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Headphone Paper Collection by Sean Olive

Created: August 28, 2015

More than 300 million pairs of headphones will be sold in 2015 as consumers everywhere are using headphones to enjoy music, infotainment and communicate with people on their mobile devices.  With this new market growth has come a resurgence in headphone research and development, much of it  focused on how to best optimize headphone sound quality for different applications.  A key problem is defining what is good headphone sound quality, and how to accurately and reliably measure it.  Some of the most fundamental research questions related to this problem include:

  1. How do we design headphone listening tests that produce accurate, reliable and valid measures of their perceived sound quality?
  2. Is there an optimal headphone target response curve that most listeners prefer?  What factors (demographics, listening experience, program, fit, etc) influence their sound quality preferences?
  3. What other acoustic performance metrics  of the headphone (phase, nonlinear distortion, etc ) influence its perceived sound quality?
  4. What is the best method (e.g. Ear coupler, binaural mannequin, probe microphone, flat plate,etc) for measuring the acoustic performance of the headphone in terms of accuracy, repeatability, and predicting its perceived sound quality?
  5. Can the perceived sound quality of a headphone be predicted based on a set of objective measurements?

Until these questions are answered, perceptually meaningful headphone performance targets, design specifications, and standards cannot be realized.

Below is  a collection of  25 headphone papers from the AES e-library  that largely focus on these five research questions.  The papers are listed in descending chronological order according to the date of publication, and cover a time span over the past  32 years.  In reading these papers, the reader should develop an increased awareness of  issues and challenges in defining and measuring headphone sound quality. Fortunately, there is good progress being made.

Unfortunately, this collection has omitted many excellent headphones papers in the AES e-library  that fall outside the scope of this narrow topic. They include important topics related to headphones used in binaural signal processing (e.g. binaural synthesis, virtual surround), acoustic modeling and simulation, headphone design, and many others. The reader is encouraged to seek out these papers in order to get a broader perspective of how headphone performance impacts these different applications.

# 1

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Nearfield Crosstalk Increases Listener Preferences for Headphone-Reproduced Stereophonic Imagery
JAES Volume 63 Issue 5 pp. 324-335; May 2015
Manor, Ella; Martens, William; Marui, Atsushi; Cabrera, Densil

Although final mixing and mastering is monitored over loudspeakers, the majority of music listeners use headphones on mobile devices. Preferences for spatial process depend on the method of reproduction. For a variety of program material using headphones, listeners often prefer a stereophonic image that is created by simulating nearfield crosstalk compared to the biphonic spatial image. This novel approach, called Nearfield Crosstalk Simulation, describes crosstalk that simulates closely located loudspeakers. Previous work used farfield crosstalk simulation in an effort to produce an enhanced stereophonic effect, but such results were less preferred. The primary difference between the more conventional farfield crosstalk and the novel nearfield crosstalk developed for this study was the introduction of a level and a time difference at low frequency, consistent with what actually occurs for sound sources very close to a listener’s head.

# 2

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Improved Measurement of Leakage Effects for Circum-Aural and Supra-Aural Headphones
Paper 9275; AES Convention 138; May 2015
Welti, Todd

Headphone leakage effects can have a profound effect on low frequency performance of headphones. A large survey, including over 2000 individual headphone measurements, was undertaken in order to compare leakage effects on test subjects and leakage effects of the same headphones measured on a test fixture. Ten different commercially available headphones were used, each measured on eight different test subjects and a test fixture with several sets of pinnae. Modifications to the pinnae were investigated to see if the leakage effects measured on the test fixture could be made to better match the real word leakage effects measured on human test subjects.

# 3

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Characterizing the Frequency Response of Headphones—A New Paradigm
Paper 9274; AES Convention 138; May 2015
Horbach, Ulrich

Traditional headphone measurements suffer from large variations if carried out on human subjects with probe microphones, and standardized couplers introduce additional biases, as concluded in a recent paper. Beyond that, there is no clear indication in literature about what the actual perceived frequency response of a headphone might be. This paper explores new measurement methods that avoid the human body as much as possible by measuring the headphone directly, in an attempt to overcome these restrictions and gain more accuracy. Design principles are described in the second part. A novel, DSP controlled, high-quality headphone is introduced that offers the ability to auto-calibrate its frequency response to the individual who is wearing it.

