# Ask A Scientist

### How fast is the speed of sound?

Asked by: Kelly Farrell
School: Seton Catholic Central High School
Grade: 9

Hobbies/Interests: Sports
Career Interest: Doctor or interior designer

### Answer from Richard Pastore

Distinguished Service Professor, Binghamton University

Research area: Human Auditory Perception and Cognition
Ph.D. school: Purdue University
Interests/hobbies: Tennis, hiking
Web page address: http://bingweb.binghamton.edu/~pastore

A great question with many answers!   Sound is the vibration of molecules in a substance such as air.  The speed of sound depends upon the density of the substance, traveling faster when the molecules are closer together.  The density of air depends upon altitude and temperature.  At sea level and a temperature of 60 degrees Fahrenheit, sound travels at 1,116 feet/second or 761 miles per hour (mph).  When air is thinner, such as at higher temperatures or (although colder) higher up in the atmosphere, sound travels more slowly. For example, seawater is denser than air, allowing sound to travel at over 3,000 mph.  In steel, sound travels at an amazing speed of over 16,000 ft/sec or over 11,000 mph. The same principles about sound explain other concepts.  We can run through air because, at our speed, the molecules can easily move out of our way, then move back after we pass.  In a denser medium, the molecules cannot move very far or very quickly and explains why it is work to push through water, but impossible to move through steel.   An airplane flying faster than the speed of sound (Mach 1) is moving too quickly for the air molecules to move out of the way.   Instead, the airplane pushes the air molecules forward against each other, creating a sonic boom, a shock wave of concentrated molecules. The speed of sound is very slow relative to the speed of light (over 186,000 miles per second).  Thus, when lightning occurs, we see the light flash almost instantaneously, but the sound, traveling at a mere 1,116 feet per second, takes 4.76 seconds to travel one mile.  This is why we can roughly estimate our distance (in miles) from the lightning by counting the seconds between seeing the lightning and hearing the thunder, then dividing by 5 (easier than dividing by 4.73). Sound reaches you both directly and after bouncing (reflecting) off solid objects (e.g., a wall). Because sound bouncing off the wall travels farther than the sound coming directly to you, the reflected sound will be delayed in time, and you sometimes hear it as an echo.  In most rooms, we are not aware of the reflected sound.  This is because our brain keeps us from consciously hearing the same sound more than once if the reflected sound is delayed by less than approximately 40 milliseconds (0.040 seconds).  This is called "echo suppression."  How far away does that wall have to be in order for you to hear an echo?   (Hint: at 1,116 feet/sec, how far does sound travel in 0.04 seconds). Finally, sound coming from directly in front of you will arrive at your ears at the same instant, but sound coming from your right has to travel around your head inches to reach your left ear, arriving there later by approximately 7 milliseconds (0.007 seconds).  Like echo suppression, you conscientiously hear only the first sound, but your brain uses the time difference to tell you the direction of the sound source.

Last Updated: 9/18/13