Stumped by zero. (newby question..Beware!)

[Ray Girard]

Greetings,
I find myself being continually confused by the '0' point on the spectrum, being both the loudest and the quietest point. I've read numerous accounts but the differentiation is just not dealt with. ( I know, ..I've obviously missed some concept.) Example: "Peak exceeds -9dB"...'exceeds' meaning moving towards '0' or away? -or- Noise Floor exceeds -55 dB.
Zero means quietest ..when? Zero ...shows up as loudest on a spectrum graph.
Has anyone a very simple way to make me see this? I'm sure there is some overview that I am missing here.

Thanks....

Ray

 

[fas42]

Welcome! ... here's a good start, https://en.wikipedia.org/wiki/Decibel

 

[amirm]

Welcome aboard Ray. Good question and even better Avatar.

As I think you know, all decibel values are ratios and references. Figuring that out can be challenging at times.

Let me explain the one I use all the time which is dBFS or Decibel relative to Full Scale. In this scenario you take the level of the source signal -- whatever level it is -- and assume it is zero (0 dbFS). Let's say you now have a distortion that has an amplitude that is 66 db lower than that peak. This is then stated as -66 dbFS.

What is cool about dBFS is that we can use it even if we don't have a real system to measure! We can take an audio file, and assign a 0dbFS to the highest digital value that can be be represented. Here is an example of music that we can analyze this way:

In this music the lowest level is -100 dbFS and the peak is around -30 dbFS. Nothing can be higher than 0 dbFS.

There are other scales such as dbu and dbv which are relative to fixed voltages. Their detail is actually not important. How we use them in relative mode, is. Take a look at this example:

We once again look at the level of our reference signal on the left. It is at about -3 dbV. That number could have been anything. I could have turned up the gain or changed the level to make that anything I wanted. What is important is how much lower the distortion products are. In a perfect system, those distortion products would not exist. But here they do. The level of the first spike is at -90 dbV. Since our source signal was at -3 dbV, this distortion is at "-90 - (-3)" or -87 dB down. It sign may also be dropped in conversation: "first distortion spike is down 87 db."

As a figure of merit then, the bigger than negative number, the better. A -90 db is better than -67 db.

So in most ways we use these numbers, we have numbers that range down from 0 and 0 being the highest.

 

[tomelex]

Its really about knowing what they are going on about, ie a background in the measurement. But, something to atleast get you started, is you must fully understand what the zero level means first, then logic or common sense will allow you to determine the db value meaning as better or worse.

 

[Ray Girard]

Thanks for taking the time, Amir. This clears up a few stumbling points. Am I correct in saying that when discussing the Noise Floor, that the '0' point is now the quietest? ...Being the 'noise' we are then dealing with, ....the distance away from the '0' will mean moving away from the quiet.

Thanks, tomelex, for the 'respectful' answer. I'm sure you know when a somewhat difficult concept...just clicks in..and you wonder what the problem was after that. The use of Limiters, Normalize, Amplify, Compression etc. is not a problem. It's just when the switch is made between the use of '0' quiet and '0' loud. I'm sure various aspects of sound in your mind, just fall into one or the other category. Thanks again.

 

[amirm]

Thanks for taking the time, Amir. This clears up a few stumbling points. Am I correct in saying that when discussing the Noise Floor, that the '0' point is now the quietest? ...Being the 'noise' we are then dealing with, ....the distance away from the '0' will mean moving away from the quiet.

My pleasure. But no, zero would not be the noise floor. Let's look at this graph again:

The noise floor is that wiggly line at the bottom. It starts at -110 dbV and goes up to -85 dbv. We would say as a shorthand that the noise floor is at -110 db to -85 db. In other words, the reference remains the loudest signal even when such a signal does not exist in the measurement.

 

[Ray Girard]

I meant when dealing with SPL, that silence is '0'. Sorry. Jumped subjects too fast.

