Negative DB scale - confusing me!

[Ciara O'Flynn]

Hi folks,

I'm a performing musician and trying to establish if the rehersals and shows are too loud to be safe for a 6 month fetus I'm carrying.

I understand the Hertz measurements but I'm finding it hard to understand the DB measurements being in negative, so right now in my quiet office it's showing background noise and me breathing at -100DB and someone on the phone as -40db.

I'm having to keep an eye out for DB levels over 70.  Can someone explain how I can read that on this negative db scale?

thanks in advance,

Ciara

[Carl Reinke]

Short answer: You need a sound level meter, not a smart phone.

Longer answer:

A decibel is a relative unit of measurement.  For example, "3 decibels" is equivalent to "twice as much power", "0 decibels" is equivalent to "the same power", and "-3 decibels" is equivalent to "half as much power".  But half as much power as what?  You need a reference level to compare it to.

In Spectroid, the decibel values are dBFS [1] (decibels relative to full scale), which means that 0 dB is defined as the maximum sound power level that your microphone could report to Spectroid.  All you can do with Spectroid's measurements is compare them to other measurements made by Spectroid on the same device.

In your case, I would guess that that you need dB SPL [2] (sound pressure level).  Spectroid does not report dB SPL because that would require microphone calibration, which Spectroid does not support.

You can find sound level meter apps that do support microphone calibration, but I personally wouldn't trust them to be accurate for anything important.

[1] https://en.wikipedia.org/wiki/DBFS

[2] https://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_level

--Carl