So today we’re going to go a lot more in-depth
into how good pro audio speakers actually work. It’s going to get really technical. We had spoken with Anika, a high schooler,
about the basics, and if you haven’t seen that video, go check it out. It’s a lot of fun. With Austin, a tech director at a local church,
we’re going to look at just the subwoofer and the low mid frequency ranges, and see
how we can have maximal smoothness in that range, and directivity of the sound. Definitely wear headphones for this video
because I used a binaural microphone in some of the sequences, and we’re listening to
bass frequencies, which you’re not going to hear out of this. Hey Austin, how’s it going? Good, how are you? Pretty good. So, I’m assuming that we share a love for
subwoofers? Yeah, that’s a pretty safe bet. Yeah, well I brought this microphone. It’s actually shaped like human ears, so
if you’re wearing headphone, you’ll be able to hear what we’re hearing. I’m going to put these on it because it’s
a little windy today. So the basics of speakers design… when we
put a speaker in a box, we can choose to have it completely sealed, or we can have a hole
to let the air out. Now, the thing is, when we have a hole in
it, the speaker typically is a little bit louder, and bigger sounding, so manufacturers
usually choose that. The reason why that is is kind of complex. When the speaker moves in, it pushes air out
of the hole, and when the speakers moves out, it sucks air into the hole. So, you’d think that those would just kind
of cancel each other out, and they actually do. But, if the speaker moves fast enough, the
direction that the air flows through the opening will actually change directions. So, instead of doing this kind of push-pull
thing where they’re opposite each other, it’ll actually change direction and they’ll
move together. So, when the speaker makes a positive air
pressure wave by moving out, at the same time there’ll be a positive air pressure wave
coming out of the opening and they’ll sort of stack up and move out together, and vice
versa in the other direction. We can actually hear this. If I take and, just for a second, plug the
opening with this piece here… so you can hear how much sound there is, right? Now I’m just going to open this up… hear
how much louder it is? Yup. So, let’s draw a speaker box here, and we’re
going to have an opening where the air can come in and out. So, I’m going to put this little speaker
icon on my hand, and that represents the speaker driver. So, the spring represents the air that’s
inside the box, and this weight represents the air that’s just kind of hanging out
as like a block inside of the opening. So at very low frequencies, you can see that
the direction of my hand and the air are going to be in opposition with each other to the
outside world. And the faster I go, there comes a point when
the direction changes and then suddenly my hand and the air are now moving in a way that
would produce sound that stacks up with each other in the outside world. Also notice how there’s this point right
around when the direction is starting to change when I have to move my hand, like, very little
for there to be a lot of air movement. And then if I move my hand really fast, my
hand moves a lot but there’s little air movement. So we can actually see this change in direction
happen if I tape this cotton ball over the opening. Now, this cabinet is tuned to about 32Hz,
so I’m going to start by playing 20Hz which is well below that. Let’s take some slow-motion footage. Ok so, let’s take a look at what that looks
like. So you hear how there was like, no bass, because
they’re cancelling each other out. Now let’s go up to like, 45Hz, which is
above the tuning frequency. Tons of bass, right? Ok, let’s see what this looks like. Alright, now check this out. Oh yeah, it changes direction. So then we’re going to go down just a little
bit in frequency, and find that point at which the driver, like my hand, didn’t have to
move very much in order to make the air in the port move a lot. So check this out. Ok so, look at that. Oh, cool. Ok so then we’re going to go up to a higher
frequency where it changes and the driver is doing most of the work and there’s very
little air coming out of the vent. So check this out. Alright, so, see how that works? Oh interesting. So, sometimes manufacturers will actually
enclose both sides of the driver in a chamber that can resonate. So, like, this chamber down here is bigger
and it resonates at a lower frequency, and this one up here resonates at a higher frequency. So they kind of work together, trading off,
handling different parts of the subwoofer frequency range, so that the full range can
experience a kind of resonance and more efficiency. So one of the basic problems with speaker
design is that, as air moves through the vent and it gets louder and louder and moves faster,
