Proceedings of 20th International Congress on Acoustics, ICA 201023-27 August 2010, Sydney, Australia

Fast near-field HRTF measurements
using reciprocal method
Noriyuki MATSUNAGA, Tatsuya HIRAHARA
Faculty of engineering, Toyama Prefectural University, Imizu, Japan

PACS: 43.58.Gn

ABSTRACT
A fast head related transfer function (HRTF) measurement system based on the Helmholtz reciprocity principle wasbuilt, and near-field HRTFs were measured using two types of miniature speakers. The HRTF measuring system canmeasure 36-channel HRTFs at a time using five word clock analog-to-digital convertors, which are PreSonus multitrack recording equipment. HRTFs can be measured with good signal-to-noise ratios from 250 Hz to 13 kHz using a
DTEC-30008 (Knowles Electronics); the miniature speaker has a high output level in the low and the mid frequency
range. HRTFs can be measured with good signal-to-noise ratios from 2 to 20 kHz using an ED-29689 (Knowles Electronics); the miniature speaker has a high output level in the high frequency range. HRTFs cannot be measured accurately below 250 or 400 Hz with either speaker because the signal-to-noise ratio of the TSP signal responses is toolow. Although the sound pressure level is higher at near-field (0.2-m distance) than at far-field (1-m distance), thesignal-to-noise ratio below 400 Hz is not sufficient in either case. These results indicate that the reciprocal HRTF
measuring system can measure HRTFs from 400 Hz to 20 kHz quickly and accurately; the frequency range, however,depends greatly on the frequency response of the miniature speaker placed in the outer ear canal.

INTRODUCTIOThe head-related transfer function (HRTF) is an acoustic
transfer function, which varies depending on the shape of thehead and ear, and sound source position. The HRTF is obtained by repeated measurements of impulse responses froma speaker, placed away from the subject’s head, to a microphone placed at the entrance of the subject’s ear and with the
speaker changing positions. This traditional HRTF measuringmethod is called the direct method. As the position of the
speaker has to be changed, HRTF measurements with thismethod take a long time. Thus, there is an increase in serious
measurement errors, because subjects cannot keep their own
heads still during such long measurements [1].
A fast HRTF method of measurement using the Helmholtz
reciprocity principal proposed by Zotkin et al. can overcome
this problem [2]. The reciprocal method is used to measure
HRTFs by placing a speaker at the ear and placing microphones around the head. This method is not commonly usedbecause there are few miniature speakers that can be inserted
into the ear canal. The low-output sound pressure level of the
speakers is another issue. We built a reciprocal HRTF measurement system. We then measured horizontal HRTFs thatwere 10 degrees apart at distance of 1 m. We tested and confirmed that the reciprocal method could measure HRTFs wellin the frequency range where the earplug speaker produces
enough sound pressure.
A number of measurements of HRTFs in the far-field havebeen done and some of the measured HRTF data sets areavailable. Very few measurements of HRTFs in the near-field,
however, have been reported thus far. Loudspeakers used formeasuring HRTFs in the far-field cannot be used as sound
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sources when measuring HRTFs in the near-field. This isbecause the speaker cannot be regarded as a point soundsource as it is too large compared with the head. A special
sound source such as a spark noise source or a very small
dodecahedral loudspeaker is required to measure HRTFs in
the near-field [3, 4].
Another way of measuring HRTFs in the near-field is tomake use of the reciprocal method. As previously described,
the reciprocal HRTF measurement system uses a small
speaker at the ear position and small microphones around thehead. Each of the microphones is sufficiently small, thus they
do not disrupt the measurement of acoustical impulse responses. The very same measurement system can be usedboth in far- and near-field HRTFs. This paper describes thereciprocal HRTF measurement system we built and the nearfield HRTFs we measured with the system.

