Are comparisons odious?

[Chrisbee]

Hi

The purchase of an ECM8000/Xenon 802 today instantly provided a chance to confirm the accuracy of the various testing devices in my possession.

Using the correct REW cal files my Galaxy 140 so closely matches my ECM8000/preamp combo as to make the latter merely desirable luxuries.

You chaps obviously did a superb job on the cal files and these devices do seem very similar from example to example.

The problems start when I try to compare my old/new RS analogue SPL meters with the above two.

There is obviously some disparity here which is quite probably the result of a simple testing error on my part: I had similar problems in matching the RS curves when I first obtained the Galaxy 140. I have tried both C-weighting and disabled. The ECM8000 and Galaxy 140 both had C-weighting disabled for their tests.

Rather than further damaging my few remaining brain cells it is very likely that you can point straight to the problem.

My old and new RS meters have near identical curves despite there being 10 years in age between them. They do not seem to share the same curves with the other two more expensive devices despite careful loading of the correct cal files for each REW test. Close matching of the position of the various mic capsules [on the tripod at the listening position] was achieved with rubber bands.



Thanks.

[Chrisbee]

Retested with all three SPL meters. One old RS , one new RS and Galaxy 140.

I didn't bother with the ECM8000 today because the Galaxy is so close.

C-ticked and not ticked for each meter in turn.

Correct cal.files loaded for each meter after clearing cal.file each time.

I downloaded the latest cal files from the download area.



The two RS meters match well despite the 10-12 year age gap. (Upper green curves)

The Galaxy 140 and ECM8000 match within a hairsbreadth.

Neither pair agrees with the other.

I started out hoping this test would be a useful comparison to save people wasting money upgrading their test equipment unnecessarily.

[brucek]

I tried to come up with a reason why the Radio Shack meters don't compare well to the better Galaxy and ECM, but I don't have any idea.

The C-Weight must be enabled on the meters themselves. The ECM is a flat mic so it has no C-Weight switch.

The C-Weight in REW is only a factor when measuring outside the range of the REW calibration file.

Most meters and microphones are quite accurate at 80Hz and require little or no compensation, In fact you'll find all the cal files are basically zero at that point. Your comparisons should be normalized to 80Hz.
My RS meter is fairly close to my ECM and so are Sonnies, so I have no answer for you. Wish I did. We have had discussions about RS changing capsule suppliers at various times, so I suppose it makes the RS meters a crap shoot.

brucek

[DougMac]

Your dilema reminds me of an old Chinese saying:

"A man with one watch always knows what time it is. A man with two watches never knows what time it is."

Doug

[Chrisbee]

brucek

Thanks.

I feel this has to be related to the cal files.

Nothing else changes when the next meter is connected.



DougMac

Abandon hope all ye who enter here! (I have a lifetime's interest in clocks)

[thewire]

My theory is that the differnet mic except the RS meters are running on a different voltage and this is why they read different. Here is an example of with a digital RS meter mic plugged in to my sound card and (green) without. Turning off the mic has a similar result.

mic and no mic.jpg

[Chrisbee]

thewire wrote:

My theory is that the differnet mic except the RS meters are running on a different voltage and this is why they read different. Here is an example of with a digital RS meter mic plugged in to my sound card and (green) without. Turning off the mic has a similar result.

I am swapping the same 9 Volt battery between the meters to exclude random variations.

Here's an new REW graph for your entertainment. RS meter V Galaxy 140.

I hope the annotation is easily understandable.

CW and AW refers to meter switch weight settings.

Cal/No Cal refers to REW meter cal file loaded under Mic/Meter. [33-2050.cal]

[3ll3d00d]

cross-post from avforums for completeness

it looks to me like that meter needs the RADIOSHACK-33-2055_4050.cal file. This would take off 6dB @ 20Hz, ~13.5dB @ 10Hz and 10dB @ 7Hz which I reckon would make it map reasonably closely to the galaxy curve.

if so then this goes back to the "how do you know which cal file to use?" issue

[Chrisbee]

3ll3d00d wrote:

cross-post from avforums for completeness

it looks to me like that meter needs the RADIOSHACK-33-2055_4050.cal file. This would take off 6dB @ 20Hz, ~13.5dB @ 10Hz and 10dB @ 7Hz which I reckon would make it map reasonably closely to the galaxy curve.

if so then this goes back to the "how do you know which cal file to use?" issue

Well done!

I was using the 33-2050 cal file for my new RS meter.

It really needed the old meter cal file. 33-2055-4050

Another senior moment!

I'll post a properly annotated graph later.

ECM8000/Galaxy 140 and New-Old RS SPL meters.

For the moment here's just the Galaxy 140 and RS meters with the correct cal. files and 1M sweep.