Do you find dB to be intuitive? - Page 1
Forum section: electronics / general

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: April 29, 2014, 01:14:09 pm
content: Maybe I'm old school, but it seems a lot has changed in electronics instruction and books since I was in school for that in the early 1980s.I don't mean the electronics technology itself. Obviously that has changed hugely, but I am talking about the way basic electronics principles--the stuff that is basically the same as always--seems to be explained nowadays.The things I struggle with:New books or webpages, etc., that attempt to explain phase relationships and reactance using mathematical formulae with j operators. We barely touched on this when I was in school, but I still find vectors and trig to be easier to deal with and understand.Books that attempt to explain electronics with very little in the way of intuitive explaination, and more or less just a string of formulas. Personally, I cannot read equations as well as english. I get far more out of books from the 1950s or so that explain what/why in english, and then quantify it all with the math examples. I do not like the trend in most(?) modern literature to give information in what seems like a secret code to me. I can do the math, but I need an idea of what it is I am doing and what is going on first.Decibels for everything. When I was in school, we covered this a bit, but I still find voltage or power levels given to be more comprehensible. I know what 3 mV is, but -50 dB means nothing intuitively to me until I convert it to power or voltage. Everwhere seems to give signal levels in dB now, whereas when I was in school, that was reserved for long-wire telecommunications technology.I guess I'm just an old, outdated fella.cheersRob

author: tom66
author_rank: Super Contributor
timestamp: April 29, 2014, 01:29:44 pm
content: Well, on its own -50dB means nothing. It's the same as saying the current is "0.3". Without units, there isn't enough information to give you a sensible value.For me using dB became easier when I remembered the (voltage) conversions 0dB, -3dB, -6dB, -20dB, and so on (1, 0.707, 0.5, 0.1, etc.) You can then multiply by each -20dB to get a ballpark estimate. For example -63dB gain is -60dB + - 3dB or 0.1*0.1*0.1*0.707 = 0.000707.To me, it makes more sense to use dB for gain than a multiplication factor because of the large possible ranges of gain. There's little info in saying your filter rejects to 0.000707x in the stopband. It makes a lot more sense to say -63dB gain (or 63dB attenuation) because you can then say I have a 10dBu signal at the stopband so my amplitude after the filter will be -53dBu. (Remember power dB is not the same as amplitude dB, this is the main thing that took a while for me to get my head around.) You can build a large system with dB gain/attenuations and work out the final result quickly; if you then desire a "real" number, you can get that.I'll admit, I don't get things like "rms jitter" or phase margins in dB... still a lot more to learn.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 29, 2014, 02:08:43 pm
content: dB is simply a ratio of two values. The only reason it's used it to avoid very large (or small) numbers. It's also very convenient to be able to add dB's instead of multipling linear gains.When I get confused about exactly what is meant I just try to remember that it's just a ratio of two numbers.When a subscript is placed after the dB such as dBm etc it just means relative to whatever. So dBm is dB relative to 1mW and dBW is dB relative to 1W.dB is often stated with no relative value mentioned. Such as sound pressure level etc. This is incorrect and basically meaningless, but happens often.It's like saying "the signal is 10 000 times greater" but not saying greater than what!

author: electr_peter
author_rank: Unknown
timestamp: April 29, 2014, 02:36:05 pm
content: I think that usefulness and readability of dB_x depends on the context.In RF related applications, where quantities vary by multiple orders of magnitude, dB is very appropriate and useful.In simple circuit design, where values are on the same order of magnitude (power dissipation, current levels, voltage drop, digital circuitry, low frequency, etc.) I find it not useful most of the times.To add to the confusion of dB usage, power and magnitude can represented by dB (factor of 2 difference). Miss power/amplitude definition near dB and you have massive error!!!I would say that dB are useful in contexts were there are:1) defined (or implied) reference/standard2) measurements are very frequent3) big order of magnitude difference in values exists.Drop any of 3 and you find yourself recalculating dB to V/A/Ohm values just to be sure.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 29, 2014, 02:48:43 pm
content: Rob , I think I know where your coming from .Most all of my electronics is self taught - so lots of gaps .Then on top of that , I swore off math at algebra in 66 in about 6th grade .So I work with simple numbers , math and principles only .Now at 60 yrs. and getting into ham radio [ every hobby I get into the building & repairing ] so now at this late age I would like to back up the train and learn the higher math .I have been in a lot of technical trades and done very well , after having enough methods to measure and make knowing the work by 2nd nature .The way I've done that in the past , like auto repair - emissions - computer control etc. , is to get enough gear to let the equipment do most of the figuring , then after 5-10 yrs. I can do most repair with principles and a multi-meter .Hoping I have enough time left to get through fixing , repairing & building ham gear & test equipment , to the same point of auto repair I had .Problem is RF electronics is much more depth than some of the other hobbies / trades , carpentry , plumbing , electrical , welding almost all types & metals , machining to .0001 , gunsmithing etc.One thing that helps me learn , is to try to teach others as I am getting a grasp of a subject , form early on I found that higher learning would leave behind talking in layman's terms so others can fallow along .I do realize the the shorthand of higher math is a time / space saver , but is speaking in plain language is left behind and or forgotten a lot .

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 29, 2014, 02:55:24 pm
content: Learning about RF , I have found the best method of displaying all the details in one glance , at least to me , is a Smith Chart .Still learning about those .https://www.youtube.com/results?search_query=cfurse+smith+charts+-+first&oq=cfurse+smith+charts+-+first&gs_l=youtube.12...12430.16045.0.17868.13.13.0.0.0.0.91.705.13.13.0...0.0...1ac.1.11.youtube.7TL9QxKLmPIhttp://www.ae6ty.com/Smith_Charts.html

author: KSP
author_rank: Regular Contributor
timestamp: April 29, 2014, 04:05:36 pm
content: It took me a while to get my head around the log scale and dB's but now I find it more useful. In the field of RF test and measurement I find dBm more useful than linear figures (obviously, depending on what it is I am measuring), it's when people try to confuse the matter by combining things... I had a specification something like x-Volts +/- x-db. Yeah, calculating a voltage change in dB can be done with a simple formula, but it's not as simple as just adding/subtracting like-for-like units.As previously mentioned, it gradually starts to make sense when you grasp the concept that dB has to be relative to a constant. It allow you to express large orders of magnitude with a relatively small figure. For example:1mW = 0dBm10mW = 10dBm100mW = 20dBm1000mW = 30dBmetc. etcDoing this linearly leads to very long numbers!

author: Rigby
author_rank: Super Contributor
timestamp: April 29, 2014, 05:26:09 pm
content: Quote from: KSP on April 29, 2014, 04:05:36 pmto me, this is what scientific notation and si units are for. No one says 0.000000001F, they say 1nF. 1 is not a long number. I don't even understand why db is around. It adds nothing and confuses everything. so what if I can add values instead of multiply. I have a calculator, and pencil & paper. I also have intuition, and intuition doesn't lead one to think of values as ratios.

