Page 114 - Master Handbook of Acoustics

In the concert hall, a full symphony orchestra is capable of producing very loud sounds, but also
soft, delicate passages. Seated in the audience, one can fully appreciate this sweep of sound due to
the great dynamic range of the human ear. The dynamic range between the loudest and the softest
passage may be 100 dB (the ear’s dynamic range is about 120 dB). To be effective, the soft passages

must still be audible above the ambient background noise in the hall, hence the emphasis on adequate
structural isolation to protect against traffic and other outside noises, and precautions to ensure that
air-handling equipment noise is low.
For those not present in the music hall, live radio or television broadcast, or recordings must
suffice. Conventional analog radio broadcasts are unable to handle the full dynamic range of an
orchestra. Noise at the lower extreme and distortion at the upper extreme introduce limitations. In

addition, broadcast regulatory restrictions that prohibit interference with adjacent channels also
introduce limitations in dynamic range.
Digital audio, ideally, has the dynamic range and signal-to-noise ratio needed to fully capture
music. The dynamic range in a digital system is directly related to the word length of binary digits
(bits) as shown in Table 5-1. A Compact Disc, for example, stores 16-bit words and thus is capable

of storing music with a 96-dB dynamic range; this can be extended if the signal is properly dithered.
Consumer formats such as Blu-ray Audio discs as well as professional audio recorders can provide
24-bit resolution and avoid the audibility of digital artifacts resulting from subsequent digital signal
processing (DSP). Digital techniques, when taking full advantage of the technology, largely transfer
dynamic range limitations from the recording medium to the playback environment. On the other hand,
digital formats such as AAC, MP3, and WMA provide dynamic range and fidelity that can be quite
variable; their quality depends on factors such as the bit rate of the recorded or streaming file.

TABLE 5-1 Theoretical Dynamic Ranges for Digital Word Lengths

Power in Speech and Music

In many applications, one must consider the power of sound sources. For conversational speech, the

average power is perhaps 20 μW, but peaks might reach 200 μW. Most of the power of speech is at
mid-to-low frequencies, with 80% below 500 Hz, yet there is very little power below 100 Hz. On the
other hand, the small amount of power at high frequencies is where consonants are and determines the
intelligibility of speech. Higher and lower frequencies outside this range add a natural quality to
speech but do not contribute to intelligibility.

Musical instruments can produce more power than the human voice. For example, a trombone
might generate a peak power of 6 W, and a full symphony orchestra’s peak power might be 70 W. The