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Old 29th January 2009, 12:19 AM   #1
pjpoes is offline pjpoes  United States
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Default Measuring a speakers efficiency

Ok please help me feel less like I'm losing it. When measuring a speakers efficiency, the 1w/1m spec is common. Voltage sensitivity, which I believe is more easy to measure, seems also very common. If I'm trying to measure it, I need to measure the voltage output of the amplifier feeding the speaker. I keep reading, and have been told, that I take this measurement with no load on the amp. This makes no sense to me, then the amp only sees the 1000 ohm+ load of the multimeter right, and the voltage delivered would be much much higher into an actual speaker load. Now, to get an accurate measurement, I figure having it hooked up to the testing load, i.e. the speaker, makes the most sense. To figure on 1 watt though is where things get trickier. Can I simply pick a frequency for which I have a known impedance (based on my impedance measurement), and measure the voltage at that frequency? Is there a more common and sensible way of doing this? Should I just use an 8 ohm dummy load to set the level, and then hook up the speaker?

How about for taking the actual measurement, should I be using a single test tone, or pink noise?
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Old 29th January 2009, 02:00 AM   #2
simon5 is offline simon5  Canada
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Well, I guess your multimeter can read current and voltage.

BTW, usually the multimeter impedance is 10 000 000 ohms. So, no impact at all when you put it in parallel with a 8 ohms speaker.

Set up with your computer or whatever, a tone generator to 1 kHz sinusoidal. Connect that signal to your amplifier, and then to the speaker you want to measure.

Place multimeter in parallel with speaker and check the voltage, place multimeter in series with speaker and check current... then you will find the impedance, so adjust until you get 1W when multiplying current and voltage... since you now know the impedance it's easy.

If you already know what's the impedance at 1 kHz, then you only need to check voltage...

Pink noise would be useful if you want to know the "average" sensivity/efficiency... but if you want to plot it, you need to know the specific sensivity/efficiency at that frequency, so a sinusoidal test tone is needed at the frequency you want to measure at.

Good luck !
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Old 29th January 2009, 03:01 AM   #3
HK26147 is offline HK26147  United States
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Am I mistaken or are you looking for reference efficiency?
If so I believe you would need to determine Fs, Qes, and Vas, and use this equation:

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Old 29th January 2009, 01:59 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Sensitivity is measured, hardly ever efficiency, which varies with impedance.

Sensitivity is per 2.83V/@1m/halfspace i.e. on a large or infinite baffle.

The amplifier is assumed to have near zero output impedance.
The amplifiers load is irrelevant to its output voltage.
A 100W amplifier is perfect into anything above ~ 1R @ 2.83V.

FWIW for a 8 ohm nominal driver sensitivity = "efficiency" as in dB/W.
For a 4ohm driver the sensitivity is often quoted as the dB/W figure
whilst in reality its dB/2.83V and dB/W is factually 3dB lower.

Real efficiency, as in actual watts per dB varies wildy in a complex
loudspeaker and can be gleaned from the frequency response
and impedance plot combined, but it does not mean a lot, e.g.
the efficiency at a sealed boxes bass resonance is very high.

/sreten.
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Old 29th January 2009, 11:51 PM   #5
Ron E is offline Ron E  United States
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Efficiency is a number between 0 and 1 that represent output / input, generally output power divided by input power. The exact number depends on impedance, and varies with frequency. It is not trivial to measure directly

Beranek, in his 1954 book "Acoustics", discusses the concept of power available efficiency, calculated from T/S parameters and using the voltage producing one watt into Rdc as a reference.

In practice it is usually just backed out from T/S parameters.
http://en.wikipedia.org/wiki/Thiele/Small
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Old 29th January 2009, 11:55 PM   #6
Ron E is offline Ron E  United States
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Quote:
Originally posted by HK26147
Click the image to open in full size.
An equation without units is worthless. In this case, I think you need Vas in cubic feet.
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Old 30th January 2009, 12:19 AM   #7
DcibeL is offline DcibeL  Canada
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Quote:
Wikipedia:
Click the image to open in full size.
The expression 4π2/c3 can be replaced by the value 9.523×10–7 s³/m³ for dry air at 25 °C. For 25 °C air with 50% relative humidity the expression evaluates to 9.438×10−7 s³/m³.
Better? It's a percent, but I think everyone already knew that.
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Old 30th January 2009, 12:27 AM   #8
pjpoes is offline pjpoes  United States
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Thanks guys. Sorry though, I mislead you, I meant to say sensitivity, not efficiency. How do you measure sensitivity, acoustically.
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Old 30th January 2009, 02:43 AM   #9
HK26147 is offline HK26147  United States
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Quote:
An equation without units is worthless.
That "equation" in one form or another has been around for a while ( in the original T/ S papers, as well ) and implemented in the 1st Box Response program, and as pointed out is expressed as a percentage.
And figures of merit have been around for a while that don't have "units".
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Old 30th January 2009, 04:06 AM   #10
HK26147 is offline HK26147  United States
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Quote:
How do you measure sensitivity, acoustically.
As sreten stated by applying 1 watt and read the SPL at 1 meter;
Obviously at the frequencies of interest and not just at 1khz - a common frequency used.
It is felt that it will lead to erroneous results when the data is used in far field calculations.
Some make a far field 10M 10watt measurement and then calculating back to a 1 watt / 1 meter SPL.
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