Silver Supporting Member
If 0db is the threshold of human hearing, what constitutes a negative decibel?
Yeah, this might be a little off-topic, but because dBu seems mysterious to a lot of people, even engineers...Actual attempt to explain in layman's terms (requires a high school education):
For Audio Applications:
0 dB - Sound is the same as the reference level. In OP's question that's a standarized value for "threshold of human hearing". This value is the same (1x).
+20 dB - 10x larger than 0 dB
-20 dB - 1/10th the value of 0 dB (0.1x)
You can have negative dB SPL values becuase sound still exists even though the "standard human" can't hear it under "standard conditions".
Note that as mentioned upthread, you have to know what the "dBs" you are talking about are refernced to. In audio applications common references are dB SPL (measurement of actual sound in atmosphere), dBu (referenced to 0.775 volts), and dBV (referenced to 1 volt). It's a enormous misnomer used by the general public to say how loud things are in "dB".
There is no negative dB unless there is a reference to go negative from. Maybe dB doesn't even exist without a reference...The biggest thing that's missing from most explanations is the fact that a decibel always starts from a ratio of two values in the same units (Volts, Watts, etc.). IOW, the measured value is compared to some reference value... if the measured value is equal to the reference, that is 0 dB. If it's less, it's a negative amount in dB, and a positive dB value means it's greater than the reference.
Example: "dBm" means "dB referenced to 1 milliwatt of power". So a measured value of 1 mW can be called, "0 dBm".
The confusing part in using "dB" to describe audio volume is that no one ever tells you what the reference level is. That's kind of OK once you get an idea of "how loud 80 dB" is. You'll know that more than 80 is louder, less than 80 is quieter.
But the "dB" used in audio does have a reference value. It's called "audiometric zero" and is a rough idea of the quietest sound a human should hear (averaged, because in reality that's different for everyone).
There's no maybe about it... dB definitely does not exist without a reference. But most people's first exposure to dB is on a stereo's volume knob. If you don't know that it's referenced to audiometric zero, you think of it as an absolute, like Watts or Volts.There is no negative dB unless there is a reference to go negative from. Maybe dB doesn't even exist without a reference...
I make my living in fiber optics and we deal in negative dB all the time. The normal value is dBm, which has 1mW of optical power as the reference (which is a lot of power in our world). I don't know how many times I have to scold techs that think you "take down the laser power" by going from -10dBm to -5dBm. Or -80dBm is "higher" than -50dBm. NO. These are usually the techs that call optical fiber "wires". That'll get you laughed at for sure.
The last hearing test I took was in one of those isolation booths. The nurse, upon leaving, told me to press the controller button whenever I heard a tone. A minute or so later she reentered the booth and asked me why I wasn’t clicking it. I told her I didn’t realize the test had started. She told me it had started when she initially left the booth. I hadn’t heard a thing!I don't know but I did pass a ear test last year and they told me I could hear -5db in the test
I was very surprised since I play a plexi into a 4x12 of EV each week without ear protection since years , anyway good thing it is