Sound intensities are typically measured
in decibels (db). A decibel is defined
as 10 times the logarithm of the power
ratio
(power ratio is the ratio of the intensity
of the sound to the intensity of an
arbitrary standard point.) Normally
a change of 1
db is the smallest volume change detectable
by the human ear.
Sound intensity is also defined in terms
of energy (erg) transmitted per second
over a 1 square centimeter surface.
This energy is proportional to the velocity
of propagation of the sound. The energy
density in erg/cm³ = 2pi²
x density in g/cm³ x frequency²
in Hz x amplitutde² in cm.
The above restrictions are based on
the Occupational Safety and Health
Act of 1970.
That Code basically states that if the
above exposures are exceeded, then hearing
protection must be worn. Note that these
are based on the "A scale"
of a standard
sound level meter at slow response and
will change if some other standard is
used.
See the OSHA Section 1910.95
for additional details on the differences.
Note that the sound level scale in decibels
is a logarthmic rather than linear scale.
A sound level change of 3 decibels is
double (or half) of the previous power
level. The ear registers this as just
noticable. A change in power level of
10 decibels is a power change of 10 times,
and the ear judges this as only twice
(or half) as loud.
These relationships do not hold true at
all power levels or at the frequencies,
as the ear is a very non-linear device.
See Fletcher-Munson hearing curves published
in books on hearing.
Some human ears can hear sounds in the
frequency range of 20Hz to 20,000Hz, however,
the hearing for most people is limited
to about 30Hz to 15,000Hz.
