I was asked to get some data on exactly what kind of noise reduction a windblocker will provide.  If you recall, many months ago (last year?) I posted that I was getting a little over 90dB in heavy Chicago Loop traffic of which I wasn't very confident accuracy wise.  I'm pretty confident in the new data since it's merely a comparison rather than a true reading. So I got my wife to drive while I took some data...she ended up getting tired so I didn't get as much data as I hoped :)

Alright, let's get on with it...
Experiment:

• Compare the dB reading with the top and windows down at various speeds to the
  dB reading with a windblocker in place and windows up.

Purpose:

• To determine if a windblocker makes a significant reduction to noise levels.

Equipment used:

• 1- Red '96 Miata
  1- Wife
  1- Windblocker (self made using KT design)
  1- Analog Sound Level Meter (Radio Shack cat# 33-2050)
  1- paved seldom used road
  1- tube of SPF-15 sunblock (noses may get burnt during experiment!)

Procedure:

• 1) Set the Sound Level Meter (dB meter) to slow response (This will give an average value reading from the input) and set the Weighting to 500-10,000 Hz (These frequencies are of greatest sensitivity to the human ear).

  2) While driving with windows down and no windblocker at a speed of 55mph, hold the dB meter next to the driver's right ear.  Estimate and record the average meter reading over a 30s interval.

  3) Repeat this step while driving in the opposite direction to compensate for wind direction.

  4) Repeat 2) and 3) using speeds of 65 and 75mph.

  5) Repeat Procedures while Windblocker is in place and windows are up.

Data:
 

extra data of interest

• 45 mph 76
   78
  75 mph passing van on I-94   83dB
  75 mph Heavy traffic I-94   88dB

Conclusion:

• Let's keep the math out of the picture for a moment.

  With the blocker in place the sound level at 75mph is equivalent to driving 45mph with it.  The sound while driving 75 w/blocker while surrounded by cars and semis is equivalent to driving 75 on a deserted back road w/o blocker.

  Ok that sounds good, but what about the math?

  The approximate 5dB reduction at 55mph doesn't seem like much does it?  Ah, but sound levels are logarithmic!  So this 5dB drop means the intensity has decreased by a factor of 3.2!  Unfortunately this intensity is NOT the same as our perception of volume.  It turns out that an increase in intensity by a factor of 10 will double the perceived sound.  Therefore an increase of 10dB means a sound is twice as loud as before.

• Looking at the data for 75mph, we can see that without the blocker the volume in the cockpit is roughly twice as loud as it would be with the blocker.  The effect of the blocker at 55mph isn't nearly as pronounced, yet it still makes a huge difference.

  In conclusion, the windblocker significantly reduces cockpit noise level (volume) to upwards of 1/2.

Error

• Trying to average the swinging of an analog meter is not very easy.  I did my best to make very conservative estimates.  The other major problem is simply the lack of more data.

After thoughts

• The meter does have an output that I'm trying to interface with.  If I can get everything calibrated I should be able to plug it into my laptop and get more accurate averages for a fixed time interval.  I'm also interested in finding
  exactly what the peak frequencies are to determine whether or not hearing loss can result from top down driving.
  Once fall rolls around I need someone to remind me to do a similar experiment with a temperature probe.  It'll be interesting to see how much warmer it is in the cockpit with the blocker up...I already know it's significant :)
   

If you've got any questions, please contact me.