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If I put my hand over the speaker of my phone like in the picture, I can clearly hear my music amplified, why does this happen?

The only cause I can think about is the fact that all the intensity doesn't have to "expand" in all directions but only towards me, so the formula intensity = source_power / area is "applied" only after as a normal spherical surface

I also wanted to know why the sound seems a little higher (the frequency) if I do this to amplify my speakers

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  • $\begingroup$ I am not absolutely sure, but if you say the sound is of higher frequency, the wavelength of the lowest harmonics of the sound gets shortened. Could there be any similarity with placing the finger on a guitar chord? $\endgroup$
    – Sofia
    Commented Jan 10, 2015 at 22:59
  • $\begingroup$ Well, guitars chords get higher frequencies the more the plectrum and the left hand finger get near, that happens because the chords vibrate at higher frequencies since the force applied to make them vibrate is constant, but as the mass of the chord pulled is lesser, the movement is enhanced $\endgroup$
    – maxpesa
    Commented Jan 10, 2015 at 23:18
  • $\begingroup$ Most probably you only redirect the sound wave towards you. This should be significant only for wavelengths smaller than the size of the aperture, or, your hand in this case: λ= 5 cm at frequency of ca. 6 kHz. Theoretically, you could even alter the radiated acoustic power if you introduced a resonance of the sound wave in front of your speaker, changing its effective acoustic impedance, but this is not likely to be noticeable without proper design of the resonant cavity. $\endgroup$
    – dominecf
    Commented Feb 3, 2016 at 7:53

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There's no amplification - the total sound energy is the same.

What is happening is that your hand is reflecting more of the sound towards you.

The reason for the change in sound quality is most likely because your hands absorb some frequencies slightly more and reflect other frequencies better.

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  • $\begingroup$ I agree. To verify the reflecting sound explanation, try doing the same experiment, but move so your ear is not in a line of sight with the speaker. The sound should now be quieter than without the tube, rather than louder, since your hand is now reflecting the sound away from your ear, rather than towards it. $\endgroup$
    – Brionius
    Commented Jan 11, 2015 at 1:01
  • $\begingroup$ This is a fair answer except that there are well demonstrated examples of amplifiers which rely on reflections (consider "Wahlstrom 1985 The Parabolic Reflector as an Acoustical Amplifier"). $\endgroup$
    – Halyn Betchkal
    Commented Sep 13 at 21:30

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