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

Discussion in 'Electronic Design' started by Skybuck Flying, Aug 30, 2009.

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  1. 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...y=&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.
     
  2. 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.
     
  3. 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 µPa.
    "

    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.
     
  4. 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:



    Bye,
    Skybuck.
     
  5. 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.
     
  6. Frank Buss

    Frank Buss Guest

    -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.
     
  7. 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.
     
  8. 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.
     
  9. 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-b...by_type.pl?product_type=Microphones:+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.
     
  10. 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.
     
  11. Frank Buss

    Frank Buss Guest

    No, use a calibrated measure microphone.
    You may need some years to learn the basics, e.g. how to develop a FIR
    filter with many taps for the filter function.
     
  12. I already have a microphone. Now it "just" needs to be calibrated lol :)

    But this raises interesting questions:

    How am I suppose to understand the law if I don't know what kind of decibels
    me, my systems, or other people's cars/trucks/busses are producing ?

    There must be a way for me to calibrate my microphone... ?!?

    So the question now becomes how to calibrate my microphone so it can be used
    to interpret the law and such...

    (I could simply follow your advice and buy a "calibrated" microphone but
    then how would I know if it was calibrated properly in according with law ?!
    The filter can be applied later... these are weights if I understand
    correctly.

    So first I would need a system without the weights... is this the SPL you
    mentioned ?

    Bye,
    Skybuck.
     
  13. Ok,

    I found some interesting information in the dutch law... maybe I can make
    some sense of it...

    I'll post it here in dutch... you'll have to use google translate to
    translate it into your language.

    However some symbols and formula's are lost in plain text so here is the
    link in case you are interested in learning more about how the dutch law
    deals with sounds/decibels and measurements ;) :)

    http://wetten.overheid.nl/BWBR0003529/geldigheidsdatum_30-08-2009


    "
    Bijlage I
    Voorschrift m.b.t. het berekenen en meten van de geluidwering
    1. Algemeen
    1.1De bepaling van de geluidwering dient te geschieden voor de octaaf-banden
    met de middenfrequenties 125 Hz, 250 Hz, 500 Hz, 1000 Hz en 2000 Hz.
    1.2Bij de bepaling van de geluidwering van de gevel dient te worden
    uitgegaan van het A-gewogen referentiespectrum, behorend bij het equivalent
    geluidsniveau buiten de woning of het andere geluidsgevoelige gebouw.
    Het A-gewogen referentiespectrum wordt zodanig weergegeven, dat het
    bijbehorend geluidsniveau 0 dB is.
    1.3Tenzij anders vermeld en gemotiveerd, wordt bij de bepaling van de
    geluidwering als referentiespectrum voor weg- of spoorweglawaai het in tabel
    1 gegeven standaardspectrum gehanteerd.
    Tabel 1. Het A-gewogen standaardspectrum in octaafbanden voor weg- of
    spoorwegverkeer
    Octaafbanden met middenfrequentie in Hz
    frequentie (Hz)12525050010002000
    Index i12345
    Ci (dB)–14–10–6–5–7

