DB with oscilloscope - Page 1
Forum section: electronics / beginners

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: March 09, 2018, 05:13:52 pm
content: hello, I'm a novice with my rigol ds1054z, I have several doubts but the main hory is how you measure decibles with an oscilloscope, if you can measure it.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2018, 05:39:21 pm
content: Decibels (dB) are merely the ratio between two values. In specific contexts, such as dBm (dB over 1 mW), the second value is specified.If you measure two voltages, the usual assumption is that the impedance/resistance where the two voltages were measured are equal, so the two power values are proportional to the voltage squared, and the impedance value cancels out when taking the ratio.Therefore, the equation isdB = 20 x log10 (V2 / V1).A 2:1 ratio in voltage therefore corresponds to a 6 dB ratio.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: March 10, 2018, 01:45:19 pm
content: So there is no quick way to measure decibels with an oscilloscope, I mean connect the output of a microphone to the probe and based on that measure?

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 10, 2018, 03:05:37 pm
content: Depends on the scope. Some have extensive math functionality that may support it.

author: drussell
author_rank: Super Contributor
timestamp: March 10, 2018, 03:09:06 pm
content: If you are trying to measure sound pressure level, you need a microphone with a known output level that increases linearly with increasing sound pressure and has a flat frequency response. Calibrated microphones for this purpose are available and will come with specifications for the particular unit from the test lab.

author: Adrian_Arg.
author_rank: Frequent Contributor
timestamp: March 10, 2018, 03:13:45 pm
content: In the case of a rigol ds1054z does it have that range? It's the one I have

author: rstofer
author_rank: Super Contributor
timestamp: March 10, 2018, 08:46:33 pm
content: Quote from: Adrian_Arg. on March 10, 2018, 03:13:45 pmCheck the User Manual. The Math button pulls up a menu that includes things like dBV and dB, but only for the FFT operator AFAICT.

author: TimFox
author_rank: Super Contributor
timestamp: March 11, 2018, 11:43:09 pm
content: Again, dB are a ratio. You need to specify the 0 dB point for your microphone, or equivalently the output voltage for a specified reference level to get meaningful dBs.

author: Brumby
author_rank: Unknown
timestamp: March 12, 2018, 03:07:50 am
content: Repeating what has been said above - dB is acomparativemeasurement. You are comparing one absolute value (such as power) against another absolute value of the same type.It's the same as asking "How many times is X louder than Y" - it's just that "Y" is a reference point that must be defined somehow because there is no absolute standards laboratory reference for it, unlike volts or amps.

author: Audioguru
author_rank: Super Contributor
timestamp: March 12, 2018, 03:11:39 am
content: A 'scope is used to see waveforms. Waveforms must be displayed linearly, not in dB's.db's are ratios of steady levels that can be seen on an audio or RF multimeter.

author: Brumby
author_rank: Unknown
timestamp: March 12, 2018, 03:16:00 am
content: There are a number of scopes which can do that math.

author: Calaverasgrande
author_rank: Contributor
timestamp: April 22, 2021, 07:54:32 pm
content: Quote from: Brumby on March 12, 2018, 03:07:50 amNot entirely true. In audio engineering we have AES and EBU reference levels. These are defined in the context of a specified load, say 600 ohms, a reference voltage .775 volts and one milliamp of current. This gives you a point of reference for 0Dbu in the standard +4Db operating level used in professional analog broadcast, public address or recording. +4dB itself is 1.23 volts in this context.Of course in the digital audio age this gets more complex, as we reference the 0DbU level as -20Dbfs or 18Dbfs or -14Dbfs depending on the audio system manufacturer reference levels.If the OP or someone else is looking for something similar, the Rigol 1054Z does have an FFT mode which scales in Dbm, which should be adequate for audio work. (though Dave who seems to love the 1054Z for an entry level scope dinged it's FFT mode, he was looking at beyond audio range performance).You can also measure the voltage from the screen and do the math to get Db for the load and voltage. But as has been said frequently elsewhere. A scope is not a DMM.

