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 havebecome 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 $45.7B by 2026, a compound annual growth rate of 19.1%(PRNewsWire, 2021). With this growth has come a renewedinterest in improving the sound quality of headphones.

recommends that professional headphones be designed to
the DF target curve to achieve best sound, but most headphone designers have rejected this suggestion and probably
for good reasons. Recent psychoacoustic investigations provide evidence that listeners prefer alternative headphonetargets 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 listeners’ loudspeaker preferences and which loudspeaker
Unfortunately, headphone sound quality has not kept pace measurements best predict them. The situation improved
with consumers’ demands and expectations. Two recent after Floyd Toole, an acoustician at the National Researchstudies have measured the variance in frequency response Council of Canada, published seminal scientific papersof more than 400 headphones and found no correlation that provided guidelines in how to measure and design
between their retail price and frequency response (Bree- loudspeakers that most listeners prefer (Toole, 1985,
baart, 2017; Olive et. al., 2018a). They included the three 1986). Later, a mathematical model was developed thatmost common types: headphones that fit around the ear could predict listeners’ preference ratings of the loud(AE), on the ear (OE), and in the ear (IE). It seems that speakers based on objective measurements alone (Olive,
headphone designers are aiming at a target frequency 2004). The science provided important answers on what
response that is as random and variable as the weather.
loudspeaker listeners prefer, design guidelines, and newmeasurement standards (American National Standards
Another telling sign that headphone sound quality has Institute/Consumer Technology Association [ANSI/
not kept pace is that headphone industry standards have CTA] Standard, 2015) that became widely accepted andnot changed fundamentally since the 1990s. The Interna- adopted throughout the industry.
tional Electrotechnical Commission (IEC) 60268-7 (2010)
standard specifies multiple ways to measure the frequency Headphone Sound Quality
response of a headphone for both free-field (FF) and In 2012, the seminal papers for headphone sound qualdiffuse-field (DF) targets, with the warning: “subjective ity did not exist, and this was reflected in the headphoneassessments are still useful because the objective methods standards and the large variance in headphone soundwhose results bear good relation to those from subjective quality. Skeptics argued that the variance in headphoneassessments are under research stage” (IEC, 2010, Section sound was explained by a need to satisfy individual tastes
in sound that vary like individual tastes in music, food,
8.6.1). This does not inspire confidence.
and preferred companions. If listeners could not agree
The International Telecommunication Union Radiocom- on what sounds good, then a single optimal frequency
munication Assembly (ITU-R) BS.708 (1990) standard response or headphone target curve could not be defined.
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https://doi.org/10.1121/AT.2022.18.1.58

These same arguments were undoubtably made about the loudspeaker. This would be repeated for several test
loudspeakers 40 years ago and until research proved lis- subjects to calculate the loudness transfer function thatteners largely agreed on what is a good loudspeaker.
defined the headphone FF target curve.
With the lessons learned from the loudspeaker industry,
the author and his colleagues embarked on a seven-year
research project to improve the consistency and soundquality of headphones. There were three fundamental
questions we hoped to answer.
(1) What is the preferred headphone target curve?
Should the reference be a loudspeaker in a FF, a
DF, or a semireflective field (SRF) found in a typicallistening room?
(2) Do listeners agree on what makes a headphonesound good? To what extent does listening experience, age, gender, and geographical locationinfluence sound quality preferences?
(3) Can listeners’ subjective ratings of headphones be
predicted based on an objective measurement?
These research questions were addressed for the threemain headphone types, but the scope of this article islargely restricted to AE and OE headphones. The preferred target curve for IE headphones is almost identicalto those for the AE and OE targets, except it has an additional 4 dB of bass (Olive et al., 2016). Each question isaddressed separately, followed by conclusions.

The Search for the Preferred Headphone
Target Curve

Over the past 50 years, headphone researchers havefocused their attention on determining what the ideal reference sound field should be for headphone reproduction
and how to measure it. Three types of reference soundfields have been proposed: a FF, a DF and a SRF thatlies somewhere between the two extremes. What thesesound fields are, how they are measured or derived, andpsychoacoustic investigations of headphone target curves
based on them are described.