# 4

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The Influence of Listeners’ Experience, Age, and Culture on Headphone Sound Quality Preferences
Paper 9177; AES Convention 137; October 2014
Olive, Sean; Welti, Todd; McMullin, Elisabeth

Double-blind headphone listening tests were conducted in four different countries (Canada, USA, China, and Germany) involving 238 listeners of different ages, gender, and listening experiences. Listeners gave comparative preference ratings for three popular headphones and a new reference headphone that were all virtually presented through a common replicator headphone equalized to match their measured frequency responses. In this way, biases related to headphone brand, price, visual appearance, and comfort were removed from listeners’ judgment of sound quality. On average, listeners preferred the reference headphone that was based on the in-room frequency response of an accurate loudspeaker in a reference listening room. This was generally true regardless of the listeners’ experience, age, gender, and culture. This new evidence suggests a headphone standard based on this new target response would satisfy the tastes of most listeners.

# 5

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The Correlation between Distortion Audibility and Listener Preference in Headphones
Paper 9118; AES Convention 137; October 2014
Temme, Steve; Olive, Sean; Tatarunis, Steve; Welti, Todd; McMullin, Elisabeth

It is well-known that the frequency response of loudspeakers and headphones has a dramatic impact on sound quality and listener preference, but what role does distortion have on perceived sound quality? To answer this question, five popular headphones with varying degrees of distortion were selected and equalized to the same frequency response. Trained listeners compared them subjectively using music as the test signal, and the distortion of each headphone was measured objectively using a well-known commercial audio test system. The correlation between subjective listener preference and objective distortion measurement is discussed.

# 6

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Effect of Headphone Equalization on Auditory Distance Perception
Paper 9149; AES Convention 137; October 2014
Sunder, Kaushik; Tan, Ee-Leng; Gan, Woon-Seng

Headphones are not acoustically transparent and thus it affects both the timbral as well as the spatial quality of the input sound source. The effect of the headphones has to be compensated by calculating the inverse of the headphones transfer function and convolving it with the binaurally synthesized audio. Headphone transfer function (HPTF) also depends on the headphone-ear coupling and thus displays high spectral variation between individuals. It has been found that the type of equalization (individual or non-individual) affects the directional perception of the virtual audio reproduced using headphones. However, little investigation has been carried out on the effect of headphone equalization on auditory distance perception. In this paper, we study in detail the perceptual effects of equalization on the auditory distance perception in the proximal region in anechoic conditions. It was found that the equalization of the headphone is critical for good distance perception. The type of equalization (individual or non-individual) did not have a significant effect on the auditory distance perception indicating that the distance perception does not depend on the idiosyncratic features. The effect of repositioning of the headphone on auditory depth perception is also studied in this work.

# 7

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Inter- and Intra-Individual Variability in the Blocked Auditory Canal Transfer Functions of Three Circum-Aural Headphones
JAES Volume 62 Issue 5 pp. 315-323; May 2014
Völk, Florian

When using headphone playback, there are many factors that contribute to variations of the intended stimuli: errors in headphone transfer functions, their inter-individual differences, and the intra-individual variability due to repeated positioning. This report provides a detailed evaluation of the blocked auditory canal transfer characteristics for one specimen of each of three different circumaural headphones frequently used in psychoacoustics and audio production; two headphones based on electrodynamic, one headphone based on an electrostatic converters. Depending on the headphone, results showed inter-individual variations up to 10 dB in the blocked auditory canal entrance headphone transfer function magnitude spectra and 0.5 ms in the corresponding group delays, especially at frequencies above 6 kHz. At lower frequencies, 2 dB and 0.1 ms were measured. This general behavior is comparable for all three headphone specimens, but each model shows specific peculiarities of the headphone transfer function magnitude and group delay. For all headphones studied, the inter-individual magnitude spectrum variability exceeded the average intra-individual variability.

# 8

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A Method for Comparison of Nonlinearities of Consumer Earphones Using Equalized Stimuli
Engineering Brief 141; AES Convention 136; April 2014
Fleischmann, Felix; Estrella, Jorgos; Plogsties, Jan

Comparison of nonlinearities between different earphone models is not directly possible due to their different sensitivities and the high dynamics in the transducer's frequency response. Also, nonlinearities are highly dependent on the level of the excitation signal. An approach to overcome these differences and allow for fair comparison is proposed. The method is based on filtering commonly used stimuli like pink sweeps with a linear correction filter. This filter is obtained by a measurement at low input levels where the transducer shows linear behavior. The nonlinear response is then measured at different levels and THD is computed. In this way the non-linearity or different transducers can be compared directly. Some examples demonstrating the performance of consumer grade earphones are presented and discussed. The results show that nonlinearities mainly appear for low frequency excitation.