 

[amirm]

Ah sorry. I was going to explain dbSPL but didn't want to confuse you. Let's look at these Fletcher-Munson curves that have lines that show equal loudness for each frequency:

As we see there, the lowest value (threshold of hearing) is actually close to -10 dbSPL. But you are close enough .

 

[Ray Girard]

If discussions would always add the SPL suffix to dB, this newby to acoustics verbiage wouldn't be confused.
Appreciated, Amir.

 

[Ken Newton]

...Zero means quietest ..when? Zero ...shows up as loudest on a spectrum graph.
Has anyone a very simple way to make me see this? I'm sure there is some overview that I am missing here.

Thanks....

Ray

Ray,

Think of dB scale measures as a number line, remember those? They run from positive valued numbers, through zero, to negative valued numbers. For example, 5, 4, 3, 2, 1, 0, -1, -2, -3, -4, -5. Level readings in dB scale are like that. Positive values are progressively greater than zero, while negative numbers are progressively less than zero.

Often, 0dB is assigned as the maximum value against which all other values will be measured. An reference level. So, for example, if 2V is arbitrarily set as the maximum level you intend to measure from some device for a given test, then 2V would typically be assigned as the 0dB reference level. All other voltages measured from the device then would then be less than 0dB. So, that means negative dB values, which is what you commonly see. Although, positive values may also be seen. The lower the output level, the greater the negative number representing it. For example, 200 millivolts would be read as -20dB, and 20 millivolts would be read as -40dB, both are relative to the 0dB level arbitrarily assigned as 2V.

An absolutely zero voltage output would mathmatically produce an infinitely negative dB reading. In reality, there is always a noise floor which sets some maximum negative dB reading, even should the device be completely switched off.

 

[RayDunzl]

I find myself being continually confused

Decibel
The decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity. One of these values is often a standard reference value, in which case the decibel is used to express the level [a] of the other value relative to this reference.

Zero
1. the figure or symbol 0, which in the Arabic notation for numbers stands for the absence of quantity; cipher.
2. the origin of any kind of measurement; line or point from which all divisions of a scale, as a thermometer, are measured in either a positive or a negative direction.
3. a mathematical value intermediate between positive and negative values.

Zero decibels seems to never mean "none" or "nothing" like an arithmetic zero, zero in decibels is always be a reference to some other value.

Examples:

Reference Sound Pressure p0 in air = 2 x 10-5 Pa = 20 µPa = 0 dB SPL

dBm : dB(mW) : power relative to 1 Milliwatt. 0 dBm = 1 mW. No reference impedance is assumed, though 600 ohms is common in audio equipment.

dBu : dB (0.775 V rms) : voltage ratio with a reference voltage of Vo = 0.7750 volt = 0 dBu, derived from a 600 ohms load dissipating 0 dBm (1 mW)

dBW : or decibel watt is a unit for the measurement of the strength of a signal expressed in decibels relative to one watt. It is used because of its capability to express both very large and very small values of power in a short range of number, e.g. 1 watt = 0 dBW, 10 watts = 10 dBW, 100 watts = 20 dBW and 1,000,000 W = 60 dBW.

There surely be more.

 

[fas42]

Sorry if I appeared to be abrupt in in my first response to you, Ray - I would have merely repeated the gist of what was in the Wikipedia article, so it made sense, to me, to point to that article.

To answer a specific question, "Peak exceeds -9dB"...'exceeds' meaning moving towards '0' or away?": as has been repeated a number of times here, 0dB is a chosen reference point which is convenient for measuring actual values against, and the zero'ness means nothing except that the value is bang on the reference level - that reference can be the biggest, the smallest, or anywhere in between - but usually picked to be meaningful, as in the maximum level before severe distortion begins. Any negative value just means less than the reference, so "peak exceeds -9" means some value a bit greater then the value which is 9dB less than the reference - thus, "exceed" means moving "upwards" towards 0, the reference.

 

[Ray Girard]

No slight taken, Frank. As well as Ken and Ray...thanks for the help. I buckled down and thought my way through, with your aids, and I got it. The problem was thinking 'zero' was static. Thanks again.