**Table of contents**

[7 Alarms, audio, and voice 7-1](#alarms-audio-and-voice-communications)

[7.1 Alarms and alerts 7-1](#__RefHeading___Toc18390728)

[7.1.1 General 7-1](#__RefHeading___Toc18390729)

[7.1.2 Alarm implementation 7-2](#alarm-implementation)

[7.1.3 Reliability 7-4](#reliability)

[7.2 Audio signals and audio alarms
7-5](#audio-signals-and-audio-alarms)

[7.2.1 General 7-5](#general)

[7.2.2 Number of signals 7-8](#number-of-signals)

[7.2.3 Differentiating signals 7-8](#differentiating-signals)

[7.2.4 Signal meaning 7-9](#signal-meaning)

[7.2.5 Periodicity 7-9](#periodicity)

[7.2.6 Duration 7-9](#duration)

[7.2.7 Frequency 7-10](#frequency)

[7.2.8 Intensity (loudness) 7-11](#intensity-loudness)

[7.2.9 Acknowledging signals 7-12](#acknowledging-signals)

[7.2.10 Presentation over headsets 7-13](#presentation-over-headsets)

[7.3 Voice signals and voice alarms
7-14](#voice-signals-and-voice-alarms)

[7.3.1 General 7-14](#general-1)

[7.3.2 Intensity 7-14](#intensity)

[7.3.3 Word selection 7-15](#word-selection)

[7.3.4 Presentation 7-15](#presentation)

[7.4 Voice communication systems 7-16](#voice-communication-systems)

[7.4.1 Speech intelligibility 7-16](#speech-intelligibility)

[7.4.2 Speech transmission equipment 7-17](#__RefHeading___Toc18390750)

[7.4.3 Speech reception equipment 7-18](#__RefHeading___Toc18390751)

[7.4.4 Design for user comfort and convenience
7-20](#design-for-user-comfort-and-convenience)

[7.4.5 Operating controls for voice communication equipment
7-20](#operating-controls-for-voice-communication-equipment)

[7.4.6 Conventional telephone systems
7-21](#conventional-telephone-systems)

[Glossary 7-23](#refheading___toc18390755)

[References 7-24](#__RefHeading___Toc18390756)

[Index 7-26](#__RefHeading___Toc18390757)

**List of** **exhibits**

[Exhibit 7.2.1.7 Characteristics and ratings of audio signals for
various uses. 7-6](#__RefHeading___Toc29965516)

[Exhibit 7.4.1.2 Speech intelligibility criteria for voice communication
systems 7-17](#__RefHeading___Toc29965517)

#  Alarms, audio, and voice communications

This section presents general criteria and rules for audio alarms and
displays, voice signals and alarms, controls for audio warning devices,
and voice communication systems.

## 7.1 Alarms and alerts

[]{#__RefHeading___Toc18390729 .anchor}7.1.1 General

-   **7.1.1.1** **When to use.** If equipment is not regularly
    monitored, an audio alarm shall be provided to indicate malfunctions
    or conditions that would cause personnel injury or equipment damage.
    \[Source: Department of Defense (MIL-STD-1472F), 1999\]

-   **7.1.1.2** **Be clear and unambiguous.** Alerting and warning
    systems shall be unambiguous, with a clear indication of the cause
    for the alert. \[Source: Billings, 1996\]

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-   **7.1.1.3** **Alarm system characteristics.** Alarms systems should

a.  alert the user to the fact that a problem exists,

b.  inform the user of the priority and nature of the problem,

c.  guide the user's initial responses, and

d.  confirm in a timely manner whether the user's response corrected the
    problem. \[Source: Nuclear Regulatory Commission (NUREG-0700),
    1981\]

-   
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-   **7.1.1.4** **Indicate degree of problem.** Alarms/alerts should
    indicate the degree of malfunction or emergency. \[Source: Wiener &
    Curry, 1980\]

-   **7.1.1.5** **Alarm in appropriate mode**. When a parameter value
    represents a fault in some modes and not in others, it should only
    be alarmed in the appropriate modes. \[Source: Nuclear Regulatory
    Commission (NUREG/CR-6105), 1994\]

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-   **7.1.1.6** **Signal loss of redundancy.** When part of a redundant
    system, unit of equipment, module, or component becomes inoperable,
    an alarm signaling the loss of redundancy shall be provided to the
    user immediately. \[Source: National Aeronautics and Space
    Administration (NASA STD 3000A), 1989\]

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-   **7.1.1.7** **Allow access to current alarm settings.** When alarm
    signals are based on user defined logic, the system should allow the
    users to access current alarm settings that are specified in terms
    of dimensions (variables) covered and which values (categories) are
    established as critical. \[Source: Smith and Mosier, 1986\]

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-   **7.1.1.8** **Provide greater probability of detection.** An
    alerting and warning system or signal shall provide the user with a
    greater probability of detecting the triggering condition than his
    or her normal observation would provide in the absence of the
    alerting or warning system or signal. \[Source: MIL-STD-1472F,
    1999\]

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-   **7.1.1.9** **Provide help.** When necessary, users shall be able to
    request help and related information for the operation and
    processing of critical and non-critical alarms, messages, and
    signals. \[Source: Department of Defense (MIL-STD-1801), 1987\]

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-   **7.1.1.10** **Areas with high ambient illumination.** Auditory as
    well as visual alarms shall be provided when the users work in an
    area with a high degree of ambient illumination. \[Source:
    Department of Energy (UCRL-15673), 1985\]

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-   **7.1.1.11** **Provide redundant visual warning.** All nonverbal
    audio signals shall be accompanied by a visual signal that defines
    the condition. \[Source: MIL-STD-1472F, 1999\]

-   **7.1.1.12** **Supplement visual displays.** When used in
    conjunction with a visual display, an audio signal shall be
    supplementary or supportive, alerting and directing the user\'s
    attention to the appropriate visual display. \[Source:
    MIL-STD-1472F, 1999\]

### 7.1.2 Alarm implementation

-   **7.1.2.1** **Prioritize presentation.** Alarms should be
    automatically organized and presented to the users in prioritized
    form, with the most significant alarms receiving the highest
    priority. \[Source: NUREG-0700, 1981\]

