|
Noise
pollution is not easily defined. Part of the difficulty lies in
the fact that in some ways it is different from other forms of pollution. |
| Understanding
Frequency & Acoustic Performance information |
| 125hz |
Low
Frequency (eg. bass sound from a speaker) |
| 250hz |
Low
Frequency (eg. bass sound associated with beat noise) |
| 500hz |
Low/Mid
Frequency (eg. sound of a car door closing) |
| 1k |
Mid
Frequency (eg. vocal noise) |
| 2k |
Mid/High
Frequency (eg. vocal noise) |
| 4k |
High
Frequency (eg. screaming) |
| |
| Terminology
used in sound proofing: |
| Acoustic
barrier |
| Solid walls or partitions, solid fences,
earth mounds, buildings, etc used to reduce noise, without
eliminating it. |
| Air-borne noise |
| This refers to noise which
is fundamentally transmitted by way of the air and can be attenuated
by the use of barriers and walls placed physically between the noise
and receiver. |
| Ambient sound |
| The totally encompassing
sound in a given situation at a given time, usually composed of sound
from all sources near and far. |
| Assessment period |
| The period in a day over
which assessments are made. |
| Assessment point |
| A point at which noise
measurements are taken or estimated. |
| Audible range |
| The limits of frequency
which are audible or heard as sound. The normal ear in young adults
detects sound having frequencies in the region 20 Hz to 20 kHz, although
it is possible for some people to detect frequencies outside these
limits. |
| Background noise |
| Background noise is the
term used to describe the noise measured in the absence of the noise
under investigation. It is described as the average of the minimum
noise levels measured on a sound level meter and is measured statistically
as the A-weighted noise level exceeded for ninety percent of a sample
period. This is represented as the L90 noise level (see below). |
| Barrier |
| See "Acoustic barrier",
a solid object used to attenuate sound. |
| Character, acoustic |
| The total of the qualities
making up the individuality of the noise. The pitch or shape of a
sound's frequency content (spectrum) dictate a sound's character. |
| Decibel [dB] |
| The level of noise is
measured objectively using a Sound Level Meter. This instrument has
been specifically developed to mimic the operation of the human ear.
The human ear responds to minute pressure variations in the air.
These pressure variations can be likened to the ripples on the surface
of water but of course cannot be seen. The pressure variations in
the air cause the eardrum to vibrate and this is heard as sound in
the brain. The stronger the pressure variations, the louder the sound
is heard.
The range of pressure variations associated with everyday living
may span over a range of a million to one. On the top range may
be the sound of a jet engine and on the bottom of the range may
be the sound of a pin dropping.
Instead of expressing pressure in units ranging from a million
to one, it is found convenient to condense this range to a scale
0 to 120 and give it the units of decibels. The following are examples
of the decibel readings of every day sounds;
|
| dB(A): A-weighted decibels |
| The ear
is not as effective in hearing low frequency sounds as it is hearing
high frequency sounds.
That is, low frequency sounds of the same dB level are not heard
as loud as high frequency sounds. The sound level meter replicates
the human response of the ear by using an electronic filter which
is called the "A" filter. A sound level measured with this
filter switched on is denoted as dB(A). Practically all noise is
measured using the A filter. The sound pressure level in dB(A) gives
a close indication of the subjective loudness of the noise. |
| Diffraction |
| The
distortion around solid obstacles of waves traveling past. |
| Fluctuating noise |
| Noise that varies continuously
and to an appreciable extent over the period of observation. It can
also include intermittent noise. As a guide, when the level varies
noticeably by more than 5 dB over a period of less than one minute,
the noise is considered to be fluctuating. |
| Frequency |
| Frequency is synonymous
to pitch. Sounds have a pitch which is peculiar to the nature of
the sound generator. For example, the sound of a tiny bell has a
high pitch and the sound of a bass drum has a low pitch. Frequency
or pitch can be measured on a scale in units of Hertz or Hz. |
| Heavy vehicle |
| Heavy
vehicles are assumed to be buses, rigid trucks and semi trailer
trucks with a tare weight
greater than 3 tones. Also heavy vehicles can be defined in terms
of length as buses, or trucks with a length exceeding 5.25 meters. |
| Impulsive noise |
| Having a high peak of
short duration or a sequence of such peaks. A sequence of impulses
in rapid succession is termed repetitive impulsive noise. |
| Intermittent noise |
| The level suddenly drops
to that of the background noise several times during the period of
observation. The time during which the noise remains at levels different
from that of the ambient is one second or more. |
| Lnn noise descriptors |
| Because noise varies with
time, a single noise value cannot adequately define the noise ambient.
