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?
How about for taking the actual measurement, should I be using a single test tone, or pink noise?
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 !
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 !
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.
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.
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
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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Joined 2003
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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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.
What I am saying is not that your equation was wrong, just that it is worthless without describing how to use it. In this case, if you were to plug Vas in Liters into your equation you would get the wrong answer....people aren't likely to mistake the units of Fs, and as you state Q has no units
Most drivers are made overseas, now, and most of the world (other than the US and Myanmar) use the metric system of units. So most T/S parameter sheets quote Vas in Liters, or sometimes cubic meters...
The equation pasted from wikipedia also has no units specified, and in order to apply it correctly you will want to read the article.
A formula is a hammer, honed for a very particular nail. For some people, when all they have is a hammer, everything looks like a nail.
Most drivers are made overseas, now, and most of the world (other than the US and Myanmar) use the metric system of units. So most T/S parameter sheets quote Vas in Liters, or sometimes cubic meters...
The equation pasted from wikipedia also has no units specified, and in order to apply it correctly you will want to read the article.
A formula is a hammer, honed for a very particular nail. For some people, when all they have is a hammer, everything looks like a nail.
HK26147 said:
Thanks, I knew that much, but the issue is in applying the 1 watt. It's common for people to take a speaker rated as 8 ohms, set the amplifiers output voltage to 2.83 volts, and measure the level of a test tone at a known distance. I need to do this for a few reasons, one of which is setting the sensitivity of the mic when making measurements.
My concern is how you measure the voltage. If I simply apply a signal with no load on the amplifier, will the voltage be accurate once I attach the speaker load?
I can't do farfield spl measurements right now, 10 watts on my speakers puts me well in excess of 100db's, which is simply too loud to be measuring indoors. I also can't get 33 feet from my speakers indoors, so such measurements would have to wait until I could take them outdoors. None the less, I don't think it's needed.
What I usually do for a nominal 8 ohm speaker is play a pink noise signal thru the speaker with a VOM set on AC volts and hooked up across the inut terminals. I set the volume level so the meter reads as close to 2.83V as possible (it will be fluctuating a bit). 2.83 is the square root of 8. With the meter reading 2.83V I take an SPL reading at 1 meter on axis.
For a 4 ohm speaker, set the level to 2 volts.
For a 4 ohm speaker, set the level to 2 volts.
I thought you did, and I'm sure you know that there is no "standard" per se, and manufacturers and not above publishing vague, unqualified, "rosy" pictures.
What speakerdoctor just detailed is one of several techniques.
If you are testing indoors you are probably also aware of the problems caused by room reflections on the measurement.
Usually this is dealt with by using a gated signal.
Popular opinion is that "Testing Loudspeakers" by Joseph D'Appolito is the authoritative book on the subject of loudspeaker testing.
( Bet you knew that already
)
pjpoes said:
Measure voltage with the loudspeaker connected.
A farfield measurement is pretty much useless unless you want a reverberant SPL level for a given input. This will have more to do with the room and the speakers directivity than anything else.
I don't understand you reason for doing this, - for mic sensitivity adjustments, the figure you back out from T/S parameters is more than good enough. Maybe describe better why you are doing this and your measurement conditions.
speakerdoctor said:
Another waste of time in a silly thread. A typical meter will not give an accurate measurement with pink noise. 99% of meters are designed to measure house current and sinusoidal signals only in the range of ~50-500Hz.
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