**Discussion**. Prioritization of alarms can be based on the immediacy
of required action and impact on overall safety. \[Source:
NUREG/CR-6105, 1994\]

-   **7.1.2.2** **Display by significance.** The display of alarms with
    higher current operational significance should automatically
    override the display of alarms with lower current operational
    significance. \[Source: NUREG-0700, 1981\]

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-   **7.1.2.3** **Simultaneous alarms.** When two or more incidents or
    malfunctions occur simultaneously, the one generating a message of
    higher priority shall be presented first. After presentation of the
    highest priority message, remaining messages shall be presented in
    descending order of priority. \[Source: MIL-STD-1472F, 1999\]

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-   **7.1.2.4** **Limit number of priority levels.** The number of
    priority levels for alarm messages should be limited to four.
    \[Source: Department of Energy (DOE STD HFAC), 1992\]

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-   **7.1.2.5** **Establish priority system.** A message priority system
    shall be established so that a more critical message shall override
    the presentation of any message having a lower priority. \[Source:
    MIL-STD-1472F, 1999\]

**Definition.**

**Caution** - A signal that indicates the existence of a condition
requiring attention but not immediate action. \[Source: Department of
Defense (MIL-STD-411), 1991\]

**Warning** - A signal that indicates the existence of a hazardous
condition requiring immediate action to prevent loss of life, equipment
damage, or a service interruption. \[Source: MIL-STD-411, 1991\]

**Advisory** - A signal that indicates a safe or normal configuration,
condition of performance, or operation of essential equipment or
attracts attention and imparts information for routine action purposes.
\[Source: MIL-STD-411, 1991\]

-   **7.1.2.6** **Using warning signals.** Warning signals shall be used
    to indicate the existence of a hazardous condition requiring
    immediate action to prevent loss of life, equipment damage, or a
    service interruption. \[Source: MIL-STD-411, 1991\]

-   **7.1.2.7** **Using caution signals.** Caution signals shall be used
    to indicate conditions requiring awareness but not necessarily
    immediate action. \[Source: MIL-STD-1472F, 1999\]

[]{.mark}

-   **7.1.2.8** **Distinguish caution signals.** Caution signals shall
    be readily distinguishable from warning signals. \[Source:
    MIL-STD-1472F, 1999\]

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-   **7.1.2.9** **Make alarms distinctive and consistent.** Alarm
    signals and messages shall be distinctive and consistent for each
    class of event. \[Source: Avery, L.W., & Bowser, S.E. (DOE HFDG
    ATCCS V2.0), 1992; Department of Defense (MIL-HDBK-761A), 1989;
    MIL-STD-1801, 1987\]

**Example.** A signal alerting a user to an incoming message would be
different from a signal alerting a user to a hazardous condition.
\[Source: DOE HFDG ATCCS V2.0, 1992; MIL-HDBK-761A, 1989; MIL-STD-1801,
1987\]

-   **7.1.2.10** **Make information simple and understandable.**
    Processed alarm information should be simple enough that users can
    easily evaluate the meaning or validity of the resulting alarm
    messages. \[Source: NUREG-0700, 1981\]

**Discussion.** Complex processing can impact the user's ability to
understand the constraints and limitations of alarm processing and the
validity of resulting alarms. Users rely on the system's information.
Thus, it is essential that the users understand the validity of the
data, how they are processed, and the limitations of the system.
\[Source: NUREG-0700, 1981\]

-   **7.1.2.11** **Status indication.** System status indication
    generally should be presented on a separate display from the alarm
    indicators. \[Source: NUREG-0700, 1981\]

**Discussion.** Status indication is not intended to alert the user to
the need for action. When status indication is presented together with
alarm information, it can increase the demands on the user. \[Source:
NUREG-0700, 1981\]

-   **7.1.2.12** **Alarm filtering.** Filtering should only be used for
    alarms that have no current operational significance. \[Source:
    NUREG-0700, 1981\]

**Discussion.** Alarm **filtering** is a technique by which unnecessary
alarms are eliminated. This differs from alarm **suppression** in which
alarm messages are not displayed but are available to the user upon
request. \[Source: NUREG-0700, 1981\]

-   **7.1.2.13** **Use of suppressed alarms.** When a single alarmed
    event invariably leads to subsequent alarmed events, the primary
    alarmed event should be shown with the subsequent events suppressed,
    as long as it does not interfere with the user's tasks. \[Source:
    NUREG-0700, 1981\]

-   **7.1.2.14** **Access to suppressed alarms.** When an alarm is
    suppressed, users should be able to access the alarm information
    that is not shown. \[Source: NUREG-0700, 1981\]

-   **7.1.2.15** **Ease of accessing suppressed alarms.** The method for
    accessing information on suppressed alarms should not be excessively
    complex. \[Source: NUREG/CR-6105, 1994\]

-   **7.1.2.16** **Training for alerts.** Training techniques should be
    devised to ensure that users are exposed to all forms of alerts and
    possible combinations of alerts and that they understand how to deal
    with them. \[Source: Wiener & Curry, 1980\]

### 7.1.3 Reliability

-   **7.1.3.1** **System failure.** In the event of a complete system
    failure, the system shall integrate messages and report the system
    failure rather than the failure of components. \[Source:
    MIL-STD-1472F, 1999\]

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-   **7.1.3.2** **False alarms.** The design of audio display devices
    and circuits shall preclude false alarms. \[Source: MIL-STD-1472F,
    1999\]

-   **7.1.3.3** **Failure.** The audio display device and circuit shall
    be designed to preclude warning signal failure in the event of
    system or equipment failure and vice versa. \[Source: MIL-STD-1472F,
    1999\]

-   **7.1.3.4** **Circuit** **test.** All audio displays shall be
    equipped with circuit test devices or other means of testing their
    operation. \[Source: MIL-STD-1472F, 1999\]

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-   **7.1.3.5** **Alarm input validation.** Alarm system inputs (such as
    sensors) should be validated to ensure that spurious alarms are not
    presented to the user. \[Source: NUREG-0700, 1981\]

-   **7.1.3.6** **Noise filtering.** Alarm systems should have the
    capability to filter out noise signals to eliminate unnecessary
    alarms. \[Source: NUREG-0700, 1981\]

**Discussion.** Spurious alarms can be generated through signals
momentarily exceeding the threshold. Time filtering and/or time delay
processing can be used to prevent these signals from generating alarms.
\[Source: NUREG-0700, 1981\]

