Thiele/Small Sensitivity Specs: When Numbers Lie (or at least don’t tell the whole truth)

Every now and then I get a question on the forum something like this: “Speaker A has a listed sensitivity (SPL) of 99-db, and speaker B is listed at 98dB, yet you say speaker B is the louder one … what’s up with that”.  Okay, that’s an excellent topic for a blog, let’s dig in!

In its pure form, Sensitivity is defined as the speakers’ ability to convert power into sound. The standard way of measuring a speakers’ sensitivity is using 1 watt/1 meter, meaning a microphone is placed 1 meter away from the speaker to measure the sound output (in decibels) with 1 watt of sound played through it.  Man, so many problems with this … where to start? 

First, the FREQUENCY! 

Most of these measurements are taken at 800Hz or 1000Hz.  That only tells you what the speaker’s efficiency is in THAT frequency (the heart of midrange).  A speaker with an enormous bottom end may have a very un-impressive looking sensitivity spec … but still be VERY impressive in actual use … because a measurement at 800-1000Hz just doesn’t address the speakers real strength.  In my perfect world, we would use white noise (consisting of all frequencies from 20Hz-20,000Hz in equal amounts) for the SPL measurement … that would help. 

Next, Voltage Sensitivity.  

Because today’s solid state amplifiers do a good job across the board of maintaining a voltage output of 2.83 volts, many companies consider this as their standard of measurement.  Here again, 2.83 volts are inputted and measured at 1 meter.  [Note: 2.83 volts into an 8 ohm load is equal to 1 watt. Ohm’s Law: Power (watts) = Voltage (V) x Current (I) or Power = V_/R (impedance in Ohms)]   In the good old days, 1-watt was always 1-watt RMS … today, ya gotta watch out, it may be 2.83 volts!  Because a speakers’ efficiency in transforming (transducer) power into sound is greatly determined by the impedance of a speaker, (see more on impedance below) 2.83 volts becomes greater … about 1.5 watts at 6 and 2 watts at 4 ohms — a 3dB increase, which to our ears sounds significantly louder. 

And yep that brings us to, Impedance. 

This is huge, because a speaker’s impedance is never a static number; it changes given the frequency it is attempting to reproduce … and it especially fluctuates in tube amps, which “reflect” the speakers impedance to the tubes and vice-versa.  Some speaker companies give frequency-specific impedance charts, but this can get confusing, and it still doesn’t address the issue fully.

And last; how many guitar players gig using about 1-watt RMS anyway?  I’m going to say absolutely none!  This is probably the most important of all my points.  You see, audio follows a logarithmic, not a linier scale.  Check this little chart out of an “AVERAGE” speaker with a rated sensitivity of 97dB:

At the bottom end, adding just ONE watt (going from 1 to 2-watts) gives you that noticeable 3-db increase in actual volume (Sound pressure level) … but by the time you get up to actual stage levels of say 115dB or so, you are needing to add an extra 60-watts to get that same 3-dB increase.  Wow.

What’s the take-away?

Well, first, a guitar player should always take all T/S specs with a grain of salt!  Beyond that, we should be more concerned with the sensitivity of a speaker at average gigging levels and at the actual entire frequency range of out instrument.  The T/S specs were birthed in 1925, and haven’t been updated since 1972.  Yes, they still are of some aid, especially when building and/or tuning a speaker cabinet to complement a given speaker … but to consider any T/S spec as the holy bible of speaker performance is a mistake

See Ya’ll next week, it’s gonna be BIG!

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