author: Rigby
author_rank: Super Contributor
timestamp: April 29, 2014, 05:33:39 pm
content: by the way, this woman explains math in ways my oddball mind understands without any effort on my part. And I remember it. the way schools teach math today is pretty much fully broken.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: April 29, 2014, 05:49:43 pm
content: I think it is the ratios I have trouble with...not thinking in absolute values. Same as Rigby. Our brains are all "wired" differently and this is my weak point regarding the dB thing.Also, I should have been more clear in saying that even with the assumption that we were talking about dBm, 1mW into 600 ohms, It doesn't send me a feeling of power level when in dB. You all posted some helpful tips, whether you know it or not.I'm nottoobad at math, I am a manufacturing engineer with a BS degree...(I always love associating my degree with B.S. LOL) so I am capable of learning, I suppose, but when I originally took electronics (long before university) they gave us a good practical education for one intensive year. I never used dB until some telecommunications study later...and then not much more than adding and subtracting dBm units to balance out the losses. For that, it was indispensable.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: April 29, 2014, 05:51:12 pm
content: Quote from: Rigby on April 29, 2014, 05:33:39 pmBookmarked.Here is another one I like on ln, Euler's number...http://betterexplained.com/articles/an-intuitive-guide-to-exponential-functions-e/

author: Mr Simpleton
author_rank: Unknown
timestamp: April 29, 2014, 07:06:33 pm
content: Once you master the dB concept you will find it immensly useful especially in radio design and evaluation. Using dB I can easily calculate sensistivity and dynamic range in a system, where it would be far too many zeros to acount for in a linear way!

author: Rigby
author_rank: Super Contributor
timestamp: April 29, 2014, 07:30:15 pm
content: well if and when I start doing that kind of thing then I'm sure I'll find db useful as a unit of measurement but as for this moment I'd be happy if people didn't just assume that db is a concept that everyone on the planet just 'gets' -- db is used wrong a lot of the time and in places where other units would make as much sense or more.

author: Mr Simpleton
author_rank: Unknown
timestamp: April 29, 2014, 07:42:17 pm
content: Oh well, for those who enjoy psi, lb feet and BTU... what can I sayYou move towards SI system inch by inchStart calculating receiver sensitivity vs. filter BW and noise figure, you will have loads of work in linear. With the dB concept is just a matter of plus and minus...

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 29, 2014, 11:15:11 pm
content: Don't forget where the dB came from - the Bel, after Alexander Graham Bell - Bell Labs. Far easier to quantify power losses in a series of long cable runs and amplifiers using decibels, where numbers can be added or subtracted rather than multiplied or divided.In fact, if you were taught logarithms properly at school, this should have been the "buy in". Long multiplication/division was much, much easier using logs, where you converted the numbers to their logarithm using log tables (remember those little books?), then adding or subtracting them, then finding the inverse-log with the same tables. Before the advent of cheap pocket calculators, log tables (and slide rules) were great...Of course, our hearing is approximately logarithmic, so decibels work especially well here. Good audio engineers think almost exclusively in decibels, but are able to convert between dB and ratios quickly in their heads. Eg, a voltage gain of x2000 is +66dB

author: w2aew
author_rank: Super Contributor
timestamp: April 30, 2014, 12:59:01 am
content: The underlying beauty of using decibels lie in the fact that they convert multiplication and division into addition and subtraction problems, and that it makes it easy to deal with wildly differing magnitudes of things. Also, remember that dB by itself is a ratio - a relative term - not and absolute value. When you follow dB with another letter, that typically implies the "reference" value used for the ratio, thus making it an absolute value.I did a video recently on dB, dBm, dBc, etc... Maybe this will help...

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 01:51:15 am
content: Everyone keeps mentioning that db is added or subtracted rather than multiplied or divided.multiplication and division ARE addition and subtraction, they're just shortcuts.so, saying db math is a shortcut for multiplication and division carries zero weight with me. "math is hard" is not a valid excuse.you all have fun with your db this and your 1st grade math that.the more experienced among you will find this reply incredulous but i implore you to think back to what went through your mind when you first saw the nonsense that is db. you have to memorize formulas to convert from regular measurements and back again, JUST SO YOU CAN ADD INSTEAD OF MULTIPLY. I honestly do not understand why people use db stuff as it is.

author: retrolefty
author_rank: Super Contributor
timestamp: April 30, 2014, 01:56:46 am
content: QuoteBecause having a log based measurement units is very useful at times. Can you imagine if spectrum analyzers were limited to using linear units for signal strength?

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 02:13:24 am
content: There is of course no reason to use dB when we have scientific notation...except that maybe its a bit more clumsy. For instance suppose some power level is attenuated by a factor of a million, well we could say the power level is down by 10-6, and that would be precise. But the only useful number in that term is the "-6" bit. The 10 occurs every time, 10-2, 10-1, 10-4, so why bother stating it?OK, so lets just say the power ratio is "-6". That's nice, except we ought to have a unit of measure to say what the "-6" means, so let's invent the bel. The power ratio is now -6 bels.However, one digit of precision is not all that good for engineering purposes, so we could make our unit a bit smaller. Let's divide it by 10 and call it the decibel. Good, now we have a gain of -60 decibels, or -60 dB. Now our ugly 10-6is starting to look a bit neater.One last thing, though. Do we really need that minus sign? If we say the signal isattenuatedby 60 dB we can drop the minus sign and let it be understood. Similarly, a ratio of +60 dB can be called anamplificationof 60 dB.So that's sweet. We turned a power gain of 10-6into an attenuation of 60 dB and our documents and our language have become much tidier.

author: ludzinc
author_rank: Unknown
timestamp: April 30, 2014, 02:14:45 am
content: Quote from: Rigby on April 30, 2014, 01:51:15 amI'd agree that adding 3dB is just as easy as multiplying by two. But dB is very useful for wide dynamic ranges. You model a system using kW transmitters and nW receivers without dB!60db is waaaay easier to fit on a chart / analyser screen that a linear 1:1,000,000 ratio...

author: Bored@Work
author_rank: Super Contributor
timestamp: April 30, 2014, 05:00:59 am
content: Quote from: Rigby on April 29, 2014, 07:30:15 pmSorry, if I would only use concepts in my daily life or just in my job everyone "gets", then all I could do is sitting there in a rocking chair and smiling silly. And even then someone would come up and complain because he doesn't get the concept of a rocking chair.When in Rome do what the Romans do. Learn the language. In this case the language of the trade. If the language of the trade contains dB and math then so be it. Learn it. End of story.What you say? They did that different 60 years ago? Really? Dude, it is called progress. Don't like progress? Here is your intuitive chisel, intuitive hammer and intuitive stone slate. Have fun with it.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 30, 2014, 07:35:06 am
content: Mr Coffee (Rob)With the help of a scientific calculator I believe that you can, in principle, get away from dB if you really want to. After all, it's just a way of expressing a ratio.However, the dB concept is so easy to understand that it's a shame that some people never embrace it.Rob, I have no doubt that with 10-15 min of reading about Decibels you will grasp it. I think to really appreciate its usefulness one needs to do a calculation using linear units and then repeat it using Decibels/logarithmic units. Only then will you appreciate the advantage. There is no doubt that RF engineers use the Decibel notation far more than other other disciplines.Embrace it and you will never look back, and it's easy.Dick