## 7.2 Audio signals and audio alarms

### 7.2.1 General

-   **7.2.1.1** **General.** Audio signals should be provided, as
    necessary, to warn personnel of impending danger, alert a user to a
    critical change in system or equipment status, and to remind a user
    of critical actions that must be taken. \[Source: MIL-STD-1472F,
    1999\]

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-   **7.2.1.2** **Use of auditory signals.** Auditory signals shall only
    be used when such signals contribute to understanding of and
    appropriate responses to the operational and task environment.
    \[Source: Ameritech Services, Inc., 1996\]

-   **7.2.1.3** **Avoid negative consequences.** Auditory signals shall
    not result in user or operator confusion, errors, or inefficiencies
    in response. \[Source: Ameritech Services, Inc., 1996\]

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-   **7.2.1.4** **Advantages to audio signals.** An audio signal should
    be provided when any of the following conditions apply.

a.  The information to be processed is short, simple, transitory, and
    requires immediate or time-based response.

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a.  The use of a visual display might be inappropriate because of
    overburdening of the visual modality, ambient light variability or
    limitation, user mobility, degradation of vision by vibration, other
    environmental considerations, or anticipated user inattention.

b.  The criticality of a response to a visual signal makes supplementary
    or redundant alerting desirable.

c.  It is desirable to warn, alert, or cue the user for subsequent or
    additional responses.

d.  Custom or usage has created anticipation of an audio display.

e.  Voice communication is necessary or desirable. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.2.1.5** **Compatible with environment.** The intensity,
    duration, and source location of audio alarms and signals shall be
    compatible with the acoustical environment of the intended receiver.
    \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.1.6** **Alarms for normal conditions.** Auditory alarms should
    not be used to indicate normal conditions. \[Source: Wiener, 1988\]

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-   **7.2.1.7** **Signal type.** When an audio signal is used, the
    particular type of signal (tone, complex sound, or speech) should be
    the best for the intended use as indicated in Exhibit 7.2.1.7.
    \[Source: MIL-STD-1472F, 1999\]

> **Exhibit 7.2.1.7** Characteristics and ratings of audio signals for
> various uses.

-   **7.2.1.8** **User evaluation of alarms.** Auditory signals shall be
    tested and evaluated for usability, operational suitability, and
    user acceptance using representative users in as near to a realistic
    operational environment as possible before the signals are
    incorporated into a system. \[Source: Ameritech Services Inc.,
    1996\]

**Discussion.** All auditory signals, including verbal signals, act as
symbols that users must learn and interpret in the light of their
operational and task environment. When designers overuse sound, the
auditory signals often are ignored and can cause annoyance,
interference, or confusion for the system users.

-   **7.2.1.9** **User setting of alarm parameters.** When appropriate
    to the task, a system or application should allow a user to set the
    parameter or condition that results in a software-generated alarm,
    alert, or status message. \[Source: DOE HFDG ATCCS V2.0, 1992;
    MIL-HDBK-761A, 1989\]

**Example and discussion.** Some examples of parameters or conditions
are priorities, percentages, and absolute values or ranges of values.
When multiple users are involved with a system, it may be the
supervisors who determine the alarm parameters, not the individual
users.

-   **7.2.1.10** **When users should not set alarm parameters.** User
    setting of parameters should not be allowed when (1) the settings by
    one user might affect the reception of alarms by another user; (2)
    the settings might affect the safety of systems, equipment, or
    personnel; or (3) alarm parameters are determined by functional,
    procedural, or legal requirements. \[Source: DOE HFDG ATCCS V2.0,
    1992; MIL-HDBK-761A, 1989\]

-   **7.2.1.11** **Alerting capabilities.** Signals with high alerting
    capacities should be provided when the system or equipment imposes a
    requirement on the user for concentration of attention. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.2.1.12** **Avoid startle.** Signals should not be so startling
    that they preclude appropriate responses or interfere with other
    functions by diverting attention away from other critical signals.
    \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.1.13** **Auditory feedback.** The most common auditory
    feedback, the system beep, should be used with other forms of
    notification such as flashing or message dialogs. \[Source:
    Microsoft Corp., 1992\]

### 7.2.2 Number of signals

-   **7.2.2.1** **Number of audio signals for** **absolute
    identification.** When absolute identification is required, the
    number of signals to be identified should not exceed four. \[Source:
    MIL-STD-1472F, 1999\]

**Discussion.** Research shows that between 4 to 7 alarms can be
acquired reasonably quickly, performance decreases dramatically for
additional alarms. The meanings associated with up to nine alarms can be
retained if the alarms are presented regularly. \[Source: Patterson,
1982; Stanton & Edworthy, 1994\]

-   **7.2.2.2** **Number of audio signals for relative identification.**
    When relative discrimination is required, the number of alarm
    signals should not exceed 12. \[Source: Stanton & Edworthy, 1994\]

-   **7.2.2.3** **Single audio signal.** A single audio signal should be
    used in conjunction with multiple visual displays only if immediate
    identification of the appropriate visual display is not critical to
    personnel safety or system performance. \[Source: MIL-STD-1472F,
    1999\]

### 7.2.3 Differentiating signals

-   **7.2.3.1** **Differentiating signals.** Auditory signals that
    require different user responses should be easily distinguishable
    from one another. \[Source: MIL-STD-1472F, 1999\]

**Example.** Varying frequency, modulation, or both can differentiate
signals. One purpose of differentiating the signals is to minimize the
user\'s search of visual displays. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.3.2** **Differentiation from routine signals.** Audio alarms
    intended to attract the user\'s attention to a malfunction or
    failure shall be different from routine signals such as bells,
    buzzers, random noises generated by air conditioning and other
    equipment and normal operation noises. \[Source: MIL-STD-1472F,
    1999\]

-   **7.2.3.3** **Multiple audio signals.** When several different audio
    signals will be used to alert a user to different conditions, the
    signals shall be distinctive in intensity, pitch, or use of beats
    and harmonics. \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.3.4** **Unsuitable auditory signals.** Auditory signals should
    not be used if they resemble sounds that can occur in the actual
    operational setting. \[Source: National Air Traffic Services, 1999\]

**Example.** Sounds that are similar to navigational signals or radio
transmissions and hisses or humming sounds similar to electrical
interference would not be good candidates for audio signals.

