Loudness in different rooms versus headphone reproduction: Is there amismatch even after careful equalization?
Michael Kohnen(1) , Florian Denk(2) , Josep Llorca-Boﬁ(1) , Michael Vorländer(1) , Birger Kollmeier(2)(1) Institute of Technical Acoustics, RWTH Aachen University, Germany, michael.kohnen@akustik.rwth-aachen.de(2) Medizinische Physik & Cluster of Excellence Hearing4All, Universität Oldenburg, Germany, ﬂorian.denk@uni-oldenburg.de

Abstract
Even though our expectation is that sound reproduced by loudspeakers in a room should exert the same perceived
loudness if reproduced by headphones with the same sound pressure level at the eardrum, ﬁndings from literature(e.g., Munson 1952[1], Fastl et al. 1985[2]) indicate a mismatch: Equal sound pressure levels at the eardrum do
not guarantee an equally perceived loudness. The aim of this study is to characterize this mismatch as a functionof room acoustics and headphone type. Subjects balanced the perceived loudness between presentations of
an identical stimulus from a loudspeaker and a headphone for three different rooms (anechoic, ofﬁce room,
reverberation chamber) and two different headphones (Sennheiser HD650 and Beyerdynamic DT770 PRO).
Additionally, a comprehensive acoustic characterization of the headphones with different measurement devices
was conducted. The results conﬁrm a loudness mismatch dependency on the room type, indicating that room
acoustics biases our loudness judgment. This is important for sound reproduction systems in rooms and thelimited ﬁdelity of room simulations even with carefully equalized headphones. The consequences of our ﬁndings
and necessary steps to pin down the origin of the observed mismatch will be discussed.
Keywords: loudness, room acoustics, sound reproduction

1

INTRODUCTION

Audio reproduction for plausible or even authentic scene representation is demanded in modern 3-D virtualand augmented reality systems. Especially in augmented reality applications, the user compares the perceived
sound image to an environment that is real and familiar from everyday life, lowering accepted tolerances formismatches. This investigation focuses on the perception of correct loudness in headphone reproduction as one
part of these mismatches. Loudness encodes information about the distance to a source and its geometricalattributes. For room auralizations it relates to the applied materials in the room in terms of reﬂection energy. If
a certain force is the cause of sound generation, loudness relates to the amount of that force. Finally, for virtualpresentations of humans, changes in loudness shifts the perception of the intended expression and respectively
mood to be emulated. Wrong loudness in realistic virtual and augmented reality therefore distracts the userfrom a full immersion into the scene. And ﬁnally, long term listening over headphones using higher gains tocompensate mismatches can be more hazardous than listening over loudspeaker.
Having a calculated reference SPL at the eardrum for the headphone reproduction, it might still differ in its perceived loudness compared to sound generated over the loudspeakers that results in the same measurable eardrum
SPL. This mismatch (also known as the missing 6 dB [1]) is known for over 70 years [3] now in the area of
loudness balancing. In blind comparison of headphone and free-ﬁeld listening and when the same waveform isreproduced at the ears using individual BRIR convolution and headphone compensation, no mismatch occurrs[4, 5]. However, in this study we assess the loudness mismatch in "classic" headphone listening without BRIR
convolution and headphone compensation, and in a non-blind comparison to loudspeaker presentation. As themismatch is investigated in different set-ups by different groups, one additional question is: How reproducible
are the fundamental measurements on which such a study relies? This work investigates the differences due todifferent measurement devices (namely a headphone test ﬁxture and KEMAR head and torso simulator). It ﬁrst
investigates the headphone measurements as a basis for a listening test done using these headphones. In the end

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results are analyzed, discussed and concluded. As part of the listening test, the inﬂuence of binaural correlation
was investigated. Evaluation for this can be found in [6].

2

HEADPHONE MEASUREMENTS

Figure 1. Picture of the KEMAR with Sennheiser HDA300 (left) and headphone test ﬁxture with Beyerdynamic
DT770 PRO (right).
To understand the inﬂuence of headphones, three different headphone types were measured on two different
devices with different ear simulators. Headphones measured were the Sennheiser HDA300 and the two headphones used for the listening test, the Sennheiser HD650 and the Beyerdynamic DT770 PRO. The ﬁrst one is afully closed reference headphone used for audiometric measurements and subject screening. The second one isan open headphone known for a ﬂat frequency response. The last one is characterized by its closed design. The

Figure 2. Frequency responses of the high-resolution ear simulator as used in the headphone test ﬁxture and the
low-noise ear simulator as used in the KEMAR artiﬁcal head and torso. The curves are divided by the soundpressure level at a frequency of 500 Hz and show an identical behavior ( ±0.65 dB ) for frequencies below 9kHz.
measurements for the eardrum sound pressure level were done on the one hand using a KEMAR 45BB-12 head
and torso simulator with anthropometric pinna and a low-noise ear simulator and on the other hand using the7880

GRAS headphone test ﬁxture 45CA-9 with high-resolution ear simulator and the same anthropometric pinna.
The main difference between the low-noise ear simulator and the high-resolution one is the frequency response
above 10 kHz. In ﬁg. 2 the frequency responses are shown for both ear simulators divided by their soundpressure level at 500 Hz. The high-resolution ear simulator has a suppressed resonance at 13.5 kHz (compared
to a standard IEC-711 ear simulator) while the low-noise has a double sloped resonance.

