Hi everyone, Dennis Foley from Acoustic Fields.
Today we're going to distinguish between sound pressure and sound reflections because a lot
of people mix these two concepts and terms up so let's walk through some definitions.
Sound pressure is wave energy so it's like the ocean, oscillating wave after oscillating
wave.
Now what distinguish pressure from reflection are the length, height and width of those
waves so their pressure activated Sound Ocean waves roll and oscillate to the shore and
then diminish.
So it's a wave and the energy is energy that's below 100 cycles so that's a good definition
of pressure, anything below 100 cycles we're going to consider in the realm of a small
room acoustics as pressure.
Reflections on the other hand would be energy that's above 100 cycles so that would be rays,
like sunshine so they're shorter in distance, smaller, not as thick, so those are your two
general categories of energy that we deal with in acoustics.
Let's take a look at some of those waves and their lengths: thirty cycle wave thirty -seven
foot long, forty cycle wave - twenty eight foot and a 50 Hertz wave is twenty-three foot
long so think of these dimensions and match them with the frequency and then when you
come up with an issue say alright I have a forty cycle problem in my room, I know the
energy that's causing that problem is twenty eight foot long in each sequence so what am
I going to use to deal with that?
Am I going to put a curtain up to stop something twenty eight foot in length?
No, so I don't tell people to use common sense a lot in acoustics because what's common for
one person may not be sensible for another so the bottom line is look at the length of
the energy you're dealing with and then just say to yourself is a curtain going to be able
to stop something 37 foot long?
Probably not so we're going to have to use other devices to do that.
Reflections on the other hand is energy that's above 100 cycles so if we look at a 200 Hertz
wave or ray, 5.6 foot long, smaller okay can a curtain deal with that?
Probably, smaller energy, not as long so we can manage it that way.
300 cycles, 3.8 feet, 400 cycles 2.8 feet and as we go up they go smaller a thousand
cycles, a thousand Hz is about a foot long.
So how do we treat those?
Pressure technologies and here's where the confusion lies in the treatment, I think people
kind of understand the definition but it's the treatment.
Any energy below 100 cycles and is a pressure activated technology needs to be treated with
pressure activated devices so Helmholtz, diaphragmatic and membrane and we've done countless videos
on those so we won't go into it but these are pressure activated device.
This is the one we favor (diaphragmatic) because it's the most powerful; it's also one of the
most difficult to get right.
So Helmholtz, we all know that is more frequency specific and I'm going to do a video on Helmholtz
so you get an idea of that.
Membrane is a cousin to diaphragmatic and we can talk about that later.
Now reflection technologies, energy above 100 cycles that's ray oriented.
foam is good, limp mass, curtains, things like that building insulation and diffusion,
don't forget about diffusion because diffusion will reduce the impact and spread it out more
evenly and cause less of an issue, cause less of a reflection if you will from the room
boundary surface.
So there's a little background on sound pressure versus sound reflection and I hope
that helps.