speaker measurement questions

speaker measuement questions

So I finally got my Radio Shack dB meter, and Rives test CD and have started a little testing. I've certainly got some results already. However I noticed a bit of discrepancy between measureing the two different speakers (as much as 4-5dB difference), even with speaker and meter in exactly the same places. I then noticed that the meter can really change depending on where I'm standing. This obviously happens to a greater extent at higher frequencies, but it seems to be effected as low as 250Hz. Is there any one place that is best to stand in a situation like this? I try and stand diagonally back behind the meter so as not to stand directly behind or to the side. Also my "listening" room is pretty small and also serves as my kitchen, dinning room, living room,and chicken brooding house for a couple of weeks. In other words, a little clutered. Would I get better results if I took them out onto the back deck? I live in the country and it is ussually pretty quiet. Obviously this would remove any room gain, but would it also be more accurate in the upper octaves as well. Thanks in advance.
Joe
 
You're trying to do electron microscopy with the magnifying glass attached to your Swiss Army knife.

To do what you want to do, you need to make the small investment for a good sound card (if you don't already have one), an electret-based test mike, an easy-to-build test jig, and free software like Speaker Workshop. Figure a $200 budget and a few weekends getting it all hooked up and working. But you'll have a great and versatile tool.

Search "Speaker Workshop" on the forum and you'll have tons of info to start sifting through.
 
Yeah, I thought about getting it all set up on my computer, but I ended up cheaping out. I'm running a Mac right now and I checked out Fuzz Measure, and mics and what not, but ended up cheaping out and getting the RS dB meter and test CD. So maybe I'll eventually have to break down and get a real measurement system, but for now I want to get the most out of what I have.
Joe
 

ScottG

Member
2003-02-04 12:23 am
US
bzdang said:
I think that I read somewhere that the RS spl meter has an output jack and can be used as a microphone.
?
If so, then get a cable and some software and go!
I use audiotester because I am able to make it work without overheating my brain.

Audiotester appears to be quite capable (..nothing to be ashamed about here).

in particular under the listed features of 1.4h (which I'm sure is included in the newer version) you have:
"Correction files to make the frequency response linear ( soundcard, microphon ... )"

Here is a calibration file for correcting the freq. response of the RS analog sound meter:

http://www.etfacoustic.com/micpreamp.html

If that doesn't work then you can always try to figure out the level differences for the freq.s on the rives test CD and manually adjust the curve in Audiotester.

If you want a MUCH better mic then consider the Behringer ECM8000 (which at around 50 US is a bargain). It isn't perfectly flat above 10kHz and it does need proper connecting plug/cord to go with your sound card. (or purchase one of their mic pre-amps.)

Also you can make your own from a calibrated panasonic mic capsule and some brass tubing and wire, solder, etc..
 
I have built a microphone with integrated preamp, and stereo, for music recording, based on the MCE-2000 ECM capsule which has a flat response from 20Hz to 22khz. In fact, it is a Panasonic ECM, I don't remember the number. All the parts, the shielded cable and the 3.5mm jack cost me 33 euros here in France for a stereo version, and it may cost as low as 25 dollars in the US !

The preamp is built arround a NE5532AN op-amp, and I used 2.2mfd MKT decoupling capacitors. Its purpose is to increase the gain by 1.2 , 2.5, or 60 for line-in plugin.

What I can say is I am really bluffed by its performance : the ECM capsules have a 60dB SNR, and as the preamp is placed 2cm behind the capsules, and shielded with aluminium sheet, there is almost no noise, and the dynamic range is excellent, due to the 2*9V dual rail power supply.

I plug it into my computer's soundcard, microphone level set on the minimum so I get no noise, and I use TrueRTA3 which is not free to obtain spectrums. It works really well !

I'll post photos later cause I'm at school right now :D
 
I noticed that you need a mic pre-amp to make it all work. Does anybody have any schematics on an easy to build mic pre-amp, or suggestions on a cheap well built one? Also the RS dB meter does have an out, if I used this would I just plug this into my computer, or do I need a mic pre-amp for that as well?
thanks for the education
Joe
 
No problem ! here is the schematic : http://www.geocities.com/ferocious_1999/md/micpreamp2.html

It's really simple to build : I only changed a few values because the gain was not strong enough for line-in driving, and I use 3 positions for gain instead of 2. I built it double supply rail but in fact it's not needed.
I used a philips NE5532AN which is an audio circuit, low noise opamp.

The preamp is not needed as far as you don't have noise issues. Using an external high quality preamp avoids using the soundcard's one which is not very good, and to avoid carrying a weak signal in a long cable.

I can give you more infos on my mic if you are interested :)

here's a photo, without the front panel which hasn't been built yet. : http://youyoung21147.site.voila.fr/DSCF2746.JPG
 

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
Use your RS spl meter via RCA out. Setting is C. Fast or slow setting is irrelevant.
The RCA output is healthy. Use a tripod. Set 100dB scale.
It will lead you very nicely to safe conclusions if you use my compensation file.
Extracted it after measuring with my calibrated mics through dedicated preamp vs RS set as above in WinMLS. But I recommend an RTA in your case.

"Radio Shack low cost condenser mic (Salas)"
"Hz" "Data"
20.0, 25
40.0, -2.5
50.0, -1.5
63.0, -1.5
80.0, -1.5
100.0, -2.0
110.0, -2.0
200.0, -1.5
1500.0, -1.5
1800.0, -1.0
2000.0, -1.0
2700.0, 3.5
2900.0, 3.5
3500.0, 4.0
4000.0, 3.5
5000.0, 3.5
6000.0, 3.5
7500.0, 1.8
8000.0, 1.3
9000.0, 0.5
10000.0, -0.3
11000.0, -1.5
12000.0, -2.5
13000.0, -4.0
14000.0, -6.5
15000.0, -7.5
17000.0, -5.5
19000.0 -8.0
20000.0 -12.0
 
Thanks everyone for all the input. I downloaded a copy of FuzzMeasure for the Mac, so I just need to get a couple more cables to get it all hooked up. I measured the built in speakers on my computer with the RS dB meter and it seems to work alright. I might go ahead though and build myself a preamp and mic. I looked around for the Mighty Mike project but could not find it. Here is a mic that looked like it works well http://www.speakerbuilder.net/web_files/Articles/diymic/diymicmain.htm and I might build the preamp that you built youyoung.
GM I checked out the link that you posted. I'm afraid that my electronics knowledge is pretty limited, I can build from a circuit diagram, but beyond that.... pretty limited. The opamp that you linked to looks like it is built just for this purpose, but I have not figured out how you would lay out a circuit involving this opamp. I still reading though.
thanks
Joe
 
couple of last questions. In my searching there seem to be quite a few references to Eric Wallin's mod's of the RadioShack meter, however I believe this is for an older model than I have. Also Eric Wallin's site does not seem to be up any more. So does anybody know if there are any modifications for the new RadioShack analog meter #33-4050?
Also Salas are the correction tables you posted for the new analog meter #33-4050? Also do you know what the corrections are for 20 and 25Hz?
Thanks
Joe