Theile (1986) conducted formal listening tests and foundthe DF target to be preferred to the FF target, which
produced an unnatural timbre and in-head localization
effects. Although the FF target fell out of favor beginningin the 1980s, it remains part of the current headphone
IEC (2010) standard today.
Diffuse-Field Headphone Equalizations(1980s to Present)
A DF occurs when a sound source is placed in a reverberation room with little or no absorption, so the listenerreceives a random and equal distribution of sounds fromall directions. The headphones are calibrated to the DF
using a subjective loudness procedure or alternative
methods. In one method, a probe microphone is placedin the ear canals of the listener to measure and thenmatch the transfer function of the headphone to that of
the sound field (Theile, 1986).
A second approach is to substitute the listener with a head
and torso simulator (HATS); this produces faster, more
reproducible, and safer measurements than putting probe
microphones in the listeners’ ears. A third option is to use aheadphone known to be DF calibrated as the reference andcompare its performance with the headphone under test.

Møller et al. (1995) derived a headphone target curve
based on different sound fields using a large set of headrelated transfer functions (HRTFs) measured at theblocked ear canal. HRTFs define the transfer functions,both the frequency and phase responses at the entrance
to the ear, for each direction and distance of a soundsource. They capture both interaural time (ITD) andintensity (IID) differences and spectral cues that humans
use to localize sound sources in space (Blauert, 1983).
Free-Field Headphone Target Curve (1970s)
By selecting HRTFs from the appropriate directions and
The reference FF was generated by placing a loud- distances and integrating them, Møller et al. (1995) werespeaker in front of the listener in a reflection-free room. able to derive transfer functions of reference sound fields
A tedious subjective loudness-matching procedure was ranging from the FF to the DF and anything in between.
used where a test subject would listen to narrow bands This method eliminated the need for a physical referenceof noise at different frequencies alternately with the FF sound field, making headphone calibration more practi(with the headphone removed) and then with the head- cal and reproducible. A headphone could be measured
phone. While listening to the headphones, the levels for and equalized to the DF target curve using a calibrated
each band would be adjusted to match the loudness of dummy head or ear simulator.
Spring 2022 • Acoustics Today

59

HEADPHONE SOUND QUALITYThe DF target was not seriously challenged until Lorho(2009) reported 80 listeners (25% audio engineers, 25%music students, and 50% naive listeners) on average preferred a significantly modified version of the DF target
where its main feature, a wide 12 dB peak at 3 kHz, wasreduced to just 3 dB. This paper sparked new interest tofind better alternative headphone target curves to theones recommended in the current headphone standards.

