Daqarta
Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows
Science with your Sound Card!
The following is from the Daqarta Help system:

Features:

Oscilloscope

Spectrum Analyzer

8-Channel
Signal Generator

(Absolutely FREE!)

Spectrogram

Pitch Tracker

Pitch-to-MIDI

DaqMusiq Generator
(Free Music... Forever!)

Engine Simulator

LCR Meter

Remote Operation

DC Measurements

True RMS Voltmeter

Sound Level Meter

Frequency Counter
    Period
    Event
    Spectral Event

    Temperature
    Pressure
    MHz Frequencies

Data Logger

Waveform Averager

Histogram

Post-Stimulus Time
Histogram (PSTH)

THD Meter

IMD Meter

Precision Phase Meter

Pulse Meter

Macro System

Multi-Trace Arrays

Trigger Controls

Auto-Calibration

Spectral Peak Track

Spectrum Limit Testing

Direct-to-Disk Recording

Accessibility

Applications:

Frequency response

Distortion measurement

Speech and music

Microphone calibration

Loudspeaker test

Auditory phenomena

Musical instrument tuning

Animal sound

Evoked potentials

Rotating machinery

Automotive

Product test

Contact us about
your application!

dB From Voltages

Although dB is technically a ratio of power levels, it is commonly used with voltages by assuming that both voltages are driving a constant resistive load, such that:

power = voltage^2 / resistance

So if we have 2 different voltages V1 and V0 and resistance R, the R drops out:

 
dB = 10 * log10((V1^2 / R) / (V0^2 / R))

   = 10 * log10(V1^2 / V0^2)

   = 10 * log10(V1 / V0)^2
 

Then using the property that the square of a log is equal to twice the log:

 
dB = 10 * 2 * log10(V1 / V0)

   = 20 * log10(V1 / V0)
 

We can rearrange the dB formula to get the voltage ratio from a specified dB value:

 
V1 / V0 = 10^(dB / 20)
 

This assumption of constant resistance may not always be true, such as when the voltage is driving a complex load like a loudspeaker whose impedance changes with frequency. But it is still proper, for example, for a manufacturer to assume constant resistance when discussing sound card amplifier performance, since there are no standards for loudspeaker impedance versus frequency... and there are a lot of different loudspeakers on the market.


See also dB, Typical dB Applications, RMS "Sum" of dB Values, Working With dB, Absolute dB (SPL, etc.)

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