-   **7.2.3.5** **Noninterference.** Audio warning signals shall not
    interfere with any other critical functions or warning signals or
    mask any other critical audio signals. \[Source: MIL-STD-1472F,
    1999\]

### 7.2.4 Signal meaning

-   **7.2.4.1** **Consistent signals.** The meaning of audio warning
    signals selected for a particular function in a system should be
    consistent with warning signal meanings already established for that
    function. \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.4.2** **New meanings for standard signals.** Standard signals
    shall not be used to convey new meanings. \[Source: MIL-STD-1472F,
    1999\]

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-   **7.2.4.3** **Established signals.** Established signals should be
    used provided they are compatible with the acoustic environment and
    voice communication systems. \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.4.4** **Consistent meanings.** The meaning of audio warning
    signals selected for a particular function in a system should be
    consistent with warning signal meanings already established for that
    function. \[Source: MIL-STD-1472F, 1999\]

### 7.2.5 Periodicity

-   **7.2.5.1** **Intermittence.** Auditory signals should be
    intermittent rather than continuous. \[Source: MIL-HDBK-761A, 1989\]

**Discussion.** Continuous tones are the most easily confused signals,
even if they vary considerably in pitch. Furthermore, the human auditory
system quickly adapts to continuous auditory stimulation. \[Source:
Merideth & Edworthy, 1994\]

-   **7.2.5.2** **Nature of signals.** Audio warning signals should
    consist of two elements, an alerting signal and an identifying or
    action signal. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.5.3** **Modulated signals.** Modulated warning signals should
    either be modulated from 1-8 beeps per second or warbling with a
    pitch rise and fall of 1-3 cycles per second. \[Source: Sanders &
    McCormick, 1993\]

### 7.2.6 Duration

-   **7.2.6.1** **Two-element signals.** When reaction time is critical
    and a two-element signal is used, an alerting signal of 0.5 seconds
    duration shall be provided followed by an identifying or action
    signal with all essential information being transmitted in the first
    2 seconds of the identifying or action signal. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.2.6.2** **Reaction time critical.** When reaction time is
    critical, signals shall be of short duration. \[Source:
    MIL-STD-1472F, 1999\]

[]{.mark}

-   **7.2.6.3** **Single-element signal.** When a single-element signal
    is used, all essential information shall be transmitted in the first
    0.5 seconds. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.6.4** **Duration.** Audio warning signal duration shall be at
    least 0.5 seconds and may continue until the appropriate response is
    made. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.6.5** **Duration limitations.** Signals that persist or
    increase progressively in loudness shall not be used if manual
    shutoff may interfere with the corrective action required. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.2.6.6** **Signal termination.** Completion of a corrective
    action by the user or by other means shall automatically terminate
    the signal. \[Source: MIL-STD-1472F, 1999\]

### 7.2.7 Frequency

-   **7.2.7.1** **Audibility.** An alarm/warning signal shall provide an
    audio level in at least one octave band between 200 and 5,000 Hertz
    such that the signal is at least 10 dBA SPL (sound pressure level)
    above the ambient noise level, or 20 dBA SPL above the amplitude of
    the masked threshold, or at such a level that assures personnel are
    adequately alerted to the danger or status so as to take the
    appropriate response, when measured within 1 foot of the
    responder\'s ear, or at more than 2 feet from the alarm. \[Source:
    MIL STD 1472F, 1999; NASA-STD-3000, 1995\]

ould

-   **7.2.7.2** **Frequency range.** The frequency range of a warning
    signal shall be between 200 and 5,000 Hz, preferably between 500 and
    3,000 Hz. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.7.3** **Frequency for long distances.** When a signal must be
    audible at a distance of 300 m (985 ft) or more, the frequency shall
    be below 1,000 Hz. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.7.4** **Signals around obstacles.** When the signal must be
    heard around obstacles or through partitions, the frequency shall be
    below 500 Hz. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.7.5** **Frequencies differing from background.** The selected
    frequency band shall differ from the most intense background
    frequencies**.** \[Source: MIL-STD-1472F, 1999\]

-   **7.2.7.6** **Spurious signals.** The frequency of a warning tone
    shall be different from that of the electric power employed in the
    system to preclude the possibility that a minor equipment failure
    might generate a spurious signal. \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.7.7** **Frequencies difficult to localize.** Mid-frequencies
    (1500-3000 Hz) should not be used for auditory alarms that require
    localization. \[Source: Sanders & McCormick, 1993\]

### 7.2.8 Intensity (loudness)

-   **7.2.8.1** **Environmental compatibility.** The intensity,
    duration, and source location of an auditory signal should be
    compatible with the acoustic environment of the intended receiver as
    well as with the requirements of other personnel within acoustic
    range of the signal. \[Source: MIL-HDBK-761A, 1989\]

**Discussion.** Avoid the use of loud sounds unless the task requires it
(e.g., the environment is loud and it is critical that the user hear the
sounds).

-   **7.2.8.2** **Compatibility with clothing and equipment.** When the
    audio alarms and signals must be heard and understood through
    equipment or garments (e.g., parka hoods and hearing protective
    devices covering the ears of a listener), audio signals shall be
    loud enough to compensate for the attenuation characteristics of the
    garments without exceeding 115 dB(A) for emergency signals and 90
    dB(A) for other signals**.** \[Source: MIL-STD-1472F, 1999\]

-   **7.2.8.3** **Exceeding ambient noise.** Auditory signals shall
    exceed the prevailing ambient noise level by at least 10 dB(A) or
    any maximum sound level with a duration of 30 seconds by at least 5
    dB(A), whichever is louder, without exceeding 115 dB(A)for emergency
    signals or 90 dB(A) for other signals. \[Source: MIL-STD-1472F,
    1999; NUREG-0700, 1981\]

-   **7.2.8.4** **Maximum intensity.** The intensity of evacuation and
    emergency signals shall not exceed 115 dB(A). The intensity of other
    signals shall not exceed 90 dB(A). \[Source: MIL-STD-1472F, 1999;
    NUREG-0700, 1981\]