A similar study (Olive et al., 2013a) reported evidence that
listeners strongly preferred headphones equalized to SRF
targets to, in descending order of preference, two DF targets (Möller et al., 1995); two high-quality headphones;
the Lorho target; and the FF target. The trained listeners
described both DF targets as having too much emphasisin the upper midrange (2-4 kHz) and lacking bass. The
Lorho target had too little energy at 2-4 kHz, which madeinstruments sound “muffled and dull.” The FF target was
Semireflective Field Headphonestrongly criticized for its strong emphasis between 2 and 4
Equalizations (2012 to Present)
kHz, lack of bass, and harsh and nasal colorations. ListenBecause stereo recordings are optimized for reproduc- ers described the highest rated the SRF target as havingtion through loudspeakers in semireflective rooms, they “good bass with an even spectral balance.” The measured
should sound best through headphones that emulate this frequency responses of the headphone targets correlate tosound field. Sank (1980) made similar proposals three and confirm listeners’ descriptions of their sound quality
decades earlier but never conducted formal listening tests (see Olive et al., 2013b, Figure 2). The highest rated target
that compared these targets with the DF target.
curve in this study soon became known in the audio industry as the Harman target curve and is widely influencing
Loudspeakers with flat on-axis and smooth off-axis fre- the design, testing, and review of headphones.
quency responses tend to produce the highest subjective
ratings in formal listening tests (Toole, 2018). When Do Listeners Agree on What Makes aplaced in a typical room, they produce a uniform quality Headphone Sound Good?
of direct, early, and late reflected sounds that in summa- Although the initial test results of the Harman target
tion produce the steady-state in-room response of the curve were encouraging, they were based on a small
loudspeaker. Due to the frequency-dependent directiv- sample of 10 trained listeners. To better understand if
ity of the loudspeaker and absorption characteristics of certain demographic factors influence the acceptance ofthe room, the in-room response will not be flat like the the curve, it was tested using a larger number of listeners
FF response nor the same as the DF response where the from a broad range of ages, listening experiences, androom absorption has been removed. Instead, the in-room geographic regions.
response gently falls about 1 dB per octave from 20 Hz
The target curve was benchmarked against three headto 20 kHz.
phones considered industry references at the time in
Fleishmann et al. (2012) reported the first formal listen- terms of sound quality or commercial sales (Olive eting test results where three SRF headphone targets were al., 2014). They ranged in price from $269 to $1,500
evaluated. The targets were based on measurements of and included dynamic and magnetic planar transducer
the steady-state in-room response of a 5.1-channel loud- designs. A total of 283 listeners participated from four
speaker setup in a standard listening room and then different countries (Canada, United States, Germany,equalized by three expert listeners to match the timbre and China) and included a broad range of ages, listenof the speakers. Two of the SRF targets were found to ing experiences, and genders. Most of the participants
be slightly preferred to the DF target, depending on were Harman employees.
the music programs. Other targets included the Lorho
target, a flat target, and three unequalized headphones A novel virtual headphone test methodology allowedthat generally received lower ratings than the two SRF controlled, rapid, double-blind comparisons among thetargets. Unfortunately, no measurements or details of the different headphones. Virtual versions of the different
loudspeakers and the three SRF targets were given. The headphones were reproduced over a single high-quality
conclusions were that the SRF targets were equal to or replicator headphone by equalizing it to match the meabetter than the DF target, but the Lorho target was not. sured frequency response of each headphone. This removed
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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 Harmantarget curve and HP2 and HP3 are high-quality, high-priced headphones. HP4 was the most popular headphone in terms ofsales (Olive et al., 2014).

any potential biases related to visual (brand, model, price,design) and tactile (weight, clamping force, feel of materials)
cues that might cloud their judgments of sound quality. A
prior validation study confirmed that subjective ratings ofvirtual versus actual headphones (with the listener unawareof the headphone brand, model or appearance) had a correlation of 0.86 to 0.99 depending on the headphone type
(Olive et al., 2013b). A limitation of the method is that it
does not reproduce nonlinear distortions in the headphones.
However, the high correlations between virtual and actual
headphone comparisons and evidence from other studies
indicate that these distortions are generally below masked
thresholds (Temme et al., 2014).

less bass can help improve intelligibility. More researchis needed to provide definitive answers.

Preferred Level of Bass and Treble
in Headphones

The same group of listeners participated in a second
experiment where they adjusted the bass and treble
levels of the headphone (Olive and Welti, 2015) several times according to taste using different samples ofmusic. The listeners’ preferred levels were influenced byseveral factors, including the music program, as well as
by the subject’s age, gender, and prior listening experience (see Figure 2). The program interactions between
preferred bass and treble levels are expected due to variThe results show that headphone preferences were ability in the quality of music recordings; often theyremarkably consistent across the 11 test locations for require adjustments in bass and treble on playback toboth trained and untrained listeners (Figure 1). As restore a proper balance. Toole (2018) refers to these
expected, the trained listeners were more discriminat- errors as audio’s “circle of confusion.” The confusion
arises from not knowing the source of these errors:
ing and consistent than the untrained listeners.
the recording, the loudspeaker, or its interaction with
Headphone preferences were also relatively consistent the room acoustics. The solution is a meaningful loudacross different age groups and the four countries. The speaker standard common to both the professional andexception was listeners in the 55+-year age category who consumer audio industries.
tended to prefer HP2, a brighter headphone with less bass
than the Harman target curve. A possible explanation Female listeners preferred less bass and treble than their
could be age related hearing loss; additional treble and male counterparts. Younger and less experienced listeners
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HEADPHONE SOUND QUALITY