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 30, 2014, 09:34:48 am
content: Quote from: Rigby on April 30, 2014, 01:51:15 amYou simply lack experience in a branch of engineering where decibels are incredibly useful. Nothing wrong with that, of course, but speaking so strongly from a position of ignorance is not exactly the best planMy main specialism is audio engineering. I can tell you that I use decibels a lot. I *hear* decibels, not voltage or power.You're wrong to assert that people today use decibels to avoid long multiplication or long division - apart from quick mental calculations, no-one does that. Very simply, people use decibels because they are a useful and meaningful metric in many areas of engineering.But do remember that the decibel first came into use in the early 1930s - some 40 years before scientific calculators became available, and perhaps around 50 years before they were affordable to the masses. In those days, using log tables to quickly convert to decibelsdidmake some things easier.OK, ifyourwork isn't audio - or any of the other specialisms that use decibels - then sure,youmight not need them. But you're utterly wrong to state that decibels are "nonsense".

author: KJDS
author_rank: Super Contributor
timestamp: April 30, 2014, 09:54:48 am
content: If I look at the block diagram of a receiver then I have the followingInput filter 1dB loss 0.83 linearInput amplifier 18dB gain 72 linearfirst mixer 6dB loss 0.5 lineardiplexer 1dB loss 0.83 linearIF amplifier 16dB gain 43 linearand so onI can tell at a glance what the total gain is in dB, I can probe along the strip looking at the result on a spectrum analyzer and see what I have in practice. Doing that in linear is a pain.

author: jpb
author_rank: Super Contributor
timestamp: April 30, 2014, 10:09:42 am
content: Quote from: KD0CAC John on April 29, 2014, 02:55:24 pmWe did Smith Charts as part of microwave engineering at university and then I went into working on microwave integrated circuits scattering parameters were generally displayed on Smith charts but even in the 1980s (which is when this was) I felt they were rather dated. They are a useful tool for hand matching but the only engineers who were doing this were those who had started working in the field before computers, designs were generally done in software and simulated. There were (are) good programs to do the matching and doing it by hand if you have to use anything much beyond a single stub it starts to get tricky.Smith charts are intrinsically attractive to look at though and I think they will hang around a long time as a means of display in a similar way to the way that VSWR is used as a parameter but I suspect that no one measures it directly with a probe in a wave guide anymore!For simply displaying the information they are essentially a polar plot, and if you're not using them as a tool (i.e. for matching/impedance transformation) the additional curves don't add very much information (in my view). (Of course the axis and the unit circle are needed for scale.)

author: richard.cs
author_rank: Super Contributor
timestamp: April 30, 2014, 10:18:26 am
content: Quote from: IanB on April 30, 2014, 02:13:24 amI'm with this. Yes, there's no actual need to use dB when we have scientific notation but it's equivilent to it, to the point where (for power) it's just a different way of writing it. For amplitude there's an extra 2 that comes from the power=amplitude2relationship. I find dB useful for power and gain, I have less time for things like dBuV but only because I end up converting it to power anyway.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2014, 10:30:03 am
content: One thing thatisa bit annoying is when people quote gain in dB in voltage terms where both readings are at different impedances.20log v1/v2 is really only legitimate where you measure the voltage across the same impedance,making it a shortcut so you don't have to work out the power in each case,then apply 10log P1/P2.If your next stage "bridges" across a low output impedance output,you can get away with the voltage formula for the gain of a stage, but itisa "fiddle, & not really correct.Using dB gain in systems allow easy determination of required drive power levels for amplifiers of known power gain.For instance, big Tetrodes in TV Output stage service commonly have a power gain of around 13 dB,which is ,to say,a gain of 20.To get 10kW out of the Output stage,you need to drive it with 500w,so the drive stage/s need to provide that output.In practice,it is easy to slide back & forth between dB & power in watts,as required.A scientific calculator allows messier gain & loss figures to be easily found.

author: KJDS
author_rank: Super Contributor
timestamp: April 30, 2014, 10:40:45 am
content: Quote from: jpb on April 30, 2014, 10:09:42 amI can't even begin to think about how to match to the input impedance of a typical high power RF transistor. Trying to get from 50 ohms to 0.2 -4.3j without a smith chart to guide the way would be somewhat tricky.

author: tszaboo
author_rank: Super Contributor
timestamp: April 30, 2014, 12:34:28 pm
content: Quote from: retrolefty on April 30, 2014, 01:56:46 amIt is also incredibly useful for precision analog, and mixed signal systems. On the other hand when I write documentation, I usually end up using uV, PPM, LSB and dB at the same time, and I have to write it there, otherwise half of the people supposed to read it wouldn't understand.

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 12:47:51 pm
content: Quote from: Bored@Work on April 30, 2014, 05:00:59 amthat would make a lot of people here very happy, myself included. you do not smile enough. or, rather, you smile at us instead of with us, which doesn't make me consider you a credible participant any any thread on this forum.I get logarithms. I get why the log scale exists. Logarithms are not the issue. Decibels are the issue. Why is 1/10th of a bel a unit of measurement? Why not just the bel? Why is 3db (or 6db) halving or doubling, and why are they different for power and amplitude measurements? It's not even 2x or 1/2, it's 1.995x or 1/1.995. this REEKS of a stale, non-sensical Ye Olde Measuremente Systeme like inches, yards, furlongs, rods or cubits. It's like some dude made some shit up, said "you can add instead of multiply" and everyone just accepted it instead of maybe refining it a bit and making it a simple concept that doesn't need explaining.so, to the topic, do I find db to be intuitive? Nope. I taught myself to calculate n-th roots on paper for fun, and decibels don't make sense to me. I GET THE MATH. I don't get the unit itself or why it's used. there are better ways to represent those differences in magnitude.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 30, 2014, 01:04:17 pm
content: Quote from: Rigby on April 30, 2014, 12:47:51 pmThe story I've read is that engineers at Bell labs decided that rather than considering just the ratio (Pout/Pin), taking the log of the ratio gave results that were meaningful and easier to work with. Remembering that they were dealing with audio, of course.But the Bel was a bit "chunky". They were talking about 0.6 Bels, 0.5Bels, 1.2Bels and so on. So making the decibel was simply the next logical step. Remember, in audio, you can just about perceive a 1dB change - so the decibel is a very nice and convenient audio metric.QuoteNo, it does not. 20 log (2) is 6.02. That's just what the log function does - you said you understood that.In the same vein, 20 log (10) is 20. That's a precise result - go figureQuoteSorry, but you really couldn't be more wrong about this.Go work with audio for a few weeks. Then come back to us.

author: robrenz
author_rank: Super Contributor
timestamp: April 30, 2014, 01:17:34 pm
content: @ Rigby You may want to consider your "Learning. Very new at this." in your attitude toward other forum members who have probably forgotten more than you know about electronics. Your forum style could be considered just as abrasive as those you are throwing stones at.

