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

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

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:

Power in watts

Volume in dB

1

97

2

100

4

103

10

107

20

110

40

113

100

117

200

120

400

123

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!

email Vaughn     About Vaughn Skow 

kcohagen
08/08/2015 12:46am

Vaughn,

When you build a full range Stereo HiFi type cabinet Theil Small parameters come into play when tuning the cabinet to match the woofer's low end capabilities in order to get a flat and extended bass response out of the speaker. When tuning guitar cabinets with an open back the builder is clearly looking to build the cabinet like it wasn't an infinite baffle. But when building a closed cabinet how important is it to build it according to the speakers TS parameters? It would seem to me that you would want to optimize the bass response at at least 60HZ, given some room for detaining. Otherwise tuning the cabinet for 80HZ would make sense since low E is 82.2HZ. What do you recommend? 

kcohagen
12/20/2015 5:37pm

I have a Genz Benz G-Flex 2X12 cab and it's tuned to 60 HZ at the bottom end. I use it with a GB El Diablo 100/50 W amp and it is extremely loud. For practice and smaller venues I bought a Valveking II 20w combo. It has an ET90 in it that I bought out of your clearance department. I was looking to buy the ET65, but it was a good deal and I couldn't pass it up. Anyway, I want to build an extension cab fo this amp and since the combo is open I want to build a closed cab to compliment it. So do I tune it to 60hz like the G-Flex? What else would you suggest?

arcticbreaze
01/10/2016 6:44pm

having just read vaughns blog on the sbject, which i must say answered a few questions i had on the subject. maybe WGS could have there own measuring system, much as Vaughn discribes in the blog 50 watts of white noise at 10 meters maybe or 20 watts for smaller stuff. give us a bit more idea of what we are buying, put g12c/s into a 15 watt tube amp and was a bit dissapointted at the lack of volume from a high spl speaker. does sound good tho