Figure 2. The mean bass and treble levels and 95% confidence intervals for a headphone calibrated to match a flat in-room
loudspeaker response. Each graph shows the interaction effect between the preferred levels and program, gender, listening
experience, age, and the country of the test location (Olive and Welti, 2015).

preferred more bass and treble than their older, more experienced counterparts. The older listeners (55+ years) werethe exception here, preferring significantly more treble andless bass, consistent with their preference for headphone
HP2. Altogether, these findings suggest that a single headphone target may not be sufficient to satisfy variations in
the recordings, individual tastes, listening experience, andhearing loss. A simple solution for headphone personalization is to provide a simple bass and treble control thatallows listeners to compensate for these variations.

The results establish that, on average, both trained anduntrained listeners preferred the headphone equalized
to the Harman target in 28 of the models tested. Fourmodels with frequency responses close to the Harmantarget were equally preferred.

Segmentation of Listeners Based on
Preferred Headphone Sound Profiles

Although the study established that listeners, on average, preferred the Harman target to other headphones
tested, it had not explored whether segments or classes
Testing the Harman Target with Largerof listeners exist based on similarities in their headphone
Sample of Headphones
preferences and what those sound quality features or
The next goal was to test the Harman target using a larger profiles are. Also, it did not identify possible underlying
population of headphones. A total of 31 different head- demographic factors that might predict membership inphone models from 18 manufacturers were evaluated each class. There was already prior evidence that younger
by 130 listeners, with an approximately equal number males and less experienced listeners preferred higher
trained and untrained (Olive et al., 2018a). The head- levels of bass and treble in their headphones comparedphones ranged in price from $60 to $4,000, including with females, experienced, and older listeners (Olive etopen and closed back designs with dynamic or magnetic al., 2013a; Olive and Welti, 2015). A reasonable hypothplanar drivers. The same virtual headphone double-blind esis was that segmentation of headphone preferences may
method was used to eliminate biases from visual and relate to bass and treble levels, possibly predicted by thesetactile cues.
demographic factors.
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Acoustics Today • Spring 2022

A statistical method known as agglomerative hierarchicalclustering exposed three different segments or classes oflisteners based on similarities in their headphone preferences. By calculating the average response of the top
five preferred headphones in each class, it was clear thatthe preferred bass level is the main feature that defines
membership in a class. Class 1 includes most listeners(64%) who prefer headphones that closely comply with
the Harman target. Class 2 listeners (15%) prefer theHarman target with 4-6 dB more bass. Class 3 listeners(21%) prefer the Harman target curve with 2 dB less bass.