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 01:48:34 pm
content: Quote from: robrenz on April 30, 2014, 01:17:34 pmedit: @robrenz: I changed my text. Don't think for one moment that it isn't accurate. I hear it every day from my own mind and the mouths of others. Imagine what I must be like in person to trigger people to actually SAY this to my face on a daily basis. end edit.Everyone keeps telling me things I already know about the dB scale in attempts to explain it, and they're getting more and more indignant about it. I haven't the same privileges?If it's my fault, I understand. My entire life everything that has ever happened to me has been my own fault. All the abuse I've received throughout my life, childhood to adulthood has always been my fault. I'm not being sarcastic. I try to communicate frustration with the scale, and somehow I phrase it so that you all think I'm attacking you. Story of my fucking life.So, now, I'm getting a divorce, having a shit period in my life, and even when I'm in a good mood I can't express myself effectively. My soon to be ex-wife says I'm always angry. I'm not. I'm really not. But, because I somehow grew up without learning how to APPEAR happy, everyone gets mad at me. Fine.dB is perfect and no one should ever question anything in electronics ever, ESPECIALLY on a forum oriented towards discussion of those exact topics. I understand, now.

author: tszaboo
author_rank: Super Contributor
timestamp: April 30, 2014, 02:01:38 pm
content: Quote from: Rigby on April 30, 2014, 01:48:34 pmWell, shit happens. I wish you the best.dB exist because some dude started using it. The term deci means "one tenth of", but we could have started using centibells or millibells. It doesnt really matter, it stays scientific. It makes 60dB more sense than the imperial system.

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 02:12:30 pm
content: Quote from: NANDBlog on April 30, 2014, 02:01:38 pm60dBWHAT. You have exemplified the issues I have with dB perfectly. Is "sense" as you use it a power measurement or amplitude?

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 02:57:21 pm
content: Quote from: Rigby on April 30, 2014, 02:12:30 pmPower. It's always power. Decibels are used to measure power ratios.For example, if I increase a signal amplitude by 3.2x that would be gain of 10 dB, telling us the power has gone up by 10x.

author: madshaman
author_rank: Frequent Contributor
timestamp: April 30, 2014, 03:11:16 pm
content: Woah, can't believe this is such a hot topic. w2aew's explanation is spot on.For my own two cents, I'll say that if logarithmic scales are troubling, vector spaces over multidimensional complex fields will be worse, frequency-domain vs spacial or time-domain will be more worse, and bizarre mathematical isomorphisms where you transform theorems in one formal system to another will bury you.Things are usually talked about and represented in terms which are the *easiest* to work with. Unfortunately, easy only gets so easy.To the OP, I'd say your grief is a combination of unfamiliarity and maybe some holes in mathematical background. The good news is that everyone on the planet faces this initially and it can be overcome; just don't be intimidated. It's really no different than learning the rules to a boardgame you've never played or have hardly played.Trust that there's usually a good reason for the way things are expressed in a given field.

author: Mr Simpleton
author_rank: Unknown
timestamp: April 30, 2014, 03:22:21 pm
content: If some prefer the "old school" why not use MSC, Miles of Standard Cable, where 1 MSC corresponded to the loss of power over a 1 mile (approximately 1.6 km) length of standard telephone cable at a frequency of 5000 radians per second (795.8 Hz), and roughly matched the smallest attenuation detectable to the average listener. Not sure why one would ever think of replacing such an intuitive unitJust to obfuscate Bell Labs came up with Transmission Unit, TU. 1 TU approximately equaled 1 MSC (specifically, 1.056 TU = 1 MSC, still pretty clear eh?Now they really went ballistic and renamed TU into Bel, B. And as this was not enough they found it way to big so had to split it into smaller chunks deciBel dB.I think I'll go back to MSC, much much clearer than any new snobbish dB!Now I have never heard one complain that dB was useless and overly complicated until this thread, honestly! dB simplify my work everyday, period.

author: jpb
author_rank: Super Contributor
timestamp: April 30, 2014, 03:36:12 pm
content: Quote from: KJDS on April 30, 2014, 10:40:45 amIt is off the topic of the thread, and I don't want to deny the usefulness of Smith charts for non-computer-aided circuit design. The point I was trying to make is that for the sort of circuits we worked with you'd only make a first stab at design/matching on paper and then optimise it using a circuit simulator. You would have a pretty good idea of topology and approximate circuit values (perhaps using a Smith chart or perhaps just knowing the device equivalent circuit) and then the computer could improve on it. There were also specialist software packages that were designed to find optimal matches as well.On the topic of dB, I'm in agreement with the general consensus in that dB make life much easier when combining power and loss (and noise) over stages. They can be very confusing when trying to compare spec. sheets.For example I've been looking at AWGs and some specs give flatness in dB and some give it as a %. What is being measured is the flatness of a voltage sine wave relative to 1kHz say but the dB figure is given as dB of power gain/loss assuming 50 ohms. The % figure is given as a voltage amplitude ratio. The dBs need to be divided by 20 (not 10) before being applied as an exponential to 10 and then subtracted from 1 and multiplied by 100 to give a figure to compare with the %!e.g. for the Agilent 33522A the amplitude flatness from 20 to 30MHz is given as +/- 0.40dB. For the Tabor WW1072 the amplitude flatness from 1MHz to 25MHz is given as 5%. 100%*(10^(0.40/20)-1) = 4.71% so the figures are very similar but it is not very obvious without doing the calculation.

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 04:04:23 pm
content: Quote from: Mr Simpleton on April 30, 2014, 03:22:21 pmThat's awesome.All dB does for me every day is get people to tell me I'm wrong and HOW DARE I even suggest that something they use all the time isn't perfect.the very best thing about any hobby is the community.the very worst thing about any hobby is the community.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2014, 04:12:07 pm
content: You could always go on a sailing forum & gripe about nautical miles!

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 04:14:15 pm
content: Quote from: vk6zgo on April 30, 2014, 04:12:07 pmI feel rejected enough already. I need to get over this one before I decide to pick up yet another hobby.

author: John Coloccia
author_rank: Super Contributor
timestamp: April 30, 2014, 04:22:25 pm
content: dB is useful because it gives you a linear scale to compare the power of various signals. In the engineering world, you're often dealing with signals that are of wildly varying power. You might be looking for a signal that's at -50dBu, with other signals riding on top that are at -10dBu, and maybe a noise floor of -60dBu. If you don't use log units, you will quickly have a mess of numbers, some teeny tiny, and some rather large, and it's very difficult to get a feel for what you're actually looking at because you're forever counting decimal points and exponents. It's much easier to talk about it in log units.It sort of reminds me of the difference between when an EE tries to write software vs an actual software engineer. The SE will "over-complicate" with all sorts of confusing and non-intuitive coding structures whereas the EE will typically just start coding and make it work. Guess who's software I'd rather maintain 6 months from now.Sometimes you need to have a bit of complexity up front and then reap the rewards later on. If you're a hobbyist, it doesn't matter. If you're a pro, and you come to me with a pile of numbers that look like, "2.5X10^-12mW .....3*10^2mW ....1*10^-6mW", I'm probably going to be very annoyed with you and the first thing we're going to do is convert everything to dBu so I can understand just what the hell you're talking about.But there's no reason to use dB for everything. Use it where appropriate and where it makes things easier. I think it makes a lot of sense for amplifier and filter design. For example, without plotting in log units, how on earth are you ever going to look at a frequency response plot and figure out what's going on? You're going to do the conversion to a log scale in your head? The plot would make absolutely no sense. You wouldn't be able to have any feel for whether that little dip at 1000kHz is huge or tiny. Other stuff? Maybe not so much.

