Term-LAB Precision
Sensors VS Microphones
CONDUCTIVE ENERGY - Competition vehicles these days are loud. Really
loud. When a competitor "burps" their system, everything
inside of the vehicle vibrates. This includes traditional mic stands and the
newer, custom made mic holders that attach to the windshield. This vibration
adversely affects the accuracy of the measurement system because the mechanical
energy from the vibration is coupled to the microphone diaphragm. To understand
why this takes place, you need to think back to what you learned in high school
physics. One of
You can verify this
yourself. Take a microphone and measure the SPL of a paint shaker. Next, clamp
your microphone to the paint shaker and turn it on. You will see a significant
difference in readings. Next, try the same test on a Term-LAB system. You will
notice that the Term-LAB readings are not significantly affected. This is
because Term-LAB doesn't really have any moving parts. (It uses an integrated
strain-gauge on the substrate of a silicon die.)
TEMPERATURE
- Microphone diaphragms are significantly affected by temperature. I don't know
if any of you play the drums, but I can tell you that when a drum head gets
cold, it gets very tight. When it gets warm, it gets loose. This is because
temperature affects the tension of the drum head. Tension variations of the mic
diaphragm will adversely affect SPL measurements.
Term-LAB has integrated
temperature compensation. Temperature variations of more than 200 deg F will
not significantly effect the reading of a Term-LAB
system.
HANGOVER -
Whenever a loud vehicle is measured with a microphone, the microphone diaphragm
is altered in some way. Although I do not have the ability to determine why
this is the case, I do have a hypothesis. Mic diaphragms must be very, very
light in order to minimize their mass. This results in a very thin membrane
material. I believe that exposure to high SPL's actually distorts or
"stretches" the diaphragm.
You can try this yourself.
Use a pistonphone calibrator to calibrate your
microphone. Put the mic in a loud vehicle and then "burp" it. Next,
immediately place the mic back in the calibrator and test it again. You will
notice that the reading is off by as much as 1 dB. Continue to check the
calibration every minute. You should see a 0.2 dB recovery per minute. After
6-8 minutes, the mic should read "close" to what it was originally
calibrated for. In most cases, it won't be what it was when you first
calibrated the mic.
Term-LAB doesn't require any
calibration. In addition, you can burp it repeatedly and it will produce the
same reading. (The sensor in Term-LAB is rated for SPL's in excess of 200 dB
although the sensor assembly is currently configured for SPL's to 180 dB.)
IMPACT -
Microphones are easily damaged if you drop them or bang them against something
hard.
You can try this yourself.
Drop your mic. On second thought, DON'T drop your mic or you may ruin it. You
can drop your Term-LAB Sensor if you like. In fact, kick it across the floor.
Throw it against the wall. It will read exactly the same. Actually, kicking the
sensor across the floor doesn't subject the unit to near as much stress as
attaching it to the windshield of a vehicle. Imagine the windshield moving back
and forth 1/2 inch 60 times a second! That is some SERIOUS G-Force. The Sensor
PCB is conformal coated. It's like encapsulating the board in an epoxy resin.
This prevents fractures in the electrical connections due to vibration and
stress. This is the same process that is used to protect the PCB's in military
aircraft. So far, we haven't had a single sensor failure!
LINEARITY
- Microphones are non-linear. In fact, they are inherently non-linear. This is
because the mic capsule acts like a spring. (Imagine the excursion on a
woofer.) Have you ever pulled a bow (as in bow & arrow)? You will notice
that it takes more and more effort to pull the bow string the farther you pull
it from its resting position. Mic diaphragms and woofers suffer from the same
problem. In fact, microphone non-linearities are one
of the biggest problems we had in dB Drag Racing. You could take 2 mics and
calibrate them together. Next, you could put them in the same car and measure
that car at different SPL's. Guess what? The readings were always different.
This is because no matter how hard we tried to match up microphones, they
always had different linearity characteristics.
Term-LAB sensors are
perfectly linear. In fact, the linearity error of the sensor is less than 0.1%
over the entire measurement range! The typical linearity error is less than
0.01% over the entire measurement range.
AIRFLOW-
Microphones can be adversely affected by airflow. If you direct a stream of air
(like that from a port) onto the mic’s diaphragm, it will greatly affect the
measurement.
Term-LAB is not
significantly affected by airflow. Try it yourself. Take an air hose and blow
air onto a microphone. Notice the effect the airflow has on the measurement.
Next, do the same thing on a Term-LAB sensor. You won't see an appreciable
change in the reading.
SYMMETRY -
Microphones produce a non-symmetrical output waveform at high SPL's. I won't
get into the technical discussion regarding why, but you can look for the info
in Auto Sound 2000. (
Test it yourself. Connect a
scope to the line output of your SPL meter. Play your system at high levels.
You will see that the negative side of the waveform is not symmetrical with
respect to the positive side of the waveform. Next, do the same thing on a
Term-LAB system. Notice that the waveform is perfectly symmetrical.