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Fisica - Onde e suono

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Breve presentazione dell'argomento con le relative leggi fisiche. (4° Liceo Scientifico) - Roberto Testa - IV^H Liceo Scientifico "Leonardo" - Giarre (CT).
Le onde e il suono, la frequenza, la lunghezza d'onda, la velocità, onde armoniche, riflessione, onde sonore, ultrasuoni, intensità..

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Fisica - Onde e suono

  1. 1. ONDE E SUONO
  2. 2. Le onde  Fisicamente, l’onda è una perturbazione che si propaga da un punto ad un altro, nello spazio e nel tempo, trasportando energia o quantità di moto, senza comportare un associato spostamento di materia.  Le onde, in base alle loro caratteristiche, si classificano in vari modi, tra cui, secondo la loro direzione (vettoriale) di propagazione in Onde Trasversali e Onde Longitudinali.
  3. 3.  Onde Trasversali : Lo spostamento delle singole particelle è perpendicolare alla direzione di propagazione dell’onda. Es. : Luce visibile (onda elettromagnetica); Corda oscillante;  Onde Longitudinali : Lo spostamento delle particelle è parallelo alla direzione di propagazione dell’onda. Es. : Onde Sonore.
  4. 4. Lunghezza d’onda, frequenza e velocità.  L’onda è una perturbazione regolare che si ripete periodicamente : i punti del massimo spostamento verso l’alto sono le creste, i punti più bassi sono i ventri. La distanza tra un punto massimo o minimo e il successivo, è detta lunghezza d’onda e si indica con λ (lambda).  λ = distanza dopo la quale un’onda si ripete.  Il tempo necessario affinché un’onda si ripeta, è chiamato periodo (T), e la frequenza (come nel moto armonico) è f = 1/T.  Dato che spazio/tempo = velocità, λ (spazio) / T (tempo) = v (velocità). La velocità si può anche avere con la formula inversa v = λf
  5. 5. Onde su una corda  La velocità di propagazione di un’onda dipende dalle proprietà del mezzo attraverso cui si propaga. In una corda di lunghezza L, la velocità dell’onda è determinata dalla sua tensione e dalla sua massa.  F = Tensione : forza trasmessa attraverso la corda.  Affinché un’onda si propaghi su una corda, deve esserci una tensione nella corda. Se la tensione aumenta (mantenendo la corda più tesa), le onde viaggiano più velocemente lungo la corda.  Maggiore è la massa della corda, minore sarà la velocità di propagazione dell’onda; ma ciò che importa è la massa per unità di lunghezza (densità lineare, che indichiamo con “μ” -miu-), quindi il rapporto m/L.  V aumenta con la tensione F e diminuisce con la densità lineare μ. V = sqrt F/ μ
  6. 6. Riflessione  Quando l’onda raggiunge l’altro estremo della corda, può essere invertita o no, in base alla modalità con la quale avviene la riflessione.
  7. 7. Funzione d’onda armonica
  8. 8. Onde sonore  Il suono è un’onda che si propaga attraverso l’aria (con T.a. = 20°C) ad una velocità di 343 m/s. N.B.: La velocità di propagazione del suono è determinata dalle proprietà del mezzo attraverso il quale si propaga e, applicando la teoria cinetica dei basi, è collegata alla velocità delle molecole.  Se l’aria viene riscaldata, le molecole si muovono più rapidamente, e quindi la velocità di propagazione del suono aumenta.  In un materiale, la velocità di propagazione del suono è determinata dalla rigidità del materiale : più rigido è il materiale, più veloce è l’onda sonora.
  9. 9. L’onda sonora provoca una perturbazione nella densità e nella pressione dell’aria. Quando la densità è elevata, anche la pressione è elevata; quando la densità è bassa, anche la pressione è bassa (il grafico è lo stesso).
  10. 10. La frequenza di un’onda sonora  La frequenza determina il tono di un suono.  Ad esempio, i tasti di un pianoforte producono suoni con frequenze che vanno da 55Hz (il più grave) fino a 4187 Hz (il più acuto).  L’intervallo di frequenze udibili dall’essere umano è tra i 20 Hz e i 20000 Hz : i suoni che hanno frequenze maggiori di 20000 sono detti ultrasuoni, quelli con frequenze minori di 20, infrasuoni.
  11. 11. Esempi di ultrasuoni Gli ultrasuoni, ad esempio, vengono prodotti dai pipistrelli, grazie ai quali riescono a muoversi nel loro ambiente e ad individuare le prede. Gli ultrasuoni sono pure utilizzati nelle applicazioni mediche, come l’ecografia o piccole operazioni, come la litotripsia a onda d’urto, dove il corpo è bombardato da ultrasuoni per l’eliminazione di un calcolo renale. Gli elefanti, invece, possono comunicare tra loro attraverso degli infrasuoni alla frequenza di 15Hz. Alcune meteore ‘invisibili’, possono essere localizzate appunto attraverso ultrasuoni, come accadde in Messico nel 1997 con una meteora che attraversò il cielo.
  12. 12. Intensità del suono  La quantità di energia che passa attraverso una data area in un definito intervallo di tempo è l’intensità del suono.  I = E / A*t  P (potenza) = E (energia) / t (tempo)  I = P / A (area)  Questo concetto è valido per tutte le onde.
  13. 13.  Quando ci si allontana dalla sorgente sonora, l’intensità del suono diminuisce, perché l’energia emessa dalla sorgente si distribuisce su un’area maggiore.  Area di una sfera di raggio r : A = 4 π r2  Intensità in funzione della distanza da una sorgente puntiforme : I = P / 4 π r2
  14. 14. La percezione umana del suono  Intervallo di frequenze udibili : 20 Hz – 20000 Hz  Se il ‘volume’ di un suono aumenta, aumenta la sua intensità, quindi il volume è misurato attraverso il livello d’intensità dell’onda.  β(livello di intensità) = (10 dB) log (I/Io)  dB = decibel. Minimo valore dell’intensità di un suono : 10-12 W/m2  Dato che il bel è un’unità piuttosto grande, si utilizza solitamente il decibel (dB) che è uguale a 1/10 b.  Il più piccolo aumento di intensità percepibile dall’orecchio umano è di circa 1 dB.

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