Telecommunications Industry Association
TR41.3.3-08-05-009-L
Document Cover Sheet
Project Number
Document Title
Free Field vs HATS - Handsfree Receive Spectral Response
Source
Texas Instruments
Contact
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Tom Harley
Texas Instruments
20450 Century Boulevard
Germantown, MD 20874
Phone: 301-407-9513
Fax: 301-515-7954
Email: tharley@ti..com
TR-41.3.3
X
For Incorporation Into TIA Publication
For Information
Other (describe) – For Posting on TIA TR-41 Web Page
The document to which this cover statement is attached is submitted to a Formulating Group or
sub-element thereof of the Telecommunications Industry Association (TIA) in accordance with the
provisions of Sections 6.4.1–6.4.6 inclusive of the TIA Engineering Manual dated March 2005, all of
which provisions are hereby incorporated by reference.
Abstract
Data is given for an exemplary narrowband handsfree IP-phone that complies with TIA-810B for a free
field measurement is shown. The same IP-phone, however, violates the TIA-810B handsfree receive
spectral mask when measured using a HATS with 3.4 type ear, including DRP to FF correction.
v1.0 – 20050426
Telecommunications Industry Association
TR41.3.3-08-05-009-L
Influence of P.340 Test Position on Spectral Characteristics
– Comparison of FF Microphone and Free-field equalized HATS Figure 1 shows two frequency response curves
measured during the SQTE on the XXXXXX.
The black curve was determined using the
artificial head’s right ear (free-field equalized).
This curve represents the “official” result,


the artificial heads microphone is
calibrated and equalized,
the HATS is in the ITU-T P.340 position
relative to the phone.
For comparison the red curve shows the
frequency
response
of
an
additional
measurement using a free-field microphone.
Note that the position of the artificial head was
not
changed
during
this
additional
measurement.


The microphone was positioned beside
the artificial head and the phone was
shifted on the table in order to obtain a
position acc. to ITU-T P.340 between
the phone and the microphone. The
measured curve for the free-field
microphone therefore includes potential
reflections from the HATS.
The microphone was not calibrated
because it only substituted the HATS
microphone for this informational test.
Figure 1: Frequency response of artificial head’s
right ear (black) and free-field
microphone (red)
Considering this calibration (offline) leads to the
curves shown in figure 2. The following
conclusion can be drawn:
Figure 2: Comparison of curves after considering
microphone calibration (offline)



Both curves are similar in the frequency range between 200 and 800 Hz. The maximum
difference can be measured to 2.5 dB around 500 Hz.
Around 1200 Hz the frequency response measured with the artificial head shows a resonance.
The frequency response of the free-field microphone shows a slight resonance around 2400 Hz.
These two curves are still influenced by parameters like reflections from the HATS (for the measurement
with free-field microphone), the test room characteristics (typical office room, reflections,…) and the
characteristics from the HFT (loudspeaker, directivity,…). The HFT is positioned on a table (acc. to ITU-T
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Telecommunications Industry Association
TR41.3.3-08-05-009-L
P.340) and not in front of the HATS in the horizontal plane, typically used for defining free-field
equalization for HATS.
In order to evaluated and exclude the influence of
the XXXXXX HFT loudspeaker the hands-free
phone was substituted by a small HiFi
loudspeaker. This loudspeaker was positioned in
the same test room and on the same table as the
phone during the SQTE (position acc. to ITU-T
Rec. P.340).
The same frequency response tests were carried
out, but the HATS was now removed when using
the free-field microphone. The resulting curves
are shown in figure 3 (transformation, not 1/3
octave). Again the black curve represents the
artificial head’s right ear response and the red
one the free-field microphone measurement.
Figure 3: Frequency response of artificial head’s
right ear (black) and free-field
microphone (red) for simulation setup
The same principles as shown above in figure 2 can be seen here:
 the responses are very similar in the frequency range between 200 and 800 Hz,

the resonance can be analyzed in the curve of the artificial head’s microphone. This resonance may
be introduced by the reflections and resonances of the artificial head, when the HFT loudspeaker
(positioned acc. ITU-T P.340 relative to the HATS) is not in the horizontal plane used for the definition
of free-field equalization of HATS.
Comparing the measured curves in figure 2 and 3 for the XXXXXX HFT and the HiFi loudspeaker the
following conclusions can be drawn:


Both measurements show the same characteristic effects. The resonance is introduced by the
reflections and resonances of the artificial head, when the phone and the HFT loudspeaker are
positioned acc. ITU-T P.340 relative to the HATS.
Slight deviations around 200 Hz and 4000 Hz are caused by the band limitation in the XXXXXX
HFT (narrow band telephony, G.711 speech coder, see blue arrows).
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Telecommunications Industry Association
TR41.3.3-08-05-009-L
The test of the HiFi loudspeaker uses a
wideband test signal. Figure 4 shows the result
from 100 to 10 kHz (for information).
Figure 4: Wideband frequency response of
artificial head’s right ear (black) and
free-field microphone (red) for simulation setup
In order to further investigate the resonance in the middle frequency range (around 1200 Hz for the HATS
analysis) the frequency responses were measured in an anechoic chamber for the free-field equalized
HATS and the free-field microphone. The same HiFi loudspeaker was used. The speaker was positioned
axial 3 m in front of the artificial head (horizontal plane).
Figure 5 and 6 show the frequency responses in a transformation and a 3 rd octave analysis for the freefield microphone (red) and the artificial head (black). The maximum peak-to-peak difference is less 3 dB.
The resonance shown in the curves above do not occur here. The notches which can be detected in
figure 5 are introduced by the size and geometry of the anechoic chamber. The loudspeaker and the
HATS respectively the measurement microphone needed to be positioned in one corner each in order to
achieve a 3 m distance.
Figure 5: Wideband frequency response of
artificial head’s right ear (black) and
free-field microphone (red) for simulation setup in anechoic chamber, 3 m
Figure 6: Wideband frequency response of artificial
head’s right ear (black) and free-field
microphone (red) for simulation setup in
anechoic chamber, 3 m distance, 3rd
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Telecommunications Industry Association
TR41.3.3-08-05-009-L
distance, trans-formation
octave.
An additional analysis was then carried out in the
anechoic by positioning the loudspeaker out of
the horizontal plane (similar to ITU-T P.340, but
without table). The measured curves shown in
figure 7 show now the formation of two
resonances,

one resonance around 1200 Hz for the
HATS analysis (black arrow) and

one resonance around 2400 Hz for the
free-field microphone (red arrow).
Figure 7: Frequency
response
measured
in
anechoic chamber, loudspeaker out of
horizontal plane
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Telecommunications Industry Association
TR41.3.3-08-05-009-L
These measurements lead to the following conclusions:

Free-field equalized HATS recordings and free-field microphone recordings lead to comparable
results under free-field conditions, if the source is positioned in the horizontal plane of the HATS.

Resonances are formed by the direction of the source relative to the HATS, e.g. the test position
acc. to ITU-T P.340. This could be expected due to the head and ear related reflections and
diffractions.

The resonances around 1200 Hz for the HATS recording is introduced by the source position
(phone position acc. ITU-T P.340 position).

This effect is probably intensified by the table, on which the phone is positioned and –perhaps additional room reflections.
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