MEASUREMENT SYSTEM
Reciprocal HRTF measurement system
The reciprocal HRTF measurement system consists of a microphone array with fixed microphone sockets 10 degrees
apart and an earplug speaker inserted into a dummy-head’s
ear canal (Figure 1(a)). The microphone array consists of acircular piano wire (1.5-mm diameter) with a radius of 0.2 m
and 36 microphones. The microphones were bound to the
piano wire with vinyl covered wire, and arranged in a circle
with a 0.2-m radius, as shown in Figure 1(b). A highsensitivity small electret condenser microphone (Primo,
EM133) was used as the system’s microphone. Miniature
speaker (Knowles Electronics, DTEC-30008 or ED-29689)
was used as the earplug speaker. The speaker was embedded
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The following electroacoustic equipment was used in themeasurement system (Figure 2). The time-stretched pulse(TSP) signal was digital-to-analog converted with a USBaudio interface (EDIROL, UA-101), then fed to the earplug
speaker through a headphone amplifier (audio-technica, ATHA20). The TSP response signals received by the microphones were amplified by 33 dB (audio-technica, AT-MA2),
then fed to the analog-to-digital converters (PreSonus,
DIGIMAX FS) connected to a FireWire-audio interface(PreSonus, FireStudio Lightpipe). The command to start
recording was sent from PC1 to PC2 and 3 using TCP/IP. A
reference signal was put into the analog-to-digital convertors
3 and 4 to adjust the time of recording. A word clock wasused to synchronize the analog-to-digital convertors and the
FireWire-audio interfaces.
The sampling frequency of the analog-to-digital and the digital-to-analog converters was 48 kHz. The TSP signal was a65,536-sample-long optimized Aoshima’s time-stretchedpulse (OATSP) [6]. The measurement room was a soundproof room (3.24×3.54×2.30 m). The A-weighting level ofbackground noise was 16.5 dB. As one measurement session
consisted of the TSP signal repeated 20 times, the total measurement time was about 30 s.
Earplug speakers
The miniature speakers, DTEC-30008 and ED-29689, werebalanced-armature electro-acoustics transducers. The transducers were originally designed for the driver-units of inserttype earphones. Thus, they did not generate large sound pressure levels in open-air space. The DTEC-30008 was
7.87×4.09×5.61 mm and the input impedance was 39 Ω at 1
kHz. The ED-29689 was 6.32×4.31×2.99 mm and the input
impedance was 10.2 Ω at 1 kHz.
Figure 3 shows the frontal frequency responses of the two
speakers and noise-floor spectrum level. The red and blue
thick lines plot the responses of the DTEC-30008 and ED29689 measured with a microphone placed at a distance 0.2
m from the front. The dotted lines are those measured at a
distance 1 m from the front. The sound pressure level produced by the speakers was about 12 dB more at the distanceof 0.2 m than that at 1 m. Both speakers produced larger
sound pressures than the noise-floor level except for the low
frequency range. The signal-to-noise (SNR) of the DTEC30008 was over 20 dB above 250 Hz for the 0.2-m distanceand above 500 Hz for the 1-m distance. It produced sufficientsound pressure in the higher-frequency range for both distances. The SNR of the ED-29689 was over 20 dB above 400
Hz for the 0.2-m distance and 700 Hz for the 1-m distance. It
produced a higher sound pressure than the DTEC-30008 in
the higher-frequency range for both distances. The DTEC30008 had a preference for low frequencies and the ED29689 had a preference for high frequencies.
Figure 4 shows variations in frequency responses for 12 horizontal directions that are 30-degree apart. Both speakers wereomnidirectional below 8 kHz but not omnidirectional above 8
kHz. They radiated more intense sound from their front thantheir rear. The DTEC-30008 had several deep notches in thefrequency responses at certain angles. A deep notch appeared
at 13 kHz in a rear position response. A deep notch also appeared around 16 to 17 kHz in several frontal position responses. This directivity of radiation by speakers could adversely affect HRTF measurements.
2

(a)

(b)

(c)

Figure 1: Reciprocal system for HRTF measurement (a)Overview of system, (b) Close up of microphone, and (c)earplug speaker.

LAN
PC1

ADC1
ADC2

DAC

A/I

PC2

A/I

PC3

ADC3
ADC4

Reference
signalADC5

Figure 2: Block diagram of electroacoustic equipment.

80
Sound Pressure Level [dBSPL]

in a silicon impression material, as shown in Figure 1(c). The
dummy-head was an epoxy-based resin made in the shape ofa human’s with a rapid prototyping system and filled with an
acoustic absorbent [5].

Proceedings of 20th International Congress on Acoustics, ICA 2010

0.2 m
1m

60
40

DTEC-30008
ED-29689200Noise floor
-20
0.1

0.5 1
5
Frequency [kHz]10

20

Figure 3: Frequency responses of earplug speaker in nearfield.

