Soaking test results

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

SOAKED HARPS TEST +5 DB LOUDER THAN DRY ONES

CONTENTS

1.   SUMMARY
1.1  STD DISCLAIMERS
1.2  MY TEST
2.   SEMI-TECHNICAL DISCUSSION OF DECIBELS
2.1  WHAT IS A DECIBEL?


1. SUMMARY

I did a scientific test to prove or disprove my empirical guess that a
soaked harp was roughly 10 dB louder than an unsoaked one.  The results
were the soaked harp was about 5 dB louder than the same unsoaked one -
not quite my guess of 10 dB, but certainly a significant improvement in
volume.  If you have feedback problems, this 5 dB _might_ give you the
additional volume you need. 

Test conditions, results, commentary, and a brief, elementary treatise on
decibel basics follows. It's about 150 lines total.  If you're not
interested, pressed for time, have "Short Attention Span Syndrome", etc.,
please quit this message now and move on to the next one. 


1.1  STD DISCLAIMERS

SOAKING HARPS WILL PREMATURELY AGE THEM AND WEAR THEM OUT.

SOAKING WOOD BODIED HARPS MAY BE HAZARDOUS TO YOUR LIPS.

SOAKING HARPS CAN BE MESSY.

WET HANDS CAN INCREASE THE RISK OF SERIOUS ELECTROCUTION HAZARD WITH
IMPROPERLY GROUNDED AMPLIFICATION EQUIPMENT.


1.2 MY TEST

I used my harmonica pickup - the same one I perform with, and therefore
the most valid test for me.  It's a sealed tube with a Shure Brothers R-47
controlled magnetic microphone cartridge and a glass pimiento jar as a
back enclosure, wired for high impedance.  Because the harp sits in a
fixed position, there is negligible margin for error.  It gets the same
"fat" sound as a tightly hand-cupped Green Bullet - that "honkin' hard
blues" sound. "Tight seal" is not an issue with this pickup. 

I ran this into the 1/4" input of my Tascam 244 Portastudio 4 track
cassette recorder (1/4" is always high impedance on pro audio equipment.)
I put a brand new Lee Oskar (C) with windsavers (but that shouldn't affect
the test) into the holder slot, firmly seated it, and blew as hard as I
could on 10 blow.  I set the Tascam 244 VU meter to zero dB using this
note as reference. 

I then removed the harp and soaked it in ice water for something like 30
seconds, shook it out, and dried it with a towel.  I replaced the harp in
the holder and once again blew #10 as hard as I could.  It pegged the
meter. 

I then set the VU meter to 0 dB using the "soaked" harp. 

Next, I took a second Oskar "C" (low mileage used) and measured it "dry". 
I read minus 5 dB.  I then dipped this one for about 10 seconds, shook and
dried it, and tested it.  It read a little over 0 dB. 

While typing in this textfile, I let about 20 minutes elapse, then
retested the harp.  It was STILL nearly as loud, dropping a fraction of a
dB. 

While 5 dB is not quite my initial guesstimate of 10 dB, it is definitely
and substantially louder.  It was also brighter, which could easily
account for my perception of 10 dB.  The R-47 has a poor high frequency
response, and would not measure additional high frequencies.  Of course,
when I'm using this, my amplifier would never see these, either, nor would
my audience. Nonetheless, it would be most interesting to do a spectral
analysis. 

2. SEMI-TECHNICAL DISCUSSION OF DECIBELS

I'd like to clear up a few misconceptions about logarithms, decibels, and
auditory response.  Because our instruments make sound, sound related
topics apply.  If you're not into tech stuff, you might want to kill the
rest of this.  I've tried to keep it readable to the layman (some may feel
I'm "condescending to the primitives", but that is not my intent;
criticisms in this vein will _not_ be received graciously), and have taken
a few liberties I would not take if addressing a body of fellow engineers. 
Nevertheless, this is reasonably accurate. 

2.1 WHAT IS A DECIBEL? 

One tenth of a Bel, of course :-)

The Bel was named after Alexander Graham Bell, husband of Ma Bell.  It is
based upon the physical laws that govern our ears, eyes, etc.  We don't
perceive sound linearly.  Something twice as loud actually has ten times
the energy!  This allows us to hear a MUCH greater range of sound than a
truly linear auditory system would, where two watts would be perceived as
twice as loud as one watt. 

A Bel is a ratio of one power level to another that is perceived as being
twice as loud if positive and half as loud if negative.  It is a function
of logarithmic proportions.  Logarithms are numbers that can be added to
produce multiplied results, and in fact, this is one use of logarithmic
tables.  The infamous Richter Scale (earthquakes) is logarithmic. 

A rough "conversion" rule is: 1 dB is about 1.25 times the power, 3 dB is
about twice the power; 6 dB is about 4 times the power, 9 dB is about 8
times the power, and 10 dB is 10 times the power.  "Power" is "work done",
and in electronics and acoustics is often measured in "Watts". 

One tenth of a Bel is a decibel.  If we increase sound by one decibel, it
is a just perceptible increase in volume.  However, it's about 25% more
power!  If you take your 100 watt amp and increase it to 125 watts, you'll
just barely be able to tell the difference on an AB comparison! 

If you double the power (about +3 dB), it is slightly louder.  The easiest
way to "hear" a 3 dB change in volume for yourself is to take your home
stereo system and play a mono recording, using both speakers.  Note the
volume.  Now turn off one speaker (the balance control should suffice) and
note the decreased volume.  It's half power, but just a little softer to
your logarithmic-responsive ear. 

If you want to double your volume, you must increase it by 10 dB, or about
ten times!  Your 100 watts will need to be 1000 watts to double its volume
to your ears (if you have any left :-)

Also, a Decibel is not an absolute measurement of sound, i.e. 120 dB
damages hearing, etc.  This is a mistake, and should be dBm, or decibels
referenced to one milliwatt of acoustic energy.  Remember - dB is a RATIO
(i.e. twice as loud - but as WHAT?).  By anchoring it to a standard (dBm,
or "twice as loud as 1 milliwatt"), we now have a way to express absolute
power. 

So 5 dB is not quite twice as loud to our ears - but is something on the
order of a threefold increase in acoustic power. 


 -- mike