Figure 3. Measurements of the headphone transfer function for three different headphone types: Sennheiser
HDA300 and HD650 as well as Beyerdynamic DT770 PRO. Gain describes the factor from input voltage tomeasured sound pressure at the eardrum. The gray lines indicate single measurements. On each measurement
device the measurement was repeated eight times, each time the headphones were re-ﬁtted on the measurement
device. The mean values of the measurements are plotted as shown in the legend: red indicates the KEMAR
measurement, blue the headphone test ﬁxture measurement and solid lines the left, dotted lines the right ear.
For each measurement, the headphone was put on and off the measurement device to achieve different ﬁttings.
Eight repetitions (as suggested in [7]) were made using a exponential sweep of 2.7 seconds. The transfer
functions for the HDA300 can be found at the bottom of ﬁg. 3. Four different HDA300 were measured but
only two are plotted exemplary as the results do not differ much. The high absolute values are due to avery small impedance of the headphones. The results show a good match for left and right channel for eachmeasurement device. At frequencies above 10 kHz the results differ as can be expected due to the different
ear simulator types. Two main differences between the devices can be observed for all headphones of the type
HDA300: For low frequencies the measured level drops for the KEMAR and between about 1 kHz to 2 kHz
the behavior is inverse. The low-frequency behavior can be explained due to different coupling. The headphone7881

test ﬁxture as a plane surface around the ears where coupling is assumed to be best, as long as circum-aural
headphones are used. The same effect can be seen for the closed DT770. As the HD650 are open headphones
there is no such effect as can be seen in the upper part of ﬁg. 3. The difference in the mid-frequency regionis yet unsolved but might relate to the fact that the HDA300 do press on the ear and not fully surround it andlead to resonance due to a speciﬁc volume coupled to a leakage. The middle row of ﬁg. 3 shows the frequency
responses for the DT770. Here, only two devices were measured. Again a low-frequency difference can beobserved and a slight offset up to 10 kHz. Above 10 kHz again both ear simulators result different levels but
also seem to be less consistent between the two devices and measurements (e.g. notch depth at 15 kHz). Also
misalignment between left an right ear is minimal. In general both measurement devices seem to result thesame frequency response.
The HD650 is an open headphone and the measured frequency responses are shown in the upper row of ﬁg. 3.
Again, four different headphones were measured and two are shown exemplary. Left and right ear are aligned
as well as the differences between the measurement devices is smaller than the differences between different
measurements. The open design eliminates differences due to coupling. The only consistent difference is aslightly higher, ﬂat peak around 5 to 6 kHz.

3

METHODS

To pin down the mismatch in perceived loudness between headphone and loudspeaker reproduction, a listening
test was conducted. During the listening test, participants had to rate whether they perceived the loudspeaker(a GENELEC 8020C) or the headphone reproduction as louder, and the presentation level of the headphone
was adapted to equal loudness. As possible inﬂuence, different rooms as well as two different types of headphones were investigated. Additionally different stimuli were used to investigate the frequency dependence ofthe perception.
3.1 Listening test design
The listening test was designed as a 1-up-1-down adaptive 2-AFC procedure using the AFC Toolbox [8, 9].
The step-size was adapted to 10, 5 and 1 dB, with 10 dB being the step size until the ﬁrst reversal, and 1
dB in the measurement phase. Each stimuli was repeated three times and each block was done two times ineach room, once using the HD650 headphone and once with the DT770. The stimuli order, room order andheadphone order was randomized. As input device a pedal switch was used to start the stimulus and to respondwhether the loudspeaker or the headphone reproduction was perceived as louder. The pedals were labeled with’loudspeaker louder’, ’headphone louder’ and ’next’. Using the pedals the participants had both hands free tohold and put on and off the headphones. To reduce time and effort for putting on and off the headphones theorder of stimulus presentation was loudspeaker vs. headphone and then headphone vs. loudspeaker (and fromthe beginning). The stimuli were each calibrated using an omni-directional free-ﬁeld microphone (B&K 4190)
to a level of 70 dB (re 20μPa). The level was calculated excluding the reverberant tail of the stimuli recording.
3.2 Stimuli
To investigate the frequency dependency of the mismatch, different one third octave band noise (tbn) stimuli
were presented with mid-band frequencies of 250, 1000 and 4000 Hz using diotic playback for the headphones.
The 250 Hz stimulus represents low-frequency features with higher binaural interaction and a mismatch should
deﬁnitely be observable according to literature. 1000 Hz is the beginning of the transition from perceptionof phase cues to perception of level cues while 4000 kHz is above this transition and no phase interactioninﬂuences the perception. Additionally, the choice of a 1000 Hz tbn plus-minus 2 octaves make the ﬁndings
more comparable to other studies. As narrow-band excitation in different regions of binaural interaction might
neglect interaction of these effects, a uniﬁed excitation noise [10] with the same energy in the 17 auditory bands
around 1 kHz was used that covers the whole range of binaural interaction. The overall length of each stimulus
was one second inclunding 20 ms hanning ramps.

