# Tagged Questions

Anything related to the decibel (dB) and similar logarithmic units of measurements (dBm, dBW, etc.).

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### conversion dB into dB SPL

I'm using a sound calibrator (1kHz, 114dB SPL, GRAS Type 42AB). When I got my data from my scope, the signal has 5mV. In spectrum mode, it is -50dB at the peak of 1kHz. I'm using this preamp: GRAS ...
135 views

### Exact meaning of dB?

I know that when people say that a gain of amplifier is X dB, they mean that 10*log(Pout/Pin). However, How can I interpret it when they say that "noise floor is -120dB"? How can I derive voltage ...
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### Relation between bels and decibles

Following is a sreenshot from "Fundamentals of electric circuits by Alexander and Sadiku". In 14.4 he defines "bels" and then in 14.5 the "decibels". I don't understand the line just above 14.5 that ...
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### Understanding microphone sensitivity

My goal is to figure out a relation between output voltage and input SPL: $$dB(SPL) = f(V_{out})$$ First, I get the sensitivity in volts from this formula: Sensitivity_{dB(V)} = 20 * log_{10} (...
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### How to lower volume on microphone input before outputting through headphones? (Using a DSP) [closed]

As part of a senior engineering course, my partner and I decided on tackling the problem of hearing loss in youth, and went with a more preventative approach, trying to reduce the amount of cases ...
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### Beacon: dBm power decrease over distance

I have beacons which advertise their configured data periodically through Bluetooth. The vendor specifies 8 different power options 0 to 7: https://support.kontakt.io/hc/en-gb/articles/201621521-...
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### Unity Op amp Question Gain and Resistances

The Question asks what is the gain in dB and also what are the input and output resistances I know the gain is 1 so dB that would be equal to 0 dB But I am confused on how to find the input and ...
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### Formula for amplitude of volatge, decibell

There is a formula for power 1 and power 2 to convert to decibell. 10*log(P1/P2), but when I have a amplitude of voltage in DC, formula is 2*10*log(U1max/U2max). Why is there a 2? Thanks.
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### average of dBs or dB of average?

In several papers, books etc. I have encountered to expression of "average gain of the path is ... dB", now I wonder this is the average of dBs or dB of averages? The sources have not mentioned ...
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### How to get output voltage of microphone?

I am really confused at this moment. I have the following microphone: http://www.farnell.com/datasheets/1660938.pdf What I want to do is make a actual dB meter with this sensor. So I would net to ...
767 views

### Types of decibels

Why we use dBA or any other type of decibel unit instead of basic decibels unit (dB)? Why we have so many types of this unit?
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### Is it voltage or power of the signal amplified when the opamp amplification is given in dB unit?

Today, one of my colleague said that he entered a preliminary examination in a company after his job application. One of the questions is exactly "Design a non-inverting amplifier with 20dB gain.". ...
217 views

### Are ten 2W speakers as loud as two 10W speakers?

If all things are the same (e.g. total magnet weight, manufacturer, cone material) and I have two 10W speakers as compared to ten 2W speakers would the decibel level be the same?
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### Frequency to dB

As I understand for a sound wave the intensity is determined by the amplitude, frequency and properties of the space, but for amplifiers I have seen that the gain is expressed in dB and is a function ...
1k views

### Is the meaning of a Decibel different when applied to Voltage vs Watts?

If I have an amplifier that provides +3dB, or an attenuator that provides -20dB, then how do I know if that applies to Voltage or power? In other words, if I have a 0dBV signal and I run it though a +...
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### How dBuV value changes with distance?