-   **7.2.8.5** **Control of volume.** The user, the sensing mechanism,
    or both shall control the volume (loudness) of an audio warning
    signal depending upon the operational situation and personnel
    safety. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.8.6** **Volume limits.** Volume control movement shall be
    restricted to prevent reducing the volume to an inaudible level or
    increasing it to an unacceptably high level. \[Source:
    MIL-STD-1472F, 1999\]

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-   **7.2.8.7** **Appropriate use.** Auditory coding should be used

a.  to alert users to critical conditions or operations;

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a.  to supplement visual signals;

b.  to present information in situations in which visual presentation is
    not feasible; and

c.  to provide feedback for control actuation, data entry, or the
    completion of timing cycles and sequences. \[Source: MIL-HDBK-761A,
    1989\]

-   **7.2.8.8** **Inappropriate use.** Auditory coding should not be
    used when ambient noise prevents effective listening. \[Source:
    National Air Traffic Services, 1999\]

### 7.2.9 Acknowledging signals

-   **7.2.9.1** **Special acknowledgement of critical alarms.** When a
    user must acknowledge a special or critical alarm in a unique way
    (e.g., with a special combination of key strokes), this special
    acknowledgement shall not inhibit or slow the response to the
    condition initiating the alarm. \[Source: DOE HFDG ATCCS V2.0, 1992;
    MIL-HDBK-761A, 1989\]

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-   **7.2.9.2** **Alarm reset.** A system or application shall provide
    users with a simple means for turning off non-critical auditory
    alarms without erasing any displayed message that accompanies the
    auditory signal. \[Source: DOE HFDG ATCCS V2.0, 1992; MIL-HDBK-761A,
    1989; MIL-STD-1801, 1987\]

-   **7.2.9.3** **Acknowledging and terminating alarms.** A system or
    application shall provide users with a means of acknowledging alarms
    and of turning off alarm signals once the alarms have been
    acknowledged or the condition generating the alarm has been
    corrected. \[Source: MIL-STD-1801, 1987\]

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-   **7.2.9.4** **Procedures for acknowledging alarms.** Procedures for
    acknowledgment and termination shall not decrease the speed and
    accuracy of operator reaction to the alerting situation. \[Source:
    MIL-STD-1801, 1987\]

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-   **7.2.9.5** **Consistent means of acknowledging.** A simple,
    consistent means of acknowledging auditory signals shall be
    provided. \[Source: MIL-HDBK-761A, 1989\]

-   **7.2.9.6** **Acknowledging** **non-critical signals.** When the
    signal is non-critical, the acknowledgement action shall also turn
    the signal off. \[Source: MIL-HDBK-761A, 1989\]

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-   **7.2.9.7** **Acknowledging alarms indicating loss of redundancy.**
    Users shall be able to acknowledge an alarm signaling the loss of
    redundancy, with the lack of available redundancy continuously
    displayed until the redundant system, equipment, module, or
    component becomes operable again. \[Source: NASA STD 3000A, 1989\]

-   **7.2.9.8** **Automatic and manual shutoff.** If an audio signal is
    designed to persist as long as it contributes useful information, a
    shutoff switch controllable by the user, the sensing mechanism, or
    both, shall be provided consistent with the operational situation
    and personnel safety. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.9.9** **Automatic reset.** An automatic reset function for
    audio signals shall be provided, whether the signals are designed to
    terminate automatically, manually, or both. The automatic reset
    function shall be controlled by a sensing mechanism that recycles
    the signal system to a specified condition as a function of time or
    the state of the signaling system so that the warning device can
    sound again if the condition reappears. \[Source: MIL-STD-1472F,
    1999\]

-   **7.2.9.10** **Ganging to mode switches.** Volume controls may be
    ganged to mode switches to provide maximum output during operational
    phases in which intense noise can occur and to provide reduced
    volume at other times. This ganging shall not be done if there is a
    possibility that intense noise could occur in an emergency situation
    during a phase in which the volume would be decreased below an
    audible level. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.9.11** **Caution signal controls.** Audio caution signals
    shall be provided with manual reset and volume controls. \[Source:
    MIL-STD-1472F, 1999\]

### 7.2.10 Presentation over headsets

-   **7.2.10.1** **Headset.** When the user will wear earphones covering
    both ears during normal equipment operation, the audio warning
    signal shall be directed to the user\'s headset as well as to the
    work area. \[Source: MIL-STD-1472F, 1999\]

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-   **7.2.10.2** **When not to use headsets.** Binaural headsets should
    not be used in any operational environment with ambient noise below
    85 dB(A) if that environment contains sounds that provide the user
    with useful information and that information cannot be directed to
    the user\'s headset. \[Source: MIL-STD-1472F, 1999\]

**Discussion.** Such sounds may include voices, machine noise that
indicates wear or malfunctions, and other auditory indications of system
performance or mission status. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.10.3** **Separate channels.** When feasible, a warning signal
    delivered to a headset that might mask another essential audio
    signal should be delivered to one ear and the other signal to the
    other ear. \[Source: MIL-STD-1472F, 1999\]

-   **7.2.10.4** **Dichotic presentation.** When earphones will be worn
    in an operational environment, a dichotic presentation should be
    used whenever feasible, with the signal alternating from one ear to
    the other by means of a dual-channel headset**.** \[Source:
    MIL-STD-1472F, 1999\]

## 7.3 Voice signals and voice alarms

### 7.3.1 General

-   **7.3.1.1** **When to use.** Voice signals should be used

a.  to supplement visual displays when communication flexibility is
    necessary,

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a.  when coded signal meanings are numerous or may be forgotten,

b.  for presentation of complex directions or instructions,

c.  when ambient noise may mask simple tonal signals,

d.  in conjunction with tonal signals, and

e.  for presentation of continuous information when the rate of change
    is low. \[Source: MIL-HDBK-761A, 1989\]

-   **7.3.1.2** **Nature of signals.** Voice signals shall consist of a
    brief, standardized speech signal (e.g., a verbal message) to
    identify the specific condition and suggest an appropriate action.
    \[Source: MIL-STD-1472F, 1999\]

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-   **7.3.1.3** **Presentation of voice signals.** Verbal signals should
    be preceded by an initial non-speech alerting signal to attract the
    users attention if the verbal warning signals are used for other
    types of information as well as warnings. \[Source: Sanders &
    McCormick, 1993\]

