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t0mm1 6th April 2012 08:57 AM

Measure sensitivity
 
Hi,

Some newbie questions:

I don't find any specs for my old 4" drivers and I need to measure the sensitivity since I'm going to replace them.

The impedance is 4 ohm.

I have a multimeter, a dB-meter and a 1 kHz sine wave sound source.

If I have done my math correctly, I have to measure 2V with my multimeter to get 1W out from my driver.

Two questions:

1. Should I measure DC voltage or AC voltage?
2. Should the driver be in its cabinet or outside while measuring?

Thanks!

t0mm1 6th April 2012 09:03 AM

Maybe it is 1.414V DC or 2V AC I should measure?

d a o 6th April 2012 09:06 AM

Hi t0mm1,

You need at Sound SPL meter, Lin, A or C for 1 kHz
Always AC in Audio
A ISO baffle but can't find a link

t0mm1 6th April 2012 09:40 AM

Quote:

Originally Posted by d a o (Post 2974792)
Hi t0mm1,

You need at Sound SPL meter, Lin, A or C for 1 kHz
Always AC in Audio
A ISO baffle but can't find a link

OK. AC then. Have I done my math correctly; 2V?

t0mm1 6th April 2012 11:30 AM

I measured 2V rms with my oscilloscope (since I think multimeters are designed for 50Hz)... and I only got about 50dB on 1 meter. I'm doing something really wrong I believe ;-)

AndrewT 6th April 2012 11:48 AM

2Vrms on an oscilloscope?
How?

Oscilloscope displays waveform.
The waveform amplitude is generally read as a peak to peak voltage (Vpp).

For a sinewave signal the Vpp to Vrms conversion factor is 2*sqrt(2) ~ 2.83.
That 2.83 factor is equivalent to 9dB

If you read 2Vpp from the scope then you need to add 9dB to your SPL reading to give a sensitivity @ 1kHz of ~59dB/2V @ your monitoring distance.

Alternatively, increase your test signal from 2Vpp to 5.7Vpp

t0mm1 6th April 2012 11:55 AM

Quote:

Originally Posted by AndrewT (Post 2974932)
2Vrms on an oscilloscope?
How?

Oscilloscope displays waveform.
Thge waveform amplitude is generally read as a peak to peak voltage (Vpp).

My HP 54600B calculates V p-p, V rms and V avg.
So, having a 1kHz sine wave and measuring the line I adjusted the volume until the scope said 2V rms.

Maybe I'm doing it wrong?

AndrewT 6th April 2012 11:55 AM

Wideband signal rather than a single frequency test signal.

But then you need and rms reading voltage meter to set the input signal.

t0mm1 6th April 2012 12:23 PM

I think it might be better to let some professionals measure it instead... there are still too many factors that can ruin the results for a newbie like me ;-)

d a o 6th April 2012 01:42 PM

2 Attachment(s)
Quote:

Originally Posted by d a o (Post 2974792)
Hi t0mm1,

You need at Sound SPL meter, Lin, A or C for 1 kHz
Always AC in Audio
A ISO baffle but can't find a link

Watt = P = U^2 / R
Watt = P = U * I
Watt = P = R * I^2

You need at True RMS AC volt meter to atleast 1 kHz
IEC Baffle and a SPL meter placed 1 meter in front of speaker

To get the RMS out of a Peak 2 Peak SINUS value = a / SQR(2) : Where "a" is amplitude (peak value)
http://en.wikipedia.org/wiki/Root_mean_square

It should actually do it for you


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