Aryuserius?

How do I measure gain in dB?
« on: January 24, 2006, 12:52:36 AM »
I built a pair of 12*72s about 6 months ago and have never been happy with the gain I can get out of them.  Is there a simple way to measure how much gain in dB they actually put out?  

-Mike


gyraf

How do I measure gain in dB?
« Reply #1 on: January 24, 2006, 02:59:51 AM »
20*log(Vout/Vin)
..note to self: don't let Harman run your company..

bcarso

How do I measure gain in dB?
« Reply #2 on: January 24, 2006, 05:04:57 AM »
To be able to effectively use gyraf's equation, you need to have a known small a.c. voltage and apply it to the input.  Most meters don'r work well at low levels so start with something you can measure and divide it down with some appropriate resistors.  For example, take, say, a 1V rms 1kHz signal, as measured with your certified a.c. voltmeter, and feed it to a 10k resistor in series with a 10 ohm resistor to common.  At the junction of the two resistors you have 1 mV rms, which is at a low enough impedance (about 10 ohms) that you can safely assume it doesn't change much when you connect it to your input.  If it's a balanced input you can tie one side to common.

Measure the output of the unit under test with the same voltmeter.  Per gyraf's formula, divide that reading by (for this example) 1 mV.  Take the (base ten) logarithm and multiply by 20.  This is the gain expressed in dB.

Samuel Groner

    Z├╝rich, Switzerland
  • Posts: 2935
How do I measure gain in dB?
« Reply #3 on: January 24, 2006, 05:48:51 AM »
May I suggest that the source impedance should be around 200 ohm for that measurement? It might happen that the input impedance somehow got too low and we'd like to measure that.

Samuel

kvintus

How do I measure gain in dB?
« Reply #4 on: January 24, 2006, 01:46:09 PM »
Quote
...with your certified a.c. voltmeter...


This is important. Remember that most cheap multimeters can't measure high frequencies. Start with, say 400 Hz or something if you're not sure.

/Anders
...not perfect, but close enough for rock 'n' roll...


 

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