author: rstofer
author_rank: Super Contributor
timestamp: April 23, 2021, 03:04:38 pm
content: Short answer: No, the DS1054Z does not have dB as a vertical scale in any function other than FFT.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 23, 2021, 05:03:11 pm
content: Quote from: Audioguru on March 12, 2018, 03:11:39 amI think this ^ is very good point (about linear vs logarithmic measurements) in a great thread about an important topic (dBs).Here is another attempt at helping the OP - others should feel to correct whatever I get wrong.dBs are useful in making measurements that have relatively large dynamic range, ie lots of difference between the largest and smallest values being measured or considered. dBs become most valuable when the dynamic range being examined needs to be expressed in orders of magnitude (factors of 10, ie 10x, 100x, 1000x, etc). To make dBs conveniently useful, as stated in other posts in this thread, dBs are often/generally (but not always explicitly) referenced to some specific value such 0dBm = 1mW and therefore we can quickly recognize that 30dBm = 1 watt. (The point made above about impedance assumptions in the use of dB for power is a great example of some assumptions being present but not always explicit, which can lead to confusion when first getting a grip on the various uses of dB; just something to be aware of.)Back to oscilloscopes. When using an oscilloscope we might be looking at somewhat small values such as 1uV or somewhat large values such a 100V but we typically don’t use a scope for a task such as looking see if something is 100.000001V, or if one signal is a 100V vs another at 1uV. In fact we generally use probes that are specified within an order of magnitude such as 10x (or sometimes 1x or 100x).So while an oscilloscope is a fantastically useful piece of test equipment (occasionally even referred to as the king of test equipment) it is not one of the types of test equipment most likely to be used for measurements of values frequently designated with the dB nomenclature.To be sure an oscilloscope might be used to measure in ranges from, for example, seconds to picoseconds on the (horizontal) time axis, but in terms of the (vertical) amplitude scale dBs don’t get used much (hardly at all?).In contrast, in the case of spectrum analyzers we can look also at the horizontal scale in terms of sweeps of full seconds to small fractions of a seconds as we look at relatively larger or smaller swaths of bandwidth, and we might greatly reduce the span of frequency to see with more detailed resolution. So again, time might range from seconds to billionths or maybe trillionths of seconds. However on a spectrum analyzer we might look at 10 orders of magnitude in amplitude (or more depending on how the reference graticule is set). So on a spectrum analyzer dBs get used regularly on the vertical scale. With a spectrum analyzer we might be measuring and comparing amplitudes in the range of +30dB to -160dB.In summary, a key concept to help make dBs come into better focus is “dynamic range”, ie the need to not only measure but also readily compare values (of whatever is being measured) across multiple orders of magnitude.The more you get your head around dBs the more profoundly you can get your head around the enormous potential for dynamic range and scale for various realms in various systems. Kinda metaphorically (so similar to but not exactly) like trying to quantify and compare how big is an electron relative to an atom or an atom compared to a solar system or a solar system compared to the Universe.

author: ebastler
author_rank: Super Contributor
timestamp: April 23, 2021, 05:07:47 pm
content: Quote from: rstofer on April 23, 2021, 03:04:38 pmRight. Software features aside, let's not forget that it only has an 8-bit AD converter. Not much fun to be had there with a logarithmic scale...

author: Michael Rempel
author_rank: Contributor
timestamp: May 29, 2021, 07:02:35 am
content: This seemed to me to be a good post to interject on. I dont know the depths of the blog yet so please forgive if this is covered elsewhere.What most people seem to want when they measure db in audio is loudness to the human ear. You will never win that battle no matter how much math you have.A decibel is a creature of electronics history and legend named after no less a person than Alexander Graham Bell himself. The problem Mr Bell solved, was to figure out how much voltage to use for efficient transmission. He promptly and arbitrarily chose the biggest voltage at his disposal, which happend to be 100 volts. He then figured out how far he could push that 100 volts through the excellent high impedance wire, in the cold, heat, rain and dry of weather. The length varried according to circumstances but he eventually figured out that 1 volt was his minimum useable signal strength. So the trick was to have a 100 volt power supply amplify a 1 volt signal. And TADA the Bell was invented. It turned out to be a whole lot more useful in fact, than the simple use Bell put it to, but they soon discovered a Bell was way too big for most people. They wanted a more modern number, none of this 0.03 stuff, we want a better way to express that ratio. so the decibel was invented with that weird formula you see in the explaination given in other posts.For audio we are measuring the power gain ratio of sound. Is that dbA for audio or is it dbV for voltage? It never says, but you can bet it means dbA because they always talk about it in terms of audio. But audio isnt a single frequency and a single loudness. So how do you measure it? Actually there are a bunch of official legal ways to do it depending on the source. There is A weighted, C weighted, and variants fast and slow. Now cops use those ranges to test SPL or Sound Pressure Level, but that aint loudness folks. It just isnt a valid measure of the human perception system at all.I have gone on a bit in this post, so for more reading delights look here:https://www.audioholics.com/room-acoustics/the-decibel-dband examine their related articles to your heart's content. If you read far enough you will see the different ways manufacturers cheat on their wattage measures and all kinds of messy fun stuff. After all that your toe will be wet for the next part which is way way bigger and more important than the miniscule electronics issues. That is what happens when sound leaves a speaker and enters human space? You wii find out what it takes to spend 300 years and the entire city budget to build a great cathedral. Yes light is important, yes bigness is vital and the flying arch is the champion of those things, but the real reason is the sound. And not one electron was wrangled in the maufacture of that magnificent sound anywhere.

author: TimFox
author_rank: Super Contributor
timestamp: May 29, 2021, 02:05:07 pm
content: The decibel (dB) is defined as ten times the base-ten logarithm of a power ratio, which is twenty times the log of a voltage ratio (with equal impedances at each voltage).It is an electrical measurement. There is often a suffix (such as in "dBm") after dB to indicate the denominator of the ratio (mW in this case). To measure audio power (not sensation), the dB(SPL) uses a specific acoustic level (a sound pressure of 20 uPa, roughly the human threshold of hearing); since pressure is an amplitude, the factor of 20 is used to compute dB.When measuring acoustic noise or other health-related acoustic levels, it is common to apply a weighting function before measuring the RMS amplitude (or mean power) and calculating the dB value. The common weighting functions give us "dBA" for "A-weighting" and "dBC" for "C-weighting", which you may look up in the literature. These weighting functions are used in health and safety regulations. "dBA" is more commonly used in this application.This is all physics and engineering. When you pass into the realm of the psycho-physical apparatus of human hearing, dB are an approximation to the Weber-Fechner Lawhttps://en.wikipedia.org/wiki/Weber%E2%80%93Fechner_law, which is an approximate description of the relationship between sensation ("loudness") and physical stimulus (acoustic pressure).