    1.3.aAls in de maatgevende periode op de betreffende spoorweg geen
    railvoertuigen passeren behorende tot de categorieën 4 of 5, bedoeld in
    artikel 1 van het Reken- en meetvoorschrift railverkeerslawaai, kan de met
    behulp van het standaardspectrum bepaalde geluidwering met 3 dB(A) worden
    vermeerderd.
    1.4De geluidwering van de gevel wordt genormeerd voor de nagalmtijd in de
    geluidsgevoelige ruimte. Voor geluidsgevoelige ruimten in woningen bedraagt
    de referentienagalmtijd To 0,5 seconde voor de onder 1.1. genoemde
    octaafbanden. Voor geluidsgevoelige ruimten, anders dan woonruimten, in
    andere geluidsgevoelige gebouwen dan woningen bedraagt de
    referentienagalmtijd To 0,8 seconde voor de onder 1.1. genoemde
    octaafbanden.
    1.5Indien door een aanwezige gevel, anders dan door het openen van ramen,
    ventilatie kan plaatsvinden, zoals door kieren, ventilatieroosters of
    ventilatieklepjes, dan wordt de geluidwering bepaald voor de situatie dat
    desbetreffende ventilatieopeningen zijn gesloten en afgedicht. Ter
    compensatie wordt een opening in de gevel in rekening gebracht ter grootte
    van de helft van de volgens de bouwverordening te bepalen ventilatie-opening
    in de gevel, met een geluidsisolatiewaarde van 0 dB(A) voor het netto
    oppervlak van de opening.
    Indien evenwel ventilatie-openingen in de gevel aanwezig zijn of worden
    aangebracht waaraan een hogere geluidsisolatiewaarde wordt toegekend dan 0
    dB(A), dan worden deze bij de bepaling van de geluidwering geopend
    respectievelijk geopend geacht.
    2. Meetmethode
    2.1Ter bepaling van de waarde van de geluidwering van de gevel worden
    metingen uitgevoerd per begrenzingsvlak met behulp van een luidspreker of
    een luidsprekerscombinatie op zodanige wijze dat de metingen voor wat
    betreft de invalsrichtingen representatief kunnen worden geacht.
    Daarbij wordt uitgegaan van een geluidsbron op één of meer vaste posities.
    Het begrenzingsvlak wordt daartoe onder een hoek van 45° ± 5° ten opzichte
    van de normaal aangestraald, zodanig dat de geluidsbelasting op de andere
    van belang zijnde begrenzingsvlakken te verwaarlozen is. De afstand van de
    bron tot het begrenzingsvlak gemeten vanuit de geluidsgevoelige ruimte is
    ten minste tweemaal de gevelbreedte (van die ruimte). De bron dient een
    signaal voort te brengen met het karakter van een ruis en met een breedte
    van tenminste één octaaf waarvan de middenfrequentie overeenkomt met de
    middenfrequentie van het te meten octaafgeluiddrukniveau. De
    frequentiekarakteristiek moet vlak zijn binnen een hoek van ca. 40°.
    De door de bron op de meetplaatsen veroorzaakte geluidsniveaus dienen ten
    minste 10 dB hoger te zijn dan het achtergrondgeluid, dat wil zeggen alle
    geluiden die niet van de bron afkomstig zijn.
    2.2Het geluiddrukniveau buiten het gebouw L2, i wordt per octaafband bepaald
    op 2 meter afstand van het begrenzingsvlak. Het (ruimtelijk gemiddelde)
    geluiddrukniveau binnen de ruimte Lb, i wordt per octaafband bepaald. Voor
    indicatieve doeleinden kan voor de bepaling van het geluiddrukniveau binnen
    de geluidgevoelige ruimte – Lb, i – eventueel worden volstaan met één meting
    op 1 à 1,5 meter van het gevelvlak, ter hoogte van het middel van de gevel.
    Ter compensatie van een systematische fout dient het aldus gemeten
    geluiddrukniveau met 1 dB te worden verlaagd.
    2.3In de geluidsgevoelige ruimte dient per octaafband met de
    middenfrequenties 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, de nagalmtijd Ti
    te worden bepaald. Uit de waarden L2, i, L b, i en Ti en de
    referentienagalmtijd To, (zie onder 1.4) wordt het partiële
    geluidsniveauverschil van de gevel Di bepaald volgens de formule:
    Di= L2, i – Lb, i + 10 log (Ti/To) (dB) (1)
    2.4In verband met reflecties van het geluid op de gevel dient het partiële
    geluidsniveauverschil gecorrigeerd te worden voor de bepaling van de
    partiële geluidwering volgens de formule:
    Gi = Di–Ci (dB) (2)
    waarin Ci = o of 3 dB.
    Uit de partiele geluidwering Gi (voor de octaafbanden met de
    middenfrequenties 125 Hz, 250 Hz, 500 Hz, 1000 Hz, en 2000 Hz) wordt
    rekening houdende met het standaardspectrum de waarde van de geluidwering
    van de gevel bepaald volgens de formule (5) (zie onder 3.5.).
    2.5De bij de bepaling van de geluiddrukniveaus te gebruiken apparatuur dient
    zodanig te zijn dat de meetketen voldoet aan IEC-publicatie 651, uitgave
    1979, type 2.
    3. Berekeningsmethode
    3.1De partiele geluidwering (Gi) van een gevel – dat wil zeggen de
    geluidwering in één octaafband (i) – wordt berekend uit de geluidsisolaties
    van de totale, uit te onderscheiden elementen samengestelde, gevel met de
    formule:

    waarin:
    Ri=de luchtgeluidsisolatiewaarde van de gevel in de octaafband i in dB.
    Cr=correctieterm in verband met de gevelreflectie.
    V =het volume van de geluidsgevoelige ruimte.
    To=de referentienagalmtijd: 0,5 s, tenzij anders is aangegeven.
    S =totale geveloppervlakte in m² (gezien vanuit de geluidgevoelige ruimte).

