In last week’s post, we had a general discussion about how to set levels, mostly around what one source should be relative to another. This week, we’ll look at the primary way we measure how loud something is when we hear it, its Sound Pressure Level or SPL. SPL is how we objectively know how loud sound is, if you know how to use these units.
To measure the SPL of music or speech in a space, you’re going to need an SPL meter. These can vary in price from a free app on your phone to over $200 dollars. A “good enough” meter will cost between $20 and $50 and is absolutely worth it. For the casual user, the cost of the most expensive SPL meters aren’t justified.
Android or iOS SPL apps will get you in the right ballpark, but are far too inaccurate for anything but general use. Because of the limited hardware options, iOS apps are usually more accurate than their Android versions. Android phones have such varied manufacture that the calibration of the apps is all over the place!
Now I Know My ACZs
Once you have a meter, how do you read it? First, it’s important to realize that our ears don’t hear sound the same at different frequencies. Our ears hear best between 3kHz and 4kHz. Outside of that range, low and high frequency sounds require more energy to sound as loud. This difference becomes more significant the quieter the overall SPL is. (This is why some stereo systems have a “loudness” feature. It boosts low and high frequencies to sound more natural at low listening levels.)
To compensate for this, most SPL measurments are “weighted” to counteract our ears’ bias. The most common weights are “A”, “C”, and “Z” weighting. There are also “B” and “D” weights, but these are no longer commonly used.
The dBA weighting, as it’s often notated, approximates human hearing at lower (below 60 or 70 dB SPL) audio levels. A-weighting is much less sensitive to low frequency audio, and somewhat less sensitive to high frequency audio. A-weighting is handy for measuring the level of human speech but is not at all suited for measuring music. A dbA measurement of music will report an inaccurately low SPL, as it will mostly disregard the low frequency content of music at any functional level above what would be used for background music. A-weighting is also the measurement used by OSHA for hearing loss calculations — more on that later.
dBC is more suited to louder content, like music, than dBA. While C-weighting has the same high-frequency sensitivity as A-weighting, its low frequency is much less contoured. This makes for a much more accurate measurement at the SPLs used for music. On the other hand, dBC will report an inaccurately high SPL when used at lower loudness levels, because of how the ear hears. It’s up to the user to know which weighting is most appropriate in what situations to get an accurate measurement.
The third common weighting is dBZ. This is a linear weighting, which means there is no adjustment for how the ear behaves. While this setting is much less helpful for audio purposes, many meters will have a Z-weighted setting to give an accurate reading across all frequencies.
The Law of Averages
Averaging is the other important variable to understand about SPL measurements. This is how the meter “smooths” the SPL recorded to give a better impression of the audio level. “Slow” averaging reflects the past full second of measured SPL. This ignores most transients and provides a broad representative of the overall SPL. “Fast” averaging reflects the past 125 ms, or an eighth-second. With fast averages you’ll get a good representation of the overall SPL of music, keeping some transients but smoothing out others. “Peak” averaging really isn’t averaging at all. This is the fastest response, showing exactly what is happening at any given time.
Why Does This Matter?
There are two main reasons why knowing these different SPL measurements is important. First and most important is hearing loss. Consistent exposure to loud SPLs leads to reduced hearing. In the US at least, OSHA’s website gives detailed information into their standards for appropriate workplace noise as it relates to hearing loss. While this doesn’t directly relate to live sound, it’s important to be aware of the OSHA standards. They give us a reference point to ensure we aren’t causing harm to those attending our event. This chart copied from the OSHA website gives the time at each SPL before hearing loss occurs:
|Duration per day||SPL dBA (slow)|
|8 hours||90 dBA|
|6 hours||92 dBA|
|4 hours||95 dBA|
|2 hours||100 dBA|
|1 hour||105 dBA|
|30 minutes||110 dBA|
|15 minutes||115 dBA|
The other main reason to understand these varies SPL measurements is those events where the venue has an SPL limit. Sometimes these limits are carefully spelled out, such as “90 dBA slow, 100 dBC peak”, but most of the time they aren’t. Always check in with whoever monitors the SPL of the event to ensure you each have the same set of units. This leads to fewer problems during the event.
Consider the Details
That should give you a general overview of SPL measurement. But a raw SPL number doesn’t give the whole story. An 80 dBC fast measurement of a “bad” mix can sound “louder” to the average person than a 90 dBC fast measurement of a “good” mix — just because it sounds bad! While it’s important to find objective ways to discuss what we do, sound mixing is still an art. We can’t get so caught up in measurement that we ignore what sounds good, and we can’t get so caught up in what sounds good that we ignore the measurements.