Click here to purchase the entire book in PDF format. Phons Most of the world measures things in dBspl. This is a good thing because it's a valid measurement of pressure referenced to some fixed amount of pressure. As Fletcher and Munson discovered, though, those numbers have fairly little to do with how loud things sound to us. So someone decided that it would be a really good idea to come up with a system which was related to dBspl but ``fixed'' according to how we perceive things. The system they came up with measures the amplitude of sounds in something called phons. 6.1 Here's how to find a value in phons for a given measured amplitude. Measure the amplitude of the sound in dBspl Check the frequency of the sound in Hz Plot the intersection of the two values on the chart of the equal loudness contours in Figure 5.8 Find the nearest curve contour and check what the value of that curve is at 1 kHz That value is the amplitude of the sound in phons The idea is that all sounds along a single equal loudness contour have the same apparent loudness level, and therefore are given the same value in phons. If you look at Figure 5.8 you'll notice that the black curves are numbered. These numbers match the value of the curve where it intersects 1 kHz, consequently, they indicate the value of a frequency in phons if you know the value in dBspl. For example, if you have a sinusoidal tone at 100 Hz and a level of 70 dBspl, it's on the 60 phon curve. Therefore that tone will sound the same loudness as a 1 kHz sinuoidal tone with a level of 60 dBspl. Since all the points on this curve will sound the same loudness to us, the curves are called equal loudness contours.