[lsiberian]

I asked you in my build thread to explain to me how to do it, and I'd see what I could do.

I'm not familiar with all the different types of measurement.

In my thread, I think I've proved these measurements are as accurate as I can do with the equipment I have. I re-measured everything at the cone, and at seating position, and it came up pretty close.

So how do I do a ground plane measurement?

Basically if you have a flat patch of land. you lay a microphone at 2m in front of the sub on an open area of ground.

The difficulty is getting a quiet enough area to do the measurements. I didn't see your other post so please forgive the miss. :whistling: Illka does this type of measurement sometimes.

http://www.audioholics.com/education/loudspeaker-basics/subwoofer-measurements/page-2

shows how to setup a ground plane test if interested. :T

I'm not doubting your data just interested in seeing how it would compare to some of the other beasts.

 

[Lonely Raven]

Basically if you have a flat patch of land. you lay a microphone at 2m in front of the sub on an open area of ground.

The difficulty is getting a quiet enough area to do the measurements. I didn't see your other post so please forgive the miss. :whistling: Illka does this type of measurement sometimes.

http://www.audioholics.com/education/loudspeaker-basics/subwoofer-measurements/page-2

shows how to setup a ground plane test if interested. :T

I'm not doubting your data just interested in seeing how it would compare to some of the other beasts.

Ok, I've seen his measurements.

I'd have to figure out how to make my setup portable, and I'd have to be able to fit the box in my car...it's cutting it kinda close. Plus I'd need to find a parking lot that happens to have a power outlet I can borrow.

Being winter, I'm doubting I'll get this done anytime soon...sorry.

 

[Kevin Haskins]

Ok, I've seen his measurements.

I'd have to figure out how to make my setup portable, and I'd have to be able to fit the box in my car...it's cutting it kinda close. Plus I'd need to find a parking lot that happens to have a power outlet I can borrow.

Being winter, I'm doubting I'll get this done anytime soon...sorry.

You also have to find a parking lot that is a good distance from any nearby road. You can easily pick up significant amounts of noise at low frequency that isn't audible but will show up in the measurement. I cannot do distortion measurements in my parking lot due to the highway 0.25 miles away. I can sit and see 60-70dB noise at 30-35Hz. Hard to get good distortion measurements with that type of background noise.

Kevin Haskins
Exodus Audio

 

[Ryan Anderson]

hahaha I cant wait to see a vid of you taking measurments of this in a parking lot... oh you gotta!

 

[Ricci]

You also have to find a parking lot that is a good distance from any nearby road. You can easily pick up significant amounts of noise at low frequency that isn't audible but will show up in the measurement. I cannot do distortion measurements in my parking lot due to the highway 0.25 miles away. I can sit and see 60-70dB noise at 30-35Hz. Hard to get good distortion measurements with that type of background noise.

Kevin Haskins
Exodus Audio

I get that sort of thing in my home and my practice area too. I've got noise issues period. That's why I always do my sweeps at a solid drive level of 95-100db to get up out of the background garbage as much as possible.

 

[DanWiggins]

I get that sort of thing in my home and my practice area too. I've got noise issues period. That's why I always do my sweeps at a solid drive level of 95-100db to get up out of the background garbage as much as possible.

Note that with a background noise level of 70 dB, you will have a minimum 3% THD in any measurement made at 100 dB SPL. For every 20 dB of SNR you cut the N portion of THD+N (what we're really measuring) by a factor of 10. So noise that's 20 dB lower than the signal is 10% THD, 40 dB lower is 1% THD, and 60 dB lower is 0.1% THD. You really need a very quiet environment - a background level hopefully no louder than 35 dBA - to do accurate THD measurements.

 

[Ryan Anderson]

Note that with a background noise level of 70 dB, you will have a minimum 3% THD in any measurement made at 100 dB SPL. For every 20 dB of SNR you cut the N portion of THD+N (what we're really measuring) by a factor of 10. So noise that's 20 dB lower than the signal is 10% THD, 40 dB lower is 1% THD, and 60 dB lower is 0.1% THD. You really need a very quiet environment - a background level hopefully no louder than 35 dBA - to do accurate THD measurements.

Hi Dan!

 

[bjs]

Note that with a background noise level of 70 dB, you will have a minimum 3% THD in any measurement made at 100 dB SPL. <snip> You really need a very quiet environment - a background level hopefully no louder than 35 dBA - to do accurate THD measurements.

There are many different algorithms (stimulus/processing) for measuring distortion. Only 30db of SNR requires one to use an algorithm optimised for low SNR enviroments. ARTA's stepped sine is excellent for this and is not limited to a minimum of 3% THD in the above scenario.

 

[DanWiggins]

If you're using pure tones - stepped or swept - you are getting a THD+N measurement; if you're using a sampled output (such as ARTA or any soundcard based system) you also get aliasing of the harmonics which will increase the susceptibility to noise.