    3.23.2. In formule (1) geldt:

    waarin:
    n =het aantal te onderscheiden elementen waaruit de gevel is opgebouwd.
    Sj =het oppervlak van het gevelelement j in m².
    Rji=de (laboratorium) luchtgeluidisolatiewaarde van het gevelelement j in de
    octaafband i in dB.
    K =kierterm. Indien speciale dubbele kierdichting worden toegepast, kan K op
    10–5 worden gesteld, in de overige gevallen op 3.10–4.
    Indien een afwijkende kierterm wordt toegepast, wordt deze afwijking nader
    gemotiveerd.

    3.3De correctieterm Cr (gevelreflectie) bedraagt 3 dB indien de hoek tussen
    de kortste verbindingslijn geluidbron – gebouw en de normaal op de gevel of
    het gevelvlak 70° of minder is en bedraagt 0 dB indien deze hoek meer dan
    70° is. Bij de bepaling van de partiële geluidwering van een gevel volgens
    formule (3) wordt gerekend met de gebruikelijke gevelstructuren. Indien er
    sprake is van een bijzondere gevelstructuur kan een correctie worden
    toegepast welke wordt gemotiveerd.
    3.4Indien de waarden Rj gegeven zijn voor het A-gewogen
    standaardreferentiespectrum – dus in dB(A) –, geven de formules (3) en (4)
    direct de geluidwering van de gevel in dB(A).
    3.5Uit de partiële geluidwering van de gevel (Gi) wordt de geluidwering van
    de gevel berekend met de formule:

    waarin:
    Ci = standaard-referentiespectrum in dB (zie onder 1.2 en 1.3.).
    "

    Bye,
    Skybuck.
     
  14. Oops... I just noticed this is for inside buildings... that could be
    interesting as well...

    But first I want to know formula's/calibration for outside buildings...

    So me go search some more ! ;)

    Bye,
    Skybuck.

    <snip>
     
  15. Ok,

    I am now one step further :) I like taking it a step at a time lol.

    This is the dutch law called "soundobstruction" in english ;) or something
    like that ;) :)

    (In dutch: "wet geluidhinder" wet = law. geluid = sound. hinder =
    obstruction.)

    (Wgh in short)

    http://wetten.overheid.nl/BWBR0003227/geldigheidsdatum_30-08-2009

    (Again use google translate if you want to understand some if it if you
    can't understand dutch :):):))

    Bye,
    Skybuck.
     
  16. The dutch law mentions international "huppledepup" lol.

    dB(a) as determined by "Internationale Electrotechnische Commissie".

    Searching google takes me to this website in dutch which says Netherlands is
    pretty much at the forefront of "sound laws" ;)

    http://s1006.nxs.nl/ski/afdrukken.asp?ID=WGHBI.1

    And via this website I come at this document which explains how to do a
    measurement...

    It's again in dutch though but could be interesting:

    http://www.vrom.nl/docs/milieu/handleiding_meten_en_rekenen_industrielawaai_module_b.pdf

    Bye,
    Skybuck.
     
  17. Some more interesting links:

    (Less interesting, too many links, too much information, too many subjects,
    but it did have a further link to the link at the bottom):

    http://www.vrom.nl/pagina.html?id=10054

    According to website above, website below has details on law about road
    traffic and measurements here in the Netherlands:

    http://www.stillerverkeer.nl.

    Now me go check that out !

    (I am getting there... I am getting there... I hope LOL)

    Bye,
    Skybuck.
     
  18. Hmm... I forgot it's sunday... I love sundays, especially when there is no
    market... nice and quiet... bad thing is shops closed though... so it has
    ups and down :p**** LOL.

    But today I don't like it lol... just kidding still love it... peacefull and
    quiet at least right now.. ;) :)

    But I was hoping to do some measurements... but that will have to wait until
    monday and other days when there is a shitload of traffic going on ! ;)

    But for now this gives me time to prepare for * THE MEASUREMENT *

    A Skybuck movie/thriller coming to your cinema's SOON !! ;) =D

    Bye,
    Skybuck =D
     
  19. Ok,

    Starting with the fricking basics ;)

    What is a microphone ;)

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

    LOL this gonna take a while LOL.

    But not really I am speedy reader LOL.

    I just read most important/interesting parts for now otherwise I could
    indeed be spending a lifetime on this ;)

    Bye,
    Skybuck.
     
  20. I found another little bit of interesting information about the microphone
    that came with the receiver...

    The manual says it was calibrated.

    It doesn't say how it was calibrated.

    I now go send a e-mail to denon and maybe the original manufacturer... to
    see if they want to supply some information about it...

    Might come in handy ?! ;)

    Bye,
    Skybuck.
     
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