Numford

Oscillations and wavesClass 11, Waves

Dimensional formula of Sound level

[M0 L0 T0]

Symbolβ SI unitdecibel (dB)

Ten times the logarithm of an intensity ratio. A logarithm needs a ratio, so the level has no dimension.

Derivation

How to get the dimensional formula of sound level

  1. Start from the definition: sound level = wave intensity / wave intensity.
  2. velocity = length / time, so velocity is [L] / [T] = [L T-1].
  3. acceleration = velocity / time, so acceleration is [L T-1] / [T] = [L T-2].
  4. force = mass × acceleration, so force is [M] × [L T-2] = [M L T-2].
  5. displacement = length, so displacement is [L] = [L].
  6. work = force × displacement, so work is [M L T-2] × [L] = [M L2 T-2].
  7. power = work / time, so power is [M L2 T-2] / [T] = [M L2 T-3].
  8. Put in the dimension of each part: [M T-3] / [M T-3].
  9. Add the powers of each base quantity: [M0 L0 T0].
Same dimensions

Quantities with the dimensions [M⁰ L⁰ T⁰]

Equal dimensions do not make them the same quantity. Compare sound level with the others in the full grid.

Practice

Three questions on sound level

1Which of these has the same dimensions as sound level?

2Which of these has no dimensions at all?

3Which one has different dimensions from the others?

Questions

Sound level: quick answers

What is the dimensional formula of sound level?

The dimensional formula of sound level is [M⁰ L⁰ T⁰]. It comes from sound level = wave intensity / wave intensity.

What is the SI unit of sound level?

The SI unit of sound level is the decibel (dB).

Is sound level dimensionless?

Yes. Sound level is a ratio of like quantities, so every power in its dimensional formula is zero: [M⁰ L⁰ T⁰].

Open the dimensional formula tool