the air can flow either smoothly or it can kind of get disturbed near the edges… there’s like, turbulence that happens. So you’d have to create a shape for the
air to flow through that keeps the air on the outsides moving as fast as the air in
the middle, or else they’ll kind of get disrupted, and if they get too disrupted then
the port will actually stop resonating, and then the air won’t move at all, it’ll
actually just kind of stand still and be all jumbled up. So, they’ve solved this in a number of ways. So like, in this design here we can see that
the wall of the vent is actually curved as it goes back, so it keeps the air flowing
very smoothly along this kind of air aerodynamic path. With this particular product, because it gets
so loud, this company had to design a new opening that’s actually shaped like a nozzle
or something, almost like a jet engine nozzle. It gets so narrow way back in here… it’s
counter intuitive because you’d think that would constrict the air flow too much, but
actually, they did some math to determine the exact amount of curvature and it keeps
the air flowing very, very smoothly and there’s no turbulence. So, this cabinet can get very, very loud. I guess that’s why they have smart people
working at R&D departments. Ok so when we walk around the sub, you can
hear how it doesn’t matter which direction we stand… we hear the bass equally everywhere? Well, what if we wanted to figure out how
to control the direction of the bass? Well, there’s this little trick that we
can play. So I brought a second subwoofer out, and let’s
turn them on. Now, I’m going to tell the one sub to actually
do the opposite of whatever the other one’s doing. So if that one makes a positive air pressure
wave, that one’s going to make a negative air pressure wave, and vice versa. Ok, ready? Here we go. So, hear how they basically cancel out, right? So let’s go ahead and add some distance
between these subwoofers that equals a half-wavelength at this frequency. So like, this is 60Hz, so that means that’s
about just over 100 inches. So let’s go ahead and scoot them apart here. How’s that? Ok so, I’m going to take this cable and
I’m going to lay it on the ground… when we hear cancellation happening, it’s
going to be closer to the configuration, and when it sums together, we’re going to move
further out. So we’re going to see what the pattern of
the sound distribution is. So, starting in the middle because it’s
cancelling… so like, we can roughly see the sort of “figure-8”
pattern, right? Where we have cancelling anywhere along this
line which is equidistant from the subs. If we wanted to try to pick a direction that
the bass goes and cancel it all in the other direction, what we can do it to actually bring
the subs closer together and then tell one of the subs to actually wait a little bit
with its digital signal. So, we’re going to do that at one-quarter
wavelength instead of one-half which would be about 56 inches. So let’s start here with our pattern again…
we obviously can hear that the bass is summing nicely. So, basically what we did was we moved the
cancellation from these two angles to this. We just scooted them back. Now, we can kind of split the difference by
maybe not delaying this sub so much either physically or with the digital delay in the
signal. So, I’ve just done that now, so if we walk
over here, you can kind of tell that now the cancellation point is more right here, and
then we get over here and we have a little bit more of a buildup, and then we kind of
keep going… so if I were to redraw this, I’d have to
take and maybe scoot this kind of like toward here. We can actually steer those cancellation spots
depending on how much physical distance and how much delay we put to the signal. So the difficulty with cardioid setups is
that you have to pick one frequency to build the setup around, where you have perfect summing
and perfect cancelling. But, sound changes wavelengths with difference
frequencies, so the ones that are higher or lower than that, they won’t get a perfect
cardioid pattern. So, in any setup, you have this issue where
you have to tell the subs to maybe ease off at that central frequency in the range, because
otherwise you’ll have an imbalance. So let’s go inside the building and take
a look at some more speakers. Let’s do it! So, I’m going to give you these two microphones. The one in your left hand is going to be the
reference, and this one’s going to be what we're testing. So go ahead and, first of all, put them together
right in front of the speaker. We’re going to run some noise through the
speaker. Now, on the graph, you can see both lines
are flat because they’re comparing each other and they’re picking up the same sound,
so there’s nothing to report. Take the one in your right hand and move it
to the vent. So we can see the graph change. Now, let’s look at the bottom graph for
just a second. Here, below the tuning frequency of the cabinet,
