What are linear decibels (dBL) and how do they compare to standard dB SPL?

Question

I'm reading through an EIS that has a blast monitoring section where blasts are characterized by Peak Pressure Level (PPL) measured in linear decibels (dBL). I cannot find a good definition of this decibel scale, my understanding is that the dB SPL (re 20 μPa) scale is logarithmic whereas this is (based on the name) linear?

I'm trying to understand the best way to relate these two scales to each other, if it's possible, so I can assess how loud this noise is to wildlife at different distances. The information I have on wildlife hearing capabilities is in db SPL.

Answer 1

A decibel (dB) is a logarithmic scale as you correctly understand. Why someone adds a 'linear' attributes to it, is confusing. I never have come across such term.

IMHO, Without seeing the EIS, there are really three possible answers to it:

• it is simply a typo (happens very often, especially in large multi-author documents)
• they have no idea what they are saying (should happen some time)
• they invented some new metrics, where they needed a catchy name for (should then be explained in appendix (this is very rare)

Answer 2

dB(L) or dB linear is a term we use in the explosives industry for measuring air overpressure. It is the same as dB SPL and the term linear is just meant to clarify that the unit does not have a frequency weighting applied, such as dB(A) (A-weighted).

All decibels are, by definition, logarithmic. They are not just displayed on a logarithmic scale, but the unit itself is a logarithmic function of the ratio between a measured air pressure and a reference air pressure (Decibels themselves are unitless and can also be used to measure ratios between other units as well).

This is where it gets tricky. The reference air pressure for dB SPL is the threshold of human hearing (quietest noise humans can hear), 20 micro pascals. This makes dB SPL a useful metric for measuring loudness to humans. But, dB SPL does not account for frequency, and extreme high or low frequencies are not as loud to humans, even if they have high pressure. To account for this, in the 1930s we developed a weighted version of the dB SPL scale that makes extreme frequencies have less of an affect on the decibels. Many weighting curves for humans have been developed since, but the most common is the A-weighted curve, notated as dB(A). This is most often used for regulatory bodies to set maximum sound exposure limits for humans.

In blasting however, we are not just concerned with human hearing, but also animals, the environment, and structures, so, we measure the air overpressure, without regard for frequency, using the unweighted scale, which we specify by noting dB(L) or dB Linear. The microphones we use to monitor air overpressure are also setup to measure without regard for frequency. For your purpose, dB(A) would not be appropriate since it is weighted for human hearing, and animals will be sensitive to a different range of frequencies.

Just make sure that any instrumentation you use is set up to measure dB and not dB(A) and you will know it is reading a linear (unweighted) decibel. Then you can interpret the air overpressure with respect to the specific animals in the blast environment. I recommend a quick wiki read on A-weighted decibels or decibel weighting.

It is a somewhat misleading notation to call it linear, since it is logarithmic, but it just means it isn't weighted for frequency.