Four Audio Myths
Jun 6, 2011 2:51 PM, By Bob McCarthy
Misconceptions you need to know.
4. "You are too close to measure the low frequencies"
My process of tuning a sound system involves the measurement of every speaker in the system. Oftentimes people are concerned when I move my measurement mic inches away from a giant subwoofer. "Aren’t you too close in to see the low frequencies? Doesn’t it take a long distance for the low frequencies to 'develop?'" My answer is to invite them to put their ear up against the cabinet while I send in a 30Hz tone. I get few takers. Then I show them the analyzer's frequency response, which inevitably reveals a fully developed 30Hz.
Air is the medium through which sound passes. There is no way for 30Hz to get to the back of the arena without passing through the air in front of the speaker. The misunderstanding seems to stem from a variation of the cosmological construct known as "string theory"—in our case, "guitar string theory." A vibrating string has fixed points (such as the bridge of a guitar) that don't move. If we visualize our subwoofer as the end of a vibrating string, then it would seem that we would have to get away from the box to experience the large wavelength vibrations. In reality, the subwoofer is not a pressure node but is quite free to move, and putting your face in front of it removes all such doubt.
There are instances where the sound waves take some distance to fully develop their character, which helps to keep this myth alive. This happens in speaker arrays, which require an extended transmission distance before the patterns of the multiple elements have overlapped and assumed the fully combined coverage pattern. Low frequencies can be steered in such arrays. We might find ourselves in a quiet zone close by an array, but this is about directional control—not being too close to hear. The sound is somewhere around the array.
There are lots more audio urban legends out there. Personally, I love to hear the theories, but what I love most is putting them to the test under my analyzer.
Bob McCarthy is president of Alignment and Design. McCarthy specializes in the design and tuning of sound reinforcement systems and conducts trainings around the world. His book, Sound Systems: Design and Optimization, was named “2007 Sound Product of the Year” by Live Design. Read his blog.
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