When I enter the dimensions of my room
into this calculator, it calculates the
resonant frequencies.
This resonance at 42 Hz is caused by the
spacing between the
front wall and the back wall,
as you can see by these
blue and red indicators.
Let's play this frequency through a
speaker in the corner of my room,
and we'll listen through
this binaural microphone.
At the wall, this frequency is
overwhelmingly loud,
but as you walk toward
the center of the room,
the tone disappears.
The same thing happens
between the side walls at 47 Hz,
and another resonance occurs between the
ceiling and the floor at 70 Hz.
These three particular standing waves are
referred to as axial room modes,
because they occur between two surfaces,
front to back wall,
left to right wall,
and floor to ceiling.
This type of mode is the most extreme,
and is therefore one of the main focuses
when thinking about the
acoustic quality of a room.
Another way to visualize this is to use a
string and a transducer connected to a
sine tone generator.
When the transducer moves the string at
its resonant frequencies,
we'll see points along the string where
there is a lot of
movement, called antinodes,
and points where there is very little
movement, called nodes.
This demonstration illustrates what's
going on with the acoustics of my room.
Kind of.
To understand why this
particular string example is flawed,
let's look at these animations by Dr. Dan
Russell at Penn State University.
You can see nodes and antinodes, similar
to the string demonstration,
but this time we're
dealing with a longitudinal wave.
Think of these dots as air molecules that
are being bunched together in compression
and spaced apart in
rarefaction, just like sound waves.
Notice that these two
red dots are moving a lot.
They speed up, slow down, and then speed
up in the opposite direction.
Meanwhile, this red
dot doesn't move at all.
Below this animation, there
are two additional graphs.
One shows displacement, the
movement of the air molecules.
The other shows pressure, the
concentration of the air molecules.
The stationary dot
isn't displaced at all,
and is therefore aligned with a node on
the displacement graph.
However, the stationary dot is aligned
with an antinode on the pressure graph.
In fact, all of the
points of maximum pressure
line up with points of
minimum displacement,
and all of the points
of maximum displacement
line up with points of minimum pressure,
at this particular frequency.
We can see this at
various other frequencies too.
Maximum displacement, minimum pressure,
minimum displacement, maximum pressure.
During the process of editing this video,
I realized I said this incorrectly.
Remember, sound waves are a
series of pressure changes.
High relative pressure
and low relative pressure.
The center line represents
the point of equilibrium,
which is some nominal pressure level
between those two extremes.
That means the points of maximum pressure
are also the points of minimum pressure.
What I should have said is points of
maximum pressure change,
and points of minimum pressure change.
In other words, pressure
antinodes and pressure nodes.
But here's where we start to go into why
this is so important.
So let's get back to the video.
Just keep this in mind throughout the
video as I say minimum
and maximum pressure.
What I really mean to say is minimum and
maximum pressure change.
So why are these animations, in the
string example from earlier,
flawed visualizations of room modes?
Well, on one end of the
graph we see a fixed boundary,
but on the other end
there's not a fixed boundary.
Dr. Russell said it himself right here.
The particular example of a standing wave
that I want to illustrate
is a standing sound wave in a pipe that
is forced by moving piston or loudspeaker
at the left end and
closed at the right end.
You probably already know that the length
of a pipe impacts its resonant frequency.
But this idea of closed versus open on
the ends is also very important.
Just listen to the sound of this pipe
with a cap on one end,
versus the same pipe
that's open on both sides.
If you were paying extra close attention,
you might have noticed that the nodes and
antinodes in the animations
didn't really line up with the listening
example from the beginning of the video.
Remember, you heard an antinode of
pressure when you
were close to the walls.
That's not what we
see in these animations,
because in these
animations only one end is fixed.
All of the boundaries of a room are
theoretically fixed,
which results in standing waves more
similar to those you'd see from a string
that's fixed on both
sides, like a guitar string.
And this makes sense, right?
I mean, there won't be much displacement
of anything right here at the wall.
Think of the air in
the room as a big spring.
When a sound wave compresses the
particles against the wall,
there will be a lot of pressure, but very
little displacement.
Most of the displacement,
at this frequency at least,
will occur toward the center of the room.
And on the other wall, there will be
another point of max pressure,
minimum displacement.
Just like the pipe examples, there will
be additional resonances
within the room at higher frequencies.
Here's another resonance from
the front and back at 84 Hz,
which is double the frequency of the
first resonance at 42 Hz.
This time you hear a
build-up at the walls again,
and a build-up in the center,
but there's a null in between the front
wall and the center,
and another node in between
the center and the back wall.
Then there's another mode at 126 Hz,
three times the first mode at 42 Hz,
which will have more nodes in between the
front and back wall.
[music]
Let me quickly mention that these
frequency calculations are theoretical,
assuming totally reflective surfaces.
