Measuring a speakers efficiency

[Ron E]

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.

[pjpoes]

Originally posted by HK26147

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.

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.

[speakerdoctor]

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.

[HK26147]

I knew that much,

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 )

[Ron E]

Originally posted by pjpoes
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.

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.

Originally posted by speakerdoctor
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

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.