by vaughn skow August 03, 2015 3 min read
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?
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.
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.
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.
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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