-   **7.3.1.4** **Acknowledging warning signals.** The system should
    require that users acknowledge spoken warning signals. \[Source:
    MIL-HDBK-761A, 1989\]

### 7.3.2 Intensity

-   **7.3.2.1** **Intensity.** Verbal signals for critical functions
    shall be at least 20 dB above the speech interference level at the
    operating position of the intended receiver but shall not exceed 90
    dB(A). \[Source: NUREG-0700, 1981; MIL-STD-1472F, 1999\]

**Definition.** **Speech interference level** is a measure of the
effectiveness of noise in masking speech. It is the arithmetic mean of
the same pressure levels of interfering noise (in dB) in the four octave
bands centered on the frequencies 500, 1000, 2000, and 4000 Hz,
respectively. The unit of speech interference is the decibel (dB).
\[Source: NUREG-0700, 1981; MIL-STD-1472F, 1999\]

-   **7.3.2.2** **Speech intensity.** Speech intensity should be
    appropriate to the expected ambient noise environment, with a signal
    to noise ratio of at least 5:1. \[Source: MIL-HDBK-761A, 1989\]

### 

### 7.3.3 Word selection 

-   **7.3.3.1** **Word choice.** The words used in verbal signals shall
    be concise, intelligible, and appropriate to the task and the
    information presented. \[Source: MIL-HDBK-761A, 1989\]

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-   **7.3.3.2** **Word characteristic prioritization.** Word selection
    priority should all be intelligibility, descriptiveness, and
    conciseness, in that order. \[Source: MIL-STD-1472F, 1999\]

**Discussion.** Given the prioritization criteria above, if two word
choices are being considered and both are concise, intelligible, and
appropriate, but one is more intelligible, the one that is the most
intelligible of the two should be used even if the other word choice is
more concise.

-   **7.3.3.3** **Words to avoid.** To the extent possible, words that
    rhyme with other words or that sound similar in other ways should be
    avoided if these other words might be used in the same context and,
    therefore, possibly be confused with the original words. \[Source:
    MIL-HDBK-761A, 1989\]

-   **7.3.3.4** **Formal words.** Formal or correct words should be
    used; slang, jargon, and colloquial words should be avoided.
    \[Source: MIL-HDBK-761A, 1989\]

-   **7.3.3.5** **Alphabetic information.** Alphabetic information
    should be presented using a phonetic alphabet that uses words like
    alpha, bravo, and Charlie rather than the letters A, B, and C.
    \[Source: MIL-HDBK-761A, 1989\]

### 7.3.4 Presentation 

-   **7.3.4.1** **Average talker.** Spoken messages should sound like an
    average talker from the user country without a regional dialect.
    \[Source: MIL-HDBK-761A, 1989\]

-   **7.3.4.2** **Distinctive voices.** When different categories of
    voice signals are used, a different, distinctive voice should be
    used for each category of data. \[Source: MIL-HDBK-761A, 1989\]

**Example.** One voice might be used for instructional messages and
another for warnings. \[Source: MIL-HDBK-761A, 1989\]

-   **7.3.4.3** **Content.** Spoken messages should be brief,
    informative, and to the point. \[Source: MIL-HDBK-761A, 1989\]

-   **7.3.4.4** **Type of voice.** The voice used in recording verbal
    warning signals should be mature and distinct enough not to be
    confused with voice communications including radio and intercom
    communications. \[Source: MIL-STD-1472F, 1999\]

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-   **7.3.4.5** **Delivery style.** Voice signals shall be presented in
    a formal, impersonal manner. \[Source: MIL-STD-1472F, 1999\]

-   **7.3.4.6** **Repetition.** Critical warning signals shall be
    repeated with not more than a 3 sec pause between messages until the
    condition is corrected or overridden by an operator or user.
    \[Source: MIL-STD-1472F, 1999\]

-   **7.3.4.7** **Speech processing.** Verbal warning signals shall be
    processed only if necessary to increase or preserve intelligibility,
    for example, by increasing the strength of consonant sounds relative
    to vowel strength. \[Source: MIL-STD-1472F, 1999\]

**Discussion.** When a signal must be relatively intense because of high
ambient noise, peak clipping may be used to protect the listener against
auditory overload. \[Source: MIL-STD-1472F, 1999\]

## 7.4 Voice communication systems

### 7.4.1 Speech intelligibility

-   **7.4.1.1** **Evaluation method.** If information about the speech
    intelligibility of a system is needed, the most appropriate of the
    following methods should be selected.

a.  

b.  The phonetically balanced (PB) word test should be used when the
    highest accuracy and sensitivity are required. It is difficult to
    administer accurately and requires a long training time (typically
    20-40 hours) before the responses of the listeners have peaked and
    are stable.

c.  The modified rhyme test (MRT) described in ANSI 3.2 should be used
    to measure the communication performance of most military
    communication systems. It is easy to administer and requires only a
    short training time of 1-2 hours.

d.  The articulation index (AI) and/or the speech transmission index
    (STI) are predictive estimators of intelligibility. They should be
    used to estimate system performance during the concept and design
    phase but not as a substitute for intelligibility test when system
    hardware is available. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.1.2** **Intelligibility criteria.** Speech intelligibility
    shall meet the criterion in Exhibit 7.4.1.2 for the appropriate
    communication requirement and evaluation method. \[Source:
    MIL-STD-1472F, 1999\]

> []{#__RefHeading___Toc29965517 .anchor}**Exhibit 7.4.1.2** Speech
> intelligibility criteria for voice communication systems

![](media/image1.png){width="6.125694444444444in"
height="3.057638888888889in"}

[]{#__RefHeading___Toc18390750 .anchor}7.4.2 Speech transmission
equipment

-   **7.4.2.1** **Frequency range.** Microphones and associated
    system-input devices shall respond optimally to that part of the
    speech spectrum most essential to intelligibility (i.e., 200 to
    6,100 Hz). Where system engineering necessitates speech-transmission
    bandwidths narrower than 200 to 6,100 Hz, the minimum acceptable
    frequency range shall be 250 to 4,000 Hz. \[Source: MIL-STD-1472F,
    1999\]

