- [Instructor] When the surface of a speaker
beats the air, the distance the surface moves
from its resting position is what creates
the high or low pressure wave
that travels through the air to your ear.
The bigger the distance speaker moves,
the more air pressure it creates when it moves out,
and the lower the air pressure
when it moves back inside the speaker cavity.
It's this range of high pressure and low pressure air
that bounces across the room to your ear,
in much the way that ripples move across
the surface of a pond.
In your ear, there's a diaphragm
that's moved by the air pressure waves.
The more it moves, the more energy is generated
and the louder the sound that's perceived.
In technical terms, we're always talking about air pressure
unless you're filming in another medium like under water,
where the sound travels even more effectively
because the water molecules are closer together.
Air pressure follows a logical, linear scale
from low to high, but our hearing doesn't.
We hear different amplitudes
with different levels of sensitivity.
In an attempt to more accurately measure
the perception of amplitude, we use a logarithmic scale,
and there are quite a few options available.
For many years, we used the Decibel scale.
In fact, in many cases we still do.
This was originally intended to measure
signal drop off in telephone wires,
but it turns out this scale is pretty good
at measuring amplitude too.
The measurement of amplitude
usually begins at 0dB, zero Decibels,
which is the maximum amount of amplitude
the recording device can capture,
and it goes down as a minus until
the signal recorded is so quiet
it's less audible than the noise floor.
The noise floor is the background noise
that's always present.
The background hiss, the ghost in the machine,
the residual radiation from the big bang, if you like.
You'll never manage to record a sound
without a tiny amount of background hiss.
For this reason, audio level isn't measured
on a scale that goes from zero
down to some other kind of zero.
It goes from zero down to infinity,
which I suppose should be read
as effectively, perfectly silent,
because it's quieter than the noise we can't avoid.
This background noise isn't quite the same thing
as unwanted noise in the environment
when recording something.
Though we'd consider that to be noise,
it's really part of the signal recorded.
You'll find there are several systems
for measuring amplitude,
and they're all broadly comparable to the Decibel scale.
However, some are based on the amount of energy
and objective amplitude in the media
or the speakers you're using,
while others are based on
the human perception of amplitude,
which is a more precise approach
to measuring and recreating sound for our purposes.
Remember, we're making sound for people to listen to.
We're working to capture and reproduce human experience
and since our perceptions aren't particularly accurate
or linear or objective, our recording and reproduction
needs to be based on an average of objective human senses.
There are quite a few approaches to this
aptly named Loudness Scale,
and if you're working for a broadcaster,
you'll be told which system to use.
The units in the scale are based on Decibels,
but the measurement is of the total energy
that hits the ears, rather than, if you like,
electricity in the wires.