For this reason, the acoustic environment is described using a number
of noise level descriptors as follows; |
| L1 |
| The sound pressure level
that is exceeded for 1% of the time for which the given sound is
measured. |
| L10 |
| The sound pressure level
that is exceeded for 10% of the time for which the given sound is
measured. |
| L10(18hr) |
| The arithmetic average
of the L10(1hr) levels for the 18 hour period between 6am and 12
midnight on a normal working day. It was a common traffic noise descriptor.
For traffic noise it is usually about 3dB(A) higher than Leq (24
hours). |
| L90 |
| The level of noise exceeded
for 90% of the time. The bottom 10% of the sample is the L90 noise
level expressed in units of dB(A). |
| Leq |
| Equivalent sound pressure
level - the steady sound level that, over a specified period of time,
would produce the same energy equivalence as the fluctuating sound
level actually occurring. |
| Leq(1hr) |
| The Leq noise level for
a specific one-hour period. |
| Leq(8hr) |
| The continuous noise level
during any one hour period between 10pm and 6am. |
| Leq(9hr) |
| The Leq noise level for
the period 10pm to 7am. |
| Leq(15hr) |
| The Leq noise level for
the period 7am to 10pm. |
| Leq (24hr) |
| The equivalent continuous
noise level during a 24 hour period, usually from midnight to midnight. |
| Loudness |
| A rise of 10 dB in sound
level corresponds approximately to a doubling of subjective loudness.
That is, a sound of 85 dB is twice as loud as a sound of 75 dB which
is twice as loud as a sound of 65 dB and so on. That is, the sound
of 85 dB is 400% times the loudness of a sound of 65 dB. |
| Low-frequency noise |
| Containing major components
within the low frequency range (20Hz - 250Hz) of the frequency spectrum. |
| Nature, acoustic |
| The innate or essential
quality of the noise. That which makes one noise distinguishable
from another (e.g. the spoken voice from the sound of a dog barking,
a telephone ringing from the sound of a gun). |
| Reflection |
| Sound wave changed in direction
of propagation due to a solid object obscuring its path. |
| Sound absorption |
| The ability of a material
to absorb sound energy through its conversion into thermal energy. |
| Sound level meter |
| An instrument consisting
of a microphone, amplifier and indicating device, having a declared
performance and designed to measure sound pressure levels. |
| Sound pressure level |
| The level of noise, usually
expressed in decibels, as measured by a standard sound level meter
with a microphone. |
| Sound power level |
| Ten times the logarithm
to the base 10 of the ratio of the sound power of the source to the
reference sound power. |
| STC |
| Sound Transmission Class.
This is a measure of the extent of sound reduction of noise going
through a building element, presented as a rating or class. It denotes
the sound attenuation properties of walls, floors and ceilings used
to construct building spaces. The higher the STC the better the sound
reducing performance of the construction. |
| Structure-borne noise |
| This refers to noise which
is generated by vibrations induced in the ground and/or structure.
These vibrations excite walls and slabs in buildings and cause them
to radiate noise. This type of noise can not be attenuated by barriers
or walls but requires the interposition of a resilient (neoprene,
springs etc.) break between the source and the receiver. |
| Tonal noise |
| Containing
a prominent frequency and characterized by a definite pitch. |
| Products.. |
| DFM > |
| Dense
Fibre Matting (used between a stud frame work construction) |
| Wall
Panels> |
| Used
directly surface-mounted to wall or ideally in conjunction with DFM |
| Ceiling
Panels> |
| Used
directly surface-mounted to ceiling or ideally in conjunction
with DFM |
| NSS> |
| Noise
Stop Systems (this is the generic name given to a certain range
of product) |
| NSSF7> |
| Noise
Stop System Floor 7 10mm Triple Layer Rubber Laminate (for use
with floors and not walls) |
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