Using microphone to measure Db(A) and Db(C) ?

Hello,

I have two microphones:

1. A Fica CMP-202 computer microphone.

(I got it for free a long time ago from somebody.)

2. A Pyramid/Pole like microphone DM-A409 that came with my 7.1 Receiver 
(Denon AVR 1909).

(Bought this recently... seems quite an impressive microphone according to 
some charts it was able to capture ?)

Anyway I was wondering if these microphones are suited to do certain sound 
measurements in either Db(A) or Db(C). (Maybe use GoldWave for analyses ?)

I want to measure the sound level that is coming from the cars/trucks/busses 
outside.
(I want to measure the sound level outside and inside my living room).

Would these microphones be suited for the task do you think ?

I was also considering buying a special microphone like this one:

http://www1.conrad.nl/scripts/wgate/zcop_nl3/~flN0YXRlPTE5MDM3OTk1OTk=?direkt_aufriss_area=SHOP_B2C_Metering&~template=PCAT_AREA_S_browse&p_page_to_display=&catalogs_sub_id=sub12&aktiv=12&navi=oben_1

Product is called: "SL-400 decibelmeter " (in dutch)

Suppose the answer would be: "No your current microphones are not suited for 
the task..."

then I wonder what would be the difference between these microphones and a 
specialized microphone ?

Can anybody explain ?

Bye,
  Skybuck. 

Also...

I have another question:

Why is DB measured in negative/minus ?

Example:

-40 Db

(I see many examples of this for example the receiver volume level... and 
goldwave bar chart in volume/play control)

Bye,
  Skybuck. 

"Skybuck Flying" <BloodyShame@hotmail.com> wrote in message 
news:cbe57$4a99e85d$d53372a9$29846@cache2.tilbu1.nb.home.nl...
> Also...
>
> I have another question:
>
> Why is DB measured in negative/minus ?
>
> Example:
>
> -40 Db
>
> (I see many examples of this for example the receiver volume level... and 
> goldwave bar chart in volume/play control)

I am gonna gather some links here for further reading:

http://www.phys.unsw.edu.au/jw/dB.html

This link has one line about negative db... and some more... but it's a bit 
too technical for me at the moment ;)

"
It is also possible to have negative sound levels: - 20 dB would mean a 
sound with pressure 10 times smaller than the reference pressure, ie 2 &#2013266101;Pa.
"

Another link giving a technical description... so far it seems to be 
something technical, and it seems to be about "relative"... and it also 
seems quite important ! If it's not understood it could be meaningless ?!

http://www.answerbag.com/q_view/11521


"
A manufacturer may choose any method to describe their equipment. Unless you 
know what reference is being used, the results are somewhat meaningless.
"

There is also a secondary answer which is kinda interesting... it mentions 
the "dBFS" scale/measuring system.. specially for digital equipment... hmmm 
;)

And it gives a little 16 bit example...

Bye,
  Skybuck. 

Hmmm... this link starts out with writing the following:

http://arts.ucsc.edu/EMS/music/tech_background/TE-06/teces_06.html

"
The decibel (abbreviated dB) must be the most misunderstood measurement 
since the cubit. Although the term decibel always means the same thing, 
decibels may be calculated in several ways, and there are many confusing 
explanations of what they are.
"

It then goes on about great differences in measurements like 10 vs 
1.000.000.000.000.

It then goes on to explain some db system where logarithms are used for 
"convenience".

This is a great thinking-error by whoever invented it.

Human beings are famous for not understanding logarithms... so it doesn't 
really make it any more convenient... on the contrary it makes it much 
harder to understand.

Don't take my word for it... Watch this professor in action =D:

http://www.youtube.com/watch?v=F-QA2rkpBSY

Bye,
  Skybuck.

Never the less... this is case where a logarithm might make it easier for 
human beings to compare sound levels... since the ear seems to work 
logarithmetic-like as well... hihi.

But still it makes it a bit harder to compare all kinds of devices... "do 
they have a reference point or not ?" ;) "And what kind of systems are they 
using ?" ;)

Anyway wikipedia also has two interesting articles.

http://en.wikipedia.org/wiki/Decibel

One specifially about DB(A) which is used by law system here in Netherlands 
;)
(Most interesting parts are the critique on this system... and it also has a 
picture of a measuring device ;))

http://en.wikipedia.org/wiki/A-weighting

Still gotta read it... but now I must do something else... but will probably 
read this when I come back or at least have a good glance over it ! ;) :)

Bye,
  Skybuck. 

Skybuck Flying wrote:

> Also...
> 
> I have another question:
> 
> Why is DB measured in negative/minus ?
> 
> Example:
> 
> -40 Db
> 
> (I see many examples of this for example the receiver volume level... and 
> goldwave bar chart in volume/play control)

-40 dB is meaningless without specifying a reference. Take a look at this 
page, which you've already discovered:

http://en.wikipedia.org/wiki/Decibel

Another nice page with an online calculator and with some background 
information for audio levels:

http://www.sengpielaudio.com/calculator-db-volt.htm

As written in this page, e.g. the reference for dBu is 0.775 Vrms 
(sqrt(600/1000), as you can derive from the Wikipedia definition). An 
example:

-40 dBu
= 10^(-40/20) * 0.775 Vrms
= 10^(-2) * 0.775 Vrms
= 0.775/100 Vrms
= 7.75 mVrms.