it’s up around 180 degrees because they’re doing that push-pull thing, right? And then as the frequency increases it goes
through the resonant point of the cabinet, and now they are in-phase. And so with these frequencies, they’re going
to be moving in unison and they’ll stack together if they’re relatively the same
volume. But at some point, it’s interesting, we
get high enough and we reach another resonance of some kind. That’s a harmonic. So you have the fundamental tuning frequency
of the cabinet, but then at some point you have a harmonic where there’s another phase
shift that happens, back to 180. And then you have more harmonics that constantly
are shifting the phase a bunch, and it gets kind of crazy. That’s why, typically, subwoofers have a
limited range of frequencies that we can have, where you get high enough and you have to
cross it over with another speaker that takes over from there. At the fundamental, the vent seems to be producing
so much more sound than the driver. Like, it goes way up in comparison, and then
kind of levels out. Right about here, they’re like about the
same volume as each other, but then again, at the harmonic, the vent has a huge spike
in relation to the driver. If you think about it, you know, right here
you have relatively the same volume between the vent and speaker, and they’re in-phase. That means they’re going to combine well. But here, where again, these are relatively
the same volume, but you have a lot of 180 degrees of point of phase here… if those
come together, they’re going to cancel each other. So, the subwoofer becomes very unusable up
in this range. So, what I’ve done here is I’ve changed
the graph to where we’re now using the actual input signal to the subwoofer as the reference. And then, the white trace is the mic that
you have in front of the driver. And then the yellow trace is the mic that’s
in front of the vent. So it’s interesting because we can see that
we have the relative peak in the vent at the tuning frequency... you can see how the driver actually becomes
a lot less. So, that’s kind of interesting. As they undergo the phase shift here to becoming
in-phase, the driver gets less, the vent gets more, but then they kind of level out up here…
they’re in-phase... and then we can see here, this next harmonic,
again we have a dip in the driver volume while the vent takes over, but we have a phase shift. Now there's out of phase here, but in this range, they're pretty close to each other in volume. Again, as we go through other phase shifts,
we’re still pretty close in volume. Ok so, let’s try taking these microphones
and putting them kind of out into the world, like maybe put them together up here. So this is kind of what we would hear… they’re
equidistant between the driver and the vent. Ok so we can see that the combined response
of the subwoofer below the tuning frequency… they’re cancelling each other out, and so
there’s very little output from the sub. And then we hit the tuning frequency and we’ve
got a pretty smooth ride… sounds great... sounds great... we hit the first harmonic here, and we have
some cancellation that takes place. We’ve got a little more summing, and then
some more cancelling, so it just kind of gets crazy up there. So the subwoofer is really only usable up
to 200Hz maximum. Probably, we’d want to cross it over somewhere
in here anyway. And that’s just kind of the nature of vented
enclosures in general. Do you remember how, in a cardioid setup,
you have a front sub and a rear sub and you have to pick a set physical distance and a
set digital delay? And there’s one frequency that’s going
to have a perfect cardioid pattern, right? It’s going to cancel in the rear and sum
in the front, and above and below that, it kind of gets less ideal, and there’s like
this limited range of frequencies that, you know, will experience the cardioid effect
before it gets crazy? We really need to be able to have a different
physical distance and even maybe a different digital delay at every frequency. Well, this company, in this cabinet, what
they did is they had two subwoofers basically glued together. So these are actually separate. It’s this one, and this one. There’s an internal divider. So this driver utilizes this vent, and this
driver utilizes this vent. And so what happens is they applied the front
signal to this driver and the rear signal to this driver. So if you think about it, we saw earlier how
the vent handles most of the acoustic energy at the fundamental frequency, and then as
you go higher, the driver really takes over? So if you think about it, at the lower frequencies,
you have more physical distance between the sources… the front and rear sources… than you do at the higher frequencies because
they move toward the driver. So we naturally have that physical distance
change per frequency. And then what they did was they took the signal
going to the rear driver and they actually applied a different digital delay time at
