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Interior Noise
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The control of the vehicle interior noise levels is of
great importance in a passenger transit rail vehicle.
Excessive interior vehicle noise can mask important
safety functions such as emergency announcements or
passenger safety directions, as well as messages
announcing route information and stops.
Human hearing is not only sensitive to sound pressure
level, but also to frequency ranges within the audible
range. The frequency range of audible sound is
approximately between 20 Hz and 20,000 Hz. The maximum
sensitivity region for human hearing is around 3 to
4kHz, and is associated with the resonance of the
auditory canal. The ear is less sensitive to low
frequencies.
Sound levels in the average human hearing range is
measured with a sound level meter set to the "A" scale.
The "A"-weighting filters out significantly more bass
than the others, and is designed to approximate the ear
at around the 40 phon level (40db@1000Hz). It is very
useful for eliminating inaudible low frequencies. The
C-weighting does not filter out as much of the lows and
highs as the other weightings. It approximates the ear
at very high sound levels and has been used for traffic
noise surveys in noisy areas.
In normal room conversations, speech intelligibility is
reduced when the background noise is above 55 dbA.
Conversation intelligibility is significantly
diminished for background noise above 60 dbA .
Prolonged exposure to noise levels above 85 to 90 dbA
may cause deafness.
Typical sound level references are shown below for
information;
Noise Level
|
Decibels (A Scale)
|
Example of Sound
Level |
Subjective Loudness
|
| Deafening |
140 |
Jet aircraft at takeoff at 50 ft |
|
| 130 |
Threshold of pain |
|
| 120 |
Jet engine nearby |
32 times as loud |
| 110 |
Chainsaw, hand held steel grinder, rock concert |
16 times as loud |
| Very Loud |
100 |
Boiler factory, Disco |
8 times as loud |
| 90 |
Noisy factory, loud street noise |
4 times as loud |
| Loud |
80 |
Garbage disposal |
2 times as loud |
| 70 |
Average street noise, medium vacuum cleaner, noisy office
|
70 dB(A) = 1 |
| Moderate |
60 |
Average assembly factory, dishwasher, air conditioning
unit at 100 ft |
1/2 as loud |
| 50 |
Average office, noisy home, average conversation |
1/4 as loud |
| 40 |
Quiet radio, lowest limit of urban ambient sound |
|
| Faint |
30 |
Average library, quiet conversation |
|
| 20 |
Rustle of leaves, whisper |
|
| Very Faint |
10 |
Threshold of audibility, soundproof room |
|
| 0 |
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| Standard: |
ANSI S1.11 - Specifications for
Octave-Band and Fractional-Octave-Band Analog and Digital
Filters
| | Purpose: |
This standard provides performance requirements for
analog, sampled-data, and digital implementations of bandpass
filters that comprise a filter set or spectrum analyzer.
An instrument complying with the requirements of this
standard may contain any number of bandpass filters covering
any desired frequency range.
This standard is used primarily to ensure that the test
equipment falls within the acceptable accuracy range for
sound level measurements. Used as a supplement to ANSI
S1.4
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| Criteria: |
Specify compliance with standard
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| Usage: |
... shall meet the requirements of ANSI S1.11
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| Standard: |
ANSI S1.13 - Methods for the Measurement
of Sound Pressure Levels
| | Purpose: |
This standard specifies the requirements and procedures
for the measurement of sound pressure levels in the air
at a single point in space. These requirements and procedures
apply primarily to measurements performed indoors, but
may be utilized in outdoor measurements under specified
conditions. Please also refer to an alternative sound
measurement standard for interior noise; ISO 3381.
This standard gives detailed definitions and procedures
for a wide variety of sound types and conditions. Typical
measurements use the A-Weighted exposure scale. This standard
should be used in conjunction with a detailed test procedure
developed by the testing entity (Carbuilder, Consultant,
or Authority) to ensure the required sound levels are
measured.
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| Criteria: |
Specify a test plan in compliance with standard and
specified criteria
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| Usage: |
... a test plan shall be provided following the criteria
of ANSI S1.13 with (specific performance criteria - measurements,
dBa values, etc.)
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| Standard: |
ANSI S1.4 - Specification for Sound
Level Meters
| | Purpose: |
This specification defines the requirements for sound
level meters, to ensure maximum practical accuracy in any
particular sound level meter and to reduce the differences
in corresponding meter measurements obtained when various
makes and models the sound level meters are used that meet
the standard. (three types of sound level meters are defined
0, 1, and 2. 0 has the tightest tolerances, 2 has more relaxed
tolerances)
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| Criteria: |
Specify compliance with standard, meter type
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| Usage: |
The sound level meter shall comply with ANSI S1.4, Type 2
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| Standard: |
ANSI S1.6 - Preferred Frequencies,
Frequency Levels, and Band Numbers for Acoustical Measurements
| | Purpose: |
This standard defines the preferred frequencies, or nominal band center frequencies to be used for acoustic measurements. The purpose of this standard is to refer all frequency series measured, to a single reference frequency, or to specify preferred frequencies for measurements. (This is applicable to detailed noise studies)
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| Criteria: |
Variable
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| Usage: |
... a test plan shall be provided following the criteria
of ANSI S1.6 with (specific frequency criteria, etc.)
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