The sound of the Big Bang … and other matters of interest …
Last time we introduced the Decibel scale, and also mentioned the loudest sound ever recorded (the volcanic eruption on Krakatoa in 1883). While researching, we came upon the concept of the loudest sound possible … another one for the philosophers to mull over perhaps! Not unlike – at least to people who haven’t studied relativity – trying to explain why nothing can travel faster than light … how can there be such a thing as a maximum loudness of any sound? Well, here’s the answer the experts give: 194 dB. Why? Because anything louder than this won’t be a sound wave at all, but a blast wave.
Now a little something for the physicists:
Sound is associated with (relatively low amplitude) pressure waves in air. However, such vibrations are only possible under conditions where air can be approximated as a continuous fluid. This approximation breaks down when the wavelength of the wave becomes comparable with the mean free path of the molecules that make up the atmosphere. The mean free path is roughly the average distance between collisions of atmospheric molecules.
The mean free path in air at sea level is about 0.1 micron. If we take the speed of sound to be roughly 300 m/s, then the upper limit on sustainable sound frequencies is:
f_max = (300 m/s) / (10^-7 m) = 3*10^9 Hz = 3 GHz.
By the way, you may be wondering, how loud was the Big Bang, the event which (some believe) started the universe? Well, as it occurred in an airless space – as there nothing existed before it – it wouldn’t have made any sound at all … not that anyone existed to hear it …
Recommended noise levels
Below are recommended maximum noise levels for a variety of workspaces – as you’ll see considerably less than Krakatoa:
| Workspace | Recommended maximum noise level (dB) |
| Offices | 40 – 45 dB |
| Large Offices | 45 – 50 dB |
| Classrooms | 40 dB |
| Large Lecture Rooms | 35 dB |
| Music Room | 30 dB |
Here are some sound reduction guidelines:
| 20 dB | Normal speech will be readily audible |
| 25 dB | Loud speech will be clearly audible |
| 30 dB | Loud speech will be clearly audible under normal circumstances |
| 35 dB | Loud speech will be audible but difficult to distinguish |
| 40 dB | Loud speech will be faintly audible but cannot be distinguished |
| 45 dB | Shouting will be audible but cannot be distinguished |
| 50 dB | Shouting will be barely audible |
| 55 dB | Shouting will not be audible |
So how do we measure the noise reduction?
We’ve learned that the Decibel is a logarithmic unit, while the percentage scale is linear. Thus, a statement like “solution XYZ would reduce noise by N dB” is completely meaningless. And to emphasise the point, the Decibel scale has no maximum value, so it’s impossible to calculate a percentage reduction anyway!
We use instead a ‘Weighted Sound Reduction Index’, Rw, which expresses the difference between the sound intensity hitting one side of a structure and the resulting sound measured on the other side.
Here are ratings for soundproof or acoustic doors provided by Enfield Doors:
You’ll see that we have a wide range of doors, suitable for different environments.
Contact the experts
We really do know all about doors – and soundproofing generally – as well as about all sorts of related topics. If you would like more information then please view our page on sound proof doors, or contact us .
Alternatively, call us any time on 020 3199 5845 and we’ll be delighted to pass on our knowledge – and we won’t give you a physics lesson!
