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Just how fast is sound, anyway?

Learning the speed of sound...


One of the most common questions I get from aspiring audio engineers is: How fast does sound travel? It's interesting that this is relevant for musicians and sound engineers alike: any time you're working with sound, there might be potential obstacles to overcome when dealing with how sound travels. More on that later.

In our ongoing project to record the remaining few launches of the space shuttle in high-definition audio, I've noticed that the time from when we see the launch to the time we hear the launch to vary greatly depending on the weather, including direction of wind. I've also noticed that a nighttime launch is a fair amount louder than a daytime liftoff.

So, why does this happen, and how does this explain "How Fast Sound Travels"?

Have you ever watched lightning during a thuderstorm, and noticed that thunder is always a few seconds (or more!) behind the lightning? It's simple, really: light travels much faster than sound. You'll always hear something well after you see it -- even if that time is measured in nanoseconds. It doesn't matter how far your eardrums are from the source, there's always going to be a delay, due to the laws of physics.

Unfortunately, there's no one answer for how fast sound travels. Sound can be affected by a variety of outside elements. Temperature is one of the major factors; humidity is another. However, a good baseline formula to use is this: at room temperature, sound will travel at 1125 feet per second. This equals out to about one mile per five seconds, or one millisecond per foot.

Sound will always travel faster when it has a vehicle to attach itself to, so in higher humidity, sound will travel faster and with more force than if it were traveling through completely dry air.

How does this matter for recording & live sound?

When recording, you may be tasked with recording an instrument with several microphones. In the case of live recording, you'll need to sometimes have room ambiance microphones alongside your mix from the stage to help give your recording a more "live" feel. You'll need to apply a delay to the microphones further from the sound source to align your tracks correctly. When applying delay, keep in mind the temperature of the room. If it's normal room temperature (around 70 Fahrenheit), you can factor in a delay of 1 millisecond per foot. If it's hotter, add a coupe milliseconds to your calculation, doing the opposite for cold.

One of the most popular delays for live recording is the TC Electronic D-Two; if you're multitracking, you can always add the delay later in digital editing. Simply apply a digital delay, with the mix set at 100%.
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