of a majority of listeners (64%) over a broad range ofage groups, genders, and levels of listening experience.
The two smaller classes of listeners who prefer headphones with more bass or less bass can be accommodated
through a simple bass tone control on the headphoneor via an app on the audio device. The bass adjustment would also help compensate for inconsistences in
the quality of recordings that contain either too muchor not enough bass and treble. A word of caution: theresearch suggests adding too much bass beyond theHarman target may alienate many listeners given thatthe “more bass is better” segment is a small segment with
Table 1 shows the different demographic categories and little female and older listener membership. Conversely,
the distribution or percentage represented in each class. reducing the bass too much may alienate trained and
For example, 69% of the males in the study are members experienced listeners who are underrepresented in theof Class 1 (Harman Target Lovers) compared with 56% “less bass is better” segment.
of females. Class 1 has roughly equal representation fromtrained (70%) and untrained (65%) listeners. Class 2 Predicting Listener’s Headphone(More Bass Is Better) has the fewest members overall and Sound Quality Preferences
is represented by all categories except female; only 4% of Conducting controlled headphone listening tests is afemales tested want more bass than the Harman target challenging, time-consuming, and expensive proposition.
provides. Class 3 (Less Bass Is Better) members are dis- An alternative solution is to model and predict listeners’
proportionately represented by females (40% of females headphone preference ratings using objective measureare in this class versus 13% of males) and listeners over ments that are relatively faster, more reproducible, andthe age of 50 (50%). Hearing loss may be a confounding cost effective.
factor here. More research is needed to better understandthe role it plays in headphone sound quality preferences. The 31 headphones from the same study discussed by Oliveet al. (2018a) were sorted into 4 categories of sound quality
The main takeaway is that the Harman target is a good based on listeners’ preference scores: excellent (90-100%
design target for headphones because it satisfies the tastes preference rating), good (65-76%), fair (42-54%), and poor
Table 1. Distribution of listeners within each category.
Distribution of Listener Categories Within Each Class (in %)Based on Preferred Headphone Sound Profile
Class 1:
Harman Target Lovers

Class 2:More Bass Is Better

Class 3:
Less Bass Is Better

Males

0.69

0.18

0.13

Females

0.56

0.04

0.40

Trained

0.70

0.30

0.00

Untrained

0.65

0.10

0.25

20s

0.69

0.17

0.15

30s

0.74

0.13

0.13

40s

0.67

0.10

0.24

50+

0.30

0.20

0.50

CategoryAge (years)

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HEADPHONE SOUND QUALITY

Figure 3. The average frequency response (blue) and error curves (red) for 31 around the ear (AE) headphones assigned to four
categories of sound quality based on their preference rating given in controlled listening tests. The Harman target curve used to
calculate the error curve (green). Dotted curve, regression line that best fits the error curve (Olive et al., 2018a).

Figure 4. Left: average frequency response (blue), standard deviation (gray area), and error response curve (red) for AE, on the ear (OE), andin the ear (IE) headphone with the Harman target curve (green). Right: predicted preference ratings versus their retail price are plotted for theheadphones with the best-fit regression line and correlation coefficient shown (Olive et al., 2018b).

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Acoustics Today • Spring 2022