author: Mr Simpleton
author_rank: Unknown
timestamp: April 30, 2014, 04:28:32 pm
content: Quote from: Rigby on April 30, 2014, 04:14:15 pmYou got awfully soft skin... Simple fact is that the vast majority do use dB and finds it very useful. Noone forces you to use it, and you cannot really expect other so stop using it just because a few find it confusing. The logaritmic concept was not "invented" to make things harder but to simplify.I do agree that some use dB wrongly and cause all sorts of aggrivation but as with any tool, used correctly it do help. So open up your mind and try see the lightTo me electronics is all about learing new stuff, and try to understand old.BTW calculating cascaded distortion needs to be done in linear numbers but are presented in dB (logaritmic) units.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: April 30, 2014, 04:30:50 pm
content: Quote from: Rigby on April 30, 2014, 04:14:15 pmBetter than aviation. Altitude in feet, distance in kilometers (or miles, or nautical miles), speed in knots, and don't forget you have three types of speed! (IAS, ground speed, true speed). Good luck converting between them.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 04:33:43 pm
content: Quote from: Rigby on April 30, 2014, 04:04:23 pmI don't think that is what people are saying.You have been saying that in your experience the use of dB doesn't fit well with you. That's your personal thing.Other people have been saying that their experiences are different and that dB works nicely for them. That's their personal thing.It's just as if I say I like olives and you happen to say you don't like olives. Just because you and I have different tastes, is that a problem? Does it make you wrong?

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 04:42:03 pm
content: Quote from: IanB on April 30, 2014, 04:33:43 pmIt must because lots of people are telling me i'm wrong, and "when in rome" bullshit like it's a real reason for anything.

author: elgonzo
author_rank: Unknown
timestamp: April 30, 2014, 04:42:03 pm
content: Quote from: Phaedrus on April 30, 2014, 04:30:50 pmA maritime forum is actually a place not better than aviation forum. If there is still energy left after lamenting miles, one can lament over "Gross RegisterTonnage" vs. "DeadweightTonnage" and "long tons" vs. "tonnes" (also called "metric tons"). But, yeah, aviation forums are nice place too... Oh, wait, you forgot to mention calibrated/equivalent airspeed. Hmm... well, okay, aviation probably is worse...

author: FrankenPC
author_rank: Unknown
timestamp: April 30, 2014, 04:47:34 pm
content: This is a really interesting thread. I have problems adopting new concepts even though I completely understand the perimeter concepts. So, without help from people like w2aew, I may go my whole life without knowing something which is actually very simple to grasp, but no one has presented it to me in the exact way that unlocks my brains tumblers. I have what's called a learning disorder. I'm on the autism spectrum.Needless to say, going through public school taught me to hate learning, especially math. When people would freak out about my inability to learn something which seems totally obvious to them, imagine my frustration. Imagine what I thought about myself (loser, stupid). It wasn't until the advent of the internet and the intelligence aggregators like EEVBLOG and YouTube, did I actually start to grasp this stuff simply because I finally had multiple education viewpoints to choose from. I finally had Franken's Rosetta stone so to speak.Why is this all relevant? Because if I still harbored anger and resentment about learning, I wouldn't be here and I wouldn't learn anything. Instead, I now know I have a disorder and that forgives my past transgressions and give me the greelight to allow old information back in, in the proper way.In other words...anger at conceptual inconsistencies may actually be another problem in disguise.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 05:02:06 pm
content: Quote from: Rigby on April 30, 2014, 04:42:03 pmI went back over the thread. I see lots of people saying "I like dB, they are good for me". I see one person, a known abrasive personality, make the "when in Rome" comment. I don't see anyone saying "Rigby, you are wrong."But now I'm telling you you are wrong, right?

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2014, 05:02:57 pm
content: Then there are Imperial gallons versus US gallons---same name,completely different quantity of liquid!Imagine the fun US & British forces had when co-operating during WW2.

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 05:14:35 pm
content: Quote from: IanB on April 30, 2014, 05:02:06 pmeveryone always tells me I'm wrong whether they're actually telling me I'm wrong or not.Quote from: FrankenPC on April 30, 2014, 04:47:34 pmI know exactly what it is, in my case. near total social ineptitude, combined with (or caused by) both ADHD and Asperger's.

author: KJDS
author_rank: Super Contributor
timestamp: April 30, 2014, 06:21:30 pm
content: Quote from: jpb on April 30, 2014, 03:36:12 pmI can't remember the last time I used a paper smith chart, but plotting the impedance on the Smith chart as you add in one component at a time to a CAD model is a very simple way of achieving a good match.

author: SeanB
author_rank: Super Contributor
timestamp: April 30, 2014, 06:58:43 pm
content: Quote from: Rigby on April 30, 2014, 05:14:35 pmWelcome to the club, you will fit right in.Sociology is not a hobby of mine, but I have met way too many psychologists and such, and a fair number of people who were totally certifiable. You do not fit into that category, and possibly just need a little more time to consider the relationships between voltage, power and the decibel. Yes, it can be confusing, but it is useful in it's place, and learning a little about the differences and how they are related will help with this field.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: April 30, 2014, 07:28:47 pm
content: Quote from: IanB on April 30, 2014, 02:13:24 amI need to start thinking of it this way. Good post!

author: Phaedrus
author_rank: Frequent Contributor
timestamp: April 30, 2014, 08:08:23 pm
content: I'm ok with dB, it's just the voltage/power thing that always trips me up. I mean I get it, mathematically, but I don't really "grok" it I guess. I just have to slow down to consider the difference whenever I work in dB, and it seems like it's an arbitary hold-over that's only relevant in certain audio contexts.The unit itself is occasionally useful, I just think the voltage/power bit is an unnecessary complication if you aren't working in audio.

author: Rigby
author_rank: Super Contributor
timestamp: April 30, 2014, 08:48:53 pm
content: Quote from: Mr. Coffee on April 30, 2014, 07:28:47 pmYeah that does explain it very well.The 10-6thing... What's wrong with saying microvolt/amp/ohm/watt? I guess I still don't know why the decibel unit is needed. I understand why logarithms are needed, that's not what I'm after.I get the math. I get logarithms. I don't get why the dB unit exists. It seems completely superfluous to me.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 09:12:19 pm
content: Quote from: Rigby on April 30, 2014, 08:48:53 pmAnd that's quite OK. Don't stress it. If dB measurements don't crop up in your work or hobby you can happily ignore them.I went 50 years of my life with little understanding of what a dB is or why anyone used dB measurements. It's only when I watched some of Shahriar's videos that I began to realize how signal engineers use them. What used to be a mystery makes sense to me now, but only after seeing some context.