I've got an emissions source that's designed to generate a peak at a certain frequency that when measured by an antenna 3m away gives a value of X dBuV. If the emissions source if moved closer to ...
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### Why 10Log(p1/p2) and 20Log(v1/v2)? Why not just 10Log(x1/x2)? [duplicate]

Based on my very basic current understanding of decibels in electronics, it is said that I should use the db = 10Log(p1/p2) formula if I want to calculate the power ...
572 views

### Choosing sampling rate according to the bandwith reponse in accelerometer datasheet

In an accelerometer's datasheet, the bandwidth response in the x and y axis is 350 Hz. There is another information related to the internal sampling rate (shown below). I don't know how these two ...
1k views

### What is Sensitivity in an ultrasonic transducer Specification?

So far I understand that Sensitivity of the transducer is defined as the ratio of its electrical output to mechanical input. While going through ultrasonic transducer specs, they have a rated ...
715 views

### Does the VU meter on audio amplifiers measure the input or output level?

Does the VU meter on commercial audio amplifiers measure the level of the input signal or the level of the amplified signal?
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### G.711 for dummies

I am in midst of some online telephony course, and there are questions regarding which I have no one to ask but this community. G.711 volume encoding. I have some difficulty to understand how ...
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### Decibel vs Decibel milliwatts [duplicate]

What are the differences between Decibel (dB) and Decibel milliwatts (dBm)? I'm most interested in dBm with cell phone signal strength. Please give a detailed explanation. I'm not familiar with the ...
337 views

### -3dB on filters, just approximated?

I have been told that -3dB is when you get half the power or equivalently the original voltage divided by the square root of 2. Nonetheless, doing the calculations I get 3.01029dB. I figure this is ...
241 views

### mW or W when calculating decibels

In my instrumentation class we have been given the following formula to get the power in decibels: $10Log(Power(mW)/mW)$ I'd like to know if taking the power in miliwatts is something widely done ...
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### I'm confused by the formulas for power gain decibels and voltage gain decibels [duplicate]

I know that my problem with this is some flaw in my logic, I just need someone to point out to me where that flaw is. According to the Wikipedia article, there are different formulas because power ...
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### Calculating the voltage generated by an 40KHZ Ultrasonic Tranceiver

In particular, this device: http://www.hexamite.com/hx40str.htm ...
1k views

### Why do 16 bit systems have a minimum dBFS of -96?

I'm working through the example on this page: http://chimera.labs.oreilly.com/books/1234000001552/ch03.html I totally understand why the maximum level of an audio system would be 0 because the log of ...
5k views

### Why Decibels are used to measure Signal to Noise Ratio?

We just started a communications course in college and we came across SN ratio. The following is an ambiguity I am facing which my professor is unable to resolve: Signal to Noise Ratio is the ratio ...
574 views

### Relationship between FFT in dB and spectrum analyser output in dBm

I am trying to calibrate my RF daughter board in USRP software defined radio. I am trying to measure transmitted power for different frequencies at a certain set gain and transmitter amplitude. When I ...
2k views

### Confusion between Voltage gain & Voltage gain in decibels (dB)

The definition of voltage gain is $V_{\text{out}}/V_{\text{in}}$. However, I read some articles about the gain in decibels, and I have a confusion now. Here is an article about it: https://en....
751 views

### Headphones impedance and the SPL

The headphones A has an impedance of 32 Ohms. The headphones B has an impedance of 250 Ohms. Both of the headphones are similar by size. Is it right that the headphones B will play quieter by 250/...
3k views

### What does it mean for a signal to have an amplitude below 0 dB?

I am a software developer (using high level languages like .NET,C,C++ etc) trying to understand how computers work at a lower level. I understand that amplitude is always positive because it is ...
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### Power and decibels

In one of the examples in a engineering textbook, I noticed that the decibel value for the Power of a transmitter (also known as modulator) was 20DB and this was said to be equivalent to 100W. ...
766 views

### What do you call the dynamic range of a variable gain amp?

Dynamic range is the ratio of the largest signal a system can process to the noise floor of that system. If the system has variable gain, though, then you could interpret this two ways: Fixed gain ...
3k views

### Why do we use dB to represent the difference between two voltages?

For example, the passband of a LC resonant circuit is the differences of frequency at +3db and -3db. Why do we prefer dB?