Good Sound in Headphones The Harman Target Curve
By: Jaden Jang
Is there a universally good sound, and if so what makes a good sound? Questions regarding advancements in headphone technology have been relatively stagnant and largely ignored, especially when compared to the impressive growth of the audio equipment market. The global market of wireless headphones alone is estimated to have a compound annual growth of 19.1%, from an estimated $15.9 billion in 2020 to $45.7 billion in 2026 (PRNewsWire, 2021). The difficulty in identifying “good sound” might sound natural, given the ever so diverse tastes people have in music. But in 2010, scientists at Harman began research and soon created a definition of scientifically “good sound”.
With sound, the focus of “good sound” is put on ideal frequency response. Arguably the best indicator of sound quality, frequency response shows how well an audio equipment reproduces a range of frequencies or musical tones. Bass corresponds to lower sound frequencies, while treble corresponds to high sound frequencies with midrange being the frequencies in between.
Headphones in the past have been inconsistent in frequency responses, largely due to the lack of research on “good sound,” which is essentially a reference frequency response. In the 1970s, the first reference sound was projected by speakers in a free-field (FF), which is when a particular sound wave from a source is the sole wave that reaches the ear, without any reflections. FF reference quickly fell out of favor after research by Theile found that free-field equalization applied to headphones cannot produce good results, compared to diffuse-field (DF) frequency responses (Thielle, 1986). DFs are the opposite of FF, where sound waves reflect off multiple surfaces to reach the ear, resulting in an even spread of sound. However, the DF target was invalidated when Lorho’s survey revealed that listeners prefer a modified version of the sound (Lorho, 2009).
Hence, new target sounds were tested in semi-reflective fields (SRF), optimized for stereo recordings. A study conducted in 2013 tested trained listeners’ preferences between SRF, FF, and DF equalized headphones (Olive et al., 2013). This revealed a strong preference for the SRF equalized target, and this soon became what is known now as the Harman target curve.
Since then, the harman target curve has been tested in multiple studies among a wide range of listeners—experience level, nationality, gender, and age—to see whether it was truly universally appealing (Olive et al., 2014). A 2014 study tested 283 listeners with simulations of different headphones in the market, and surprisingly, they have found that a remarkably high proportion of the listeners preferred the Harman target over different frequency responses. Another study by the same researcher tested the correlation between proximity with the harman curve and the listeners’ preference scores (Olive et al., 2018). This study showed an impressive result: listeners preferred sounds the more they were close to the Harman target.
All together, we might have just figured what good sound, at large, would sound like. Despite how different we are, we all seem to gravitate to the harman sound, just like how most of us love Beethoven or Hemingway.
References
PRNewsWire (2021), Global Wireless Headphones to Reach $45.7 Billion by 2026.
Olive, S. (2022), The Perception and Measurement of Headphone Sound Quality: What Do Listeners Prefer? Theile, G. (1986), On the Standardization of the Frequency Response of High-Quality Studio Headphones. J. Audio Eng. Soc., vol. 34, no. 12, pp. 956-969.
Lorho, G. (2009). Subjective evaluation of headphone target frequency responses. Proceedings of the 126th Audio Engineering Society Convention, Munich, Germany, May 7-10, 2009. https://www.aes.org/e-lib/browse.cfm?elib=14966.
Olive, S. E., Welti, T., and McMullin, E. (2013). Listener preference for different headphone target response curves. Proceedings of the 134th Audio Engineering Society Convention, Rome, Italy, May 4-7, 2013. https://www.aes.org/e-lib/browse.cfm?elib=16768.
Olive, S. E., Welti, T., and Khonsaripour, O. (2018b). A survey and analysis of consumer and professional headphones based on their objective and subjective performances. Proceedings of the 145th Audio Engineering Society Convention, New York, NY. https://www.aes.org/e-lib/browse.cfm?elib=19774.
