![]() Sound (in air) covers roughly 340 meters each and every second, even as its amplitude shrinks. As sound travels, its amplitude decreases- but that’s not the same thing as slowing down. That means sounds must slow down and come to a stop, right? Wrong. I know what you’re thinking- how is that possible? Sounds die out as they travel, right? True. Sound does not speed up or slow down as it travels (unless the properties of the material the sound is going through changes). Which takes longer to travel the distance of a football field: the low pitched sound of a whale or the somewhat higher pitched sound of a human being?.Which sound takes longer to travel 100 meters: the sound of a snapping twig in the forest or the sound of a gunshot?.Which takes longer to cross a football field: the sound of a high pitched chirp emitted by a fruit bat or the (relatively) low pitched sound emitted by a trumpet?.That’s because the speed of sound doesn’t depend on amplitude. The rule of thumb works the same for all thunder- regardless of loudness. If the speed of sound depended on loudness, this rule of thumb would have to account for loudness- yet there is nothing in the rule about loud vs. You’ve probably heard that you can figure out how far away the lightning by counting the seconds between when you see lightning and hear thunder. The sound still takes the same amount of time to reach the listener. The amplitude of the sound does not matter- loud sounds and quiet ones travel at the same speed. If different frequencies traveled at different rates, some frequencies would arrive before others. If you’ve heard someone shout from across a field, you’ve noticed that the entire shout sound (which contains many different frequencies at once) reaches you at the same time. If you’ve ever listened to music, you’ve witnessed this- the low notes and the high notes that were made simultaneously reach you simultaneously, even if you are far from the stage. The properties of the medium are the only factors that affect the speed of sound- nothing else matters.įrequency of the sound does not matter- high frequency sounds travel at the same speed as low frequency sounds. Though speed of sound in air also depends on humidity, the effect is tiny- sound travels only about 1 m/s faster in air with 100% humidity air at 20 o C than it does in completely dry air at the same temperature. The speed of sound is 331 m/s in dry air at 0 o Celsius and increases slightly with temperature- about 0.6 m/s for every 1 o Celsius for temperatures commonly found on Earth. The speed of sound in air depends mainly on temperature. However, the high density of metals is more than offset by far greater stiffness (compared to liquids and solids). This may seem backwards- after all, metals are quite dense. In general, sound travels fastest through solids, slower through liquids and slowest through gasses. Sound travels fastest through materials that are stiff and light. ![]() The speed of sound in a material is determined mainly by two properties- the stiffness of the material and the density of the material. The physical properties of the medium are the only factors that affect the speed of sound- nothing else matters. Sound travels at different speeds though different materials. A youTube video (2:05 min) produced by the UNSW PhysClips project shows the demo with a drumming toy monkey instead of a cell phone. As the air is removed from the jar, the cell phone’s ringer gets quieter and quieter. You can show that sound requires a medium by putting a cell phone inside a glass jar connected to a vacuum pump. This material sound travels through is called the medium. Sound requires some material in which to propagate (i.e. This may seem fast, but it’s tiny compared to light, which travels roughly a million times faster than sound (roughly 300,000,000 m/s). In everyday terms, sound travels about the length of three and a half foot ball fields every second- about 50% faster than a Boeing 747 (roughly 250 m/s). The actual value depends somewhat on the temperature and humidity. The speed of sound in air is roughly 340 m/s. If you’ve sat in the outfield seats in a baseball stadium, you’ve experienced the delay between seeing the player hit the ball and the sound of the “whack.” Life experiences tell us that sound travels fast, but not nearly as fast as light does. You see lightning before you hear the thunder. However, the delay can be noticeable if the distance between source and detector is large enough. In everyday life, the delay is usually too short to notice. There’s a delay between when a sound is created and when it is heard. Traveling waves 17 How sound moves Speed of sound
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