24 Feb EQ, mixing, and the Fletcher-Munson Curves
There are many different ways to approach how one equalizes and mixes sound elements in a podcast episode. I like to start the process by keeping a few things in mind: in general, the goal is to produce a pleasing sounding podcast that entertains, informs, inspires, or all of the above. For those things to happen, our audience needs to be able to hear and enjoy all of the sounds within the episode, and particularly the human voices. To help do that, it’s important to think about the Fletcher-Munson curves, more currently known as an equal-loudness contour. Here I’ll refer you to Wikipedia for a deeper dive: https://en.wikipedia.org/wiki/Equal-loudness_contour .
Generally speaking, the Fletcher-Munson curves show us that the human ear hears certain frequencies more easily than other frequencies at equal loudness levels, and that how much more or less easily these frequencies are heard changes as sound pressure level (SPL) changes. At roughly 70 dB SPL, frequencies between 300 Hz and 6 kHz (and in particular 2 kHz and 5 kHz) are perceived as louder than surrounding frequencies, and especially those at the low end of human hearing. As sound pressure increases, those same frequencies between 300 Hz and 6 kHz are still perceived as louder than surrounding frequencies, though gradually less loud as SPL increases. In sum, mid-range frequencies are always perceived as louder than surrounding frequencies, though how much louder depends on sound pressure level.
Fletcher-Munson curves.
So how does being aware of this inform how one applies equalization, or mixes sounds in a podcast episode? We want our listeners to be able to clearly hear and understand host and guest voices, but we also want the all other sounds to be heard and understood as well. Oftentimes we need to raise the level of elements in our mix enable them to be more easily heard. But what if by doing that, the human voices start to get buried in the mix? A high-pass filter can be carefully applied to one or more of the human voices to roll off some of the low end and cause the middle frequencies to shine through and be perceived as louder. This can also be done in the case of dry voices, helping listeners hear above the noise-floor in their listening environment. Similarly, if bass notes in a music bed are missing at low playback volumes, or if the sound effect of a bus rumbling by has too much low frequency energy to be clearly understood as a bus, a high-pass filter can be gently applied to those elements as well, to help them sound louder in the mix (a multi-band compressor is also a useful tool that can help achieve the same goal, though in a much different way). I think that a reasonable conclusion one can draw is that by carefully narrowing the frequency range of certain elements in a production, we can make the whole program more listener-friendly. I’m not advocating for the creation of thin, tinny, harsh sounding mixes. I’m suggesting using what the Fletcher-Munson curves tell us to use EQ in a way that helps place sounds in a mix, creating pleasing sounding podcast episodes that entertain, inform, and inspire.
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A high-pass filter.
A multi-band compressor working on the low end.
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