newecm.cal seems wrong

[warrensomebody]

Is the newecm.cal calibration curve really correct? All my measurements spike up from the 10k-20kHz range, exactly opposite to its compensation curve. At first I just thought it was that my tweeters needed some attenuation, but today while doing some measurements I did a full range sweep with only my subs engaged, and still found the curious rise in the 10-20k range, despite that the subs can't be producing any output there. Here's an example:

[thxgoon]

Could this be noise? Have you tried looking at a spectral chart of the noise without anything playing?

[brucek]

Is the newecm.cal calibration curve really correct?

Correct for the mic that was used in the actual calibration. Probably close enough for home use for other ECM8000 mics.

I did a full range sweep with only my subs engaged, and still found the curious rise in the 10-20k range, despite that the subs can't be producing any output there.

Didn't you just answer your own question. If there is no signal at 10K-20K, there is likely a flat level of noise floor and no signal that is boosted by the calibration. The purpose of the file is to compensate for signal from the microphone being low or high, not to modify your noise floor.

brucek

[Anthony]

Also, that calibration file is for the microphone pointing straight up. If you aim it at the target and use that cal file, you will likely see the effect you are describing.

I had the same problem with some speaker design work. I was showing that tail up right where the cal file went down. Unfortunately I was also trying to design a notch filter to compensate (for an effect that wasn't there).j

So bottom line: newecm.cal -- mic pointing straight up. Other cal files (that show a more flat top octave) -- mic pointing at target.

I have a thread in the mic/meter section that discusses this.

Only way to be sure is to have yours calibrated. But I understand the hesitation there.

Good luck,
Anthony

[warrensomebody]

Ah... mic pointing straight up. That's probably my problem. I'll try again tonight. Thanks.

[Anthony]

I was going to post this tomorrow, but here's a teaser:

BoomieMCT is coming over tonight with his ECM8000 and we're going to run a quick battery of tests using one of my Magnepans as a reference:

No cal file: upright mic
No cal file: horizontal mic
newecm.cal: upright
newecm.cal: horizontal
old ecm8000.cal: upright
old ecm8000.cal: horizontal

Then repeat for his mic. If we have time, I'll also measure my Neo3PDR tweeter with the different positions and overlay the manuf. plot.

In any case, the low frequency performance seems very consistent, regardless of orientation, so for sub calibration any cal file is probably fine.

[brucek]

Ah... mic pointing straight up. That's probably my problem

No, in this case, that's not your problem. It is as I stated before. You have no output from your subwoofer at 10KHz-20KHz to measure, so the graph shows the flat noise floor, not the microphone output.

brucek

[Anthony]

yeah, sorry. I agree with Bruce, if there's no signal there then it's just your flat noise floor being artificially raised by the cal file. Nothing to worry about.

Also, I tested the mics, and for bass stuff, I saw very little difference in response based on orientation. Above 800 Hz, changes start to occur, but below that, you can pretty much do whatever is most convenient.

[glaufman]

While I agree on the noise floor assertion, ideally, the proper mic cal file when listening to just the noise floor would yield a flat response...
As a quick check, and as a reference for any future full range measurements, could he run a scan while sending no signal to the amp (cable disconnected) to see what the mic+cal file actually reads as the noise floor?

[brucek]

the proper mic cal file when listening to just the noise floor would yield a flat response

Really? and if the typical noisy soundcard had a noisefloor that was higher than the microphone output with no stimulus, would you still think that to be true?

brucek

[glaufman]

brucek wrote:

Really? and if the typical noisy soundcard had a noisefloor that was higher than the microphone output with no stimulus, would you still think that to be true?

brucek

If the soundcard had a proper cal file? Why not?

[Anthony]

In the end this is all about voltage. When fed nothing, the microphone will produce no voltage (or a steady DC voltage, but steady is the key word). Now when a 1kHz tone is played at 80dB a voltage is produced. When a 10kHz tone is played at 80dB a different voltage is produced. They should be the same, but most of the time they are not. This is why you have a microphone cal file.

So when a cal file is subtracted from a 0dB response, you get the result you are seeing.

Now you are saying, but hey, what about background noise in the room? That noise is most likely very broad-band and unless you have machinery that emits a high pitched whine all the time, the mic is likely just picking up a fairly even white noise.

If the soundcard is noisy (or the mic internals) the noise floor may be above the ambient noise in the room! Then nothing would get picked up.

Also, the soundcard cal file is only affecting the frequency response, not the noise floor.

Noise is a floor that you cannot measure below. It is random, usually flat, but not always. The important thing is that you cannot correct for it, you just make it as low as you can (insulation, better sound card, repeated measurements) and then make the signal much higher so the noise does not matter.

Frequency response is predictable and correctable. Different correction values are required for different frequencies, but because that is not random and repeatable, you can correct for it.

In this case, we're seeing no signal, so just noise, and the calibration file is being applied to the noise after the fact.

One last thing,
I have another thread about this microphone. While I did not test subs, I pretty much show that orientation does not matter for the mic for frequencies below 3k or so. Your measurements are probably good and I would not worry about it unless you want to start measuring full range speakers.

[warrensomebody]

Anthony - Thanks for your explanation.

I know that this rise in the noise floor doesn't matter when measuring my subs, but I have also done a set of measurements of my tweeters (from a few mm away) using the same mic and cal file, and I see an identical rise in the response. The fact that this rise, in both the noise floor case and the close-mic'd high-frequency case, correlates with the dip in the calibration curve says to me that it really is that the calibration curve isn't correct for my mic.

So I'm tempted to go into the newecm.cal file and flatten out those datapoints so that I don't have to look at the rise anymore... unless this is a bad assumption, and it's really not possible to know whether the rise is real or calibration-related. ?

[glaufman]

warrensomebody wrote:

Anthony - Thanks for your explanation.

I know that this rise in the noise floor doesn't matter when measuring my subs, but I have also done a set of measurements of my tweeters (from a few mm away) using the same mic and cal file, and I see an identical rise in the response. The fact that this rise, in both the noise floor case and the close-mic'd high-frequency case, correlates with the dip in the calibration curve says to me that it really is that the calibration curve isn't correct for my mic.

So I'm tempted to go into the newecm.cal file and flatten out those datapoints so that I don't have to look at the rise anymore... unless this is a bad assumption, and it's really not possible to know whether the rise is real or calibration-related. ?

If it was me, here's what I would do (you've done some of this already)...
1. Use your setup to measure the noise floor... first with amp off...
2. Then measure again with amp on, and set to reference level, but scan with no input to the amp...
If 1 shows no HF rise, but 2 does, your reading accurately the noise floor of your amp...
If 1 shows an HF rise, I would...
3. Go to another location and run the scan again...
If 3 shows no HF rise, you're accurately reading a rise in the noise in your theater...
If 3 shows the SAME rise, you can be reasonably certain the cal file is not matched to your mic, and you can either manually adjust, or get a file made for your specific mic...
Of course, in this case, I would like to test in my setup again with someone else's mic, just to double check, but I'm funny that way...

[Anthony]

Yeah, if you are doing tweeter measurements, you will need some sort of calibration.

Even if it's measuring against a known speaker or tweeter, it will help you get close to actuality. Of course, nothing is better than getting your actual mic calibrated, but I understand hesitating there ($120 calibration on a $50 mic )