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From another thread, Maxmercy posted how to do it with REW.


Quote:
Originally Posted by maxmercy /forum/post/19510010


Use REW (new beta edition).


Use the RTA Function, set Mode to Spectrum, FFT Length to 131072, Averages to 4, Window to Hann.


Use the onboard tone generator in REW, fire up a sinewave. Set the THD calculator on (It'll open up a THD tab). Make sure the record button is on.


Increase the volume until you are around 5%-10% THD. Hear what it sounds like at different distortion levels and for different frequencies, set an audibility threshold for yourself (how much THD can you stand?). Make sure you have a voltmeter across the speaker terminals to monitor power going in, to make sure you don't fry the drivers. The difference between feeding a box 250W and 500W is only 3dB....


You'll be surprised what you'll tolerate down below 20Hz, where the harmonics are more easily heard than the main tone. Set for yourself a threshold, and see which box can get louder at a certain freq before reaching that threshold. The winner is the box that can go louder for the same THD down low, IMO.


I am quite interested to see the results....


BTW - I'd do this indoors v outdoors to see how much room gain you get....


JSS
 

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My measurement bench started with a Tektronix 4 trace 100 Mhz scope and then added an HP8903B and Earthworks M30 with mixing boards from Mackie and Alto. More recently I've joined the laptop measurement fray with Sound Easy. And very recently, I've tried out HolmImpulse. For the money (how does "free" sound?
), HolmImpulse is a no brainer. While my laptop farts occasionally and spits out junk and the HP spectrum analyzer is always precisely on the money - if you are approaching this as a hobby, it's really hard to do better than HolmImpulse. There are certainly others out there that will cost you (Liberty/Praxis,DRA Labs....yada yada) but they really aren't necessary for everyone. Money is often better spent on a decent SPL meter, microphone, and USB card when free measurement software is readily available.
 

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Measure the amp too....



Easy way? Get out your oscilloscope and look at the waveform at power into an appropriate load resistor. Distortion is typically pretty obvious.


No oscilloscope??? Though I've not done it, I understand that you can use REW or similar to measure an amp's response (within the audio frequency range). It is a little more convoluted (amp drives a resistor, signal gets into REW or similar with an appropriately designed voltage divider probe protecting the sound card) but it can be done.


This is risky though. Do this wrong and lots of the magic smoke comes out of expensive things, so if you don't know what you're doing, find someone that does to help you (not me....
).
 
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