# 9

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Listener Preferences for In-Room Loudspeaker and Headphone Target Responses
Paper 8994; AES Convention 135; October 2013
Olive, Sean; Welti, Todd; McMullin, Elisabeth

Based on preference, listeners adjusted the relative bass and treble levels of three music programs reproduced through a high quality stereo loudspeaker system equalized to a flat in-room target response. The same task was repeated using a high quality circumaural headphone equalized to match the flat in-room loudspeaker response as measured at the eardrum reference point (DRP). The results show that listeners on average preferred an in-room loudspeaker target response that had 2 dB more bass and treble compared to the preferred headphone target response. There were significant variations in the preferred bass and treble levels due to differences in individual taste and listener training.

# 10

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Magnitude and Phase Response Measurement of Headphones at the Eardrum
Paper 3-3; AES Conference: 51st International Conference: Loudspeakers and Headphones; August 2013
Christensen, Anders T.; Hess, Wolfgang; Silzle, Andreas; Hammershøi, Dorte

Transfer functions of headphones are measured to verify that they meet certain requirements or to determine what equalization may make them meet an ideal target curve. The present study compares six headphones by physical measurements at the eardrums of six individuals and on a dummy head. For headphone transfer functions we are interested in the variability in the produced sound pressure at the eardrum across individuals, the similarity between dummy head and real-ear measurements, the agreement with the diffuse-field design target and, finally, the prevalence of all-pass phase.

# 11

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A Virtual Headphone Listening Test Methodology
Paper 3-5; AES Conference: 51st International Conference: Loudspeakers and Headphones; August 2013
Olive, Sean E.; Welti, Todd; McMullin, Elisabeth

Comparative listening tests on multiple headphones are challenging to conduct in a controlled, double-blind manner. One solution is to present the listener virtualized versions of the headphones through a single reference headphone that is equalized to simulate the linear magnitude response of the different headphones under test. This paper describes a method for conducting virtual headphone listening tests and presents results of a validation experiment where listener sound quality ratings from standard and virtual headphone listening tests are compared. The listening test results show good correlation between the two methods in terms of perceived spectral balance and overall preference.

# 12

Listener Preferences for Different Headphone Target Response Curves
Paper 8867; AES Convention 134; May 2013
Olive, Sean; Welti, Todd; McMullin, Elisabeth

# 13

Identification and Evaluation of Target Curves for Headphones
Paper 8740; AES Convention 133; October 2012
Fleischmann, Felix; Silzle, Andreas; Plogsties, Jan

# 14

The Relationship between Perception and Measurement of Headphone Sound Quality
Paper 8744; AES Convention 133; October 2012
Olive, Sean; Welti, Todd

# 15

Sound Quality Assessment of Earphone: A Subjective Assessment Procedure and an Objective Prediction Model
Paper 8-4; AES Conference: 38th International Conference: Sound Quality Evaluation; June 2010
Chon, Sang Bae; Sung, Koeng-Mo

# 16

Audibility of Headphone Positioning Variability
Paper 8147; AES Convention 128; May 2010
Paquier, Mathieu; Koehl, Vincent

# 17

Subjective Evaluation of Headphone Target Frequency Responses
Paper 7770; AES Convention 126; May 2009
Lorho, Gaëtan

# 18

Research on a Measuring Method of Headphones and Earphones Using HATS
Paper 7529; AES Convention 125; October 2008
Inanaga, Kiyofumi; Hara, Takeshi; Rasmussen, Gunnar; Riko, Yasuhiro

# 19

A Method for Objective Sound Quality Evaluation of Headphones
Paper 14; AES Conference: 32nd International Conference; September 2007
Jang, Seongcheol; Kim, Jongbae; Sung, Ho Young

# 20

Headphones Listening Tests
Paper 6890; AES Convention 121; October 2006
Opitz, Martin

# 21

Listening Test Methodology for Headphone Evaluation
Paper 5736; AES Convention 114; March 2003
Hirvonen, Toni; Vaalgamaa, Markus; Backman, Juha; Karjalainen, Matti

# 22

Design Criteria for Headphones
JAES Volume 43 Issue 4 pp. 218-232; April 1995
Møller, Henrik; Jensen, Clemen Boje; Hammershøi, Dorte; Sørensen, Michael Friis

# 23

Transfer Characteristics of Headphones Measured on Human Ears
JAES Volume 43 Issue 4 pp. 203-217; April 1995
Møller, Henrik; Hammershøi, Dorte; Jensen, Clemen Boje; Sørensen, Michael Friis

# 24

On the Standardization of the Frequency Response of High-Quality Studio Headphones
JAES Volume 34 Issue 12 pp. 956-969; December 1986
Theile, Günther

# 25

The Acoustics and Psychoacoustics of Headphones
Paper C1006; AES Conference: 2nd International Conference: The Art and Technology of Recording; May 1984
Toole, Floyd E.

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