-   **7.4.2.2 Dynamic range.** The dynamic range of a microphone used
    with a selected amplifier shall be great enough to admit variations
    in signal input of at least 50 dB. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.2.3** **Noise canceling microphones.** In very loud, low
    frequency noise environments (100 dB overall), noise canceling
    microphones shall be used and shall be capable of effecting an
    improvement of at least 10 dB peak-speech to root-mean-square noise
    ratio as compared with microphones that are not noise canceling, but
    that have equivalent transmission characteristics. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.4.2.4** **Pre-emphasis.** If necessary, speech system input
    devices shall employ frequency pre-emphasis with a positive slope
    frequency characteristic no greater than 18 dB per octave from 140
    to 1,500 Hz, and no greater than 9 dB per octave over the frequency
    range of 1,500 to 4,800 Hz when no clipping is used. \[Source:
    MIL-STD-1472F, 1999\]

    **Discussion.** If speech signals are to be transmitted over
    channels that have less than 15 dB peak-speech to root-mean-square
    noise ratios, peak-clipping of 12 to 20 dB may be employed at system
    input and can be preceded by frequency pre-emphasis. \[Source:
    MIL-STD-1472F, 1999\]

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-   **7.4.2.5** **Noise shields.** If the talker is in an intense noise
    field, the microphone should be put in a noise shield. Noise shields
    should be designed to meet the following requirements:

a.  a volume of at least 250 cu cm (15.25 cu in) to permit a pressure
    gradient microphone to function normally,

b.  a good seal against the face achieved by pressure of the hand or by
    tension straps,

c.  a hole or combination of holes covering a total area of 65 sq mm
    (0.1 sq in) in the shield to prevent pressure buildup,

d.  prevention of a standing wave pattern by shape or by use of sound
    absorbing material, and

e.  no impediment to voice effort, mouth or jaw movement, or breathing.
    \[Source: MIL-STD-1472F, 1999\]

[]{#__RefHeading___Toc18390751 .anchor}7.4.3 Speech reception equipment

-   **7.4.3.1** **Frequency range.** Headphones and loudspeakers shall
    be subject to the same frequency response restrictions as
    microphones and transmission equipment except that loudspeakers for
    use in multi-speaker installations and multiple channels fed into
    headphones (e.g., where several speech channels are to be monitored
    simultaneously) shall respond uniformly (±5 dB) from 100 to 4,800
    Hz. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.3.2** **Use of de-emphasis.** If transmission equipment
    employs pre-emphasis and peak clipping is not used, reception
    equipment shall employ frequency de-emphasis of characteristics
    complementary to those of pre-emphasis only if it improves
    intelligibility, that is, de-emphasis shall be a negative-slope
    frequency response not greater than 9 dB per octave over the
    frequency range of 140 to 4,800 Hz. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.3.3** **Monitoring of speakers.** If several channels are to
    be monitored simultaneously by means of loudspeakers, the speakers
    shall be mounted at least 10° apart in the horizontal plane frontal
    quadrant, from 45° left to 45° right of the user\'s normal
    forward-facing position. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.3.4** **Filtering of speaker signals.** If additional channel
    differentiation is required, apparent lateral separation shall be
    enhanced by applying low-pass filtering (frequency cutoff, Fc=1,800
    Hz) to signals fed to loudspeakers on one side of the central user
    position. If there are three channels involved, one channel shall be
    left unfiltered; a high-pass filter with a 1,000 Hz cutoff shall be
    provided in the second channel; and a low-pass filter with a 2,500
    Hz cutoff shall be provided in the third channel. A visual signal
    shall be provided to show which channel is in use. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.4.3.5** **Speaker and side tone.** The speaker\'s verbal input
    shall be in phase with its reproduction as heard on the headset.
    This side tone shall not be filtered or modified before it is
    received in the headset. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.3.6** **Use of binaural headsets.** If listeners will be
    working in high ambient noise (85 dB(A) or above), binaural rather
    than monaural headsets shall be provided. Unless operational
    requirements dictate otherwise, binaural headsets shall be wired so
    that the sound reaches the two ears in opposing phases. \[Source:
    MIL-STD-1472F, 1999\]

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-   **7.4.3.7** **Binaural headsets.** Binaural headsets should be
    capable of reducing the perceived ambient noise level to less than
    85 dB(A). Provisions should be incorporated to furnish the same
    protection to those who wear glasses. \[Source: MIL-STD-1472F,
    1999\]

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-   **7.4.3.8** **Speaker and side tone.** The speaker\'s verbal input
    shall be in phase with its reproduction as heard on the headset.
    This side tone shall not be filtered or modified before it is
    received in the headset. \[Source: MIL-STD-1472F, 1999\]

### 

### 7.4.4 Design for user comfort and convenience

-   **7.4.4.1** **Comfort.** Communication equipment to be worn by a
    user, such as headphones and telephone headsets, shall be designed
    to preclude user discomfort. Metal parts of a headset shall not come
    into contact with a user\'s skin. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.4.2** **Hands-free operation.** User microphones, headphones,
    and telephone headsets shall be designed to permit hands-free
    operation under normal working conditions. Specialized emergency
    equipment may be exempt from this criterion. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.4.4.3** **Accessibility of handsets.** When communication
    requirements necessitate the use of several telephone handsets, the
    accessibility of their locations when not in use shall be determined
    by operational priority, that is, the most frequently or most
    urgently needed handset shall be the most accessible. The handsets
    may be color coded if the users will be able to perceive the coding
    under normal working conditions. \[Source: MIL-STD-1472F, 1999\]

### 7.4.5 Operating controls for voice communication equipment

-   **7.4.5.1** **Volume controls.** Accessible volume or gain controls
    shall be provided for each communication receiving channel, such as
    loudspeakers or headphones, with sufficient electrical power to
    drive the sound pressure level to at least 100 dB overall, when
    using two earphones. The minimum setting of the volume control shall
    be limited to an audible level, that is, it shall be impossible to
    inadvertently disable the system using the volume control. \[Source:
    MIL-STD-1472F, 1999\]

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-   **7.4.5.2** **Separate controls for power and volume.** Separate
    controls should be provided for power (ON-OFF) and for volume
    control. \[Source: MIL-STD-1472F, 1999\]

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-   **7.4.5.3** **Combined power and volume controls.** If power and
    volume controls are combined because of space limitations, an easily
    noticeable detent position shall be provided between the OFF
    position and the lower end of the continuous range of volume
    adjustment, and the OFF position shall be labeled. \[Source:
    MIL-STD-1472F, 1999\]

