# What is Db: Definition and 62 Discussions

The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10 (approximately 1.26) or root-power ratio of 101⁄20 (approximately 1.12).The unit expresses a change in value (e.g., +1 dB or −1 dB) or an absolute value. In the latter case, the numeric value expresses the ratio of a value to a fixed reference value; when used in this way, the unit symbol is often suffixed with letter codes that indicate the reference value. For example, for the reference value of 1 volt, a common suffix is "V" (e.g., "20 dBV").Two principal types of scaling of the decibel are in common use. When expressing a power ratio, it is defined as ten times the logarithm in base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing root-power quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two, so that the related power and root-power levels change by the same value in linear systems, where power is proportional to the square of amplitude.
The definition of the decibel originated in the measurement of transmission loss and power in telephony of the early 20th century in the Bell System in the United States. The bel was named in honor of Alexander Graham Bell, but the bel is seldom used. Instead, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels.

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1. ### Microwave Oven door attenuation in dB?

I recently made a repair to the door switches/bracket on our microwave oven (MWO). All is well, but later, after reinstalling a trim piece and checking the fit, I noticed that with the door closed and latched, that there was a bit of play (1/8"?) - I could push the door a bit further closed, and...
2. ### Question on sound intensity and dB

Would we have to use the 3-dB exchange rate? How would we solve this question?
3. ### How do civil engineers use grad to measure slopes in construction?

G’day. I am here to clear my doubt about the dB scale, in general, and it’s use in calculating received power in a microwave cavity. Given that I have arrived at received power (Pr in Watts) for a given input power (Pt), I understand that 10*log (Pt/Pr) yields Pr in dB. Now, say, the...
4. ### Sound wave intensity (dB) and sound processing

Hello Forum, A sound wave intensity (pure frequency) is proportional to the square of the wave pressure amplitude, i.e. ##I \approx p_0^2##, where ##p_0## is the pressure wave amplitude: ##p_0 sin(\omega t \pm kx)##. This means that the (gauge) pressure value goes larger (positive) and lower...
5. ### Amateur Dev Q: Creating Front End for SQL Server DB

Hi All, I am kind of new to this, please be patient. I appreciate your input I have a SQL Server DB, I am trying to create a remote front end to give graphical, or at least simple access to the DB (Client-Server, of course). I would appreciate some suggestions. This is my way of breaking...
6. ### How does sound intensity affect the current produced by a mic?

I measure sound (dB) from a speaker at 0cm and 100cm using a diaphragm microphone. 0cm-100dB, 100cm-30dB diaphragm microphone cinsists of a diaphram, coil and a permanent magnet. the coil is attached to the diaphragm, when a sound wave hits the diaphragm it causes to move back and forth which...
7. ### 6 dB of IF response curve....?

Hi folks, My query is that... As one can see that in the IF response curve the picture carrier is placed 6db down the amplitude curve. Such a placement modifies the output of video detector in such a way that the picture carrier lies at half point or 50 % of the total amplitude. Now this whole...
8. ### Finding the magnitude of the voltage gain in dB

Homework Statement Hi, so in a MOSFET amplifier question(a common-source amplifier in particular), we are examining the effects of the parasitic capacitances of the MOSFET when included in the small-signal model. In particular, the gain of the amplifier with these capacitances. So I derived the...
9. ### Pitch and amplitude of sound wave Vs its volume

Hi. If I know the pitch and amplitude of a sound wave, will I be able to calculate its volume. I can understand volume of devices vary betwerb brands and other categories. For the sake of discussion, let's assume volume to be a consistent unit or if db is the right unit, let's take that.
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### Questions about finding dBm values

Homework Statement 1. A sinusoidal source with RMS voltage V = 11Volts across a resistor R = 50Ohms, determine the dBm value. Hint: you will first have to determine the power delivered and then check the formula for dBm calculations. 2. A type of coaxial cable has a loss of 12dB/100m at some...
11. ### Monitoring a 24 kHz frequency, logging a digital dB reading

Hi, I had a project in mind but I'm not sure its possible, or what I might need. Current Setup: A small loop antenna monitors a 24kHz (VLF) radio signal, and passes it to an amplifier (schematic can be found here: https://www.physicsforums.com/threads/schematic-symbol-help-supersid.709128/)...
12. ### What is the relationship between power in watts and power in dBW?