The reflective properties of
the walls, floor and ceiling
will ultimately determine the exact
frequencies and wavelengths
that will resonate in the room,
but in this case it's
pretty close to reality.
As you keep going up in frequency,
you'll see that the location of the nodes
and antinodes will vary,
sometimes a node in the center, sometimes
an antinode in the center,
but there will always be
maximum pressure at the boundaries.
Here's another room
mode calculator by Harman
that illustrates where you can expect
nodes and antinodes of pressure
throughout the room at
its axial mode frequencies.
This is important when thinking about
acoustic treatment in your room.
One of the most popular and
cost-effective acoustic treatment options
is a porous absorber,
sometimes called a velocity absorber,
like this monster
trap from GIK Acoustics.
You can find a link to this panel in the
description below the video.
These absorption panels
come in many shapes and sizes,
but the depth is particularly important.
Think about what you've
learned so far in this video.
Where were the areas of
maximum velocity in the animations?
The particles against the boundaries have
very little velocity,
remember they don't move much at all,
and the particles that move
the most will move very quickly,
then slow down as pressure builds,
and then move very quickly
in the opposite direction
until pressure builds again.
So the points of greatest velocity are
right here in the middle of those paths,
particularly the
points of minimum pressure.
If you place a velocity absorber like
this directly on a wall,
you'll be placing it in an area of
maximum pressure, not velocity.
That's one reason why there's an air gap
behind these panels in some cases,
because a porous
absorber directly against a wall
will only be effective at higher
frequencies that have
a shorter wavelength.
The thicker the panel, the more effective
it will be at lower frequencies.
A helpful guide is the
quarter-wavelength principle,
which tells us that we
need a porous absorber
with a thickness of at
least a quarter the wavelength
of the lowest
frequency we want to absorb.
This panel is about 7.5 inches,
and therefore the
quarter-wavelength principle
tells us it can effectively
absorb down to about 450 Hz,
although you can see that it's somewhat
effective below that frequency as well.
Meanwhile, this 3.6-inch panel will be
most effective down to about 1000 Hz.
That means if we use a ton
of thin panels in our room,
we'll be absorbing
mostly higher frequencies,
resulting in a build-up of
low-mid and low frequencies.
Depending on the
application, that's okay,
but if you need low-frequency control,
you'll need thicker panels.
You might ask, "Can't we just place
velocity traps away from the walls?"
That's a good question,
and it indicates that you're thinking
about this the right way.
But there are a few additional variables,
like the horizontal and vertical
dimensions of the panels
versus the wavelength of sound that will
just simply go around it.
Plus, there's a practicality factor.
I need space to move around for
efficiency and creativity
more than I need better
absorption at low frequencies.
So I've chosen to use thick panels placed
against the boundaries of my room
with a relatively
small air gap behind them.
At a certain point, you'd need panels
that are impractically thick
to absorb the lowest frequencies,
and that's why there are pressure traps
that operate in a different way
that works best in
areas of maximum pressure,
such as against the boundaries.
I'll provide links to the
panels I've shown you here,
plus a few others in the
show notes below this video,
in case you want to check
them out for your own room.
Another reason the relationship between
pressure and displacement is important
is for selecting the listening position
and the speaker location.
You may have heard of a guideline
referred to as the 38% rule.
This shouldn't be
taken as a hard-fast rule,
because again, the
wavelengths that resonate
will vary depending on the reflectiveness
of the walls, floor, and ceiling.
But it's a helpful starting point.
The 38% rule would suggest
that a good starting point
for the listening position would be about
38% into the length of your room,
as this is where some of the first few
modes in that
dimension are most balanced.
For the stereo image, we usually sit
directly in the center of the room,
and that's directly in the center of
those two side walls,
so we'll naturally be in a complete null
for some of those first few modes.
And with my 8-foot ceilings,
I unfortunately have a listening position
that's about halfway,
about 4 feet between
the floor and ceiling.
The source also matters, though.
The room will respond
to each frequency most
when the source is at a
point of maximum pressure.
Here's a helpful animation that
illustrates this concept.
The red dot is the position of the
speaker on this plane.
At the areas of max pressure,
the room has the most
extreme response to each frequency,
while the room will
theoretically not respond at all
if the speaker is in
one of the pressure nodes.
Let's experiment with that.
I'll play 95 Hz,
which should theoretically create
pressure antinodes at the
walls and in the center.
And if we place our ears in an area where
there's a pressure antinode
and move the speaker to a pressure node,
we'll hear that there's a
dip in the room's response.
But we can still hear
that frequency somewhat
because this is a three-dimensional room,
not a two-dimensional
theoretical animation.
In the next video
that's on your screen now,
I'll show you the first
places to put acoustic treatment
in your listening or mixing room
before you even start
trying to treat standing waves. I'll see you there.