author: FrankenPC
author_rank: Unknown
timestamp: April 30, 2014, 09:18:16 pm
content: QuoteIMO, he's not confused at all. He understands the parameters of the issue completely. What he doesn't understand is the arbitrary nature of the unit itself. I understand what he's saying. There's just nothing anyone can do about it. It's adopted. Reminds me of the English measurement system. Makes no sense.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 09:31:12 pm
content: Quote from: FrankenPC on April 30, 2014, 09:18:16 pmExcept...I'm sure it made sense to the people who introduced it. Pre-decimal measurement systems worked satisfactorily for trade and commerce for thousands of years. For most of that time there was no concept of zero, or of negative numbers, or of decimal fractions. If you wanted fractions you used 1/2, 1/3, 1/4, 1/5, etc. (And for finer resolution you could add them, e.g. 1/3 + 1/12, just like you could add 50 + 10 + 3 to get LXIII.)Did such a system stop making sense just because we invented an improvement? I don't think so. I think earlier measurement systems didn't stop making sense, they just became displaced by a system that is more useful in modern times.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 30, 2014, 09:53:45 pm
content: Quote from: Rigby on April 30, 2014, 08:48:53 pmI have already explained the significance of the decibel in audio engineering, but you haven't acknowledged that. Perhaps you missed it? Perhaps you didn't believe me? So rather than take my word for it, please check out these links:http://www.music-production-guide.com/decibel-scale.html- Edit: test your understanding - see if you can spot the mistakes in this linkhttp://www.audioholics.com/room-acoustics/the-decibel-dbhttp://www.soundonsound.com/sos/1994_articles/feb94/decibels.htmlhttp://www.animations.physics.unsw.edu.au/jw/dB.htmhttp://www.axiomaudio.com/whatisdbhttp://cp.literature.agilent.com/litweb/pdf/5990-4503EN.pdfhttp://sound.westhost.com/articles/fadb.htm#s20All of these were picked pretty much at random, and all explain things slightly differently, so hopefully covering a good range of learning styles.As I said earlier, it's entirely possible that you will never do audio engineering, or indeed any of the other disciplines that use decibels, in which case, the decibel might well remain superfluous toyou. But for the sake of 5 or 10 minutes spent reading one or two of those links, you will no longer be ignorant of one of the biggest applications of the decibel, and you won't ever feel the need to call it "nonsense" again.Finally, please understand that the majority of us on here are doing our best to help you. Why? Because we are passionate about the subject, and love passing on our enthusiasm, especially to people who say they want to learn. None of us are paid to be here; we are all voluntarily giving up our time to contribute. We are not trying to make you look or feel bad.At this point I feel I have done all I can - you have the information at your disposal, and what you do with it is entirely up to you. I wish you all the best for the future; whether that future includes decibels or not

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 10:10:46 pm
content: OK, I read the first link, but I did not follow one point:QuoteThis I don't follow.Suppose I am driving a loudspeaker of constant impedance and I double the input power. Then the increase in dB is 10 log 2 = 3 dB.But to achieve that doubling of input power, I must increase the (RMS) signal voltage by sqrt(2). So the increase in dB for voltage is 20 log 1.414 = 3 dB (not 6 dB).Whether I work with voltage or power, the dB measurement for the same scenario is the same value. This only makes sense. It would be completely confusing otherwise.Can you explain?

author: madires
author_rank: Super Contributor
timestamp: April 30, 2014, 10:22:15 pm
content: Yes, I love my dBs. If you're dealing with signals passing through wires and connectors you can calculate the total attenuation simply by adding all the dBs for each part. That way you'll know if a data link will work, if you need a sender with more power or how much the link margin is.

author: madires
author_rank: Super Contributor
timestamp: April 30, 2014, 10:32:49 pm
content: Quote from: IanB on April 30, 2014, 10:10:46 pmdB = 10 log (P1/P2)P = U * I with R = U /I -> P = U^2/RdB = 10 log ((U1^2/R) / (U2^2/R)) = 10 log (U1^2 / U2^2) = 20 log (U1 / U2)Hint: U = V ;-)

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 10:36:59 pm
content: Quote from: madires on April 30, 2014, 10:32:49 pmThat's exactly what I wrote in my post, just expressed in a different way.Let me ask it a different way: Can you explain why the link below seems to be wrong?http://www.music-production-guide.com/decibel-scale.html

author: Tris20
author_rank: Regular Contributor
timestamp: April 30, 2014, 10:38:08 pm
content: Studying electronics with an interest in the audio side would be hell without decibels. Essentially the best use I can think off just now is the use it has in comparing a signal with one which is thousands/millions of times bigger or smaller in magnitude without having to draw/simulate a graph the size of a house.Similarly the way logs benefit bode plots. Our hearing is roughly 20-20khz, but the most important sounds are those roughly between 20Hz and 4kHz so it makes sense to use a numbers system (and hence the unit of measurement being discussed) which will give us graphs that show this region in the greatest level of detail whilst still displaying the less essential data in some way.Note how the 10k to 20k is 10kHz in size but only a small section of the graph. Compare that to the 20Hz to 10kHz where the difference is also around 10kHz and you can see how much of a difference this system makes in displaying the data. Decibels just does this for loudness/amplitudes.

author: madshaman
author_rank: Frequent Contributor
timestamp: April 30, 2014, 10:51:20 pm
content: Apologies to the OP, I missed your post distinguishing between being okay with logarithmic scales and the evolution of the decibel (being not really standard like SI units).You do have a point there; the decibel definitely exists mostly because it's entrenched.

author: madires
author_rank: Super Contributor
timestamp: April 30, 2014, 10:56:14 pm
content: Quote from: IanB on April 30, 2014, 10:36:59 pmhttps://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_level

author: Tris20
author_rank: Regular Contributor
timestamp: April 30, 2014, 11:03:27 pm
content: Decibel is essentially just a name for the log scale of gain. Another area in which a log scale of gain is absolutely needed is in the mixing slider on an audio mixing desk. Working on a logarithmic scale of say -120 to +12 dB where 0dB will output the audio at unity (the same amplitude as the input) is far more intuitive and most people who work in audio pick this up almost instantly. Change it to the linear equivalent and people would have to spend a much greater time understanding the scale which they're using to alter the sound.Best image I could find for an audio fader/ volume control. Note how the decibel scale means this can actually be a manageable size and we can use nice round numbers too. A linear scale would be so long you would need to have very very very long arms, or perhaps even an arm extender to slide the fader.Google "Mixing desk fader" to see how these sliders are used.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 30, 2014, 11:08:39 pm
content: Quote from: IanB on April 30, 2014, 10:10:46 pmYes. They made a mistakeJust goes to show that you've got to keep on your toes with this subject - people make mistakes all the time. The first part of the article seemed pretty well-written, but I admit that I didn't read it all.Saying 3dB (or 6dB) is twice as loud is pretty misleading. A 3 dB difference is perceptible, but not a big deal. Really, a change of 10dB or thereabouts feels like a doubling in loudness subjectively.As you rightly say, to achieve that 3dB difference, you have to double the power, and to double the power, you need to increase the voltage by a factor of root-2.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 11:16:23 pm
content: (Note: just read Mark's reply above, but I'll post anyway for completeness.)Quote from: madires on April 30, 2014, 10:56:14 pmYou are still not following.Here is the quote again:QuoteIf we say "twice as loud" we have a common basis for comparison. "Twice as loud" means all experimental parameters are the same and all we are doing is making our measurement in a different way.Now then, if "twice as loud" is a 3 dB increase, then "twice as loud" is always a 3 dB increase. It cannot be 3 dB in one case and 6 dB in another case, or chaos would ensue. Misleading and badly written web pages like that one are perhaps one of the reasons why people find decibels confusing. If people trying to write about them get confused, then no wonder people reading their descriptions get confused.For another example, that same web page at one point says that "twice as loud" is a voltage gain of 10. Which in decibels would be 20 log 10 = 20 dB.So now, which is it? Is twice as loud equal to 3 dB, or 6 dB, or 20 dB?No wonder people have trouble understanding.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 30, 2014, 11:21:37 pm
content: I've just seen a mistake on the SOS page. That I didn't expect!To be fair, I think it's a copy and paste that didn't get updated, but even so, that's disappointing.I won't say any more; rather, test your own understanding by trying to find it.It's a shame I can't simply post the training documents I use at work (copyright, etc).

