February 24, 2011

038 How We Hear Different Pitches

Some people sing high, others sing low. There are so many pitches, which are the result of different sound wave frequencies.

How does the ear allow you to distinguish between these various pitches? Watch this video and listen as Leslie details the processes in the inner ear that result in us being able to tell the difference.


Transcript of this Episode

“Aaa! Aaa!  Aaa! Aaa!” (high tone to a deep tone) Hello! Welcome to another episode of Interactive Biology TV where we’re making Biology fun. My name is Leslie Samuels and I apologize for what you had to listen to at the beginning of this episode. In this episode, episode 38, I’m going to talk about how we hear different pitches. And what do mean by different pitches? I’m glad you asked. I mean, “Aaa! Aaa! Aaa!” (high to low pitch).

I’m sorry, I apologize. I shouldn’t be putting you through that. But, that is exactly what we’re going to be talking about today. I just made a few different sounds and they were different pitches. We want to look at how your brain is able to distinguish the different pitches based on what is happening inside the ear. So, “Let’s continue,” (high pitch). “Let‘s continue.” (Low pitch).

Here we’re looking at the ear. We’ve looked at this figure in the last episode and we looked at one that was similar to it in the episode before that. Where we ended off last time, we had a signal coming in, and we spoke about how the malleus, incus, and stapes are involved in transferring that signal to the cochlea. What I’m going to do now is, I’m going to take this cochlea and I’m gong to roll it out and just kind of extend it.

So, we’re not going to look at it like how it looks here, kind of like a snail. We’re going to look at it as if was just rolled out. So, let’s go to the next picture.

Here we have it. We have the cochlea that we unrolled and now it extends right here. What you’ll see is, here we have a membrane that we call the basilar membrane, and here’s the writing for that right here. This is the basilar membrane. What you’re going to notice about the basilar membrane is it’s thinner over here than it is over here.

So, at this end, it’s significantly thinner and as it goes away from the oval window where the malleus, incus, and stapes connect, as it goes away from that section, it gets thicker and thicker and thicker until it’s thickest right here at this end.

What you’re going to see here is, we have a number of frequencies that are associated with these different sections. Here we have at 25 Hz which is a low frequency, and as we come over to the thinner section, we have higher frequencies up here to 1600 Hz. And, it goes all the way up here to about 20 kHz. So, we go as low as 25Hz and as high as 20kHz.

If you’ve taken a Physics class, you know that higher pitches are the result of higher frequencies. Forgive my writing there again. So, higher pitches are the result of higher frequencies and lower pitches are a result of lower frequencies. And, we’re talking about the sound waves, the frequency of the sound wave.

If a certain sound comes into the ear, causes the tympanic membrane to vibrate, the malleus, incus, and stapes vibrate, and that causes the oval window to vibrate, that’s going to cause fluid inside the cochlea to vibrate. Now, depending on the frequency, it’s going to cause a different section of the basilar membrane to vibrate.

Is it easier to move a thinner piece of membrane or a thicker piece of membrane? The answer to that question should be quite obvious. It’s much easier to move a thinner piece of membrane than it is to move a thicker piece of membrane. So, in order for it to vibrate down here, we need more force. You’re going to get a greater force from lower frequencies. Just think about it, if you’re in front of a huge speaker, I mean, massive speaker, and there is sound coming out of that speaker, you’re playing some music, and you’re playing music that has a lot of frequencies. For example, something like this – {nice high frequency music played}.

Now, if you’re standing in front of that huge speaker that’s playing that nice little soft high-frequency music, it’s not going to have a huge effect on you. But, if you start playing something with a lot of bass, something like this – {Music played with lots of bass}.

That’s going to cause you to move. You might even feel the wind of the speaker vibrating and causing the ear to be pushed. You might actually feel that. That’s because when you have lower frequencies, the lower the frequencies, the greater the force that comes along with that frequency.

So, here, in order to cause this to vibrate, we’re going to have a lower frequency sound, which makes sense. That’s why we’re showing 25 Hz here. The closer up we go, where we have the thinner membrane, we can cause that to vibrate with a higher frequency tone. If the frequency is low enough, that might actually cause this entire basilar membrane to vibrate.

The take home message is, depending on the frequency, we’re going to get different regions of the basilar membrane vibrating. This then sends a signal to the brain. Depending on where that signal is coming from, if that signal comes from here and it goes to the brain, that is going to tell the brain that it’s coming from a low frequency and, the brain is going to interpret that as a lower pitch.

If it’s coming from over here, it’s going to the brain, and that’s going to tell the brain that it’s coming from this region which is associated with a higher frequency, and the brain is going to interpret that as a higher pitch.

So, there’s a direct relationship between where it vibrates and where in the brain is being stimulated and depending on where it’s stimulated and where the signal comes from, the brain is going to be able to distinguish between the different pitches. Now, you’re hearing me speak and me speaking right now is a result of a number of different frequencies combining together.

So, there is going to be a complex interaction here, different parts of them is going to be vibrating in different ways, and the brain is going to take all of that and paint the picture of the sound that’s coming from my voice, well, that’s coming from the speakers that you’re listening to this video on and, you can easily distinguish between my, “haa!” (high pitch) and my, “haa!” (low pitch).

I hope that wasn’t too painful and I hope it makes sense. That’s really all for this video. If you have any questions, go ahead and leave them in the comments below. And of course, you can always visit the website at, www.Interactive-Biology.com for more Biology videos and other resources. That’s it for this video and I’ll see you on the next one!

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