Telecommunications Industry Association
TR41.3.10-08-02-004
Document Cover Sheet
Project Number
PN-3-4350.120-RV3
Document Title
Preferred speakerphone listening levels
Source
Nortel Networks
Contact
Name:
Miguel De Araujo
Phone:
Complete
Address:
250 Sydney St
Fax:
Belleville Ont. Canada
Email:
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613-961-2316
[email protected]
TR-41.3.10
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For Incorporation Into TIA Publication
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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 is a version of an internal memo on the results for preferred listening levels for handsfree
terminals (speakerphones), completed in the BNR (now Nortel) Subjective Assessment Lab in
August 1986. This study examined user preference for speakerphone receive output levels in
quiet and with several levels of room noise. Recommendations for speakerphone nominal level
are given based on user response to specific levels across a range of room noise values.
Nortel is providing these data in support of the development of PN-3-4340.120-RV3, to be
published as ANSI/TIA-470.120-C “Telecommunications Telephone Terminal Equipment
Handsfree Acoustic Performance Requirements”. These results were originally contributed to
IEEE in 1995 in support of similar work, and they are consistent with TIA and ITU
recommendations for the electroacoustic levels of speakerphones.
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Telecommunications Industry Association
TR41.3.10-08-02-004
Subject : Preferred Listening Levels
Following is the text of an internal memo on the results of the preferred listening levels
experiment for speakerphone levels, completed in the BNR (now Nortel) Subjective Assessment
Lab in August 1986.
Objectives
1. To determine preferred listening levels for loudspeaker listening.
2. To examine the relationship between preferred listening .level and room noise.
Apparatus
A loudspeaker was placed on the table 31.5 cm behind the standard handsfree position, and
tilted 10 degrees upwards. Acoustically transparent foam hung, from ceiling to table height, in
front of the loudspeaker. A disconnected Northern Telecom Companion 4 terminal was placed
in the standard experimental position (40 cm in front of subject). The speech samples were
bandpass filtered at 500-3500 Hz to simulate the telephony speech band and equalized (per
sentence) for signal level. Speech levels were measured using a B&K sound pressure meter
that fed into a BT Speech Voltmeter SV6. Measurements were made using a 60 second sample
of recorded speech (same samples and talkers used in the study). Levels were measured from
the standard handsfree measuring position, 40 cm in front, and 30 cm above the handsfree
terminal. The speaker amplifier was calibrated to produce an unattenuated speech level of 89.5
dB SPL. A programmable attenuator was used to control the speech level heard on each trial.
Procedure
Subjects sat facing the loudspeaker, with the chair placed at a natural distance from the table. A
speech sample consisting of one sentence was played through the loudspeaker. Although
speech samples were presented through the loudspeaker, subjects reported that speech
appeared to come from the handsfree terminal.
Subjects were asked to rate whether speech samples were "Too Soft' , "OK", or "Too Loud",
and indicate their response using the button box. Twelve trials were randomly presented at each
of the ten speech levels shown in the table below. Three blocks of trials were given to each
subject, one at each of the three room noise levels: 35, 45, and 55 dBA. Blocks occurred in
random order. The 360 trials took approximately 20 minutes to complete. Results below are
based on responses from 20 subjects.
Preliminary Results
As room noise increases, preferred listening level increases. At 35 dBA the preferred listening
level is 70 dB SPL, at 45 dBA it is 73 dB SPL, and at 55 dBA preferred listening level is 76 dB
SPL. The rate of increase of preferred listening level with increasing room noise is
approximately .3 dB/dB, which is in line with Gardner's (1969) and Ericsson's (1977) results.
There appears to be a threshold between 79 and 82 dBSPL where speech is considered too
loud, regardless of the room noise level.
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Telecommunications Industry Association
TR41.3.10-08-02-004
Compression of the acceptable range of speech levels as room noise increases, as reported by
Gardner and others, is not readily apparent in the present results. In all room noise conditions,
there are at least three speech levels where %OK is above 80%. Substantial compression may
not occur until higher room noise levels, or maybe the compression information is lost when
data is averaged.
Recommendations
A good compromise for nominal level appears to be 73 dB SPL, since it gives over 90%
acceptance (94,97,95) at all three noise levels. An acceptable range for volume would be 61 to
82 dB SPL; 64% found 61 dB to be too low even in the quiet room, and 55% found 82 dB to be
too loud in the noisiest room. The step size should be smaller than 3 dB, since a single 3 dB
step can change opinion by over 40%.
It should be remembered that the present results were obtained in the large sound room, which
is not a very reverberant room. Since sound is absorbed readily in this room, subjects may
prefer higher listening levels in this room than in a more reverberant room. This should be taken
into account when generalizing these results to other environments.
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Telecommunications Industry Association
TR41.3.10-08-02-004
Table of Results
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