Page 58 - Spring2022

FEATURED ARTICLE
The Perception and Measurement of Headphone Sound Quality: What Do
Listeners Prefer?
Sean E. Olive
Headphones are the primary means through which we listen to music, movies, and other forms of infotainment. They have become an indispensable accessory for our mobile phones, providing a 24/7 connection to our entertainment, colleagues, and loved ones. This trend is reflected in the exponential growth in sales. The global market for wireless headphones alone was estimated at $15.9B in 2020 and is projected to rise to$45.7Bby2026,acompoundannualgrowthrateof19.1% (PRNewsWire,2021).Withthisgrowthhascomearenewed interest in improving the sound quality of headphones.
Unfortunately, headphone sound quality has not kept pace with consumers’ demands and expectations. Two recent studies have measured the variance in frequency response of more than 400 headphones and found no correlation between their retail price and frequency response (Bree- baart, 2017; Olive et. al., 2018a). They included the three most common types: headphones that fit around the ear (AE), on the ear (OE), and in the ear (IE). It seems that headphone designers are aiming at a target frequency response that is as random and variable as the weather.
Another telling sign that headphone sound quality has not kept pace is that headphone industry standards have not changed fundamentally since the 1990s. The Interna- tional Electrotechnical Commission (IEC) 60268-7 (2010) standard specifies multiple ways to measure the frequency response of a headphone for both free-field (FF) and diffuse-field (DF) targets, with the warning: “subjective assessments are still useful because the objective methods whose results bear good relation to those from subjective assessments are under research stage” (IEC, 2010, Section 8.6.1). This does not inspire confidence.
The International Telecommunication Union Radiocom- munication Assembly (ITU-R) BS.708 (1990) standard
recommends that professional headphones be designed to the DF target curve to achieve best sound, but most head- phone designers have rejected this suggestion and probably for good reasons. Recent psychoacoustic investigations pro- vide evidence that listeners prefer alternative headphone targets to DF and FF target standards (Olive et al., 2013a).
The chaos that exists within the headphone industry today is reminiscent of the loudspeaker industry 30 years ago when there was insufficient knowledge on lis- teners’ loudspeaker preferences and which loudspeaker measurements best predict them. The situation improved after Floyd Toole, an acoustician at the National Research Council of Canada, published seminal scientific papers that provided guidelines in how to measure and design loudspeakers that most listeners prefer (Toole, 1985, 1986). Later, a mathematical model was developed that could predict listeners’ preference ratings of the loud- speakers based on objective measurements alone (Olive, 2004). The science provided important answers on what loudspeaker listeners prefer, design guidelines, and new measurement standards (American National Standards Institute/Consumer Technology Association [ANSI/ CTA] Standard, 2015) that became widely accepted and adopted throughout the industry.
Headphone Sound Quality
In 2012, the seminal papers for headphone sound qual- ity did not exist, and this was reflected in the headphone standards and the large variance in headphone sound quality. Skeptics argued that the variance in headphone sound was explained by a need to satisfy individual tastes in sound that vary like individual tastes in music, food, and preferred companions. If listeners could not agree on what sounds good, then a single optimal frequency response or headphone target curve could not be defined.
©2022 Acoustical Society of America. All rights reserved.
58 Acoustics Today • Spring 2022 | Volume 18, issue 1
https://doi.org/10.1121/AT.2022.18.1.58