So let’s talk about obstacles. When we want to get a point across, and we're really determined,
we don't let anything get in our way. But when we're making sounds, we're literally
getting in our way all the time. To create all those consonants we love so much, we need
to chop up the airflow coming out of our mouths without any mercy. So today, we're going to
tell a tale of what happens to those sound waves as we move our lips and tongues around.
I’m Moti Lieberman, and this is the Ling Space.
So we've talked before about how we create the speech sounds that come out of our mouths.
It all starts with air flowing up from your lungs, through your mouth, and then out into
the world. And we've also talked about all the different ways that we can configure our vocal
tracts, the pathway that air travels through from your vocal folds to sweet freedom, in
order to shape all the different consonants and vowels that we can make. But when we're
moving everything around, what does that actually do to the shapes of the sound waves that you're
making? This is kind of a big topic, so for today, we're going to focus on just two things:
plosive or stop sounds, and fricatives.
Let's start with the fricatives. These are the hissy sounds you make when you shut off
the airflow almost but not quite completely, somewhere in your vocal tract. The air tumbles
chaotically through the tight gap, and everything gets really noisy. These are sounds like the
[f] at the beginning of “flower”, or the [s] at the beginning of Sans.
And all of this rough air creates a very different sort of acoustics for fricatives than for
other speech sounds. See, your vowels and your more mellifluous consonants like approximants
or nasals, they each have characteristic acoustics we associate them with. In making these sounds,
our vocal folds vibrate, slamming shut and getting blown open over and over again. That
cycle in our larynx creates a complex sine wave with different harmonics that get
amplified or dampened based on how we arrange our mouths. These amplified sections are known
as formants, and they tell us a ton about what sound we're hearing.
But we can't do this for fricatives! They're just made too differently. Rather than the
source of the noise being the vocal folds, with its nice, regular opening and closing,
for fricatives, it's whatever constriction along the way that the air gets forced through.
What we hear is the turbulent fuzziness. There's no clear formant pattern for us to lock onto
in the sound stream shooting our way; when you look at a spectrogram of a consonant like
[x] or [θ], you just see a lot of noise. That’s why fricatives are really easy to
spot on spectrograms. They don't look like anything else.
But even if on first glance the noise looks random, it's really not – if it were, every
sentence we hear would be heralding a bad time for communication. It's true, we may
not have clear formants, but the noise does peak around particular frequencies, depending
on the fricative. Just think about [s] and [ʃ]. Even without looking at a spectrogram
to see what each sound looks like, you can probably just hear that [s] is higher frequency. Try it yourself!
[s], [ʃ]
And that's also borne out by the data – the main part of
the noise for [s] is about 4500 Hz, whereas for [ʃ], it's down closer to 2500.
In fact, if you compare [s] and [ʃ] to other fricatives like [x] or [θ] , you can probably
hear that the first pair sound hissier and louder. That's because there's a whole extra
obstacle for [s] and [ʃ] to run into: your teeth. See, when you make a [s], you've got
that constriction right up at your alveolar ridge, behind your teeth, creating turbulence.
And then you're taking that chaotic air and just slamming it into the back of your teeth,
breaking the airflow again. Just picture a waterfall – the rushing water makes some
noise, but it's the sudden splashing at the bottom that's the noisiest. Fricatives that
are too far back, like [x] or [h], or ones where the closure is in front of the teeth,
like [ɸ] or [f], don't get this extra slamming, and so they're just not as loud. This louder
group of [s], [z], [ʃ] and [ʒ] are known as sibilants.
The non-sibilants can still have their own peaks, too, even if they’re not as loud.
Like, look at this spectrogram for [x]. You can see that it’s overall fainter than the
ones for [s] or [ʃ] from before. But you can also see that the noise in it is darker
around 1500 Hz or so. And you can see the same with any kind of fricative you look at - there’s
always a characteristic pattern in the noise to latch onto. It may be very diffuse and
hard to make out, like for [f] vs. [θ], but if there wasn’t something to latch onto
in there, they wouldn’t actually sound any different from each other to us. We wouldn’t
be able to distinguish fricatives made at each of those places.
There’s one more thing that we need to be able to tell fricatives apart, and that’s
voicing. After all, pairs like [s] and [z] are both pronounced with the tongue right
up close to the alveolar ridge, and we can still tell them apart. That’s because when
we make the sound for [z], the whole time, our vocal folds are doing their vibrating thing.
And that lends a whole low buzz to the sound. We can see that if we look at the spectrograms,
too. With [s], we just have that noisy hiss we saw before, but with [z], suddenly, we
have these extra dark beats at the bottom. Each of those vertical smudges there? That’s
one pulse of your vocal folds opening and closing. You can see them lining up at the
bottom, right there. So cool.