Homework Statement A stereo amplifier is rated at 130W output at 1150Hz. The power output drops by 10.0dB at 15.3kHz. What is the power output in watts at 15.3kHz? Homework Equations R=I/Io Io = 10^-12 w/m2 bel = log(r) The Attempt at a Solution I have no idea where to start
13. ### Algorithm for dB vs time graph for one frequency from FFT?

I'm doing a physics experiment for school, for which I am measuring the reverb time for specific frequencies in a room. What I did was record a 1000 Hz sound, and some time after it, and looked at its FFT on Audacity to see the intensity of just 1000 Hz at a given time. Manually, I can do this...
14. ### Help importing Northwind db to mysql

Hi, I am having problems importing the Northwind database https://msdn.microsoft.com/en-us/library/bb399411(v=vs.110).aspx into mysql on my machine. I'm running wamp64 and mysgl ver 5.7.14. When I download the files, I obtain instnwnd.sql file. However, when I use phpMyAdmin and attempt to...
15. ### Units of sound pressure level, example: 120 dB re 20 μPa

I am trying to do a calculation using a value of -211dB re uPa/V , are these SI units? i.e. if OCV = -211db re uPa/V and I have RL = 20log(Vrms) - OCV Do I need to convert the OCV to standard units before putting it in the equation? Wiki isn't so clear about this:
16. ### B Difference Between dB and dBA Scales Explained

Please can someone explain the difference between the decibel (dB) scale and the adjusted decibel scale (dBA scale) and why the dBA scale is used in preference to the dB scale. I've been stuck on this for ages and can't seem to understand the difference
17. ### Renaming Columns in DB Table: Apex SQL or Examine Dependenci

Hi All, Just curious as to what those with experience in creating databases believe is the best method when renaming databases: using Apex SQL , which (supposedly) automatically checks for dependencies or just sitting down , looking at the dependencies and doing the changes one by one as needed...
18. ### Big Data and RDBS (Relational DB). Do They Fit?

Hi All, I am having trouble seeing how Relational Databases (RDBS) can be used in the world of big data. The inflow of data seems to be way too fast for the database to reflect what is going on at a given moment. I understand this issue is supposed to be addressed by data warehouses. Is...
19. ### Relational DB: Examples of When 1:1 Should be Kept Seperate versus Together

Hi All, The "best practices" in RDB on 1:1 relations between entities are that the two entities E1, E2 should be merged into a single one by default, i.e., unless there is an overwhelming reason to keep them separated. Can someone provide illustrating examples of when the two entities...
20. ### Redundancy and Inconsistence in Relational DB

Hi All, I am given this relational table ROOM (RoomID, BuildingName, FloorNumber, NumberOfFloors). The underline and bold mean that RoomID is the primary key. And I am asked to show an example of how redundancies may result in inconsistencies. This is a questions someone asked, hoping for my...
21. ### Relational DB :Relation-type Algebra?

Hi, say we have entities A,B,C in a Relational Database. Is there a sort-of operational algebra that allows us to determine that if, say, A and B are in a many-many relation and B and C are in a 1-many relation, then A,C are in a certain relationship, i.e., a composition algebra between...
22. ### Mirrored pattern in antenna dB vs frequency, explanation?

Hi, my high school astronomy class designs/builds antennas to detect VLF radio for solar flare detection. Recently, and regardless of their design, we get a strange "mirrored" shape to the dB vs frequency plot. The shape of the graph before roughly mid-band is exactly mirrored about the...
23. ### Fixing Corrupt Access 2003 DB: Error "Database Engine Could Not Find MSysDb

Access 2003 database. Unable to open - error message "database engine could not find object MSysDb".Can not JetComp it; can not import tables into a new db; can not open in Access 2007; can not compact & repair.
24. ### How Do You Calculate Decibel Intensity at a Distance in Spherical Waves?