Measuring dB(A) and db(C) is more complicated, because you have to use a 
filter. See this page for reference:

http://en.wikipedia.org/wiki/A-weighted

And finally there is the sound pressure:

http://en.wikipedia.org/wiki/Sound_pressure

And as you know now already, using "dB" would be meaningless, the right 
naming is dB(SPL). There are high quality measure microphones, with 
guaranteed characteristic curve, e.g. from Behringer or EMC, with which you 
can measure dB(SPL). But then you have to calculate the filter function and 
you have to calibrate your sound card input, if you want to know dB(A) or 
dB(C) from the microphone output signal. Would be easier to just buy a good 
sound level meter.

PS: please do not crossposting to so many groups. sci.electronics.design is
not the right group (but you might get better answers in this group :-)
choose some basic group, or maybe alt.audio.equipment would be the right 
place. I've limited the posting to sci.electronics.design, because if I
made some mistakes in my explanation, I know there are people in this
newsgroup who can correct me.

PPS: next time read Wikipeda first, then post your questions, if there are 
any left.

-- 
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

And indeed, wikipedia also mentions DB(A) bad-suited for what it's used for:

http://en.wikipedia.org/wiki/A-weighting

"
The curves were originally defined for use at different average sound 
levels, but A-weighting, though originally intended only for the measurement 
of low-level sounds (around 40 phon), is now commonly used for the 
measurement of environmental noise and industrial noise, as well as when 
assessing potential hearing damage and other noise health effects at all 
sound levels; indeed,

!!!
the use of A-frequency-weighting is now mandated for all these measurements, 
although it is badly suited for these purposes, being only applicable to low 
levels so that it tends to devalue the effects of low frequency noise in 
particular[1].
!!!

"

Bye,
  Skybuck. 

Ok,

I am trying to find more information about the DM-A409 microphone that came 
with the Denon AVR 1909.

Quite funny really, I came across this website which has one little line of 
important information about it:

"Accurate microphone DM-A409 (included) custom made by Audio-Technica"

http://www.audio-net.com.ar/denonavr1909.htm

So now I go check out audio technica website at:

http://www.audio-technica.com/world_map/

Maybe it has more information about this microphone ;) :)

Or maybe it has comparable microphones or other interesting information ! ;)

Bye,
  Skybuck. 

Well the "custom made" probably says it all...

This microphone is not really for sale at their website anyway.

I did see a couple of them on e-bay and such.

Audio-technica does have a lot of microphones... but none in this shape...

I would guess these come closest to it:

http://www.audio-technica.com/cgi-bin/product_search/wired_mics/mics_by_type.pl?product_type=Microphones%3A+Lavalier

I think it's time to give this microphone a test myself and connect it to my 
soundblaster and see what happens ;) :)

Bye,
  Skybuck.

"Skybuck Flying" <BloodyShame@hotmail.com> wrote in message 
news:875c4$4a99fcce$d53372a9$28916@cache3.tilbu1.nb.home.nl...
> Ok,
>
> I am trying to find more information about the DM-A409 microphone that 
> came with the Denon AVR 1909.
>
> Quite funny really, I came across this website which has one little line 
> of important information about it:
>
> "Accurate microphone DM-A409 (included) custom made by Audio-Technica"
>
> http://www.audio-net.com.ar/denonavr1909.htm
>
> So now I go check out audio technica website at:
>
> http://www.audio-technica.com/world_map/
>
> Maybe it has more information about this microphone ;) :)
>
> Or maybe it has comparable microphones or other interesting information ! 
> ;)
>
> Bye,
>  Skybuck.
> 

Hmm this microphone seems to be very good at picking up sounds... there is a 
catch however...

The volume is very low... so to be able to hear it it must be amplified.

Fortunately goldwave (wave) editor has a effect/volume/change volume 
option/tool that allows the volume to be enhanced by 20 points whatever that 
means... and then this can be repeated one more time...

After applying the change volume effect twice it sounds pretty good ! (Even 
picking up low sounds it does pretty well).

I am already convinced this is a good microphone however now I am getting 
slightly worried that the car sounds from outside might actually damage 
it... but then again... the testing sounds from the receiver where also 
pretty loud so it can probably handle it ! :)

I wonder how much this microphone is worth LOL :) For example how much would 
a comparable microphone cost ?!?

Anyway I am hoping now that I can use this microphone to try and do some 
meaningfull measurements ?!

I am not scared to do some arithmetic/math to convert whatever it takes to 
DB(A) or DB(C)... I might actually learn something from doing the math or 
so... and get a better understanding of it... and then maybe I will also be 
more sure that it's more or less correct for my purposes and law or so ;) :)

I do have a little problem with guessing the distance to the street/cars. I 
am on 5th floor or so... so I am up in the air... I don't know if that 
matters much... it probably does matter a little bit... but the sounds are 
also reflected by the surrounding buildings...

Bye,
  Skybuck.