How loud is 130 dB, really?

The scale multiplies, it does not add

Decibels are logarithmic. Every 10 dB is ten times the sound energy, and roughly twice the loudness as a human ear judges it. So 130 dB is not 30 percent more than 100 dB, it is about a thousand times the energy and around eight times as loud to you.

That is why the last few decibels on a spec sheet cost so much, and why the jump from 100 dB to 130 dB is the difference between a noise you can talk over and one that stops the conversation.

Where 130 dB sits

Some familiar reference points, all measured close up. A conversation runs at about 60 dB, a busy street at 80 dB, a nightclub at 110 dB, and a jet engine heard from the tarmac lands around 140 dB.

At 130 dB you are above the level at which most people feel physical discomfort. That is the point: an alarm is meant to be unpleasant, because unpleasant is what makes heads turn and hands hesitate.

Distance eats the number quickly

Sound loses roughly 6 dB every time the distance doubles. An alarm rated 130 dB at one metre is about 124 dB at two metres, 118 dB at four, and near 100 dB by the time it reaches the other end of a street.

That is still loud, and still audible through a closed window. But it explains why an alarm on its own is a local tool: it works on the people who are already near you, not on the ones who are not.

A bigger number does not mean a louder alarm

Some alarms advertise 140 dB, a few claim 150 dB. On its own the figure says nothing until you know the distance it was measured at: taken at 10 cm instead of 30, the same alarm gains about ten decibels on the label without making any more noise in the street.

The second point is physical. A speaker a few centimetres across, running off a battery that fits on a keyring, has a ceiling. Past it the sound distorts rather than grows. A pocket device advertised at 150 dB is promising the energy of a jet engine at one metre, which is not what it does.

The useful figure is not the biggest one, it is the measuring distance, and whether the siren covers the street you are standing in. Compare two numbers only when both are quoted at the same distance, otherwise you are comparing two methods rather than two alarms.

• A figure without a measuring distance compares to nothing
• 10 cm instead of 30 cm adds about ten decibels to the label
• Past a certain point a small speaker distorts instead of getting louder
• Pitch cuts through background noise as much as volume does

• Compare

Loud enough is not the same as heard

A siren in an empty street is a siren nobody hears. This is the limit of noise as a safety strategy, and no decibel figure fixes it.

It is why an alarm worth carrying does something else at the same time: an automatic call, an alert to people you trust, a message with your position. The noise handles the street, the alert handles everyone who is not on it.

What these numbers give you on OYA

OYA is rated at 130 dB measured at 30 cm, and the measuring distance matters as much as the figure: an alarm sold as 130 dB without saying from how far can land ten decibels below yours once the two sit side by side.

On a pavement that means a sound which stands out clearly a few dozen metres away, enough to turn heads along a whole street. The light comes on at the same instant as the siren, because a noise nobody can place makes people hesitate rather than come.

The rest is no longer a question of decibels. Once the loop is pulled, the paired phone dials the emergency number and warns your contacts: volume works on the people already there, the alert takes care of everyone else.

Any questions?