{"id":767,"date":"2026-08-11T00:01:57","date_gmt":"2026-08-11T00:01:57","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=767"},"modified":"2026-08-24T13:03:45","modified_gmt":"2026-08-24T13:03:45","slug":"wave-speed-different-media","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/waves\/wave-speed-different-media\/","title":{"rendered":"Wave Speed in Different Media: Sound, Light and Strings"},"content":{"rendered":"\n
Definition<\/div>

\n \nWave speed is how fast a wave carries its energy through a medium, and it is set almost entirely by that medium rather than by whatever produced the wave. Sound travels at 343 m\/s in air but about 5,960 m\/s in steel, while light falls from 299,792,458 m\/s in a vacuum to roughly 124 million m\/s inside diamond.\n \n<\/p><\/div>\n \n

Watch a firework burst and you see it instantly, then wait \u2014 one, two, three \u2014 before the bang arrives. Both signals left the same point at the same moment. Only one of them was in a hurry.<\/p>\n \n

That gap is wave speed. Over a kilometre the light took about three microseconds; the sound needed nearly three seconds, because it had to shove air molecules along the way. Swap the material and both numbers change \u2014 and, strangely, they move in opposite directions.<\/p>\n \n

What Is Wave Speed?<\/h2>\n \n

Wave speed is the distance a wave crest travels each second through a medium, measured in metres per second (m\/s). It describes how fast the disturbance moves, not how fast the material itself moves.<\/p>\n \n

That distinction trips up more students than any other part of the topic. When sound crosses a room at 343 m\/s, no air molecule makes that journey \u2014 each one jiggles back and forth by a fraction of a millimetre and passes the disturbance to its neighbour.<\/p>\n \n

Think of a stadium Mexican wave. The wave sweeps around the ground in seconds; every spectator simply stands up and sits down on the spot.<\/p>\n \n

So wave speed is a property of the medium<\/em>, in the same way that density or stiffness is. Change the medium and you change the speed. Keep the medium fixed and the speed is essentially locked, no matter how the wave was made.<\/p>\n \n

Wave Speed in Different Media: The Reference Table<\/h2>\n \n

Wave speed ranges from around 100 m\/s on a slack guitar string to 299,792,458 m\/s for light in a vacuum \u2014 a span of more than six orders of magnitude. The table below collects the values you are most likely to need, with the conditions that define them.<\/p>\n \n

\n\n\n\n\n
Medium<\/th>\nWave<\/th>\nWave speed (m\/s)<\/th>\nConditions & notes<\/th>\n<\/tr>\n<\/thead>\n
Guitar low-E string<\/td>Transverse<\/td>about 107<\/strong><\/td>Tension 68 N, mass per metre 6.0 g\/m<\/td><\/tr>\n
Carbon dioxide<\/td>Sound<\/td>268<\/strong><\/td>20 \u00b0C; heavy molecules, so slower than air<\/td><\/tr>\n
Air<\/td>Sound<\/td>331<\/strong><\/td>0 \u00b0C, dry \u2014 the freezing-point value<\/td><\/tr>\n
Air<\/td>Sound<\/td>343<\/strong><\/td>20 \u00b0C, dry \u2014 the standard exam value<\/td><\/tr>\n
Helium<\/td>Sound<\/td>1,007<\/strong><\/td>20 \u00b0C; about 2.9 times faster than air<\/td><\/tr>\n
Fresh water<\/td>Sound<\/td>1,482<\/strong><\/td>20 \u00b0C; roughly 4.3 times the speed in air<\/td><\/tr>\n
Seawater<\/td>Sound<\/td>about 1,500<\/strong><\/td>Varies with depth, salinity and temperature<\/td><\/tr>\n
Soft body tissue<\/td>Ultrasound<\/td>about 1,540<\/strong><\/td>The value clinical scanners assume<\/td><\/tr>\n
Pyrex glass<\/td>Sound<\/td>about 5,640<\/strong><\/td>Bulk longitudinal wave<\/td><\/tr>\n
Steel<\/td>Sound<\/td>about 5,960<\/strong><\/td>Bulk longitudinal; about 5,050 in a thin rod<\/td><\/tr>\n
Aluminium<\/td>Sound<\/td>about 6,420<\/strong><\/td>Bulk longitudinal wave<\/td><\/tr>\n
Diamond<\/td>Light<\/td>1.24 × 108<\/sup><\/strong><\/td>Refractive index n = 2.42<\/td><\/tr>\n
Crown glass<\/td>Light<\/td>1.97 × 108<\/sup><\/strong><\/td>n = 1.52 (varies with colour)<\/td><\/tr>\n
Water<\/td>Light<\/td>2.25 × 108<\/sup><\/strong><\/td>n = 1.333<\/td><\/tr>\n
Air<\/td>Light<\/td>2.997 × 108<\/sup><\/strong><\/td>n = 1.0003; only 90 km\/s below vacuum speed<\/td><\/tr>\n
Vacuum<\/td>Light<\/td>299,792,458<\/strong><\/td>Exact by definition of the metre<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n \n

Two patterns jump out of that table. Sound gets dramatically faster<\/em> as you move from gas to liquid to solid, while light gets steadily slower<\/em> as the material gets denser.<\/p>\n \n

The same word \u2014 denser \u2014 pushes the two families in opposite directions. Working out why is the whole point of the next two sections.<\/p>\n \n

\"Bar<\/figure>\n \n

Wave speed across common media. Note the two rows use different scales \u2014 on a single axis, the light bars would be around 50,000 times longer than the steel bar.<\/p>\n \n

The Wave Speed Formulas<\/h2>\n \n

Three equations cover almost every wave-speed problem you will meet. The first is universal; the other two tell you what the medium actually contributes.<\/p>\n \n

Start with the relationship every wave obeys, whatever it is made of:<\/p>\n \n

v = f \u03bb<\/div>\n \n