## Negative dB on graph

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Calek
Newbie
Posts: 5
Joined: Thu Feb 25, 2021 3:20 pm

### Negative dB on graph

Hi all,

I have a question for you: why on NVH graph i can see negative value for dB ?

single channel + 3 Axis
- TA259 interface + TA143 accelerometer
-TA259 interface + TA144 microphone (connected on axis X with yellow cable)

Thanks
AC
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Steve Smith
Pico Staff Member
Posts: 1097
Joined: Sun Aug 25, 2013 7:22 am

### Re: Negative dB on graph

Hello and thank you for the post

When using the frequency view of our NVH software the amplitude of captured sound is displayed using a logarithmic scale using either dB (A), dB (B), dB (C) or dB (SPL) as the desired unit

The use of a logarithmic scale enables a large measurement range (such as that of the human ear to volume) to be displayed using a smaller range of useable values

In your screen shot we have a range of – 30 dB to 125.4 dB with our reference value set to 0 dB represented by the horizontal line below 4.5 dB

0 dB does not represent zero sound but a reference value to which all other dB levels are compared too

As an example, if we are to consider 0 dB as the human threshold of hearing (i.e., the quietest detectable sound by a human) then any sound below this level is considered a negative value

This is not to say it is totally inaudible, it denotes it is lower than the reference value set at 0 dB

The Decibel is a number that represents a ratio of two values and in our example, this would be the voltage output of our NVH mic (in relation to captured sound level) against a reference voltage at 0 dB

You can find a more detailed explanation here https://en.wikipedia.org/wiki/Decibel

Just reading through some material on the Decibel expression I found the following really helpful: https://www.quora.com/What-does-negativ ... scale-mean thanks to Cristiano Sadun

“It’s nothing specific with decibels - it’s just where you decide to set the “zero” (the reference level) for whatever you are measuring.

For example, measuring height it makes sense to set the zero at ground level and go up (so for example when measuring a building you don’t count the foundations).

But when you are at 15th floor of a building and accounting is on the 12th, and someone asks you “where’s accounting?”, you can well say “it’s three floors down”… you’re implicitly setting the zero at the floor you’re on, and going down 3 from that - so accounting is at -3 from where you are.”

I hope this helps, take care…..Steve