DTEC-30008
Power spectrum [dB]23-27 August 2010, Sydney, Australia

θ= -90 deg.
θ= 0, 30, 60, 150 deg.
ED-29689
20 dB
0

2

4

6
8 10 12 14 16
Frequency (linear scale) [kHz]18

20

Figure 4: 12-direction (30-degrees apart) frequency responses of two miniature speakers in horizontal plane.

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Proceedings of 20th International Congress on Acoustics, ICA 2010

RESULT

Further, there are several spurious spectral peaks in the highfrequency range around 13-17 kHz in the HRTF contour
patterns measured with the DTEC-30008 miniature speaker(Figures 5(a) and 6(a)). The spectral peaks have anomalies inamplitude and appear at certain azimuthal angles. Such spectral peaks cannot be seen in the HRTF contour pattern measured with the ED-29689 miniature speaker. Thus, there
should have been some acoustical anomalies that influenced
the HRTF measurements with DTEC-3008.

Figure 5 has typical examples of near-field horizontal HRTF
contour patterns measured with the reciprocal method using(a) DTEC-30008 and (b) ED-29689 miniature speakers. Figure 6 has typical examples of far-field horizontal HRTF contour patterns measured with the reciprocal method using (a)DTEC-30008 and (b) ED-29689 miniature speakers.
The near-field and far-field HRTF contour patterns measured
with DTEC-30008 have a familiar horizontal HRTF contour
pattern. The HRTF spectra have peaks on the ipsilateral side
(left side) and notches on the contralateral side (right side).
The notch frequencies vary according to the azimuthal angle.
They are very similar to HRTFs measured with the conventional direct method or numerically computed HRTFs [7].
There are, however, several spurious spectral peaks andnotches in the low frequency ranges.
HRTF spectra should be flat around 0 dB in the low frequency range, as the head has no effect due to the long wavelength compared to the size of the head. Thus, the spectral
peaks and notches that appeared below 250 Hz in the nearfield HRTF contour pattern (Figure 5(a)) and those that appeared below 500 Hz in the far-field HRTF contour pattern(Figure 6(a)) should be measurement errors. In the same way,
the spectral peaks and notches that appeared below 400 Hz in
the near-field HRTF contour pattern (Figure 5(b)) and below
700 Hz in the far-field HRTF contour pattern (Figure 6(b))
should be measurement errors.
In addition, spectral coloration can be seen in the mid frequency range, i.e. 400 Hz to 2 kHz, of the HRTF contour
patterns measured with the ED-29689 miniature speaker(Figures 5(b) and 6(b)). Such spectral coloration is not seenin the HRTF contour pattern measured with the DTEC-30008
miniature speaker. Thus, there should have been some acoustical reflection that influenced the HRTF measurements with
ED-29689.

DISCUSSIONAs seen in Figures 5 and 6, the near-field HRTFs could bemeasured better with the reciprocal method than the far-field
HRTFs. The closer the distance between the speaker and themicrophones, the more intense the sound pressure the TSP
signal. However, the measured near-field HRTFs still hadband-width problems. The effective band-width of the
HRTFs was limited by that of the miniature speaker used in
the measurements.
When the DTEC-30008 miniature speaker that had a preference for low frequencies was used as the earplug speaker, thenear-field HRTF could be measured without error from 250
Hz to 13 kHz. Below 250 Hz, the low SNR of the received
TSP signal affected the measurements of acoustical impulse
responses. Above 13 kHz, the speaker response suffered fromthe directivity of radiation and deep notches appeared in therear and frontal position responses. These notches turned out
to be spectral peaks due to the normalizing procedure, where
the spectrum of the TSP response received at the entrance ofthe ear was divided by that at the center position of the head.
When the ED-29689 miniature speaker with the preference
for high frequencies was used as the earplug speaker, thenear-field HRTF can be measured without error from 2 kHz
to 20 kHz. Below 400 Hz, the low SNR of the received TSP
signal affected the measurements of acoustical impulse responses. From 400 Hz to 2 kHz, coloration by an unknown
source modulated HRTFs. The microphone array may have

180

180

0

0
-10

-90

-20
-180
180

(b)

90

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-10
-20

-180
180

(b)

90

-90

-90

Figure 5: Near-field (0.2-m distance) HRTF contour patternsof left ear of dummy-head. (a) Using DTEC-30008 and (b)using ED-29689.

0

-90

0

1
10 20
Frequency [kHz]

10

0

0

-180
0.1

20

90

Azimuthal angle [deg.]

10

Power spectrum [dB]90

Azimuthal angle [deg.]