Homework Statement The intensity of a certain spherical wave is 8.0 W/m^2 at a distance of 1.0 m from the source. If the medium is isotropic and nonabsorbing, calculate the decibel intensity 100m from the source. Homework Equations β=10ln(I1/I0) The Attempt at a Solution I know how to use...
25. ### How to find the total dB of two sounds?

Homework Statement The frequency of the two sounds is the same, and the observer is equal distance from either sound source. When one speaker is turned off the loudness is 70dB, what is the total loudness when it is turned back on? Homework Equations A2∝I dB=10log(I/I0) The Attempt at a...
26. ### Solve Ohm's & dB Homework: Find Volt, Current, Load Res.

Homework Statement A power amplifier increases a signal’s voltage by 42.04 db and increases the output power by 70.0 db. the input signal is -10 dbv and the input resistance is 10kohm. a. What is the output signal amplitude in dB? b. What is the output signal amplitude in volts? c. What is the...
27. ### Why not just use B rather than dB?

Hi, My question is very simple but I have not found on google. Why Decibel unit employs Deci? I know Deci = 0.1, but why not just use B rather than dB? Is there some historical reason? Thank you, enjoyed the forum, it is my first post.
28. ### Cartesian or Polar Coordinates to store intergalactic objects in DB?

So I'm wondering, should I use Cartesian or Polar Coordinates to store intergalactic objects in DB? I'm currently prototyping a game idea that can be oversimplified as a spaceship simulator in infinite space. I'm considering grouping objects together so that they have a "parent super-space"...
29. ### Applying a gain of dB to a signal (function) help?

Homework Statement Homework Equations The ratio of the parameters \frac{E_1}{E_2} to find the gain (in dB), 10\timeslog10\frac{E_1}{E_2}. "To determine the gain factor in part 8, first think about what happens to the energy when you multiply the signal by a constant factor. Then use the...
30. ### MHB Sq. FD + sq. DB = sq. BF Almagest Ch 10 Book I Ptolemy

Hello, everyone I am reading through the Almagest by Ptolemy and I have a question I hope someone can resolve for me. In Book I Chapter 10 of the Almagest, Ptolemy sets up a semicircle ABC with two right triangles on the diameter AC. To view the text just click the link below: Claudius Ptolemy...
31. ### What is the acoustic power output of a blue whale in Watts?

I didn't think it would be this hard to find the answer to this, but 1 hour of googling has come up with nothing that I can make sense of. The main cause of my confusion is that there's about a dozen different ways of measuring sound power in decibels with different units or reference powers...
32. ### Bandwidth at resoance and peak dB level

Hi, I'm investigating what happens to bandwidth at resonance of a series RLC circuit when each component is doubled. i have simulated the circuits and created a bode plot. You can see from the photos that at resonance the gain peaks at -20.82 dB. I would like to know how to calculate this...
33. ### Confusion with dB equation - 10 or 20?

how do you know when to use dB= 20log(ratio of input/output quantities) or dB=10log(ratio of output and inout). voltage seems to be 20, power seems to be 10... but how about when doing Fourier transforms and you want to find out 3dB of maximum, for example? thanks
34. ### Writing Uncertainty in terms of dA, dB, dC

Homework Statement Write down formulae for dX in terms of dA, dB, dC if: X = [(A) (B^2)] / C Homework Equations I'm using the formula below as this corresponds to this relationship. (dX/X)^2 = (dA/A)^2 + (dB/B)^2 The Attempt at a Solution For the first line, not the...
35. ### Having trouble understanding the dB formula

I know that to work out dB the formula is: dB = 10*log(P1/P2 I just don't get what the P1 and P2 are exactly but I know that dB is a ratio that expresses 2 values? And also, how do you compare two values of dB. Say, there's 80dB and 105dB, how do you work out how many times louder is the 105dB...
36. ### Signal to Noise ratio dB levels

Hello, I have some 10 signals in noisy and nonoise(noisy+good) situations. I just calculated the power of each signal in two cases and then used SNR formula for dB(10log noisy/nonoise). Now, I got noise levels in dB's. Here, i confused that these dB levels are referred to...
37. ### Deriving a formula for calculating dB based on distance

