The Weighted dB and Ear Safety

The weighted dB scales

We saw via the equal loudness contours that our hearing changes spectrally as the dB_SPL increases. Because of this using dB_SPL to state the "loudness" of a room doesn't make a ton of sense, so we have the weighted dB scales. These scales change the measured dB value for particular frequencies to better match how we hear them.

There is dBA, dBB and dBC with A better representing lower levels and B and C representing increasingly higher level.

For example, at lower levels we hear lower frequencies worse, so they shouldn't be included in the measurements as much because they don't contribute to how "loud" it sounds as much therefore those frequencies are reduced.

Here is a graph of the 3 scales.

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Depending on your job you may be required to use these scales for safety reasons. For example in live sound often dBC is picked because it is an extremely loud environment. dBC will better reflect how we "hear" such an environment rather than dBA which would remove bass frequencies as if we were at a lower equal loudness contour. If you were to use dBA in such a place it would allow you to play the music louder because the bass would be getting a reduction in the meter reading. This is why some people push for dBA over dBC. If your job doesn't specify then picking to use dBA would let you get louder than B or C would.

OSHA (The Occupational Safety and Health Administration) only uses dBA in their standards. Your work place may require a different dB scale. Clubs and live shows are just a few examples of common places that usually use dBC. Below are the OSHA standards for how long one can listen to dBA at various levels before ear damage becomes likely. Note it is assumed that continuous listening and "hearing damage" may not be severe but it adds up over the course of a lifetime.

It is also stated in the standard that transient noises should not exceed 140 dB SPL

Lets take a look at the difference between dBA, B, and C and how to calculate such values. Here is a table showing their weighted factors for 9 frequencies.

Question

Why is the weight 0 for all 3 scales at 1 kHz?

Steps for calculating a weighted dB

1. Get the measured value at each frequency.

2. Apply the weight to each frequency for the particular scale you are using.

3. Add the decibels with the uncorrelated sound equation.

   $$Total\ dB\ =\ 10\ \log\left( 10^{\frac{x_{1} \ dB\ SPL}{10}} +10^{\frac{x_{2} \ dB\ SPL}{10}} +...+10^{\frac{x_{n} \ dB\ SPL}{10}}\right)$$

Example

Suppose I have the following measurement and wish to get the dBA scale.

Now that we have the measurement we apply the weight:

Finally we use the equation:

Questions

1. Compute the dBC value for the same measurements as the example.

2. For the following measurement compute the dBB value. If a frequency is not listed you can leave it out.

   Hz:	125	250	500	1000	2000
   Measurement in dBSPL:	80	83	88	85	78

Ear Safety

As stated in the ear lesson, once your Stereocilia are damaged thats it, no redo. Because of this we should take extra precautions to protect the ear.

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OSHA Standards

The Occupational Safety and Health Administration has set the standards for ear safety. Audio is not the only job that has to deal with a potentially very loud noise environment. Even something as mundane as driving with the windows rolled down can cause hearing loss over time. Occupational noise exposure is covered in standard 1910 Subpart G, specifically 1910.95.

In this standard are many important safety guides that you should at least be aware exist! If you are going to work somewhere loud these will help you keep your hearing healthy longer if your employer doesn't already have these implemented. If they don't you can use these standards to fight for your ear health.

Noise Induced Hearing Loss

Noise induced hearing loss is hearing loss due to damage of the structure of the ear. Hearing loss can occur from a single loud blast of noise or over time. The above listed OSHA table for dBA are the current standards for the "safe" amount of listening time at various levels.

For context driving with your windows down gives anywhere between 89 to 115 dB_SPL depending on your speed! The sound created from driving with your windows down is similar to white noise so it is broadband. Looking at dBA won't really tell the whole picture and dBC is probably a better measurement. We can see that it is generally fine for short amounts of time but if you have a habit of doing it then you can expect gradual hearing loss over your life. The ear closer to the open window will sound more dull than the other and possibly respond to transients differently. So in addition to gradual damage you also risk damaging your ability to spatially position sounds.

Another factor not considered in such studies is how loud transient moments are in such situations. Bursts and spikes can also cause permanent damage. Sometimes the damage is great is the spike is great but often it is small enough to seem ok and doesn't show up until much later in life.

In general if you're gonna be monitoring for long periods of time take regular breaks and keep the sound below 90 dB_SPL. There is nothing wrong with boosting it for a bit but bring it back down! Boosting it may be tempting because the equal loudness contours cause it to sound more full, but don't overdo it and bring it back down!