author: madires
author_rank: Super Contributor
timestamp: April 30, 2014, 11:31:58 pm
content: Quote from: Mark Hennessy on April 30, 2014, 11:08:39 pmYep, using "twice as loud" is incorrect since we know from psychoacoustics that 10dB more power is perceiced as twice as loud by humans. I think the author used "twice as loud" as analogy to explain things.

author: John Coloccia
author_rank: Super Contributor
timestamp: April 30, 2014, 11:36:04 pm
content: Quote from: IanB on April 30, 2014, 11:16:23 pmI think we should just do away with the "twice as loud" thing altogether. The real crux of it as it applies to audio is that a log scale more closely approximates how we hear things, so it's convenient to talk about log units because that roughly linearizes all of the curves and they make more intuitive sense.As it applies to other kinds of engineering, it linearizes the "difficulty" of plucking out signals. Analog engineering is all about plucking out signals...what else is there, really? It's just a convenient way of figuring out if you can get what you need to get, and maintain it out the other end so the other guy can get what he needs to get.

author: madires
author_rank: Super Contributor
timestamp: April 30, 2014, 11:38:10 pm
content: Quote from: IanB on April 30, 2014, 11:16:23 pmFor humans twice as loud is a 10dB increase in power. The author used loudness as analogy, not as a physical unit.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2014, 11:42:01 pm
content: Quote from: John Coloccia on April 30, 2014, 11:36:04 pmI don't mind what the specific example is. It could be half as loud, or three times the power or four times the voltage. But whatever example you pick you ought to be consistent in the use of that example. "X times as loud" cannot be 3 dB, 6 dB and 20 dB at one and the same time...

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: April 30, 2014, 11:48:54 pm
content: Quote from: Mark Hennessy on April 30, 2014, 09:53:45 pmOh dear. I've just spotted an error on Rod's page. Like with the SOS article, I'm quite surprised at that - Rod is normally 100%. Again, it just goes to show. And again, I'll leave it as a test for you all.I forgot to include a link to the Lindos article. For those that don't know, Lindos are a small UK manufacturer of audio test gear, and their LA100 is something of a mainstay in many professional settings. Pete Skirrow has retired now, but has contributed a lot of content to Wikipedia as well as the Lindos website.http://www.lindos.co.uk/test_and_measurement/SOURCE=Articles/SOURCE=Articles|VIEW=full|id=4

author: vk6zgo
author_rank: Super Contributor
timestamp: May 01, 2014, 12:00:07 am
content: I spent a bit of time in the Hearing industry--a 3dB increase sounds the same both in power & Sound Pressure.It isn't "twice as loud" at all--it is twice the power in each case.If it was "twice as loud" we wouldn't have to use a Logarithmic system,as we would be happy with a linear one.Voltage/power relationship:-Think about the case of 3dB power gain in a system with equal input & output impedances (& driving a matching impedance)If we look at the signal with a meter calibrated in watts,we will see a reading of 2 times the power at the output compared to the input level.(actually,it is 1.9953 according to my HP22s--but near as dammit!)If we use a meter calibrated in volts,we will see a reading at the output of 1.414 times the voltage reading at the input.(Again,not quite,but good enough for engineering work!)If we increase the gain of the DUT so we get an outputvoltagereading of 2 time the input level,you have a gain of 6dB,both in voltage and power.Sound Pressure has the same relationship to sound power as voltage has to power in the Electronics field, resulting in theformula:-dB =20log p1/p2This,of course,which just lookswrongto Electronics people,but in this case p1/p2 refers tosound pressure.By the way,the voltage formula dB=20log v1/v2 is just a short cut used when both voltages are present across equal impedances.It saves you working out the power levels P1 & P2 & applying dB=P1/P2People who should know better do misuse this formula a lot.The power formula is always valid,as you must include the impedance ,although it is normally written as R rather than Z,using the classic power formula.

author: ludzinc
author_rank: Unknown
timestamp: May 01, 2014, 12:05:38 am
content: Quote from: IanB on April 30, 2014, 10:10:46 pm3dB and 6dB are MEANINGLESS!3dBW, 6dBV make perfect sense.To double your power you need an increase of 3dBW.As P = VI, V^2/R, I^2R, and if R only remains constant, you cannot alter your power alone by altering the voltage. As V goes up so do does I.So when talking dBV (or even dBI) you need to use a x20 scalar to account for the fact you are altering two variables (that are in fixed ratios due to P=VI).TL;DRIf talking POWER, use dBW, otherwise define your reference (dBV etc).

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: May 01, 2014, 12:11:57 am
content: Quote from: vk6zgo on May 01, 2014, 12:00:07 amThis was touched on earlier, and of course, technically you are correct.But in practice, impedance is never a concern in audio until we get to the loudspeaker. Not these days. We stopped using 600 ohm matched impedances (and dBm measurements) a long time ago. It's just about possible that you might find some legacy gear at transmission sites that takes in 600 ohm feeds, for example, but relatively unlikely today...So as you follow an audio signal through a sound desk, for example, you make voltage measurements and translate those to dBu. Perhaps your DVM does it for you (e.g. off the top of my head, Fluke 187/9 or 287/9, or 8050A or 8060A). The circuit impedances at these points will be varying all over the place. But it doesn't matter because each amplifier responds to voltage, and voltage alone. Indeed, dBu is a voltage measurement; it just so happens that the voltage is the same voltage that gave 1mW into 600 ohms. The "u" in dBu means "unspecified"; i.e. the impedance isn't important.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 01, 2014, 12:15:22 am
content: Not really,a 3db increase in power between 1 microwatt to 2 microwatts is every bit as valid as the same increase between1 Megawatt to 2 Megawatts.A ratio is always a legitimate thing.Actually,dBs are always about power.Where we should not do is use dB =20log v1/v2 when we are comparing voltages across different impedances,as that is nonsense!

author: atw60444
author_rank: Contributor
timestamp: May 01, 2014, 12:19:42 am
content: Quote from: Mr. Coffee on April 29, 2014, 01:14:09 pmI agree with this. Nothing makes me glaze over quicker than reams of formula and maths examples. Explanation in real world terms make much more sense to me and early text books to seem better in this respect. Possibly because they were aimed at apprentices or forces trainees rather than academics? I am the former.Jon

