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(Page 1 of 1 pages for this article ) Wednesday, July 09, 2008Wrong, wrong!
Olympics Researchers get their physics messed up, twice. Here’s why it matters to every filmmaker.
So those researchers set up a study, which was published in the June issue of Medicine and Science in Sports and Exercise. They tested runners with gun sounds calibrated to 120 dB, 100 dB, and 80 dB, to simulate being in lanes 1 through 8. Here’s a picture of what they were trying to simulate. The red bugs represent runners in different lanes, which by international standard are 4 feet wide. The green guy is the starter. Since this is also an example of my graphic skills, you can see why it’s a good thing I specialize in sound.
Sure enough, researchers found that louder “guns” resulted in better reaction times. Their suggestion, according to the Times, is for the starter to use a silent device that sets off identical noisemakers behind each runner. As near as I can tell, this really would solve the problem. There’s only one problem: Their analysis has nothing to do with physical reality! Fortunately, they also got something else wrong. That second mistake is why their suggestion would work. Those two factors are vitally important to anyone recording sound in the field. I’ll use some basic math to explain the first, but you can understand the principles without dealing with numbers. If your eyes start to glaze over, skip to the bottom of this article.
Problem 1: Things get softer when they’re farther away. But nowhere near as much as they thought.
Sound expands in a ‘bubble’ from its source, like three-dimensional ripples in a pond. So the farther you go, the more that the sound’s original energy has to be spread around. Or: the bigger the bubble, the less energy at any one point on the bubble’s surface. (Light does a similar thing, which you see if you walk away from a flood while holding a light meter.) Decibels are ratios. When we say ‘a gun is 100 dB’, we’re really talking about dB SPL - or Sound Pressure Level - which is the ratio of a sound to a specific standard loudness. The noise in a really quiet recording studio is around 20 dB SPL. In a typical office it’s around 60 dB SPL. Movie dialog in most theaters around 85 dB SPL - about what the researches used for their quietest gun. The threshold of pain - say, a jet plane taking off and you’re standing next to the runway - is 120 dB SPL… what they used for their loudest gun. If you turn down the volume so a sound is subjectively half as loud, you’re making it about -10 dB softer… no matter whether the original sound was a clock ticking, theatrical dialog, or a jet plane. Because it’s based on ratios, the math is much easier if you use logarithms. The formula is “decibel difference equals 20 log (D1/D2)”. Those D’s are the distance from the sound source to any two measuring locations. I don’t do logs in my head, so basically I remember that every time you double the distance, the sound gets 6 dB softer. Double it again, and the sound is 12 dB softer. And so on… My copy of Excel, on the other hand, has no problem with the math. So I had it crunch the numbers, using the measurements from my drawing, and extended the results out to fifty running lanes. I don’t think any Olympic track will ever get that wide, but spreadsheets let you do that kind of thing easily. Here’s what Excel said:
The loss in Lane 1 is 0 dB: that runner hears things exactly as loud as himself (duh). Runner 2 is 12 feet from the gun - the original 8 feet to the track, and one lane that’s 4 feet wide. That’s not a doubling of Runner 1’s distance, so there’s not a lot of loss. Runner 3, however, is 16 feet from the gun, twice as far as Runner 1. As predicted, the gun seems 6 dB softer there. But to make that much difference again we’d have to be 32 feet from the gun… which takes us to Lane 7. And if we wanted 6 dB less than that, or we’d need to be at 64 feet, or Lane 15! Remember the researchers tested their gun sounds at 80 dB, 100 dB, and 120 dB… a 20 dB loss each time. They chose some very big differences. To hear 20 dB less than Runner 1, you’d have to be 18 lanes away. But if you go out to Lane 36 you’re only doubling that distance, which means 26 dB less than Runner 1. And you’d have to go to Lane 72 for another 6 dB! In fact, according to Excel’s number crunching, we’d need a track 135 lanes wide to achieve what the researchers were testing! Here’s why it’s important to filmmakers: If you can keep your mic a lot closer to the actors’s lips than it is to any sources of noise in the room, it’ll hear much more of the dialog and much less of the noise. On the other hand if your mic is much closer to a camera’s motor than it is to the actor’s lips, it’ll hear the motor better… even though the actor is louder. It’ll also hear any echoes better than the actor’s original voice, if it’s closer to walls or ceiling than it is to the lips! Start to make sense? That’s why booms or lavs, carefully placed, always sound better than a camera mic. Now that other factor? Why their ‘silent gun and individual noisemakers’ would work, in Olympic events measured in hundredths of a second? And why the reason is so important to filmmakers? I’ll let you guess for now. It uses much simpler math, and should even be intuitive. Send your guesses to me and I’ll respond by email. In a few days, I’ll also post the answer here. Disclaimers: Olympic lanes are actually 4.00262467 feet wide. Doubling the distance actually gives you a -6.020599913 dB loss. Neither of those fudges are big enough to make any difference. The standard level for 0 dB SPL is a theoretical ‘softest sound a human can hear’, defined by scientists and nerds as an energy of .0002 dynes per square centimeter.
I couldn’t find a standard for where the starter should be, so I put him 8’ from the track. I’ll update the math if someone has a more precise distance, but unless his gun is dangerously close to the runner’s ear, it won’t affect the point of this article.
Audio • Production • (Page 1 of 1 pages for this article ) 
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