MCAT Podcast Ep. 101: Next Step Full Length 10, Passage 4, Questions 18-21

Session 101

Once again, I’m joined by Bryan of Blueprint MCAT (formerly Next Step Test Prep) for some more full length 10, passage 4. Passage 4 on Next Step full-length 10 covers human hearing and sound waves.

Bryan emphasizes that taking full-length practice tests is the single most important thing you can do to get ready for the MCAT.

Passage 4: In order for humans to hear, pressure waves are transferred, funneled into the ear by the way of the auditory canal and reach the tympanic membrane. This oscillation is transferring energy to the membrane to cause to vibrate. Via resonance, these vibrations stimulate hair and associated nerve cells. And sends information to the auditory cortex for pitch and intensity.

Bryan says this is one of those classics he calls, “throat-clearing paragraphs.” The AAMC likes having passages where the opening paragraph is just backward information you should know anyway. Having said that, take a minute to just read it through to make sure you know.

Moving on, you’d find a diagram that compares decibel level to frequency for the threshold of human hearing.

Human hearing detects vibrations in the range from 20 Hz to 20 kHz. Our range of hearing shrinks with age and the upper age for hearing is the most affected. Ultrasound is not physically different from other sounds that we do hear but the brain is not capable of processing these vibrations.

Loudness is a characteristic of a pressure wave associated with amplitude. The greater the amplitude, the greater the stimulation of nerve cells. Loudness is a subjective property that depends on the individual. In an attempt to quantify loudness…

[03:55] Question 19

What is the intensity of a 70 decibel sound at a frequency of 103 Hertz?

(A) 10-12 wm2 (watts per square meter)
(B) 10-7 wm2
(C) 10-5 wm2
(D) 10-2 wm2

Bryan’s Insights:

Before walking into the MCAT, students should know the decibel equation. This should be one of those you’ve got to have off the top of your head.

In particular, that equation is decibels = 10 log of the intensity of the sound over 10-12.

And you notice right away in the answer choices that (A) is 10-12. It’s the threshold of human hearing, the quietest sound the human ear can perceive. But you might remember that and then just jump on it because it’s familiar. And you don’t want to do that as you want to do the actual calculation. Even with the numbers as answer choices, the wrong answers are going to have something tempting and a little tricky about it. But you want to know the calculations here, which would result to what in your answer choice C.

[08:04] Question 20

Based on the passage information, which of the following statements about sound waves in water is correct?

(A) Ultrasound waves are attenuated and more difficult to perceive in the water.
(B) Sound waves in water are more intense than sound waves in air.
(C) Sound waves have greater wavelengths in the water than in the air.
(D) Sound waves have higher frequency in the water than in air.

Bryan’s Insights:

You want to remember that the word “perceive” is a specific technical meaning. It’s a specific psychology term that means available to your conscious awareness. And we can’t perceive any ultrasound at all. What you want to remember about sound waves is that they speed up or slow down based on what they’re traveling through. They go the slowest in air, and faster in water. And then they go the fastest in a solid.

Sound waves are just molecules bumping into each other. So if the molecules are right next to each other then they bump into each other really quickly and fast. Then the sound waves just jet along. It makes sense then that the sound waves really quickly through a solid.

But it’s going faster, something has to be changing about the width. There’s an equation, velocity = frequency x wavelength. Or if it’s a light wave, then c = λf.

This is a very simple but very important equation in sound or waves in general. If the left side of the equation goes up, if v goes up, it’s faster in the water, then either f goes up or λ goes up. So one of them has to change.

In this case, you should walk into the test knowing that when a wave, light sound, anything at all, goes from one medium to another, frequency is constant. So if the sound is going faster through water, v goes up then λ goes up as well. The wavelength has to be going larger. This leads us then to answer choice C.

[12:24] Insights into Using Equations

Very important in Physics is that you’ve got to know your equations. In this case, we take these two questions because #19 is an example of an equation where you have to do the math. While #20 is an example where you have to know the equation but you just have to deal with the conceptual, no need to actually do that math.

[13:33] Blueprint MCAT (formerly Next Step Test Prep)

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