Reverberation Time Calculator (Sabine and Eyring) and Helmholtz Resonator Frequency

Find how long sound lingers in a room and plan the absorption.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the length, width and height of the room, the absorption coefficient of the floor, ceiling and walls and any extra absorption from people and furniture.

The calculator finds the volume and surface area, the total absorption in sabins, and the reverberation time by the Sabine equation 0.161 V over A, and by the Eyring equation.

For a Helmholtz resonator, enter the neck area and length and the cavity volume to find its resonant frequency.

Formulas used

  • RT_{60} = \frac{0.161\,V}{A}
  • A = \sum S_i\alpha_i
  • RT_{60} = \frac{0.161\,V}{-S\ln(1 - \bar\alpha)}
  • f_0 = \frac{c}{2\pi}\sqrt{\frac{S}{V L_{eff}}}

Worked example

With the default numbers in the calculator above:

  1. Volume of the room and the total surface area: 240.00 m3 and 268.00 m2
  2. Total absorption in sabins (area times absorption coefficient, added up): 74.80 m2 of perfect absorber
  3. Reverberation time by the Sabine equation (the time for the sound to fall by 60 dB): 0.52 s
  4. Reverberation time by the Eyring equation (better when the absorption is high): 0.44 s (average absorption 0.28)

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is RT60?

The time for the sound in a room to fall by 60 decibels after the source stops.

What reverberation time is good?

About 0.4 to 0.6 s for speech in an office or classroom, 1 to 2 s for music, and more for a church.

Why use Eyring as well as Sabine?

The Sabine equation overestimates the time when the room has a lot of absorption. Eyring is better above an average absorption of about 0.2.

How does a Helmholtz resonator work?

The air in the neck acts as a mass on the spring of the air in the cavity, and it absorbs strongly at its resonant frequency.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Kinsler et al, Fundamentals of Acoustics; AS/NZS 2107 Acoustics: recommended design sound levels and reverberation times for building interiors.

Need a calculator we do not have?

Tell us what you would like to calculate. We read every request and build the most asked-for ones.

We store your request (and your email if you give it) to decide what to build. We do not store the numbers you type into calculators.

Embed this calculator on your own page

Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.

Need this drawn, designed or checked?

Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.

Need help with a real project?

Talk to Drafting Central →

← All engineering calculators