Apple MA261Z/A User Manual - Page 583

VIII In practice, a bel is a bit too large to use for measuring sound, so a one-tenth unit called the decibel is used instead. The reason for using decibels instead of bels is no different than the reason for measuring shoe size in, say, centimeters instead of meters; it is a more practical unit. Number of decibels 0 1 3 10 20 30 50 100 Relative increase in power 1 1.26 2 10 100 1000 100,000 10,000,000,000 Decibel Units Audio meters measure sound level using decibels. Since decibels describe the ratio between two signals, audio meters always display the incoming signal as if it is being compared to an assumed reference signal. Several reference levels have been used in audio meters over the years, starting with the invention of the telephone and evolving to present day systems. Some of these units are only applicable to older equipment. Today, dBu is used for most professional equipment, and dBV is used for most consumer equipment. dBFS is used for digital meters. Â dBm: The m stands for milliwatt (mW), which is a unit for measuring electrical power. (Power is different from electrical voltage and current, though it is related to both.) This was the standard used since the early days of telephone technology, and remained the professional audio standard for years. Â dBu: This reference level measures voltage instead of power, and uses a reference level of 0.775 volts. dBu has mostly replaced dBm on professional audio equipment. The u stands for unloaded, because the electrical load in an audio circuit is no longer as relevant as it was in the early days of audio equipment. Â dBV: This also uses a reference voltage like dBu, but in this case the reference is 1 volt, which is more convenient than 0.775 in dBu. dBV is often used on consumer and semiprofessional devices. Â dBFS: This scale is very different from the others because it is used for measuring digital audio levels. FS stands for full-scale, which is used because, unlike analog audio signals that have an optimum signal voltage, the entire range of digital values is equally acceptable when using digital audio. 0 dBFS is the absolute highest possible digital audio signal you can record without distortion. Unlike analog audio scales like dbV and dBu, there is no headroom past 0 dBFS. For more information about digital audio metering, see "About Audio Meters" on page 593. Chapter 42 Audio Fundamentals 583