Measuring speaker with mic

johnr66 · 22nd September 2011, 05:37 PM

I took an 8 ohm speaker (in cab) and measured the voltage from a mic & op amp circuit @1m. Now, I placed a second identical speaker next to the first and connected it in parallel so the amp is outputting double the power (4 ohm total load). The output from the measuring circuit has nearly doubled in voltage.

With two speakers, the sound pressure level has doubled which means +6db over the single 8ohm speaker. 2x of the voltage reading would seem to indicate a +3db increase. To my ears, there was a significant increase in sound. Okay, so what am I missing?

kipman725 · 24th September 2011, 06:34 PM

Your thinking of dB in power terms your result does show a 6dB level increase. formula for voltage gain is 20*log10(vout/vin) therefore your doubling in voltage level is +6dB. This is because your microphone doesn't measure power but the amplitude of the sound pressure.

Decibel - Wikipedia, the free encyclopedia

paulspencer · 25th September 2011, 12:34 AM

John,
You're seeing a textbook increase. 3db due to double the cone area - an efficiency increase. 3db due to twice the power.

22nd September 2011, 05:37 PM	#1
johnr66 diyAudio Member Join Date: Mar 2009	Measuring speaker with mic I took an 8 ohm speaker (in cab) and measured the voltage from a mic & op amp circuit @1m. Now, I placed a second identical speaker next to the first and connected it in parallel so the amp is outputting double the power (4 ohm total load). The output from the measuring circuit has nearly doubled in voltage. With two speakers, the sound pressure level has doubled which means +6db over the single 8ohm speaker. 2x of the voltage reading would seem to indicate a +3db increase. To my ears, there was a significant increase in sound. Okay, so what am I missing?

24th September 2011, 06:34 PM	#2
kipman725 diyAudio Member Join Date: Jun 2007 Location: London/N.lincs	Your thinking of dB in power terms your result does show a 6dB level increase. formula for voltage gain is 20*log10(vout/vin) therefore your doubling in voltage level is +6dB. This is because your microphone doesn't measure power but the amplitude of the sound pressure. Decibel - Wikipedia, the free encyclopedia __________________ CHR-70+Subs

25th September 2011, 12:34 AM	#3
paulspencer diyAudio Member Join Date: Sep 2003 Location: Melbourne, Australia	John, You're seeing a textbook increase. 3db due to double the cone area - an efficiency increase. 3db due to twice the power. __________________ AUDIO BLOG \| Bass integration guide My work: www.redspade.com.au web design studio

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