Spartan 821/721 Manual
Glossary
C-9
L
min
The minimum value, expressed in dB, of the Frequency and Exponential-Time Weighted
Sound Level (
L
ωτ
) in a given time interval. For the dosimeters, an L
max
value is recorded
for 2 different time intervals. The first records L
max
for the entire record’s time. The
second records L
max
for each individual time history sample.
Ln Value
The
Ln
value is the sound level that has exceeded n% over the total measurement time.
For example, if n=90%, a displayed value of 35dB for L90 means that for 90% of the
measurement period the dB level was at or above 35dB. These statistical values are
commonly used to describe the characteristics of non-steady sound such as
environmental noise.
To calculate Ln values, the 721 or 821 creates an amplitude distribution table over the
range zero to 200 dB in amplitude increments of 0.1 dB. This data permits the
calculation of Ln values for any value of n in the range 00.01 to 99.99%.
Ln Values can be defined on the 721 or 821 by selecting
Tools
Settings
Ln
Settings
.
Measurement Duration (T or t)
The time period of measurement. It applies to hearing damage risk and is generally
expressed in hours. Standard: ANSI S12.19
Microphone Guidelines
Microphone - Types
: A device for detecting the presence of sound. Most often it
converts the changing pressure associated with sound into an electrical voltage that
duplicates the changes. It can be composed of one of the following types:
Capacitor (Condenser)
: A microphone that uses the motion of a thin diaphragm caused
by the sound to change the capacitance of an electrical circuit and thereby to create a
signal. For high sensitivity, this device has a voltage applied across the diaphragm from
an internal source.
Electret
: A microphone that uses the motion of a thin diaphragm caused by the sound to
change the capacitance of an electrical circuit and thereby to create a signal. The
voltage across the diaphragm is caused by the charge embedded in the electret material
so no internal source is needed.
Microphone - Uses
: The frequency response of microphones can be adjusted to be used
in specific applications. Among those used are:
Frontal incidence (Free Field)
: The microphone has been adjusted to have an
essentially flat frequency response when in a space relatively free of reflections and
when pointed at the source of the sound.
Random incidence
: The microphone has been adjusted to have an essentially flat
frequency response for sound waves impinging on the microphone from all directions.
Pressure
: The microphone has not been adjusted to have an essentially flat frequency
response for sound waves impinging on the microphone from all directions.
