called Fletcher–Munson curves. Because subjective loudness was difficult to measure, the Fletcher–Munson curves were averaged over many subjects. Robinson and Dadson refined the process in 1956 to obtain a new set of equal-loudness curves for a frontal sound source measured in an anechoic chamber. The Robinson-Dadson curves were standardized as ISO 226 in 1986. In 2003, was revised as equal-loudness contour using data collected from 12 international studies. Sound localization Sound localization is the process of determining the location of a sound source. The brain utilizes subtle differences in loudness, tone and timing between the two ears to allow us to localize sound sources. Localization can be described in terms of three-dimensional position: the azimuth or horizontal angle, the zenith or vertical angle, and the distance (for static sounds) or velocity (for moving sounds). Humans, as most four-legged animals, are adept at detecting direction in the horizontal, but less so in the vertical directions due to the ears being placed symmetrically. Some species of owls have their ears placed asymmetrically and can detect sound in all three planes, an adaption to hunt small mammals in the dark. Masking effects Suppose a listener can hear a given acoustical signal under silent conditions. When a signal is playing while another sound is being played (a masker), the signal has to be stronger for the listener to hear it. The masker does not need to have the frequency components of the original signal for masking to happen. A masked signal can be heard even though it is weaker than the masker. Masking happens when a signal and a masker are played together—for instance, when one person whispers while another person shouts—and the listener doesn't hear the weaker signal as it has been masked by the louder masker. Masking can also happen to a signal before a masker starts or after a masker stops. For example, a single sudden loud clap sound can make sounds that immediately precede or follow inaudible. The effects of backward masking is weaker than forward masking. The masking effect has been widely studied in psychoacoustical research. One can change the level of the masker and measure the threshold, then create a diagram of a psychophysical tuning curve that will reveal similar features. Masking effects are also used in lossy audio encoding, such as MP3. Missing fundamental When presented with a harmonic series of frequencies in the relationship 2f, 3f,