-   **7.4.5.4** **Squelch control.** If communication channels are to be
    continuously monitored, each channel shall be provided with a
    signal-activated switching device (squelch control) to suppress
    channel noise during no-signal periods. A manually operated ON-OFF
    switch shall be provided to deactivate squelching during the
    reception of weak signals. \[Source: MIL-STD-1472F, 1999\]

-   **7.4.5.5 Foot-operated controls.** When normal working conditions
    will permit the operator to remain seated at the working position
    and require access to \"talk-listen\" or \"send-receive\" control
    switches, or if console operation requires the use of both hands,
    foot-operated controls shall be provided. \[Source: MIL-STD-1472F,
    1999\]

-   **7.4.5.6 Duplicate emergency controls.** Hand-operated controls for
    the same functions as foot operated controls shall be provided for
    emergency use and for use when the operator may need to move from
    one position to another. \[Source: MIL-STD-1472F, 1999\]

### 7.4.6 Conventional telephone systems

-   **7.4.6.1** **General.** In special environments such as control
    rooms, selection and placement of conventional telephone systems may
    be more critical than in a normal office environment. Within such
    specialized environments, systems selected for use shall provide a
    good frequency response in that portion of the spectrum essential
    for speech intelligibility. The standard telephone band pass of 200
    to 3,300 Hz is acceptable. Handsets shall be compatible with users\'
    hand sizes and mouth-to-ear distances. Again, the standard telephone
    dimensions are acceptable. Handsets shall provide firm ear contact.
    \[Source: NUREG-0700, 1981\]

-   **7.4.6.2** **Cords.** Cords shall be non-kinking or self-retracting
    and of sufficient length to permit reasonable user mobility. Cords
    shall be positioned to avoid entangling critical controls or
    becoming entangled with passing people or objects. \[Source:
    NUREG-0700, 1981\]

-   **7.4.6.3** **Handset cradles.** Vertically mounted handset cradles
    shall be designed and located to prevent the handset from being
    knocked out of the cradle by passing people or objects. \[Source:
    NUREG-0700, 1981\]

-   **7.4.6.4** **Multiple telephones.** If several telephones are
    located close to each other, they shall be coded to indicate circuit
    or function. \[Source: NUREG-0700, 1981\]

-   **7.4.6.5** **Press-to-talk button.** If a press-to-talk button is
    used, the button shall be convenient to both left- and right-handed
    people. \[Source: NUREG-0700, 1981\]

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-   **7.4.6.6** **Switching.** Switching should be designed and
    programmed to minimize delay in making desired connections under
    both normal and emergency conditions. \[Source: NUREG-0700, 1981\]

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-   **7.4.6.7** **Priority.** Switching shall be programmed to give the
    control room and critical functions automatic priority of access to
    the switching system. \[Source: NUREG-0700, 1981\]

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-   **7.4.6.8** **Noisy environments.** In noisy environments, volume
    controls should be provided for loudness of ringing and speaker
    output. \[Source: NUREG-0700, 1981\]

[]{#refheading___toc18390755 .anchor}Glossary

**Advisory** - A signal that indicates a safe or normal configuration,
condition of performance, or operation of essential equipment, or
attracts attention and imparts information for routine action purposes.

**Alarm** **filtering** - A technique by which unnecessary alarms are
eliminated.

**Alarm suppression** - Alarm messages are not displayed but are
available to the user upon request.

**Caution** - A signal that indicates the existence of a condition
requiring attention but not immediate action.

**Speech interference level** - A measure of the effectiveness of noise
in masking speech. It is the arithmetic mean of the same pressure levels
of interfering noise in the four octave bands centered on the
frequencies 500, 1000, 2000, and 4000 Hz, respectively. The unit of
speech interference is the decibel (dB).

**Warning** - A signal that indicates the existence of a hazardous
condition requiring immediate action to prevent loss of life, equipment
damage, or a service interruption.

[]{#__RefHeading___Toc18390756 .anchor}References

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Index

A

Alarm parameters 7-8

Audio alarm 7-1

Audio alarms 7-5

Acknowledgement 7-12

Alarm filtering 7-4

Alarm implementation 7-2

Alarm reset 7-12

Alarm system characteristics 7-1

Alarms and alerts 7-1

Audio signals 7-5

advantages of 7-5

use of 7-5

Auditory feedback 7-8

Auditory signals

absolute identification 7-8

ambient noise 7-12

noninterference 7-9

relative identification 7-8

type 7-6

unsuitable 7-9

volume control 7-12

Automatic shutoff 7-13

B

Binaural headsets 7-19

C

Caution signals 7-3

Circuit 7-5

D

Differentiating signals 7-9

Duplicate emergency controls 7-21

Duration 7-10

F

Failure 7-4

False alarms 7-4

Foot-operated controls 7-21

Frequency 7-11

H

Headsets 7-14

Help 7-2

I

Intelligibility criteria 7-17

Intensity 7-11, 7-15

M

Manual shutoff 7-13

Multiple signals 7-9

N

Noise filtering 7-5

Noise shields 7-18

Number of signals 7-8

O

Operating controls 7-20

P

Periodicity 7-10

Probability of detection 7-2

Presentation 7-16

Priority levels 7-3

R

Reliability 7-4

S

Signal meaning 7-9

Simultaneous alarms 7-2

Speech intelligibility 7-17

Speech intensity 7-15

Speech interference level 7-15

Speech reception equipment 7-19

de-emphasis, use of 7-19

frequency range 7-19

speaker monitoring 7-19

speaker signals, filtering of 7-19

Speech transmission equipment 7-18

dynamic range 7-18

frequency range 7-18

pre-emphasis 7-18

Squelch control 7-21

Startle 7-8

Status indication 7-4

Suppressed alarms 7-4

T

Telephone systems 7-21

Terminating alarms 7-13

U

User comfort 7-20

V

Verbal signals 7-7, 7-15

Voice communication 7-16

Voice signals 7-14

Volume controls 7-20, 7-22

Visual warning 7-2

W

Warning signals 7-3, 7-15

Word choice 7-15

Warning signals 7-3, 7-14

Warning signals 3

Warning signals 1, 16