every frequency, which is just really cool because now we get both. So at the widest range of frequencies, we
have the best cardioid effect because we’re changing the physical distance and the digital
delay at each frequency to get the best cardioid effect possible. So, you hear how much bass there is right
now? Like, it’s really amazing. So let’s walk around what is the back side
of the cardioid setup. So as we start to get around to the back,
do you hear how it’s getting lower and lower? And by the time we get back here, I mean,
we’re standing three feet from it and it’s just like… gone. There’s almost no bass. It’s really, really effective. So, earlier we saw how, in a typical vented
enclosure, you have the fundamental frequency, and then you have a usable range, and then
you have the harmonic that kind of screws things up above that? Well, the problem is, you know, subwoofers
can cross over then with a driver like this. But, this has to be able to function all the
way up into the midrange frequencies… much, much higher, because it doesn’t get
to cross over until it reaches the frequencies that the horn handles, which are really, really
high. So, in typical designs, you just kind of had
to deal with the fact that there’s, you know, some cancellations and some weird stuff
that happens, and try to make the best of it. So this is an older speaker design by this
company that exhibits this behavior. So if we play some noise through it… go
ahead and put the microphones there. We can see that we have our fundamental frequency
here, we have an in-phase period, and then we have the harmonic, and the vent and the
driver are relatively the same volume here, but we have an almost 180 degree phase shift
throughout the entire range. So there’s going to be some cancellations
and disturbances that happen in that range that are just sort of typical of these speaker
designs since forever. So when we hold the microphones out away from
the speaker, which is what we would hear, you can hear how the combination of the vent
and the driver produces a relatively smooth response up until that first harmonic, and
then we get a pretty uneven response in this upper-midrange here. So, um, that’s, you know, not the best,
but it’s kind of the best that anybody could do back in the day. So with the introduction of the new series
of this same box, they found that if you have this V-shaped cut in the internal wall of
the vent, it kind of stops the harmonic from being able to fully form and cause disturbances,
so it keeps the phase relationship between the vent and the driver much more linear through
a higher frequency range. So we can actually see this phenomenon on
the graph, so, hold the microphones up to the driver and the vent… so we can see the normal behavior… tuning
frequency… we have a phase-linear response. But, look at the phase above the harmonic. It remains pretty linear all the way up to
here… even this isn’t so bad. And the relative volume of the port, even
though, like, it’s still about the same as the driver up in here… because the phase
is so linear, they’re going to combine nicely instead of fight each other and cause cancellations. So, holding the mics out away from the speaker
again, we can hear the combination of the vent and the driver, which is what we hear,
and we can see the frequency response is very, very smooth all the way up through this midrange
area here. I mean, there’s very, very few disturbances
because almost all the frequencies are summing together nicely. And that contributes to the very smooth nature
of the sound of this box, as opposed to the older series. It’s a really cool little technology. So, with this cabinet here, which also has
the same technology built into the vent, we can see that the phase response… although it drifts a little bit during this
range here… it’s pretty linear up here again where the volume is relatively the same. During the section where it drifts even a
little bit, the relative volume of the vent to the driver is very, very low. So, it would other cancel, but because it’s
so low in comparison, it’s kind of out of the way, it doesn’t really matter that much. So again, we’re holding the mics out away
from the speaker, which is what we hear in reality, and again, we can see that the total
response of the speaker is very, very smooth all the way through that critical midrange. That’s why these speakers sound so good
these days from this particular company. So Austin, thanks again for doing this. I had a lot of fun… what did you think? Yeah! Um, I definitely learned so much more about
these products, especially with the ones that we use here. The correlation between the unique cardioid
design and the unique vent design surprises me... it puts out a pretty smooth frequency
response. And then the total directional control of
the low-end… it’s just pretty rad. Awesome. Alright man, thanks so much.
