Decibels in EE - Page 1
Forum section: electronics / projects

author: drcobol2000
author_rank: Newbie
timestamp: November 07, 2015, 06:13:03 am
content: Okay, I am a bit of a geek (leaning toward business management and programming languages rather than circuit design) but decibels in my parlance is a unit of measure regarding howloudsomething is. Can anyone with more knowledge than I out there describe for me how decibels are practically used in circuit design/electronics engineering? This concept is something I'm having a little difficulty wrapping my brain aroundand I cannot find any simple description in the ether that will help me to understand what on earth this measurement would be used for other than what I've indicated previously.

author: ivaylo
author_rank: Frequent Contributor
timestamp: November 07, 2015, 06:36:29 am
content: It may take you a while before it makes intuitive sense... It's the relationship of two things which is expressed by that measure. It is good you've seen the loudness thing. It is the relationship between the minimum loudness percievable and what is observed. Now enter the formulas...

author: Tim F
author_rank: Regular Contributor
timestamp: November 07, 2015, 06:38:03 am
content: Decibels aren't a unit of measurement, they are simply a logarithmic ratio. When you measure sound level it is in "dB SPL" where 'SPL' is the unit of measurement. You could just measure in 'SPL' (linear) if you really wanted to but since sound levels typically cover a huge dynamic range (many orders of magnitude), representing them on a logarithmic scale tends to be easier to interpret.You can use a dB ratio with any unit of measurement - e.g. 'dB Volts', 'dB Milliwatts' etc. Logarithmic scales useful for representing measurements where the distribution of data points is typically logarithmic - e.g. plotting data [0.1 100 and 100000] on a linear scale might not be intuitive to interpret. Converting to a log/dB scale would give you [-20 40 100] instead.

author: IanB
author_rank: Super Contributor
timestamp: November 07, 2015, 06:45:03 am
content: Decibels measure a ratio between power levels, or most commonly the ratio between some measured power level and a reference power level.This works nicely with sound because the loudness of sound is related to how much power is in the sound waves. The reference level for sound measurements is the quietest sound that a normal person supposedly can hear at 0 dB.The decibel scale says that every 10 dB is ten times more power. So a sound at 20 dB is 10 x 10 = 100 times more powerful than a sound at 0 dB. 50 dB is 105or 100,000x more powerful.In electronics it is just the same, except the reference power is usually taken to be 1 watt (dBw) or 1 milliwatt (dBm).So a signal of 30 dBw is 103watts or 1 kW (!). This is why dBm is more common for typical signal levels.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 07, 2015, 06:56:45 am
content: dB is nothing more than a concise way to express a wide range of ratios. The "dB" that refers to sound volume is actually dBA, referenced to the accepted "noise floor" of the human ear. In electronics, dBm is a very common unit and that refers to dB referenced to a milliwatt. Also very common is dB used to express gain or attenuation. That's handy when it comes to calculating levels as it lets you use simple addition rater than multiplication. For example, if you put a 10dBm signal into an amplifier with a 10dB gain, you get a 20dBm signal out.As for an example where dBA and dBm meet, let's take a look at some Xiaomi Piston 3 headphones. They have a sensitivity factor of 98dB, meaning that you'll get a very loud 98dB sound if you put in 0dBm or 1mW. Or you can put in -98dBm (that's a *tiny* signal!) and get 0dBA or a sound that theoretically can just be heard. (It's a little more complex than that since the "A" is not a constant but rather a curve that tries to approximate the frequency response of the human ear. Usually, the sensitivity factor is measured at 1kHz.)https://en.wikipedia.org/wiki/Decibel

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2015, 08:23:22 am
content: You might find this site helpful for quick calculations / estimations: How to Cheat with dBshttp://w4neq.com/htm/dbtalk.htm

author: bookaboo
author_rank: Frequent Contributor
timestamp: November 07, 2015, 10:03:36 am
content: This guy has a great youtube channel with a video dedicated to the subject, no idea who he is though:

author: tggzzz
author_rank: Super Contributor
timestamp: November 07, 2015, 10:17:07 am
content: Quote from: drcobol2000 on November 07, 2015, 06:13:03 amThat's easy to answer. Seehttp://lmgtfy.com/?q=decibel+simple+explanation

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2015, 10:22:11 am
content: dBs are used exactly as you have been using them: a logarithmic measure of intensity. If you've been using acoustic dB, that's probably referenced to 0dB == threshold of hearing, give or take (which is some convenient acoustic pressure or power level I forget).More generally, dB are a ratio. dB with electrical references are common: dBm (1mW at whatever the system impedance is -- if you're measuring voltage or current instead, the impedance matters!), dBuV (0 dBuV == 1 microvolt, note this does not include system impedance so is not also a measure of power), and so on.Typical system impedances are 600 ohms (telco, audio), 75 ohms (video, CATV..) and 50 ohms (radio, test equipment, general?), but others might be snuck in: be careful and inspect what the aim of the circuit/system is. (Audio systems typically do not match impedances, using a low source impedance, and a >= 10kohm load impedance. dB volts would make more sense in such a case. Video and radio do match impedance, so it matters what impedance you're using, and that it be matched correctly as well.)BTW, for the acoustic analogy of impedance, consider the product and ratio of velocity and pressure (the electrical analogs of current and voltage). The product is power (more specifically, power density: acoustic intensity). A high pressure with a low velocity will give the same power as a low pressure with a high velocity. But the ratio of the two is an acoustic impedance (which has units of momentum per volume, I guess). Free air has a given impedance; any material that has a different impedance will tend to reflect waves (hence solid material is a good reflector). A speaker is an impedance matching system, because the voice coil can deliver pretty good pressure, but not much velocity, or area. A cone transmits the force to a greater area = lower pressure, but suffers from flex in its material. A horn type speaker provides a continuous matching between coil and air, which is why it gets the highest efficiency.Tim

author: dom0
author_rank: Super Contributor
timestamp: November 07, 2015, 10:43:06 am
content: QuoteAnd there's an important difference: A 3 dB difference for a power quantity means double/half the power (depending on sign). A 3 dB difference for a field quantity (e.g. voltage, current) means 2^-.5 / 2^.5 times the quantity. Why? To get power, you always need to square the field quantity(*). 2^-.5 squared is 2 ; thus a dB difference always relates to the samepowerratio in the end.(*) and multiply it with a system constant, e.g. system impedance.

author: TimFox
author_rank: Super Contributor
timestamp: November 07, 2015, 03:01:50 pm
content: Decibels are better than percentages to express changes.For example, if you increase a signal by 2 dB, then decrease it by 2 dB, you are back to where you started.However, if you increase a signal by 25%, then decrease it by 25%, you are back to (0.75) x (1.25) = 0.9375 of where you started.

author: drcobol2000
author_rank: Newbie
timestamp: November 07, 2015, 05:09:37 pm
content: Many thanks to all! I can now put this concept into perspective.

author: Neilm
author_rank: Super Contributor
timestamp: November 07, 2015, 05:52:51 pm
content: They are also great for winding up your manager if he fancies himself as an engineer. Nothing like the look on their face when he comes back from holiday and you tell them your product has failed EMC by 6dB, but you have managed to halve the power at that frequency..... so there is only 3dB to go.

author: drcobol2000
author_rank: Newbie
timestamp: November 08, 2015, 05:12:53 am
content: Quote from: Neilm on November 07, 2015, 05:52:51 pmI hate management and would rather be down in the weeds doing line-by-line work, you cheeky little monkey!