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: May 01, 2014, 12:29:27 am
content: Quote from: ludzinc on May 01, 2014, 12:05:38 amHonestly, I think you've got yourself confused here.dB on its own is a ratio of two quantities.dB with a suffix - such as dBW - is an absolute measurement. The suffix indicates the reference level - e.g. 1 watt in the case of dBW, 1mW in the case of dBm, 1 volt in the case of dBV, or 20µPa in the case of dB(SPL).3dB simply means a doubling in power, or an increase in voltage of x1.414 (the square-root of 2).So, 3dBW is actually +3dB relative to the reference of 1 watt, which is 2 watts.Similarly, 6dB means a doubling in voltage, or an increase in power of 4 times.So, 6dBV means +6dB relative to the reference of 1 volt. Which is 2 volts.It's non-syntactical to say "an increase of 3dBW". But "an increase of 3dB" makes complete sense.I normally say that if you understand percentages, you'll understand decibels. After all, both are just different ways of expressing a ratio. If I say I'm going to give you a 10% pay rise, you'll understand what that means because you know your current pay, which is the reference level. But if I offer you a new job, and say I'll pay you 110%, you would rightly reply "110% of what???". Because a percentage on its own - without a reference - cannot describe an absolute quantity.

author: IanB
author_rank: Super Contributor
timestamp: May 01, 2014, 12:38:57 am
content: Quote from: vk6zgo on May 01, 2014, 12:00:07 amNot quite so. A doubling in the sound pressure is a quadrupling of the transmitted power (since wave power is proportional to the square of the amplitude).Hence a 3 dB increase in sound pressure is given by 20 log p2/p1, while a 3 dB increase in sound intensity is given by 10 log I2/I1, where I2/I1= (p2/p1)2.It's the same 3 dB in both cases (and hence sounds the same), but the ratios of sound pressure and sound intensity (wave energy) are different.

author: ludzinc
author_rank: Unknown
timestamp: May 01, 2014, 12:57:30 am
content: Quote from: Mark Hennessy on May 01, 2014, 12:29:27 amYes and no....If you tell me to increase something by 3dB, I have no idea what you want. Double Power? SQRT2 Volts?If you imply the relationship i.e. increase power by 3dB, or if you say increase from 1 to 2 uW you are implicitly telling me to refer to a power ratio.So if you listen to someone's explanation, read their sentence, look at their schematic, you usually can determine what reference they are relating to.But to state 'xdB' without any extra content, is meaningless.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 01, 2014, 01:12:53 am
content: Quote from: IanB on May 01, 2014, 12:38:57 amI think that's is what I said!--- "a 3dB increase sounds the same both in power & Sound Pressure.It isn't "twice as loud" at all--it istwice the powerin each case"At no point did I refer to a doubling of Sound Pressure--multiplying SP by 1.414 causes an increase in sound power of 3dB.Interestingly,about the time I left the Industry,there were moves to change measurements to "Sound Power" (not intensity).After all,power is "rate of doing work" & 'WORK' is what wiggles our eardrums!I'm not sure what happened,but I think the idea came to nothing.

author: IanB
author_rank: Super Contributor
timestamp: May 01, 2014, 01:16:48 am
content: Quote from: vk6zgo on May 01, 2014, 01:12:53 amI see, sorry about that. I misread what you wrote.

author: stitch
author_rank: Regular Contributor
timestamp: May 01, 2014, 01:51:22 am
content: Quote from: IanB on April 30, 2014, 02:13:24 amThat's the best explanation I have ever heard!

author: vk6zgo
author_rank: Super Contributor
timestamp: May 01, 2014, 01:52:48 am
content: Quote from: Mark Hennessy on May 01, 2014, 12:11:57 amI wrote a long convoluted answer to this,then accidentally hit the wrong key & sent it all off to cyber purgatory!This is a much reduced version,but I'd like to make a few comments:-Yes,the "low output Z,High input Z" stuff was relatively new when I left Broadcasting,but we had some racks using this method.If you really think about it,dBm on an old style N & D Set is a voltage reading---the meter is just calibrated as dBm!Plugging in an N&D set on its dBm range,into a device output gave the same value reading in both dBm & dBu,so it was pretty transparent.The only difference was that 600 Ohms & "Bridging" on the meter gave "near as dammit" the same reading,whereas bridging an unterminated 600 Ohm output gives a high reading.The voltage level for 0dBu doesn't "just happen to be" the same as for 1mW in 600 Ohms,it was designed to be so!"each amplifier responds to voltage, and voltage alone"----Not really,they are still power amplifiers,it is just that power gain is easy to come by with modern solid state components,without the necessity of using matched impedances.The use of dB in this circumstance is a legitimate technique but it is used with the express knowledge that itisa technique,& cannot be applied "willy nilly" to other quite different circumstances.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 01, 2014, 03:20:04 am
content: Oh man, it got into SPL and perceptible volume :-)

author: kony
author_rank: Regular Contributor
timestamp: May 01, 2014, 04:00:10 am
content: OT:It's just a neat log scale, nothing more - how can you find it not intuitive ?And it can be applied almost anywhere, see ?

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 01, 2014, 02:02:45 pm
content: Quote from: Rigby on April 30, 2014, 08:48:53 pmOh I probably will always say it in terms of input and output millivlots of microwatts, etc. It's a habit.I'm not against the dB usage; having worked some with adding and subtracting dBm, 600 ohm in communications, it was really good to have. I'm just saying in my original post that I have struggled with it being intuitive to me for looking at electronic amplification or attenuation--and that I see the usage of dB more and more nowadays--and I didn't really learn it that way back it the stone age when I went to school for electronics.

author: Mr Simpleton
author_rank: Unknown
timestamp: May 03, 2014, 09:40:26 pm
content: As long as you keep in mind that dB (or Bel for that matter) is the ratio between two power levels there should be no confusion!3dB --> is twice the power6 dB --> four times the power but if youmeasure voltage, twice the voltage! (Keeping resistance fixed)You can have both voltage and resistance vary, as long as you convert back to power when taking the ratio.Now what may be problematic is that Wikipedia says:"The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, often power or intensity."This may imply that other physical quantities may be used other than power (or intensity), which may be mathematically correct, but for us engineers, I have never encountered any other meaning that power ratio.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 03, 2014, 10:40:14 pm
content: The most common usage I've come across is sound pressure. But I'm a different kind of engineer, and that would be relevant to factory noise level safety concerns.

author: IanB
author_rank: Super Contributor
timestamp: May 03, 2014, 11:10:46 pm
content: Quote from: Mr Simpleton on May 03, 2014, 09:40:26 pmIf there should be no confusion, why do people keep choosing these awkward examples like 3 dB and twice the power?Illustrate with 10 dB is 10x the power, 20 dB is 100x the power and 30 dB is 1000x the power and it becomes much clearer what is going on.

author: robrenz
author_rank: Super Contributor
timestamp: May 12, 2014, 03:27:07 pm
content: Nice article on dBhere in EDN

author: electr_peter
author_rank: Unknown
timestamp: May 15, 2014, 12:50:51 pm
content: I will come back to my point that dB is intuitive enough is you use them often (RF, audio etc.) AND youKNOW in advanceunderlying assumptions (fixed resistance, some predefined resistance for audio circuits, positive or negative attenuation, etc.). Also, you have to remember to use base 10 logarithm, not natural base (this is, again, "silent" assumption).On top of that, dB doesn't measure anything particular - it is just another way of expressing proportions (sometimes useful, sometimes not). I can say that I use 2 power supplies and IV multimeters (both arabic and roman numerals) - this is simple expression, but there is no need to use 2 different counting systems in the same context, it just confuses everyone. Same goes for dB (in some cases).Add these aspects together and you get confusion over simple topic of proportions.