TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
Proposal to Revise the Acoustic Limits in UL 60950-1
RATIONALE
Proposal submitted by: Allen Woo, Plantronics, on behalf of TIA TR47.1 and supported
by the Technical Harmonization Committee.
Rationale for Changes to NAD.3, Short-duration Impulses
The current 136 dB limit at the ERP does not agree with the OSHA regulations from
which it was derived. The proposed change brings the limit back to the original 140 dB
limit and makes it clear that the limit is based on a diffuse field measurement. This
means that it is necessary to translate all measurements made at the couplers to the
diffuse field as specified in IEEE 269 and ETSI EG 202 518 which aligns with the
requirements in the OSHA regulation and EU Directive 2003/10/EC.
The Original Limit:
The original limit was 140 dB based on the OSHA limit of 140 dB measured in the open
field 1. This was identified as conservative. In an EIA report to UL dated November
1984, they stated:
"The OSHA limit of 140 dB peak (and 100 exposures per day) for impact or impulse noise
is recommended without change. Criterion for repeated gunfire noise has recommended
limits of 152 dB peak for simple peaks (telephone clicks fall in this category) and 140 dB
for oscillatory decay. Although 140 dB is quite conservative, it is a valid criterion because
impulse noise is common on telephone lines."
1
OSHA does not specifically identify the 140 dB limit as "free field", "open field" or
"diffuse field". However, their measurement method describes actual noise exposure
received by a person in an "open-field" environment. An "open-field" environment is an
environment where the sound or noise sources are at a distance from a person's ear.
The sound or noise environment can be a combination of more acoustic fields, i.e. free,
partially reflected, diffused and reverberant fields. For measurement purposes it is not
possible to recreate a true "open field" environment. A diffuse field measurement is
considered the closest representation of an open field environment.
Diffuse Field Measurement:
The original peak limit in UL 1459, where acoustic limiting was first introduced for
telephones, was 140 dB. This was based on OSHA limits for similar impulse noise
measured in a diffuse field. However, translating the measured value from the ERP
(where the measurement is actually made with the 318 coupler) to the open field (where
the requirement came from) was overlooked. The error was further made worse when
the in-ear coupler (Type 2 coupler) was added for insert earphones which measures at
DRP. There was never an attempt to translate measurements made with either coupler
(which are different from each other, one ERP, one DRP) to the open field as specified
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
by the OSHA requirements. A translation to the diffuse field was never included as it
should have been. See Figure NAD.2 and NAD.4 for the translations.
The Change to 136 dB:
In researching the change to 136 dB, TR41.7 found the following regarding when the
change was made and why.
In July 28, 1995, (more than 10 years after the original requirements were established)
UL published a "Working Document for the Standard for Safety of Information
Technology Equipment, Including Electrical Business Equipment; CSA C22.2 No. 950 *
UL 1950 - Third Edition. In it under deviation No. 317 the original 140 dB limit was still
included, with the source document coming from the TIA that were essentially the
original UL 1459 requirements; 140 dB using the two different couplers. There was an
associated note that stated:
This deviation is contained in the IEC proposal mentioned in deviation No. 316. At the
October 1993 meeting, it was decided to replace this deviation with the revised proposal
being pursued through the IEC. See deviation No. 317a.
Deviation No. 317a included the text from an IEC proposal on acoustic limiting that
contained the 136 dB limit. It appears that the change was made to the proposed UL
1950 to try and harmonize with anticipated requirements for IEC 950. Even though the
IEC never adopted acoustic requirements, the 136 dB stuck for UL 1950 and was later
carried over to UL 60950.
Regarding the current requirements in UL 60950-1 that do mention the ERP, since the
original requirements in UL 1459, UL 1950 or UL 60950 did not mention open field, ERP
or DRP, and the original reference ear coupler was the 318 (Type 1), it was natural to
assume that the measurement and limit was at the ERP and this is what was included
into the present UL 60950-1 requirement.
The current ETSI (European Telecommunication Standard Institute) EG 202 518 sets
the Peak Limit the same as the Directive 2003/10/EC at the equivalent diffused field:
Peak Sound Pressure level (Ppeak)
Exposure limits
140 dBSPL(C)
Upper action values
137 dBSPL(C)
Lower action values
135 dBSPL(C)
"The noise exposures from handset and headset terminal equipments are different from
the "open-field" environment. They are localized at or inside of users' ear... ...