But what’s even cooler is how we deal with plosives, or stops. When we make these sounds,
the airflow through our mouth totally closes for a bit. And with no airflow, there’s
no sound wave for us to listen to. So there should be no way for us to tell the difference
between a [p] or a [t] or a [k]. And in fact, in the middle of each sound,
they’re acoustically exactly the same: just pure silence. Here, listen: [p] [t] [k].
But obviously, we can tell the difference. Like, we know “Papyrus” has two [p]s,
and it’s not like “Kakyrus” or something. And that’s because of how incredibly sensitive
we are to little changes over the course of hundredths or even thousandths of a second.
The way one sound transitions into the next tells us a lot about what sounds we’re hearing.
For stops, those changes are the only clues we get.
Specifically, there are two main clues as to where the stop we just heard was pronounced.
The first is known as the burst. See, our mouths don’t have any teleporting shortcuts
to get our tongues or lips from one place to another. When we want to go from making
a stop to another sound, we move through a space where the closure that’s left is really
close to being totally shut, a tiny brief period where the air tumbles chaotically through.
Now, you might recognize “a near-closure with air chaotically tumbling through” from
a few minutes ago. Basically, the burst shows up in the sound wave as a fricative-y looking
piece. And because each fricative is associated with its own noise pattern, we can fish that
tiny burst out of the sound stream to figure out where the stop got pronounced, and work
out what we were just listening to.
The other thing that can tell us what stop we just heard is formant transitions. So the
idea here is similar: when we come out of a stop, we’re moving towards a new sound,
but we can’t just jump to the new one. That means we see the lingering effects of the
stop on the following sound. Most of the time, a stop will burst into a beautiful sonorous
sound, like a vowel or an approximant. And both of those have formants, the characteristic
resonance patterns that we’ve talked about before. So if we see where the formants are
coming from on their way into stabilizing themselves as a vowel, we can tell where the
stop we just heard was pronounced.
So since there are a lot of places you can make stops, and a lot of vowels they can go
into, we’re not going to unpack all of this in the video; we’ll put a link down in the
description for where you can look at more.
But let’s check out a couple of examples. When you’re transitioning out of a labial
stop like [b], into a vowel like [æ], as with [bæ], all of the formants quickly rise
up, so it looks like they have little tails coming under where they begin. And this is
a hallmark of labial stops: when you go down into one or up out of one, there’s that
tail trailing off the formants towards the stop. It’s more pronounced with some stops
than others - the tails are sharper in [be] than [bu]. But they’re there.
But maybe instead of sheep sounds like [bæ], you want to talk about bad jokes, like [gæg].
Here, as we go from the [g] into the [æ], we see that the second and third formants,
or F2 and F3, are pretty close together. As we get further into the sound, F2 goes down
and F3 goes up - they move apart.
And then as we get to the [g] at the end, the space between those two formants narrows
again. That movement is known as the velar pinch, and it’s a big signal for us that
what we’re hearing is a velar sound like [g].
Of course, just like with fricatives, the other thing that we need to tell us exactly
what stops we’re hearing is voicing. But because there’s no real airflow through
your vocal tract for stops, and because you kinda need airflow for voicing, that gets
complicated enough that it’s going to merit its own episode later. For now, though, if
you’re curious, we’ll also be talking about it some back on our website.
Stops and fricatives may be less acoustically robust than loud showy sounds like vowels
or approximants, but that doesn’t mean we can’t work out what we’re hearing. If
you listen closely, and you’re determined, there’s enough information in the sound
stream to overcome all those obstacles to communication.
So we’ve reached the end of the Ling Space for this week. If you didn’t sink under
the turbulent waves, you learned that consonants like stops and fricatives don’t have the
same kinds of acoustic structures that vowels have; that instead, we can tell what fricatives
we’re hearing by the particular spectrum of hissy noise that they create; and that
even if stops are silent themselves, we can hear what they are by paying attention to
how they influence the sounds around them.
The Ling Space is produced by me, Moti Lieberman. It’s directed by Adèle-Elise Prévost,
and it’s written by both of us. Our editor is Georges Coulombe, our production assistant
is Stephan Hurtubise, our music is by Shane Turner, and our graphics team is atelierMUSE.
We’re down in the comments below, or you can bring the discussion back over to our
website, where we'll have some extra material on this topic. Also, try dropping by our store,
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We’re taking a break for a couple of weeks, but we’ll be back with a new episode on
June 1. We’ll see you then! Pachhi bhetaulaa!