Homework Statement Decibel level from source at 1.0 m is 60.0 dB Find dB level at distance 2.0 m. Homework Equations dB = 10 log10 (I/I0) <- this one I know but couldn't see an application. Forgot to add that I0 is equal to 1.0 x 10^-12 W/m^2 dB = dBi - 20 log10 (r2/r1)...
38. ### Relation between dB loss and percentage loss

I want to know generally what the relation between dB loss and percentage loss is. If I want to know what percentage of a signals strength is lost knowing what its dB loss is, how would I accomplish that. Thanks.
39. ### Noise Power Calculations in dB or dBm

The ourput Noise Power is given as No = kTsBG + kTeBG ...= k.(Ts+Te).B.G But when it is asked in dB or dBm, How to calculate it something like 10Log10(Value) or 20Log10(Value)
40. ### Convert cm^-1 to dB: Confirm Formula

Hi, i have found a formula (in Waveguide Loss Measurement Using the Reﬂection Spectrum, PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 16, 2008) to calculate losses in semiconductor structures such as waveguide. I want to compare the result with another method (described in Photonic crystal...
41. ### Converting sound intensity to dB

Homework Statement The sound at a concert is measured to be 0.5 W/m^2. How many decibels is this? Homework Equations β = 10ln(I/I_0) I/I_0 = 10^(β/10) The Attempt at a Solution I_0 = 1*10^-12 W/m^2 I = 0.5 W/m^2 Therefore, β = 10ln(0.5/10^-12) β = 269.378 dB However, .5/10^12 = 10^(β/10)...
42. ### Comparing Sounds: 80 dB, 30 dB, and 50 dB

Homework Statement How many times quiter or louder is a sound at 1. 80 dB 2. 30 dB than a sound 50 dB Homework Equations dB2=dB1-dB2? The Attempt at a Solution Would this just mean 1. 30 dB louder 2. 2 dB softer? Or how would I finds "times louder". Sorry for bad...
43. ### Understanding the Relationship Between Voltage Gain and Power Gain in dB

Why is voltage gain in db given by 20 Log (v2/v1) but power gain is straight forward : 10 Log (p2/p1).
44. ### Find the Ratio of Sound Intensity with 40 dB Reduction | DB Ear Plug Guide

[b]1. When you buy earplugs at the store, they have a "dB rating". What it tells you is the reduction of SIL that the plugs will provide, if properly worn. Suppose you're helping some friends, working the stage at a rock concert wearing earplugs that provide a reduction of 40 dB. What is the...
45. ### How Does Voltage Affect Decibel Levels?

does dB increases when voltage increases?can sum1 state or declare a formula or proof?
46. ### Converting dBm to dB: A Simple Guide

How do I convert a value in dBm to dB?
47. ### DB loss for a 3GHZ EM wave traveling through 2 meters of a medium

Homework Statement What is the dB loss for a 3GHZ EM wave traveling through 2 meters of a medium with ϵ=1.5ϵ_0 and loss tangent = 9E-4? Homework Equations Umm...I'm actually not sure. I can't find anything really relating these things at all. The Attempt at a Solution My first...
48. ### How Do You Convert dB to W/m^2 for a Test?

Homework Statement I have a test coming up and I need to be able to convert dB to W/m^2 and vise versa. Could someone please explain to me how to do this? Homework Equations B=(10dB) log_10(I/I_0) I_0 = 1.0*10^-12W/m^2 The Attempt at a Solution Google, came up with nothing
49. ### Calculating Total Sound Intensity and Combined Sound Level: Ice Show Example"

Homework Statement A family ice show is held at an enclosed arena. The skaters perform to music with level 80.0 dB. This is too loud for your baby, who yells at 75.0 dB. (a) What total sound intensity engulfs you? (b) What is the combined sound level?Homework Equations dB = 10log(I/1E-12)...
50. ### Calculating Sound Levels in dB: How to Convert Intensity to Decibels

Homework Statement Calculate the sound level in decibels of a sound wave that has an intensity of 4.00u "W/m2. Homework Equations dB = 10log(I/Io) The Attempt at a Solution dB = 10log(4/1E-12) = 126.02 db Book says 66db. It vaguely covers this topic, not sure if I had missed...