(0-39%). Frequency response measurements of the head- approximates the preferred in-room response of a fullphones were made using an ear simulator according to IEC range loudspeaker calibrated in a typical listening room.
60318-1 (2009) equipped with a custom pinna that better
Conclusions
simulates headphone leakage on humans.
Our understanding of the perception and measurement
In each category, the average frequency response for the of headphone sound quality has not kept pace withheadphones is plotted with the Harman target curve and consumer demand and expectations. Two independent
the error curve that is the difference between the two (see studies measured over 400 headphones and came to
Figure 3). Figure 3, black dotted line, is a regression line similar conclusions: there is little correlation between
that best fits the error curve. The relationship between the price of a headphone and its frequency response, theobjective and subjective headphone measurements seems single best indicator of its sound quality. Most profesclear: the more the frequency response of a headphone sional and consumer headphone designs today do notdeviates from the Harman target curve, the lower the lis- comply with the FF and DF targets recommended by current headphone standards, which warns “the objective
teners rated its sound quality.
methods whose results bear good relation to those fromA linear model was developed that predicts headphone pref- subjective assessments are under research stage” (seeerence ratings using two variables based on the standard IEC 60268-7, 2010, Section 8.6.1). The research stage isdeviation and the absolute slope of the error curve. The largely completed, the results are in, and the headphonecorrelation between the predicted and measured ratings is standards need to be updated.
0.86, with an error of 6.7 ratings on a 100-point scale. A
similar model was developed for IE headphones that pro- Listeners largely agree on what makes a headphone soundduces slightly better (r = 0.91) predictions (Olive et al., 2016). good. For stereo reproduction, the preferred headphonetarget approximates the in-room response of an accurate
The two models were used to predict preference rat- loudspeaker calibrated in a semireflective room. This
ings for 158 headphones, including AE, OE, and IE makes perfect sense because stereo recordings are intended
types (Olive et al., 2018b). Figure 4, left, shows the aver- to sound best through accurate loudspeakers in semireflecage magnitude response, standard deviation, Harman tive rooms. What makes a headphone sound good is thetarget, and error response curve for each headphone type. same as what makes a loudspeaker sound good.
Figure 4, right, plots the retail price versus the predicted
preference rating for each headphone model tested. On
Figure 5. Proposed headphone target curves normalized at 500
average, the AE headphones come closest to the Harman
Hz: the Harman AE headphone target (green), two diffuse-fieldtarget and produce the highest preference ratings, the OE(DF; orange and black) and free-field (FF; dashed) targets
headphones are the worst, and the IE headphones fall in(Møller et al., 1995), and the average frequency response (blue)
between. The retail price of a headphone is not a goodof 82 different models of AE headphones (Olive et al., 2018b).
indicator of its sound quality based on the relatively low
correlation values shown here.
These findings are generally in agreement with thosereported by Breebaart (2017). The two studies together
provide evidence that headphone designers are aiming
at a target curve that is closer to the Harman target thanthe DF or FF target curves recommended by the current headphone standards. Figure 5 shows the average
response of the 82 AE headphones in Figure 4 comparedwith the Harman, DF, and FF target curves (Møller etal., 1995). Although the DF and FF targets specify a flat
response below 200 Hz, the average AE headphone andHarman targets have 5-6 dB more bass, which better
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HEADPHONE SOUND QUALITYThe Harman target curve is one example that is preferred
by a majority (64%) of listeners from a broad range of ages,
listening experiences, and genders. Slight adjustments inthe bass and treble levels may be necessary to compensate
for variance in the quality of recordings and to satisfy individual tastes. The Less Bass Is Better class (21% of listeners)
includes a disproportionate percentage of females andolder listeners and none of the trained listeners. The More
Bass Is Better class is skewed toward males versus females
by a factor of 4 to 1. There is no evidence that sound quality preferences are geographically influenced. Recognitionof good sound reproduction seems to be universal.
Objective measurements of the headphones using standard ear simulators can predict how good they sound.
The further the frequency response a headphone deviatesfrom the Harman target response, the lower its perceived
sound quality will be. A simple linear model based onthese deviations can predict how listeners would rate itin controlled listening tests.
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 thedesign, testing, and review of many headphones frommultiple manufacturers, providing a much needed
new reference or benchmark for testing and evaluating
headphones. Several headphone review sites provide
frequency response measurements of headphones showing the extent to which they comply with the Harmantarget (Vafaei, 2018; Audio Science Review, 2020); incases where they fall short, corrective equalizations areoften provided.
As expected, there are also critics whose headphone tastes
in sound may not agree with the research. The Harmantarget is intended as a guideline and is not the last wordon what makes a headphone sound good. One legitimate
criticism is the limited number of headphones, programs,
female listeners tested, and questions raised about theconfluence of variables like hearing loss and its effecton headphone preference. Future studies will hopefully
address this. Finally, I hope that this article encourages
others to continue the research and improve our knowledge of the perception and measurement of headphonesound quality. Although listeners largely agree on whatmakes a headphone sound good, there are still many
unanswered questions and more to learn.
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Acoustics Today • Spring 2022

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About the Author
Sean E. Olive
sean.olive@harman.com
Harman International
8500 Balboa BoulevardNorthridge, California 91329, USA
Sean E. Olive studied music at the University of Toronto, Toronto, ON, Canada,and sound recording at McGill University, Montreal, QC,
Canada, where he received master’s and PhD degrees. He isa senior fellow in acoustic research at Harman International,
Northridge, California, focused on research in sound qualityof reproduced sound. He has published over 50 research
papers for which he received Fellow of the Audio Engineering Society (AES), two AES publications awards, and the
ALMA Titanium Award. Sean is the past president of AES.

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