... ...
... ...It is necessary to transfer the sound pressure level measured at the DRP to the
"open-field" ... ...
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
It is recommended to use a real time filter to carry out the DRP to "open-field" head
related transfer function (HRTF) translation. This is especially important for peak
measurements as both magnitude and phase of the measurement must be best
preserved in order to accurately evaluate the results. A minimum phase filter shall be
used.
The HRTF filter translation could be accomplished mathematically using a software
algorithm to process digitally sampled and stored waveforms. It is the responsibility of
the test house to ensure such methods accurately emulate the preferred real time filter
method."
Rationale for Changes to NAD.4, Long-duration Disturbances
This proposal establishes the limit of 125 dB for handsets and 118 dB for headsets (e.g.
over the ear, in-ear, insert earphones) with all measurement results referenced to ERP.
This provides consistency no matter what type of coupler is used for the measurement.
Original Requirements for Hand-Held Receivers
The original limit established for UL 1459 was 125 dB for hand held receivers using the
IEC 318 Coupler (Type 1) that measures at the ERP.
Receiver Sound Level
A. The maximum telephone receive level shall not exceed 125 dBA
steady state with the telephone in any operating state or with a 1000 Hz
signal of 10 volts rms or greater applied between the tip and ring
terminals of the telephone. Response is measured using an IEC coupler
for supra-aural earphones per ANSI S3.7-1973 with a type M
microphone per ANSI S1.12-1967 (R1977). The sound pressure within
the artificial ear is measured with a sound level meter set to use an Aweighted slow response.
Steady state is defined as an acoustic response having a duration of 500
milliseconds or longer. This criterion is based on the assumption that the
telephone receiver will not be held against the ear for longer than 2
seconds under these conditions.
See the document "841015 EIA Receiver Sound Level.pdf" from October 1984.
[Note from the STP Project Manager: To view this document, click on the
magnifying glass icon next to "Supporting Documentation" under the Quick View
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Original Requirements for Headsets and Earphones:
From the December 14, 1987 UL 1459 Meeting report on revised requirements:
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
Headsets and earphones should have greater limiting because of the higher usage
associated with these devices. A level of 118 dBA is based on the capability of headsets
that have been used successfully for many years. Since the ear canal produces about 3
dB of amplification, an earphone producing 121 dBA creates the same sound pressure
on the eardrum as a headset at 118 dBA. Therefore, headsets and earphones have
effectively the same criterion.
This added 118 dB for headsets based on a longer than 2 second exposure since it
takes a user longer to remove a headset than to pull a handset away in the event of a
loud noise. This was still measured using the IEC 318 (Type 1) coupler that measures
at the ERP.
It also added a limit of 121 dB for insert earphones which was derived at the time from
the 118 dB ERP limit translated to a measurement made with an insert earpiece coupler
at the drum reference point (DRP).
Using a single coupler (Type 3.3 from IEEE 269) for on-ear and insert ear couplers
invalidates the different requirements for insert ear pieces and on-ear headsets when all
of the measurements are referenced back to the ERP. Although different couplers are
still allowed where appropriate, IEEE 269 instructs that all of the measurements,
regardless of the coupler used, should be referenced back to the ERP. The original limit
for measurements taken at the ERP is 118 dB.
Although the difference in the requirements for handsets and headsets (based on time
of exposure) remain valid, because of the variations of headphones, earpieces, etc, it is
also no longer valid or practical to base the requirements themselves on the type of
earpiece or coupler used.
PROPOSAL
Note: All of Annex NAD is a D2 national difference. However, underlining to indicate text
added to IEC 60950-1 is not used in this annex. Proposed changes are indicated with
(NEW) and (CURRENT) and (PROPOSED) headings.
(NEW)
NAD.1.2 Definitions
NAD.1.2.13.20 DIFFUSE FIELD: A sound field with a high number of reflections that, at
any given point in the diffuse field, sound arrives from all angles in a uniform manner.
(CURRENT)
NAD.3 Short-duration impulses
The peak acoustic pressure measured at the earpiece or receiver of the
communications handset or headset shall be limited to reduce the risk of permanent
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
hearing damage due to short-duration impulses ( 0,5 s) that can occur under normal
operation.
In addition, the equipment also shall be checked for self-generated acoustic impulses
such as those produced by operation of the hook switch or by dialing.
