Trying to get my head around noise floor

When I use an app I downloaded onto my iPhone (NIOSH SLM) to measure the noise floor of my recording space it shows 27.1 db. But I have always been told that the noise floor would be measured as a negative number like -30 db.

Can someone please help me get a better understanding? is 27.1 bad, good, or great? And why doesn’t the app show it as a negative value?

Thank you

“dB” is a “ratio”, not a “unit”.
Unlike units such as grams, or seconds, or miles, a “dB” is not a fixed quantity, but the ratio of one quantity compared to another. “dB” is meaningless without a reference point.

When dealing with audio signals, it is usual to measure signals with reference to “full scale”. This is called “dBFS”, though because it is so common, it is very often abbreviated to “dB”. In Audacity, “Full Scale” refers to the track height, shown on the track’s vertical scale as +/- 1.0. When a signal is “full scale”, that is the 0 dB reference level. Signals smaller than full scale are therefore negative (less than zero), and signals greater than full scale (if that is possible) are positive.

With regard to your iPhone, “27.1 dB” is meaningless, unless you know what the referenced level is (what the 0 dB level represents).

How do I determine what the referenced level is (what the 0 dB level represents) of my space?

Thank you

To measure “sound pressure level”, you need an SPL meter (Sound Pressure Level):
You can get a cheap one from ebay for under $20, but an SPL meter that is accurate for low level noise isn’t cheap (often costing hundreds or thousands of $)

Can we go way back to the beginning and tell us what the actual job is? Are you going to record for Audiobooks?

You are trying to measure dBSPL Sound Pressure Level. Doesn’t it say that in your software or manual anywhere?

The only way to really know what the phone is doing is compare it to a real dBSPL meter.

This is my old one.

There is no single dB Sound Pressure Level. You also have to say which specification and weighing curve you’re using. “A” Curve is the setting most legal, hazardous environment and law enforcement people use. It only measures the stuff you can actually hear. This is the one that says how many minutes you can be exposed to a jet engine before you go deaf.

This is where we find out what your job is. ACX Audiobooks doesn’t use the “A” curve.


iPhone (NIOSH SLM) to measure the noise floor of my recording space it shows 27.1 db

If that’s C Weight, that’s actually pretty good. My meter won’t reliably go any further down than about 55dB or so before I lose it. Somewhere down there, or even further is my studio which has no problem producing quiet, well-behaved recordings (unless a Metrobus goes by).

I have four known noisemakers.

– The power supply of my analog mixer hums to any microphone nearby.
– My bass cabinet hums and does not turn off.
– My wall clock clicks in all recordings.
– My internet terminal equipment screams.


Oh, and to bring this around. The object of your measurements is to make sure you have a quiet room so room, microphone, preamplifier, mixer, and analog to digital converter noise is a thousand times quieter than your voice. That’s the audiobook and broadcast specification. -60dBFS.

That’s when you change into the other measurement method. 0dB is maximum loudness in a recording. That’s where the digital system runs out of numbers and starts eating the sound. It works down in negative numbers -3dB is the loudest voice peaks are allowed, -18dB to -23dB is the performance loudness range and -60dB is the loudest background noise is allowed to be.