TR41.9-07-05-014
Document Cover Sheet
Project Number
PN-3-3602-RV3
Document Title
TIA TSB31-C Rationale and Measurement Guidelines for U.S. Network Protection
Source
MWM Acoustics
Contact
Name:
Complete
Address:
Glenn Hess
Phone:
317-596-1721
Suite 520
Fax:
317-849-8178
Email:
hess@mwmacoustics.com
6602 East 75th Street
Indianapolis, IN 46250
Distribution
Intended Purpose
of Document
(Select one)
TR-41.9
X
For Incorporation Into TIA Publication
For Information
Other (describe) -
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
This document recommends changes to the hearing aid compatibility test procedures in section 15.1.5 of
TIA TSB31-C Draft 17f for magnetic field intensity. Sections 15.1.5.1 and 15.1.5.2 define test levels to
use at 1000 Hz based on the handset receiver’s acoustic output. Unfortunately, these procedures fall short
in specifying the artificial ear to use for measuring this acoustic level.
I propose using the Type 3.3 artificial ear described in ITU-T Recommendation P.57 for all proprietary,
special use, and IP-based telephones. The artificial ear is item (69) in the suggested equipment list of
TSB31-C section 5.5. This addition would be consistent with the artificial ear defined in TIA 470.110-C,
810-B, and the latest 920-A draft for measuring transmission performance of analog and digital wireline
telephones.
A second proposal would be expanding the frequency band used for measuring acoustic output in setting
test levels. Because of the response irregularity that occurs in the 1000 Hz region with handset telephones,
limiting this test to 1000 Hz only may produce erroneous results. I recommend a wider bandwidth
measurement from 200 to 4000 Hz with a broadband acoustic level determined.
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TR41.9-07-05-014
TIA TSB31-C section 15.1 was studied regarding test procedures for measuring hearing aid compatibility.
Draft 17f section 15.1.5.1 and 15.1.5.2 defined test levels based on the handset receiver’s acoustic output
at 1000 Hz. However, it fell short in stating the artificial ear to use when testing proprietary, special use,
and IP-based telephones.
In my experience measuring handsets and handset telephones, the artificial ear significantly affects
acoustic performance on traditional telephone handsets and newer handset designs. Our work at MWM
Acoustics has shown that handset telephones can pass or fail TIA and FCC requirements depending on
the artificial ear and test fixture used.
This acoustic performance variability is shown in measurement data collected on two handsets produced
and sold in the North American market. The AT&T K2M2 handset was designed over twenty years and
used on numerous consumer telephone products. More recently, the Avaya S1K1 handset is used with
wideband IP business phones sold in the marketplace today.
The measurement data presented below shows frequency response as well as 1000 Hz levels. Acoustic
testing was limited to the handset only with measurements performed on the IEC 318 coupler (Type 1)
and B&K HATS soft pinna (Type 3.3). All tests were conducted per IEEE 269-2006 and ITU-T
Recommendation P.57.
20
AT&T K2M2 HANDSET
RECEIVE FREQUENCY RESPONSE
Level (dBPa)
10
0
-10
-20
-30
100
Frequency (Hz)
50mV Input at Receiver
IEC 318 = 0.24 dBPa @ 1 kHz
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Type 3.3 5.33 dBPa @ 1 kHz
10000
TR41.9-07-05-014
20
AVAYA S1K1 HANDSET
RECEIVE FREQUENCY RESPONSE
Level (dBPa)
10
0
-10
-20
-30
100
Frequency (Hz)
50mV Input at Receiver
IEC 318 = -5.14 dBPa @ 1 kHz
10000
Type 3.3 -11.63 dBPa @ 1 kHz
These results clearly show how acoustic performance changes depending on artificial ear. Both frequency
response and level are affected in varying degrees from the older acoustic design to newer handsets
available today. These results are not surprising and vary based on the transducers selected and acoustic
design implemented.
If the focus is limited to the 1000 Hz test frequency as prescribed in TSB31-C, acoustic output levels
differ between artificial ears by 5-6 dB in the handsets selected for this experiment. Typically, traditional
handsets like the K2M2 are sensitive to the acoustic leakage conditions. Of course, the 1000 Hz results
shown above have direct implications on the test level to use for subsequent HAC magnetic testing.
Because of the problems revealed in the graphs, I propose additional steps be taken to define one artificial
ear in conjunction with a broadband acoustic output measurement from 200 to 4000 Hz. I recommend the
Type 3.3 artificial ear be used exclusively for measuring acoustic output in setting the magnetic intensity
test level. This ear simulator is also prescribed for TIA 470.110-C, 810-B, and latest 920-A draft.
The receive frequency response data shows both handsets having peaks and irregular responses across
frequency, including the 1000 Hz point where test levels are based. Here, I propose an overall broadband
acoustic measurement from 200 to 4000 Hz. This frequency range was chosen to be identical to the one
defined for calculating loudness ratings in the TIA standards referenced above.
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