Compliance is checked by following the methods described in NAD.3.1 or NAD.3.2.
During the above tests, the peak acoustic pressure level measured in the artificial ear or
coupler shall not exceed 136 dB (relative to 20 µPa) at ear reference point (ERP).
(PROPOSED)
NAD.3 Short-duration impulses
The peak sound pressure level (SPL) of short duration impulses (< 0.5 s) from an
earpiece or receiver of a communication handset or headset shall not exceed an
equivalent diffuse field SPL of 140 dB; relative to: 20 µPa.
Compliance is checked by the measurement methods described in NAD.3.1 or
NAD.3.2. The methods utilize artificial ears for testing. Artificial ears measure SPL at
eardrum reference point (DRP) or ear reference point (ERP) depends on the type of the
artificial ear. If either the DRP or ERP measured SPLs are below the 140 dB limit,
compliance can be assumed.
NOTE 1 Compliance can be assumed because a measured equivalent diffuse field SPL by nature of it being an open field measurement is
always lower than a measured DRP or ERP SPL.
If either the DRP or ERP measured SPLs exceed the 140 dB limit, the measured SPL
shall be translated to the equivalent diffuse field and compared to the limit again.
Appropriate correction factors in Table NAD.1 or Table NAD.2 shall be used for the
translation, e.g. SPL(diffuse field) = SPL(DRP) + SDDff (correction factor for DRP to
diffuse field) or SPL(diffuse field) = SPL(ERP) + SEDff (correction factor for ERP to
diffuse field).
It is recommended to apply the corrections with a real time (minimum phase) filter.
NOTE 2 The correction outputs from the filter should be as close as possible to the appropriate graph in Figure NAD.2.
The test setup is illustrated in Figure NAD.3.
Alternatively, although not recommended, the DRP or ERP translations to the diffuse
field may be accomplished mathematically using a software algorithm to process
digitally sampled and stored waveforms. If this method is used, the method shall
accurately emulate the preferred real time filter method.
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
NOTE 3 A given amplitude response can in theory be produced by an infinite set of filters, each of which creates a different phase response.
A phase response that is a non-linear function of frequency smears the response in time. The filter that produces the amplitude response
with the least time-smearing is the minimum phase filter of the set. Most simple equalizers are minimum phase filters.
NOTE 4 For equalizers (e.g. programmable digital equalizer) with broader frequency bandwidth than the given correction factors’ frequency
bandwidth in Tables NAD.1 and NAD.2, the setting of the equalizer at outside of the correction factors’ bandwidth should be 12 dB per 1/3
Octave attenuation. For example, if the highest frequency for the ERP to Diffuse Field correction factor is 8 kHz (0.9 dB), the setting at the
next 1/3 frequency, 10 kHz, should be -11.1 dB.
Table NAD.1 - Correction factors to convert DRP SPL to diffuse field SPL
Frequency, Hz
SDDff
Frequency, Hz
SDDff
100
0.0
1250
-5.5
125
0.0
1600
-7.5
160
-0.0
2000
-11.0
200
-0.0
2500
-14.0
250
-0.0
3150
-14.5
315
-0.5
4000
-13.5
400
-1.0
5000
-11.5
500
-1.5
6300
-10.0
630
-2.5
8000
-11.0
800
-3.5
10 000
-11.0
1000
-4.0
NOTE 6
SDDff The correction from DRP to Diffuse Field
SDDff = 20 log10(PDff/PD)
Where: PDff is SPL at diffuse field
PD is SPL at DRP
Table NAD.2 - Correction factors to convert ERP SPL to diffuse field SPL
Frequency, Hz
SEDff
Frequency, Hz
SEDff
100
0.0
1000
-2.5
125
0.0
1250
-3.0
160
0.0
1600
-3.5
200
-0.0
2000
-4.5
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
250
-0.0
2500
-4.5
315
-0.5
3150
-4.0
400
-0.5
4000
-7.0
500
-1.0
5000
-8.5
630
-1.5
6300
-7.0
800
-2.0
8000
0.5
NOTE 7
SEDff The correction from ERP to Diffuse Field
SEDff = 20 log10(PDff/PE)
Where: PDff is SPL at diffuse field
PE is SPL at ERP
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
Figure NAD.2 - Corrections to Convert DRP and ERP Sound Pressures to
Diffuse Field Sound Pressure
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
Figure NAD.3 - Test setup
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
(CURRENT)
NAD.4 Long-duration disturbances
The maximum steady-state A-weighted sound pressure measured at the ear simulator
for the communications handset or headset shall be limited to reduce the risk of
permanent hearing damage due to long-duration disturbances (> 0,5 s) that can occur
under normal operation.
