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octave. Human hearing encompasses a ten-octave frequency range. Although human hearing spans from 20 - 20,000 Hz, human speech does not. Human speech ranges from about 200 - 6,000 Hz for a man and about 400 - 8,000 for a woman. Vowels are low frequency sounds that give a voice its character, richness and sexiness. Consonants are high frequency sounds that provide diction and clarity. response for an audio file. So, a 48,000 sample rate yields a maximum frequency response of 24,000 Hz; which encompasses the full range of human hearing. However, audio is more than just frequencies, it also varies in loudness. That’s where bit-depth comes in. Bit-depth determines the range between the softest and loudest portions of a digital audio clip. (I’d explain how, but I have a 1,000 word limit in this article. Trust me, it does.) For instance, say the letters “F” and “S.” Both are formed the same way, with air hissing between the tip of the tongue and the roof of the mouth. If you can hear the hiss, it’s an “S.” If you can’t, it’s an “F.” For a man, that hiss is located about 6,000 Hz, for a woman, its closer to 8,000 Hz. OTHER COOL THINGS TO KNOW NOTE: Many individuals with hearing loss hear low frequency sounds perfectly fine, but have trouble with higher frequencies. This means that they can’t tell the difference between an “F” and an “S,” because they can’t hear the frequencies in an “S,” making both letters sound the same. For every 6 dB you lower peak audio levels, the perceived volume of the audio is cut in half. So, reducing peak levels to -6 dB reduces perceived volume to 50%. Lowering levels another 6 dB reduces perceived volume to 25% of maximum. And so on in 6 dB increments. If you are creating a program for pre-schoolers, you have a wide latitude in your audio mix, because those kids can hear anything. If, on the other hand, you’re creating programs for the retired set, you’d be well- advised to boost the higher frequencies to help them follow the dialog more easily. CONNECTING A COMPUTER Now, in the real world, audio waves work just fine. But not for computers. This is principally because computers don’t have ears. Instead, then need to convert all these pressure waves into something a computer can understand and store. Worse, computers don’t like things like waves that have smooth curves and infinite variations. They like measuring and storing things in chunks. Engineers measure audio pressure waves as a variation in voltage from -1 to +1 volt. If you think of a sine wave, the low point of the wave is -1 volt, while the high point of the wave is +1 volt. Samples are used to convert waves into chunks of data that the computer can store. A sample measures the average voltage for a very short period of time; for example, 1/48000 of a second. This sample is then stored by the computer for later playback. The cool part about this is that the Nyqvist- Shannon sampling theorem states that dividing the sample rate by 2 equals the maximum frequency Just as audio frequencies are logarithmic, so also are audio levels. 0 dB in a digital system represents the loudest your audio can be. Let’s call that audio level 100%. NOTE: The absolute #1 rule of audio is that peak audio levels during export must not exceed 0 dB. Doing so causes audio distortion which is really, really difficult to fix once the damage has been done. TYING IT ALL TOGETHER So, in summary, here’s how this applies to us: - Sample rate determines frequency response. - Bit-depth determines the range between the softest and loudest portions of digital audio clip - The human voice is a subset of the range of human hearing. - Doubling the audio frequency raises the pitch by one octave - Reducing levels by 6 dB cuts the perceived volume in half We can use this knowledge to improve our mixes, boost intelligibility, integrate music without making narration hard to understand, prevent distortion, and keep our audience glued to their seats. I’ll write more about audio in future issues. KITPLUS - THE TV-BAY MAGAZINE: ISSUE 101 MAY 2015 | 49