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3.3 Rooms
Three different rooms were used for the listening test. The rooms are located at the Institute of Technical
Acoustics in Aachen. The ﬁrst one is a hemi-anechoic chamber, the second one an empty ofﬁce room with akitchenette and the third one a reverberation chamber. All three rooms can be seen in ﬁg. 4.

Figure 4. The pictures show the three different rooms in which the listening test was conducted. From left toright: Hemi-anechoic chamber, ofﬁce room and reverberation chamber.
While the choice for the hemi-anechoic room and reverberation chamber as extrema was straightforward, theselection of the ofﬁce room underlies some preceding thoughts. The room should not be to dry, so that adifference to the anechoic chamber can be observed, yet it should not have a too long reverberation time or
distinct reﬂections such as ﬂutter echoes or focus points that lead to an unnatural room perception. Therefore,
an ofﬁce room that is almost empty was chosen. It has a reverberation time of 0.6 seconds (T20, averaged
between 0.5 and 1 kHz). Additionally, the room with kitchenette was chosen because it features some diffusion
and therefore avoids symmetric effects of the wall. Room impulse response measurements were done witha Genelec 8020C loudspeaker at a distance of 2.25 meters (as in the listening test). In the hemi-anechoic
chamber the ﬂoor was covered with absorbers to suppress ﬂoor reﬂections. The room is mainly characterized
by its volume of about 300 m3 and its lower cut-off frequency at 100 Hz due to the length of the wedges. The
reverberation time for the reverberation chamber is 4.3 seconds.
3.4 Results
Eight test subjects (6 male, 2 female) participated in the listening test (except for the reverberation chamberwhere one person is missing due to logistical reasons). The reference sound pressure levels were measured with
the KEMAR and low-noise ear simulators mounted. For loudspeaker reproduction, the KEMAR was positioned
at the point where the participants were seated. Each calibrated stimulus was played back and recorded ﬁvetimes and averaged. The stimuli used in headphone reproduction were measured with the same KEMAR in
the anechoic room using the two different headphone types. The measured stimuli were afterwards adjusted to
the results of the 2-AFC of equal loudness. The difference between the headphone level and the loudspeaker
level at the eardrum are analyzed, where positive values stand for a higher eardrum pressure needed during
headphone reproduction to perceive the same loudness. A three-way repeated-measures analysis of variance(ANOVA) was performed, predicting the mismatch values by the factors ’room’ (at the levels ’anechoic’, ’ofﬁce’
and ’reverberant), ’stimulus’ (at the levels ’tbn250’, ’tbn1000’, ’tbn4000’ and ’uen17’) and ’headphone type’ (atthe levels ’HD650’ and ’DT770’). Non-signiﬁcant results of Shapiro-Wilk tests (p > .05) and visual inspectionof model residuals through quantile-quantile plots suggested that the assumption of normal distribution was
fulﬁlled. Signiﬁcant main effects of Room [F(2,12) = 26.24, p < .001] and Stimulus [F(3,18) = 14.7, p < .001]
were observed, while the latter must be interpreted in presence of a signiﬁcant interaction between Stimulusand HP [F(3,18) = 11.02, p < .001]. The results can be found in ﬁg. 5 where the mismatch is plotted asmean values together with two times the standard error. In the upper left corner of ﬁg. 5 we see the inﬂuenceof the room on the mismatch of headphone and loudspeaker reproduction. Mismatches in the ofﬁce room aresigniﬁcantly different from those of the anechoic room (p = .006) and the reverberation chamber (p = .001).

