Ultrazvuk

Slika ljudskog fetusa s 14 tjedana pomoću ultrazvuka

Ultrazvuk je zvuk čija je frekvencija iznad gornje granice čujnosti za normalno ljudsko uho, a koja iznosi 20 kHz (20000 herca).[1]

Neke životinje (npr. psi, dupini, šišmiši, miševi, itd.) mogu čuti ultrazvuk jer imaju višu gornju graničnu frekvenciju od čovjeka.[2][3][4] Mlađe osobe, a posebno djeca, mogu čuti neke zvukove visokih frekvencija. Što je čovjek stariji, gornja granica čujnosti mu pada, što znači da sve slabije čuje zvukove visokih frekvencija. Visoke zvučne frekvencije sastavni su dio spektra frekvencija koje proizvodi neki izvor zvuka, a spektar zvučnih frekvencija čini boju zvuka. Opadanjem čujnosti visokih frekvencija starenjem, starijim ljudima se mijenjaju i boje zvuka, što znači da simfonijski orkestar ili zvuk violine drugačije čuje dijete od šest, odrastao čovjek od 30 ili starac od 80 godina.

Snimak djeteta u tridesetom tjednu trudnoće četverodimenzionalnim ultrazvukom.

Najpoznatija primjena ultrazvuka je u medicini - ultrazvučna dijagnostika (npr. Transkranijalni dopler),[5] no koristi se i u mnoge druge svrhe (otkrivanje jata riba i podmornica, tzv. sonar). Princip korištenja je vrlo jednostavan: odašilje se ultrazvučni val, koji se odbija od prepreke te se prema vremenu potrebnom da se val vrati određuje udaljenost i oblik objekta.

Načini proizvodnje ultrazvuka

Elektrostrikcija

Najrasprostranjeniji generator ultrazvuka je kvarcni generator, čiji je rad zasnovan na tzv. piezoelektričnom efektu. To je pojava kada se na nekim kristalima (kvarc, turmalin i dr.), pogodno odrezanima, javlja električna polarizacija kada se silom izvrši elastična deformacija. Umjesto da se djeluje silom, može se postići obrnut efekt - elektrostrikcija - stavljanjem pločica u izmjenično električno polje vrlo visoke frekvencije. Kada se frekvencija izmjeničnog napona poklopi sa svojstvenom (vlastitom) frekvencijom kvarcne pločice, nastupa rezonancija. Uslijed rezonancije, pločica jako titra i proizvodi ultrazvučne valove znatne amplitude. Upotrebom turmalina mogu se dobiti ultrazvučne asilacije i do nekoliko stotina MHz.

Magnetostrikcija

Glavni članak: Magnetostrikcija

Ova metoda dobivanja ultrazvuka zasnovana je na svojstvu nekih materijala da djelovanjem magnetnog polja mijenjaju dimenzije (željezo, kobalt, nikal itd.).[6] Ovi se materijali u jačem magnetnom polju uglavnom skraćuju, te je ova pojava nazvana magnetostrikcijom (strictura - stezanje, sužavanje). Ako se kratak štap od ovih materijala nalazi u promjenjivom magnetnom polju, onda se dovodi u rezonantno logitudinalno titranje, kad se kroz elektromagnet propušta izmjenična struja odgovarajuće frekvencije.

Veze

Izvori

  1. ↑ Corso, J. F. (1963). „Bone-conduction thresholds for sonic and ultrasonic frequencies”. Journal of the Acoustical Society of America 35 (11): 1738–1743. Bibcode 1963ASAJ...35.1738C. DOI:10.1121/1.1918804. 
  2. ↑ Novelline 1997: str. 34–35
  3. ↑ Matt Kaplan (17. 7. 2009.). „Moths Jam Bat Sonar, Throw the Predators Off Course”. National Geographic News. 
  4. ↑ Some Moths Escape Bats By Jamming Sonar (video)
  5. ↑ Emmanuel P. Papadakis (ed) Ultrasonic Instruments & Devices, Academic Press, 1999 ISBN 0-12-531951-7 str. 752
  6. ↑ K.H.J. Buschow et al, (ed), Encyclopedia of Materials Elsevier 2001 ISBN 0-08-043152-6 p. 5990

Literatura

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