Have you ever noticed that the frequency balance of your mix sounds completely different at different listening levels? Learn how to compensate for it and find out how it can help you create better sounding mixes. They say you should mix around 85 dB, but remembering to listen at a range of mixing volumes helps you keep your mix in check. After all, check out the video tip on the subject from Bobby Owsinski.

But why does that work? Why is that a thing? Well, it just so happens that there are multiple ways to define and interpret loudness. You’ve probably noticed that the frequency balance in your mix will change at different volumes. Understanding the way the human ear responds to different frequencies at different volumes will help you tailor the listening experience. Knowing how the human ear responds differently than a machine will help you understand why you get different experiences at different listening levels, even though the computer is showing you the same spectrograph. In fact, the way this balance changes is actually scientifically documented, and predictable. So let’s dig in and learn how this works so that we can use its predictable nature to our advantage!

Decibel (dB)

Not many people know this, but a decibel is actually one tenth of a “bel”, which is a unit named after Alexander Graham Bell (inventor of the telephone). It measures the magnitude of relative signal strength. It’s a logarithmic unit of measurement that expresses a ratio of two different physical quantities. These quantities can be power, sound pressure, intensity or voltage, etc. Logarithmic units are good for measuring a giant range of quantities, and usually involve fairly arbitrary reference points. Decibels are used in areas outside acoustics and audio, such as electronics and optics. So, they’re not always used for measuring sound pressure level. In the audio world though, decibels measure sound pressure level, and use the generally accepted threshold for human perception as a reference point.

Equal Loudness Curves

Measuring sound pressure level to get a scientific reading of dB is all well and good, but our ears don’t hear all frequencies equally. Anyone that’s been annoyed by the piercing cry of a baby, or has felt the bass more than they’ve heard it would understand why this is true. This phenomenon was originally studied and compiled into a handy chart by researchers Fletcher and Munson in 1933. Their work has since been updated by newer research, and updated standards. But, equal loudness curves are still often called “Fletcher-Munson” curves nonetheless.
The chart shows frequency along the X axis, and sound pressure level (dB SPL) along the Y axis. What is charted within the graph is how many “phons” we hear at each combination of frequency and dB SPL. It is a unit of perceived loudness, so it’s very much dependent on the human ear as the tool for measurement. As you can see, it takes a higher sound pressure level for the human ear to hear certain frequencies.

So how does this apply to music and mixing? Well, if you’re mixing or listening to a song, you’ll have a range of frequencies within the track. Equal loudness curves help engineers conceptualize why the frequency balance of a mix will sound different at different volumes. You can see this when you note that different lines representing phon values are not completely parallel to each other. This means that when you crank your mixing volume, you’ll actually hear a different balance of frequencies than when you lower it to a whisper. This can be the difference between an instrument overwhelming the listener or being inaudible at different listening volumes. Since consumers will listen at varying volumes, it’s important to factor that in while mixing, and listening at a variety of volumes.

Weighting Filters: dBA and Others

Ok, so here’s where it gets a little complicated. Because the human perception in phons doesn’t perfectly line up with a mathematical measurement of SPL, we sometimes apply a weighting filter to our measurements of decibels. This type of filter (dBA) usually takes a scientific reading of SPL from a measurement device, and applies more weight to the frequencies to which human ears are most sensitive, such as around 2 to 4 kHz. This skews the mechanical readings of SPL so that they more closely resemble the readings from a human ear. There are also other weighting filters that behave differently, for varying purposes (dB B through D, among others). Here’s a chart that visualizes dBA through D weighting filters:

dBA is the most commonly used weighting filter for SPL, and was developed as a response to the work of Fletcher and Munson. There are also dB(C), B and G filters, among others. The dBA filter applies weight around 1kHz, and makes the SPL meter less sensitive at very high and very low frequencies, like the human ear. If you see a measurement being reported at dBA, then you know it has this type of weight applied to it. The dB(C) weighting scale is similar, but applies the weight over several octaves, and is more useful for very high levels. For all you visual learners, here are some graphs depicting the dBA and dB(C) filters. dBA measurements are better served for low level sounds. This filter is often applied to measurements of noise in audio gear, so it’s an important variable to understand. Even though it’s not good for high sound levels, it’s also used when measuring industrial and environmental noise, such as when determining public health effects of noise. In some areas of Europe, a different weighting system is used that’s called ITU-R 468 noise weighting. This system takes into account that the dBA through C systems are based on single tones, and our ears respond differently to a pure tone than to random noise.

The dB(G) weighting system is an example of a weighting filter that isn’t applied to mimic the human response. It gives weight to very low frequencies (1 to 20 Hz), and thus is very sensitive to frequencies that are below the generally accepted range of human hearing.

So that’s it! Next time you’re mixing, using an RTA, using a SPL meter, or selecting gear, make sure to factor in your units of measurement, and follow Owsinski’s advice! After all, it’s based on science!

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