Osnovi audiotehnike Audio signali

Signal govora

Dinamika Normalan glas (64 dB / 1m, snaga 10 uW) – prosečne vršne vrednosti su 12 dB iznad i 18 dB ispod prosečne dugovremene vrednosti – dinamika 30 dB kada je nepromenjen dugovremeni prosečni nivo. Podignut glas (govor pred skupom) – 6 do 10 dB iznad normalnog – obično se daje 74 dB/1m, 100 uW. Vika – 300 uW (79-80 dB) - ako se doda 12 dB na ime vršnih vrednosti dobija se da je snaga najjačih komponenata veća od 1 mW (blizu 5 mW).

Jačina glasa, prosečna dugovremena vrednost Nivo, db/m Normalan64 Podignut74 Vika 80 Vršne vrednosti iznad prosečne12 Vršne vrednosti ispod prosečne18

Šapat – snaga vrlo mala Snaga vikanja / snaga šaptanja = 10 6 : 1 Za prenos signala govora u telekomunikacijama zahteva se dinamički opseg od 60 dB.

Odnos signal - šum Razumljivost govora je prihvatljiva (65%) ako je prosečan nivo govora samo nekoliko dB iznad nivoa šuma. Razumljivost pada na nulu ako je nivo šuma za 25 dB viši od nivoa glasa. Pokazuje se da je pri normalnom nivou glasa od 64 dB ovaj uslov identičan uslovu da se spektar vršnih vrednosti govora u svim tačkama nalazi ispod spektra šuma. Šum ne utiče na razumljivost (razumljivost je 100 % usled uticaja šuma) ako je u svim tačkama spektar dugovremene vrednosti glasa 18 dB iznad spektra šuma.

Frekvencijski opseg Razumljivost govora prihvatljiva (65%) – preneti frekvencije ispod 1500 Hz. Dovoljna razumljivost govora + prepoznavanje govornika – 300 Hz – 3400 Hz (telefonija)

Signal muzike

Prosečne vršne vrednosti signala muzike simfonijskog orkestra su oko 15 dB iznad i oko 35 dB ispod dugovremenog proseka. Prosečna jačina zvuka u pojedinim deonicama jednog muzičkog dela se takođe menja – razlika po nivou između najjačeg signala orkestra (fortissimo) i najtišeg signala violine (pianissimo) je veća od 70 dB. U realnim uslovima kod jednog orkesta ove razlike su nešto manje i iznose 50 do 60 dB, izuzetno 65 dB. Kod drugih vrsta muzike ove razlike su manje, kod džeza recimo, znatno manje. Simf. orkestar od 75 izvođača ima prosečnu snagu oko 10 mW, u fortissimu ona može da bude veća od 1 W. Dinamika

Odnos signal šum Za kvalitetan prenos muzike nivo šuma mora biti za oko 20 dB ispod nivoa najtiših delova muzike.

Frekvencijski opseg 30 do 40 Hz – 15 kHz – visoka vernost reprodukcije (FM radio, Hi-Fi uređaji) Prihvatljiv opseg 80 Hz – 8 kHz (obezbeđuje 70 % kvaliteta muzičkog signala) Stari mehanički gramofoni 400 Hz – 2 kHz

Prilozi

Even if a 120 dB dynamic range were possible in audio equipment, would it be desirable - and useful? A listener in a domestic setting enjoying the exhilarating effects of a 96 dB Sound Pressure Level will almost certainly be causing his neighbour a significant amount of annoyance, if not distress! At the other end of the dynamic scale, a typical ambient noise level of at least 40 dB SPL precludes the use of very quiet levels in recorded or broadcast sound media. Almost always, it is necessary to compress the dynamic range of natural sounds to fit them into a window suitable both for the equipment and for comfortable listening. Although the dynamic range of the programme

material must fit into a 30 to 40 dB window, there is another factor in the equation - noise. All the sound that we want to listen to, whether natural or electronically processed, are accompanied by a certain amount of unwanted background noise - tape hiss, or the rustle of musicians turning the pages of the score in the studio. Even if, in the final programme, the noise level is below the ambient noise of the listening area, it may still be heard and is therefore undesirable. The Compressor/Limiter and the Expander are valuable tools for the control of dynamic range and reduction of noise. Compression reduces the dynamic range of any instrument or programme source. Expansion has the effect of subjectively reducing or eliminating background noise.

Compression and Expansion have their artistic uses too. The sounds of instruments and voices can be altered. A mix of instruments can be compressed to give a ‘tighter’ dynamic effect. Or a previously over- compressed recording can have life put back into it by the subtle uses of expansion. The Compressor/Limiter/Expander offers in a compact unit virtually complete control over dynamic range, for corrective and for artistic purposes. BSS audio

VThere is an optimum operating range for a speech reinforcement system. For those systems in very quiet surroundings a normal level of 65 to 75 dB SPL is ideal. In progressively noisier environments the system operating level should be raised so that the signal-to-noise ratio is at least 15 dB. Typical here would be a transportation terminal at peak travel times, where noise levels in the 60 to 65 dB(A) range would call for system operation at peak levels of 80 dB SPL for greatest intelligibility. Sports venues often present high crowd noise levels in the range of 85 to 95 dB SPL, and under these conditions it is virtually impossible for a speech reinforcement system to work at all. It is better to wait until crowd noise subsides before making announcements. By John Eargle & Chris Foreman