Speed of Sound
Robert: Professor, Classroom: Discussion of the Speed of Sound
“Let's start out with the basics. Who can tell me what sound actually is?” I peered out at the half-circle of multi-level rows that filled the auditorium. There were at least a seventy-five students present that day, and yet not one raised a hand.
“Fine, I'll start,” I grimaced, feigning hesitation and earning a titter of laughter. “Sound is a vibration of molecules traveling away from a source. Can you name several fluids that sound travels through?”
A few students called out their answers: “Water. Air … “
“Those are correct; both are fluids. Now, how does sound travel?”
“Waves,” a few students responded uncertainly.
“Correct … mostly. Molecules become 'excited' – and I don't mean happy,” earning a groan from the crowd. (My inner geek was still alive and kicking.) “And then they vibrate back and forth and bump into each other. The stronger the energy source, the further the molecules away from the epicenter are affected. This particle motion manifests itself as waves. So let me ask this: What is the sound barrier?”
“Maybe 800 mph?” one of the students ventured.
“No, what is it?”
A nervous boy with curly red hair fidgeted with his pen before raising it. I caught the movement and pointed to him.
“It's the fastest that sound can travel,” he said finally.
“So sound can't go faster than a certain speed? There's a limit?”
He nodded his head, “Yes.”
“OK. So can anyone tell me exactly what the speed limit for sound is?” I gestured to the student who'd guessed before.
More confidently this time, he answered, “About 760 mph.”
“Yes, very close. It's normally considered 761 mph, with some caveats that we'll discuss in a minute. It's pretty common to refer to the sound barrier as the 'speed of sound'. Did you know that once a jet reaches the sound barrier, there's a tremendous boom … and then complete silence?”
“None at all?” a guy in the front row questioned.
“Quiet as a tomb,” I confirmed. “Of course, if the pilot speaks, there's sound inside the cockpit, but the sound generated from his aircraft is still behind him trying to catch up. Have you ever heard military jets flying overhead? When you look up towards the sound and only see the white contrails that the jets leave behind? Where the jets leave those contrails of frozen condensation? The sound can't keep up.” The students grew quiet as they remembered seeing the very same thing. “Now, what is the unit they use to quantify the speed of sound?”
“Mach,” a number of students responded together.
“Good. So we're familiar with that one. Mach units,” I continued, “help represent the point when the speed of sound is broken – in other words, when the sound can't go any faster. Any time we measure sound, it is a fraction or a factor of Mach. 3.5 times the speed of sound is Mach 3.5. Technically, we're really supposed to say '3.5 Mach'. Hardly anyone does it that way except for scientists and pilots, though,” I admitted. “What's the speed of sound in Space?” I asked. “Anyone?” I surveyed the audience.
A student near the back row raised his hand. “I don’t remember reading that there is sound in space.”
“Why not?” Blank stares all around. Someone spoke up. “Sound has to have molecules to travel through, right?”
“Correct.” I felt relieved when that answer was given. Technically, in Space there were areas of gas that could still conduct sound, but in those cases, there were molecules to conduct it. However once the particles no longer were present, the sound wouldn’t have anything to travel through. “The speed of sound is dependent on whatever fluid it is traveling through. There's even a different Mach for speed under water – in contrast with the Mach for air. So normally, the atmosphere is the sound's 'fluid'. But even in air, the temperature has a direct impact at what speed Mach is achieved."
"Do you think that sound barrier at the Arctic is higher or lower than in Hawaii?”
A young freshman girl raised her hand. “I would bet it's lower at the Arctic.” Several students shook their heads in disagreement.
“Go on,” I said, urging her to complete her thought. “Why?”
“Because the air is thicker.”
“Why?” I prompted.
“It's colder, more dense,” she concluded.
“Correct! Just so we're clear, we're saying that the sound barrier is broken at a lower speed at the Arctic than in Hawaii,” I reiterated. “If the sound barrier is lower at the Arctic, what does that say about how fast the sound itself is actually traveling?”
A girl in the front row looked up while absentmindedly chewing on a pen that appeared as if it had been ravaged by wild dogs. “If the barrier – or the 'finish line' – is reached sooner, then wouldn't the sound have to travel faster too?”
“Excellent!” Pointing to the freshman girl who'd previously responded, I asked, “And why does sound travel faster at lower temperatures?”
She smiled and stated, “Because the molecules are closer together. Easier for them to bump into each other before they get tired out.”
“Good illustration,” I nodded. “Most of you are familiar with the equation, a=F/m, Acceleration = Force divided by Mass. Normally, more force is required to accelerate an object when it is pushed through a fluid that is more dense … like running through water versus running on dry ground, for example. But with sound, the molecules are more densely compacted and don't rebound as much, meaning that they more easily transfer their movement to adjoining molecules. So getting back to our question about sound in Space. Can there be any sound in a vacuum?” I pointed to the guy who had originally answered the question.
“Um, If there’s no fluid or atmospheric particles to travel through, then no sound exists.”
Nodding, I asked, “And what does that tell you?”
“No sound barrier either?” I nodded. “Except for the conditions I described earlier. And what does that tell you about sound effects in the Star Wars movies?” The students laughed, reflecting on the sounds they'd heard from TIE Fighters and exploding Death Stars.