Headphones, initially developed to support military radio communications, have undergone a remarkable transformation since their inception. While they remain pivotal for the military, they have evolved into a ubiquitous device used by people worldwide in their daily lives. Whether we are immersing ourselves in music or movies, engaging in school or business meetings, or simply seeking a private audio sanctuary, headphones provide a remarkably convenient and intimate gateway to connect with the world around us.

AKG’s first headphone, the K120 DYN, was an over-ear design; because foils were not available, it had diaphragms made from pressed granules of trolitul.

As headphone demand has skyrocketed, sound quality has become an increasingly critical design consideration. Listeners are yearning for audio experiences that transcend mediocrity. In professional audio, facilitating accurate mixing and editing decisions is paramount to ensuring sound is experienced exactly as artists and producers intended.

Recognizing the need for a well-defined target, Dr. Sean Olive, Senior Fellow of Acoustic Research and Mr. Todd Welti, Distinguished Engineer, as well as several additional members of the HARMAN X Innovation Team embarked on a research endeavor to study and survey the perception of headphone sound quality. Their combined research established the Harman Target Curve, which was the basis of the AKG Reference Response acoustics target. AKG K371 Series Professional Studio Headphones, available in wired and Bluetooth models, are precision-engineered to match this target to reproduce natural, balanced audio in extraordinary detail. They are often regarded as the reference monitoring standard for true neutrality in headphone sound reproduction.

Dr. Sean E. Olive, Senior Fellow of Acoustic Research at HARMAN International

With over 30 years of research, over 50 published papers, and having served and led a variety of industry associations, including serving as President of the Audio Engineering Society (AES), Dr. Olive is considered one of the world’s principal authorities on acoustics. He shared the story of how he and his team developed an industry benchmark and how it is being used today to elevate the professional headphone experience.

The Journey That Led to the Research
“Up until 2012, I’d mostly focused my research on loudspeakers, but I started listening to and measuring different headphones. And what impressed me was the huge variance in sound quality among the different brands and even models within the same brand. They were all over the map. I started talking to headphone designers, and no one basically agreed on what makes the headphone sound good or how to measure it… we undertook this seven-year project into the perception of measurement of headphone sound quality,” said Dr. Olive2.

Identifying the Research Parameters
Dr. Olive sought to answer three fundamental questions in his research:
“(1.) What is the preferred headphone target curve?
(2.) Do listeners agree on what makes a headphone sound good and to what extent does listening experience, age, gender, and geographical location influence sound quality preferences?
(3.) Can listeners’ subjective ratings of headphones be predicted based on an objective measurement?”1

The Science Behind AKG Reference Response Acoustics

Defining the Parameters to Develop the Target Curve
“A headphone target curve is defined as the frequency response of a headphone that’s been shown to satisfy the preferences of the majority of listeners. Although the initial test results of the Harman Target Curve were encouraging, they were based on a small sample of 10 trained listeners. To better understand if certain demographic factors influence the acceptance of the curve, it was tested using a larger number of listeners from a broad range of ages, listening experiences, and geographic regions. The target curve was benchmarked against three headphones considered industry references at the time in terms of sound quality or commercial sales (Olive et al., 20141).

Figure 1. The mean preference ratings are shown for 11 different groups of listeners categorized as trained (left) and untrained (right). The tests were administered in four different countries: Canada, United States, Germany, and China. HP1 is the Harman target curve and HP2 and HP3 are high-quality, high-priced headphones. HP4 was the most popular headphone in terms of sales (Olive et al., 20141).

They ranged in price from $269 to $1,500 and included dynamic and magnetic planar transducer designs. A total of 283 listeners participated from four different countries (Canada, United States, Germany, and China) and included a broad range of ages, listening experiences, and genders. Most of the participants were Harman employees. The results show that headphone preferences were remarkably consistent across the 11 test locations for both trained and untrained listeners (see Figure 11).

The same group of listeners participated in a second experiment where they adjusted the bass and treble levels of the headphone (Olive and Welti, 20151) several times according to taste using different samples of music. The listeners’ preferred levels were influenced by several factors, including the music program, as well as by the subject’s age, gender, and prior listening experience,” said Dr. Olive1.

“There is evidence that the Harman Target Curve is widely influencing the design, testing, and review of many headphones from multiple manufacturers, providing much-needed new references or benchmarks for testing, and evaluating headphones.” – Dr. Sean E. Olive, Senior Fellow of Acoustic Research at HARMAN International

Next Phase: Expanding the Survey
“The next goal was to test the Harman target using a larger population of headphones. A total of 31 different headphone models from 18 manufacturers were evaluated by 130 listeners, with an approximately equal number of trained and untrained listeners (Olive et al., 2018a1). The headphones ranged in price from $60 to $4,000, including open and closed-back designs with dynamic or magnetic planar drivers. The same virtual headphone double-blind method was used to eliminate biases from visual and tactile cues. The results establish that, on average, both trained and untrained listeners preferred the headphone equalized to the Harman Target in 28 of the models tested.

Table 1. Distribution of listeners within each category.

Four models with frequency responses close to the Harman Target were equally preferred. The main takeaway is that the Harman Target is a good design target for headphones because it satisfies the tastes of most listeners (64%) over a broad range of age groups, genders, and levels of listening experience (see Table 11),” Dr. Olive continued1.

Industry Response to the Research Findings
“The Harman Target Curve is one example that is preferred by a majority of listeners from a broad range of ages, listening experiences, and genders. Slight adjustments in the bass and treble levels may be necessary to compensate for variance in the quality of recordings and to satisfy individual tastes. The further the frequency response a headphone deviates from the Harman Target response, the lower its perceived sound quality will be.

The reaction from the headphone industry to this new research has been largely positive. There is evidence that the Harman Target Curve is widely influencing the design, testing, and review of many headphones from multiple manufacturers, providing much-needed new references or benchmarks for testing, and evaluating headphones. Several headphone review sites provide frequency response measurements of headphones showing the extent to which they comply with the Harman Target (Vavae, 2018; Audio Science Review, 20201),” added Dr. Olive1.

AKG K371 Series Professional Studio Headphones – Reference Monitoring Standards
Because of their incredibly accurate and ultra-neutral sound, AKG K371 Series Professional Studio Headphones, featuring AKG Reference Response Acoustics, are ideal for musicians, engineers, content developers, and other audio professionals.

Learn more about AKG K300 Series Professional Studio Headphones.

AKG K371-BT Headphones add the option of switchable Bluetooth wireless and wired connectivity with up to 40-hour battery life. A­­ built-in microphone empowers two-way Bluetooth communications.

K371 Series Headphones are meticulously crafted with 50mm drivers, the largest in their class, and feature titanium-coated diaphragms and pure oxygen-free copper voice coils. They also include closed-back, oval earcups to further ensure a precision-engineered listening experience.

We invite you to further explore Dr. Olive’s extensive research and story:

1 The Perception and Measurement of Headphone Sound Quality: What Do Listeners Prefer? by Dr. Sean Olive

2 Across Acoustics Podcast Interview with Dr. Sean Olive

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