The equipment shall also be checked for self-generated acoustic disturbances, such as
tone dialing signals fed back to the receiver and paging signals sent to a cordless
handset.
NOTE Typical signals considered are alerting (ringing) signals during the on-hook operating condition; and tone-type dialing, network signals
and other similar signals generated within the device that can cause excessive acoustic output during the off-hook operating condition.
Compliance is checked by following methods specified in NAD.4.1 or NAD.4.2. During
the above tests, the maximum steady-state A-weighted sound pressure coming from the
earpiece or receiver shall not exceed 125 dBA for handsets, 118 dBA for headsets, and
121 dBA for insert earphones.
(PROPOSED)
NAD.4 Long-duration disturbances
The maximum steady-state, long-duration disturbances ( 0.5 s) A-weighted sound
pressure level (SPL) from an earpiece or receiver of a communications handset or
headset shall not exceed 125 dBA for handset and 118 dBA for headset at ERP (ear
reference point).
NOTE 1 Typical signals considered are alerting (ringing) signals during the on-hook operating condition; and tone-type dialing, network
signals and other similar signals generated within the device that can cause excessive acoustic output during the off-hook operating
condition.
Compliance is checked by the measurement methods specified in NAD.4.1 or NAD.4.2.
The methods utilize artificial ears for testing. Artificial ears measure SPL at DRP
(eardrum reference point) or ERP (ear reference point) depends on the type of the
artificial ear. If the DRP measured SPL is below the above limits (125 dBA for handset
and 118 dBA for headset), compliance can be assumed.
NOTE 2 Compliance can be assumed because a measured equivalent diffuse field SPL by nature of it being an open field measurement is
always lower than a measured DRP or ERP SPL.
If the DRP measured SPL exceed the above limits, the DRP measured SPL shall be
translated to an equivalent ERP SPL and compared to the limits again. The correction
TR41.7.1-11-02-002-L-Woo-AcousticSafetyProposal-UL60950
factors in Table NAD.3 shall be used for the translation, e.g. SPL(ERP) = SPL(DRP) +
SDE(correction factor for DRP to ERP).
It is recommended to apply the corrections with a real time (minimum phase) filter.
NOTE 3 The correction outputs from the filter should be as close as to the graph in Figure NAD.4.
The test setup is illustrated in Figure NAD.3
Alternatively, although not recommended, the DRP to ERP filter translation may be
accomplished mathematically. If this method is used, the method shall accurately
emulate the preferred real time filter method.
NOTE 4 A given amplitude response can in theory be produced by an infinite set of filters, each of which creates a different phase response.
A phase response that is a non-linear function of frequency smears the response in time. The filter that produces the amplitude response
with the least time-smearing is the minimum phase filter of the set. Most simple equalizers are minimum phase filters.
NOTE 5 For equalizers (e.g. programmable digital equalizer) with broader frequency bandwidth than the given correction factors’ frequency
bandwidth in Table NAD.3, the setting of the equalizer at outside of the correction factors’ bandwidth should be 12 dB per 1/3 Octave
attenuation. For example, if the highest frequency for the DRP to ERP factor is 10 kHz (-14.4 dB), the setting at the next 1/3 frequency, 12.5
kHz, should be -26.4 dB.
Table NAD.3 - Correction factors to convert DRP SPL to ERP SPL
Frequency, Hz
SDE
Frequency, Hz
SDE
100
0.0
1250
-2.5
125
0.0
1600
-4.0
160
0.0
2000
-6.5
200
0.0
2500
-9.0
250
-0.0
3150
-10.0
315
-0.0
4000
-6.5
400
-0.5
5000
-3.0
500
-0.5
6300
-3.0
630
-0.5
8000
-16.0
800
-1.0
10 000
-14.0
1000
-1.5
NOTE 7
SDE The correction from ERP to DRP
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SDE = 20 log10(PE/PD)
Where: PE is SPL at ERP
PD is SPL at DRP
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Figure NAD.4 - Corrections to Convert DRP Sound Pressures to ERP
Sound Pressure