How Do You Read an Equal Loudness Contour Graph?

How Do You Read an Equal Loudness Contour Graph

Producing audio mixes calls on us to use studio monitors and headphones with flat-frequency responses. This helps reduce the possible sound variation resulting from the influence of equalization preferences, specific stereo systems, and listening room designs.

Did you know these devices wouldn’t be what they are today without the Fletcher Munson Curve? Let’s discuss the first of the equal-loudness contour’s numerous measurements to help you understand the graph enough to read it correctly.

Who Were Fletcher and Munson?

These two physicists changed the face of the recording, entertainment, and home studio industries so that the impact and implications are still discussed several decades later.

Both laid the groundwork that furthered the investigation on the nature of human hearing and, in turn, the development of international policies concerning the research and manufacturing of audio-related equipment and studies.

The Fletcher and Munson Curve

This is the first of what was soon known as the equal-loudness contours’ measurements. They are visual graphs depicting the effect of frequency and loudness on human hearing.

With the focus mainly on diaphragm and mechanism frequency responses that led to sound reproduction, acoustic environments, and speakers among them, no one thought to consider how biological organs might perceive sound.

Fletcher and Munson were among the first to take the problem on and measure how the human ear perceives pitch, volume, and each frequency range’s relative loudness in comparison with each other. Through this, they launched some of the earliest ever studies on psychoacoustics.

How to Interpret the Contours on the Fletcher and Munson Graph

Here’s how to read an equal-loudness contour graph correctly:
  1. Choose a one-phon curve that runs down the page, from right to left.
  2. Select an x-axis frequency and follow it up to the point where it crosses paths with the phon contour.
  3. Trace that intersection back to the y-axis. The value you find is the perceived level of phon sound pressure.
Now, let’s assign values to those variables:
  1. You chose an 80-phon curve
  2. You chose the 150 Hz frequency, following it up until it intersects with the chosen phon contour
  3. Following that point to the y-axis, you get a perceived sound pressure level of 80 dB.
Thus, a loudness level of 80-phons at 150 Hz frequency is perceived by the human ear to have a sound pressure level of 80 decibels.

What Do the Contour Shapes Show?

Generally, the actual figures plotted on the Fletcher and Munson curves don’t have any real use in home studio mixing. However, they provide an understanding of the acoustic and principal nature of the ways humans perceive frequencies at varying levels of sound pressure. That’s what’s vital in sound mixing.

Start with each contour’s general shape. While each phon curve is in a different location on the diagram, they are all similar in form. If you feel the need to memorize the curves, try doing so like this:

Imagine a pro skater who is about to launch himself down a wave ramp. The camera will show him at the very top of the ramp on your screen’s left. That’s a cornered top edge with a steep dropdown. The skater launches down, picks up speed, and rises on the other side of the wave ramp before taking a dramatic dip.

In the final part of the ride, the skater rises one more time using the last bit of momentum he has and latches on to the edge of the summit. Take a look back at the diagram while imagining this, and you’ll see what we mean.

Why Does Sound Mix Better With the Volume Cranked Up?

The Fletcher-Munson contours showcase dynamic differences between the 20-phon and 100-phon curves, the lowest and highest parts of the curve, respectively. The 100-phon curve has a flatter contour because of its much lower range than the 20-phon curve.

Thus, the 100-phon equal loudness is less variation for low, medium, and high-frequency sound pressure levels. That means human ears hear more music at higher volumes instead of lower volumes. In turn, cranked-up music is also perceived to have a complete frequency range that we humans can truly appreciate.

This is why many of us prefer listening to music at full blast. When you have your headphones on or are playing music in the car, you may appreciate music better when it’s coming at you at full force and nothing less.

The Equal Loudness Contour Graph: The Takeaway

Studying and developing audio-related products in ways that don’t affect the frequency response of their produced sounds is hard enough. However, it’s also vital to consider the way our brains and ears perceive sound across the full frequency range.

That’s what equal-loudness contours are for. They are the basis for understanding the changes that need to take place in our mixing and mastering methods and music manufacturing processes.

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