at64 dB SPL. However,humansare more than 10 times as sensitive to sound at 90 Hz than at 30
Hz. Thehonlineardistortion produced by a woofer must be weighted by the sensitivity functionofthehuman ear.
When this is done,it becomes clear that even a small percentage of harmonic distortion is unacceptable in a bass
driver .. ltis·alsoclearthat high-order (3rd,.4th, 5th, etc.) harmonic distortion products are more seriousthan·are
2nd harmonic distortion products. These stUdies have . led to the conclusion that 0.5% Is the maximum tolerable
amountofnonlil1ear distortion for a bass driver, andthe Jess distortion the driver produces, the more capable it is
of accurately reproducing live sounds.
Since most woofers today have at least 10% THD when 0.5'% Is the maximum tolerated, minor improvements
in woofer technology are essentially ineffective. Servo Series Subwoofers have about 90 times less nonlinearity
than a typical 10 inch bookshelf speaker. They have about S times less distortion due to their greater size and
power handling capability, and about 30 times less distortion due to the Servo feedback loop. The improvement is
evengreaterifoneconsidersexceptionally.loudan(j deep tones such as organ pedal notes.
An interesting result ofthis reduced distortion isthatrnuch of the bass, when played over a servo Series
Subwoofer, seems to actually be reduced. The truth, of course, is that the "missing" bass was never really there
on the source material. and was never intended to be played by the artist. Distortion products introduced by
conventional woofers actually make the entire bass spectrum sound louder.
Our Solution
The Velodyne Servo Series Subwoofer System produces sound in the same manner as any conventional
loudspeaker: a cone-shaped piston is forced to mOve by means ofaHnearmotorcomposedof a coli situated
between the poles of a very strong permanent magnet In the Servo Series,however, this linear motor is a servo-
motor, meaning thatthe motor is linked to the power amplifier as part of a control system. The basis tor the control
action is the signal obtained. from an accelerometer which is rigidly fastened to the. voice coiL The strategy of
continuously monitoring the cone motion, and using this information as the basis for the corrective action, is called
motional feedback. This feedback technique is illustrated in the system block diagram, Figure 4.
The signal into the system is a time varying voltage whichisan exact analog of the desired acoustic output
from thetoudspeaker. The accelerometer output contains suffioientinformation to deduce the actual acoustical
output oHhe woofer. The servo circuitry compares almost instantaneously how well the acoustic output matches
the command signal coming in. If and when there is a deviation, the servo circuitry modifies the drive signal to the
amplifier so thatthe acoustic output from the speaker is a true representation of the input.
In conventional loudspeakers, the drive signal into the speaker is virtually free from distortion butthe
speaker's acoustical outputmay nehighly distorted. In Servo Series Subwoofers, the speaker coil drive signal is
often highly distorted,oractually pre-distorted,inexactly such a way that the acousticalotJtput from the speaker is
free from distortion. Motional feedback has been used prevfously in an attempt to make small woofers sound like
larger,more powerful ones. At Velodyne,the use of motional feedback is applied to make what is already a large
and powerful woofer achieve a level of performance that has never before been reached in a home entertainment
product.
HGS System Components
High and Low Pass Crossovers receive the incoming audio Signals from your system pre-amp or power amp.
Frequencies above 85 Hz are diverted to your power amplifier while frequencies from the right and left channels
below Hz, are summed to monaural and passed on to subsequent stages in the subwoofer.
loop/Control Stage. Incoming bass signals pass through a variable gain amplification stage designed to protect
the subwoofer fromneingoverdriven (see Supporting Circuits below). With a bit of added filtering, the bass signal
is now ready to be compared with the feedback signal of the accelerometer. A composite signal is created which
compensates for erroneous movement of the cone.
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