Sound & Units

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SOUND & UNITS

dB(A)

(Originally written by Sean Casey to help in the sound education of his city, NO City, and its residence. Why? Check out NO Amph.

Human hearing, the faculty of perceiving sounds, is non-linear and subjective. (Repeat: human hearing is non-linear and subjective.) Overlapping fields of scientific study on hearing are clustered heavily in the health sciences (that sounds sensible). Primary branches include psychology, audiology, neurobiology, and acoustics generally. Because our hearing is subjective and non-linear, studies have led researchers to contrast and compare using a combination of subjective and objective methods, yielding models that attempt to make sense of, and predict, how humans will interpret sound experiences, and what might cause harm. Scientists studying sound and hearing agree on two general points. Our ears and brains are the final arbiter of what sounds loud/soft, bad/good, disruptive/peaceable—the subjective; and the objective reality that sound effects our physical and psychological health, and is the main cause of hearing loss.

So the decibel [dB]… most all of us playing or working in audio and sound think we know what we are saying when we use that term. My experience however leads me to think that the vast majority of us are simply familiar and we have tied it to our experience, which then places the unit into the subjective. [Hit pause, replay, listen…. Yep, we’re getting ahead of ourselves.]

There are many audio units in use, several are tied to a physically defined functional decibel unit and others are tied to loudness, the term conveys the subjective nature of hearing. In the subjective realm we have two common and historical units, the sone and phon. [If you know these units you are board stiff reading this—do yourself a favor and move on, the small amount of humor you might run into here is not worth your time, I’m not that funny.] As those units are essentially absent from the professional, commercial and home audio market's jargon, we’ll turn our attention to the subjective units, the ones we think we relate to, decibels—the empirical and ever sighted dee-bee, thrown around like it's the sonic equivalent to watts. Another unit we use all the time in audio and think we know. We don’t, not really. Okay, likely the HAM guys and the five or six HAM girls do, they’re a smart and educated bunch.

Most of us sound freeks grew up with a handheld sound level meter, my first was a common Radio Shack, with a two position weighting switch A/C, a two position response switch slow/fast, and then an amplitude scale dial. Better ones had more options, like A, B, C and Linear. Now, we use our phones for this, and most have the option of, A, B, C, Z; and as we assume they are calibrated and accurate, we reference the number displayed, “looks like it measures 79.4 dB [continued nonsensical talk].” Now if you are a sound mavin that also digs the physics, and certainly a lot of us do, you know what the weighting kinda means and have set your phone to the Z-weighting unit, for a reference that might make a bit more sense to music. Those of us that really want to expand that nearly infinitely compressed data point and really see what is happening, we’ll download an app that has spectrum, decay, and impulse display options. Yes, you should be sensing the sarcastic coriolis drift. [Hit pause again. No, stop. Find the tape labeled Decibel/dBA. Load. Punch play.]

Decibel (dB) is a logarithmic unit used to express a ratio between two physical properties. Without reference it has no meaning. Yes, it can be and is commonly used to express a change in or specific value, but to do that requires a defined reference. And to define a precise value we have to use a precise language, we have to do some math. And no, let’s not argue about whether or not math is a language.

Sound Pressure Level (SPL) is the most common method in relating to and measuring sound, referenced as:

SPL = pressure over reference pressure x log base 20.

When the decibel unit is combined with SPL (making simplifications and assumptions about airborne sound) and making the zero reference the rough threshold of human hearing (a standardized reference) our zero position is:

0 dBSPL = 2x10^-5 pascals [20 µPa]

With a defined reference (yes we took some shortcuts to get there) we can now reference sound using the dBSPL units. This is for sound pressure, and sound in air is a pressure wave, there is no mechanism for shear waves in air. Also, dBSPL is not a power reference and cannot be converted to a power without a whole lot of additional measures and math. Yes, you can use engineering tricks like an index (I am guilty there) to approximate and convert sound power and sound pressure. But powers of sound is way outside the scope of this letter.

So what’s a dBA? We reference it all the time.

dBA also dB-A dB(A) and noted as L subscript A, is a unit that is weighted to relate to how we hear speech and speech-level sounds. Say it with me, dBA is built for speech and speech-level sounds. While this weighted sound level measure has solid relevance for speech and speech-level sounds including environmental acoustics at speech levels, the dBA unit has little relevance to music, film and the performing arts and is rarely used. Why? because it was build around how humans here speech and relate to speech. A-weighting filters frequencies below 1000Hz, with 20Hz being attenuated by 50+dB! A-weighting frequency characteristics were designed to mirror the 40-phon equal loudness-level contour based on the 1933 Fletcher-Munson data. If you didn't understand that last sentence and you need to look at sound and noise and impact, please do everyone a favor and just hire a vetted professional.

Here’s the A-weighting correction graph:

Correction to Octave-band levels to convert to dB-SPL A-weighted band levels

Sean Casey1 Comment