(a)

20

Power spectrum [dB]

(a)

-180
0.1

1
10 20
Frequency [kHz]Figure 6: Far-field (1-m distance) HRTF contour patterns ofleft ear of dummy-head. (a) Using DTEC-30008 and (b) using ED-29689.
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Proceedings of 20th International Congress on Acoustics, ICA 2010

CONCLUSION

Azimuthal angle [deg.]

Power spectrum [dB]
-20 0 20 40 60
180

0

-180

(a)
0.1

1

1
10 0.1Frequency [kHz](b)
10

Figure 7: Near-field transfer function contour patterns of (a)HEar and (b) HCenter measured with ED-29689.
caused reflection. In Figure 7(a), we can see the impulseresponse-spectrum contour pattern with the head, i.e., headrelated impulse response spectra HEar and in (b) those without
the head, i.e., head-center impulse response spectra HCenter at
the 0.2-m distance. Figure 5 (b) was obtained by dividing
Figure 7(a) by Figure 7(b) in the complex frequency domain,
HEar/ HCenter. Although the same microphone array was used
to measure both HEar and HCenter, coloration is only seen inHEar but not in HCenter. Consequently, the microphone array
did not cause the reflection. The horizontal stripes in HEar and
HCenter are due to differences in the microphone sensitivity
and microphone amplifier gain for each channel. We need tosearch for the cause of coloration in future studies.
Usually, HRTFs have been measured over a wide frequency
range from several hundred Hz to the Nyquist frequency, i.e.,
half the system sampling frequency. Even in the HRTF
measurements with the direct method, accurate measurements of acoustical impulse responses in the high frequency
range are not very easy as small geometrical differences affect the results. Subjects can hardly continue to hold theirhead position still during long HRTF measurement sessions[8]. Is such a wide frequency range necessary in practicaluse?
The physical acoustic indicated that the HRTF spectra should
be flat around 0 dB at low frequency. There is no need tomeasure HRTF accurately below several hundred hertz. The
psychophysical acoustics indicated that mid frequency spectral components are important to localize sound sources.
Morimoto et al. reported that spectral components from 4.8
to 9.6 kHz are necessary to carry out sound localization in the
median plane [9]. Morikawa et al. found that spectral components from 2 to 12 kHz are necessary to conduct soundlocalization in the horizontal plane [10]. There seems to be
no need to measure HRTF accurately above a dozen kilohertz
and below a few kilohertz.
The reciprocal system for HRTF measurements we built
could measure 36 channel HRTFs in 30 s. The measuring
time theoretically does not depend on the number of channels.
The more microphones and analog-to-digital converters wehave, the more channels of HRTF can be measured in 30 s.
These very fast HRTF measurements can avoid the problems
with conventional HRTF measurements systems using thedirect method. We are now free of measurement error causedby subject head movements and changes in temperature andhumidity during measurements. This is the greatest advantageof reciprocal HRTF measurements. The narrow-frequency
range of the earplug speaker is the most serious issue withreciprocal HRTF measurements. We need to search for verysmall loudspeakers that can produce sufficient sound pressure
over a wider frequency range in future work.
4

We measured near-field HRTFs with the reciprocal method
using two types of miniature speakers. We obtained six main
results.
1
The near-field HRTFs could be measured better with thereciprocal method than with far-field HRTFs.
2
HRTFs could be measured better with a good SNR from250 Hz to 13 kHz using the DTEC-30008, which is aminiature speaker with a preference for low frequencies.
The directivity of the DTEC-30008 was the cause of the
spurious spectral peak that appears in HRTFs above 13
kHz.
3
HRTFs could be measured with a good SNR from 2 to
20 kHz using the ED-29689, which is a miniature
speaker with a preference for high frequencies. The
cause of coloration that appeared in the HRTFs from400 Hz to 2 kHz remains unknown.
4
The reciprocal measured HRTF involved large measurement errors below 250 or 400 Hz in both speakers
because of the low SNR of the TSP signal. The SNR of
the TSP signal received by microphones should be 20
dB or more to measure HRTFs without errors.
5
The effective frequency range of the reciprocal HRTF
measurements depended greatly on the frequency response of the miniature speaker used as the earplug
speaker.
6
The effective frequency range of the reciprocally measured HRTFs was not very wide but it was sufficient tocarry out sound localization in practice.
Acknowledgement
Part of this work was supported by KAKENHI (22300061).

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2
3
4
5
6
7

8
9

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