### Video Transcript

Which of the following sound waves is an example of ultrasound? Is it a) 5555 hertz, b) 3.5 times 10 to the power of three hertz, c) 0.01 hertz, d) 1.5 times 10 to the power of two kilohertz, or e) 7.5 kilohertz?

So in this question, we’re asked to find out which one of the sound waves is an example of ultrasound. So what is ultrasound? Ultrasound is sound which has a higher frequency than what humans can hear. In other words, it’s so high frequency that we can’t even hear it. Now, the upper limit of human hearing is about 20000 hertz. So anything which has a frequency larger than 20000 hertz is ultrasound. Which means that we need to go through options a to e and work out which one has a frequency greater than 20000 hertz.

So let’s start with option a. We’ve got 5555 hertz. Is this greater than 20000 hertz? No, it’s less than 20000 hertz. Which means that this is not the answer to our question. So let’s move on to option b.

We’ve got 3.5 times 10 to the power of three hertz. Well, 10 to the power of three is the same as 1000. So 3.5 times 10 to the power of three is the same as 3.5 times 1000. And that is the same as 3500 hertz. So is this greater than 20000 hertz? Once again, no. 3500 is less than 20000. So option b is not our answer either.

Option c: 0.01 hertz. Well, this is clearly less than 20000 hertz. So we can move on to option d.

Option d is 1.5 times 10 to the power of two kilohertz. Now 10 to the power of two is 100. So 1.5 times 10 to the power of two is the same as 150. But remember that this is in kilohertz. And we want it in hertz. We can recall the conversion that one kilohertz is equal to 1000 hertz. So 150 kilohertz is the same as 150 times 1000 hertz, which ends up being 150000 hertz. Now, this value is greater than 20000 hertz. So it looks like option d is our answer. But let’s check option e, just to make sure that it’s not our answer either.

Option e is 7.5 kilohertz. And as we’ve seen already, a kilohertz is the same as 1000 hertz. And so 7.5 kilohertz is the same as 7.5 times 1000 hertz, which ends up being 7500 hertz. And as we expected, 7500 is less than 20000. So option e is not our answer.

And so, happily, we can say that option d is our answer. 1.5 times 10 to the power of two kilohertz is an example of ultrasound.