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HD650
DT770

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Figure 5. Results of the three way repeated measures ANVOA. Plotted are the mean values and two times
standard error of each variable averaged over the other two factors (in the lower right only over the different rooms). The y-axis shows the mismatch of level when both playbacks (headphone and loudspeaker) wereperceived at same loudness, positive values mean that the eardrum level was higher for the headphone reproduction. Signiﬁcance levels are indicate at 0.001, 0.01 and 0.05 with three, two and one stars. The lower rightplot shows the results for a simple effect analysis, where the signiﬁcance level in blue (those on the horizontalaxis) only show differences between the stimuli for the DT770. The black (vertical) line indicates a signiﬁcantdifference between the headphones.
The upper right corner of ﬁg. 5 shows the inﬂuence of the stimuli. For low frequencies (250 Hz), an increased
mismatch can be seen that differs signiﬁcantly from all other (tbn1000: p = .002, tbn4000: p = .048, uen17:
p < .001). Anyhow, these ﬁndings have to be seen in an interaction with the headphone type. While the plot
in the lower left corner suggest a tendency for the DT770 to be perceived less loud (and therefore needing
more ampliﬁcation to excite the same loudness), the difference as such is not signiﬁcant but can be further
divided by the stimuli used. The lower right corner of ﬁg. 5 shows the interaction between headphone type andstimulus. A follow up simple effect analysis divided into the effects headphone type and stimulus was done toinvestigate the interaction between headphone type and stimulus using a univariate analysis of variance. Whileno signiﬁcant difference between the stimuli using the HD650 was found the use of a DT770 [F(3,86) = 8.63,
p < .001] does. A tukey post-hoc-test was done to ﬁnd that only the 250 Hz stimulus is signiﬁcantly different

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from all other stimuli using the DT770 (tbn1000: p < .001, tbn4000: p = .003, uen17: p = .002). Between theheadphones only a signiﬁcant difference [F(1,43) = 8.61, p = .005] can be found for the 250 Hz stimulus.
3.5 Discussion
The results for the stimulus and headphone interaction are mainly in accordance with those found by Fastl etal. [2] in terms of higher differences for low-frequency narrow band excitation as well as higher ampliﬁcationneeded for a closed headphone design to perceive the same loudness compared to an open design. Yet, theresults for a 1 kHz stimulus differ. This might be due to the fact that this work investigates narrow-bandnoise and not pure tones. Additionally, Munson and Wiener [1] also found a higher mismatch for the closed
headphone design and low-frequency excitation. The results for the reverberation chamber are not as expected.
Feedback from participants indicated an ’unnatural’ perception of the reverberation chamber, a disturbance bynoise they made (especially clicking of the pedal switch) and whether they should concentrate on the playback
itself or at the decaying process or both. Additionally, the pause between headphone and loudspeaker playback
was increased, as the decaying process needs time and participants should listen to the whole decay. Therefore,
the reverberant room might be unsuitable for loudness balancing as it introduces an additional dimension to theterm of loudness: a separate perception of playback and reverberation and an unwanted pause. It should be
noted that results indicate no interaction between the factor ’room’ and other factors, and the statistical results
for the other factors are not inﬂuenced by taking out the data for the reverberation chamber. Nevertheless, there
is a signiﬁcant difference between the mismatch in the anechoic and ofﬁce room, yet more data is needed toquantify and explain the difference in future studies.
3.6 Conclusion and outlook
A listening test was conducted to a) show that the missing 6 dB can be found using the setup and to b)
investigate whether the room has an inﬂuence or not. To explain the results, different methods to measure the
SPL at the eardrum when using headphones were investigated and especially the ﬁtting of the headphone seems
to affect low-frequency responses of closed headphones. The results indicate a good consensus with literature
data where low frequency and closed headphones lead to lower perceived loudness and additionally the results
suggest a signiﬁcant inﬂuence of the room, although results taken in the reverberation chamber can not fully be
explained. Yet, the inﬂuence of the room cannot be neglected and should be taken into account for headphone
reproduction of virtual or augmented realities.
Further listening tests are in progress at two facilities in Aachen and Oldenburg. At both sites, an anechoic
chamber is used and an ofﬁce-like room that differs between both sites. Therefore, two rooms apart from theanechoic rooms will be included. Furthermore, a limited number of test subjects will participate at both sites
in the listening test. The eardrum levels are measured using probe tube microphones to account for individual
characteristics (including the individual HRTFs) and screen individual headphone ﬁtting. Additionally, inﬂuenceof different source width and coloration perception is taken into account when comparing headphone to loudspeaker reproduction. To further investigate different reproduction methods in-ear transducer will be included.
And ﬁnally, a higher number of participants will increase the statistical power of the listening test.

ACKNOWLEDGEMENTS
The authors like to thank everybody who participated in the listening test voluntarily. The project was fundedby the Head Genuit Foundation under the project ID P-16/4-W and by the Deutsche Forschungsgemeinschaft(DFG, German Research Foundation) – Projektnummer 352015383 – SFB 1330 A4.

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