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Guest
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 Posted: Fri Nov 14, 2008 11:17 am Post subject: Negative SPL |
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What exactly is the interpretation of negative SPL (dB)? An SPL near 0
just means the sound references the ambient SPL (reference point), but
how can you go below the threshold of which your reference point is? |
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Randy Yates
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| Posted: Fri Nov 14, 2008 7:16 pm Post subject: Re: Negative SPL |
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Casper.Ptrsn@gmail.com writes:
| Quote: | What exactly is the interpretation of negative SPL (dB)? An SPL near 0
just means the sound references the ambient SPL (reference point), but
how can you go below the threshold of which your reference point is?
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Hi Casper,
Do you know that the logarithm is negative whenever its argument
is less than one? Thus if a dB SPL is negative, then P/Pref in
the expression 10*log(P/Pref) must be less than one, i.e.,
P is less than Pref.
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <yates@ieee.org> % 'Big Wheels', *Out of the Blue*, ELO
http://www.digitalsignallabs.com |
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Paul Russell
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| Posted: Fri Nov 14, 2008 8:50 pm Post subject: Re: Negative SPL |
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Casper.Ptrsn@gmail.com wrote:
| Quote: | What exactly is the interpretation of negative SPL (dB)? An SPL near 0
just means the sound references the ambient SPL (reference point), but
how can you go below the threshold of which your reference point is?
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0dB SPL is not "ambient" - it's just an arbitrary reference point which
happens to be somewhat close to the absolute threshold of human hearing
(give or take 20 dB or so).
Paul |
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Jerry Avins
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| Posted: Fri Nov 14, 2008 9:07 pm Post subject: Re: Negative SPL |
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Casper.Ptrsn@gmail.com wrote:
| Quote: | What exactly is the interpretation of negative SPL (dB)? An SPL near 0
just means the sound references the ambient SPL (reference point), but
how can you go below the threshold of which your reference point is?
|
By going to someplace quieter.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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Fred Marshall
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| Posted: Sat Nov 15, 2008 3:53 am Post subject: Re: Negative SPL |
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Casper.Ptrsn@gmail.com wrote:
| Quote: | What exactly is the interpretation of negative SPL (dB)? An SPL near 0
just means the sound references the ambient SPL (reference point), but
how can you go below the threshold of which your reference point is?
|
As Paul mentions, the reference is an absolute value. It's different in air
and water - so be careful which reference you use for which application.
In air the reference is: 20 micropascals
In water the reference is: 1 micropascal
Historically, SPL in water used to be referenced relative to 1 microbar
which is 10^5 micropascals. So, the conversion from the old system to the
new system is to subtract 100dB <> 10^5.
Fred |
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DigitalSignal
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| Posted: Sat Nov 15, 2008 5:20 am Post subject: Re: Negative SPL |
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Realistically, how quiet a sound chamber can be made? Ever heard of
anything below 0dB SPL?
James
www.go-ci.com |
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Kari Pesonen
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| Posted: Sat Nov 15, 2008 9:06 am Post subject: Re: Negative SPL |
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"DigitalSignal" <digitalsignal999@yahoo.com> wrote in message
news:0bfac652-4d36-4f00-a388-5dba63c76cd0@r37g2000prr.googlegroups.com...
| Quote: | Realistically, how quiet a sound chamber can be made? Ever heard of
anything below 0dB SPL?
James
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SPLs achievable depend on frequency. Usually level at (very) low frequencies
are higher than levels at medium and higher frequencies.
Levels near 0 dB and bellow are difficult to measure because of electrical
noise produced by microphones and instruments. Very low levels can
be reached in deep mines (when ventilation and other noise sources shut
down).
I know one underground acoustical laboratory where background noise level
is ca. -10 dB(A) (when ventilation and other noise sources are shut down).
Kari Pesonen |
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DigitalSignal
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| Posted: Sun Nov 16, 2008 5:01 pm Post subject: Re: Negative SPL |
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On Nov 15, 1:15 am, "Kari Pesonen" <Kari.Peso...@rem.welho.com> wrote:
| Quote: | "DigitalSignal" <digitalsignal...@yahoo.com> wrote in message
news:0bfac652-4d36-4f00-a388-5dba63c76cd0@r37g2000prr.googlegroups.com...
Realistically, how quiet a sound chamber can be made? Ever heard of
anything below 0dB SPL?
James
SPLs achievable depend on frequency. Usually level at (very) low frequencies
are higher than levels at medium and higher frequencies.
Levels near 0 dB and bellow are difficult to measure because of electrical
noise produced by microphones and instruments. Very low levels can
be reached in deep mines (when ventilation and other noise sources shut
down).
I know one underground acoustical laboratory where background noise level
is ca. -10 dB(A) (when ventilation and other noise sources are shut down)..
Kari Pesonen
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Kari, It is good to know. Thanks.
James
www.go-ci.com |
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Paul Russell
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| Posted: Mon Nov 17, 2008 4:04 pm Post subject: Re: Negative SPL |
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DigitalSignal wrote:
| Quote: | On Nov 15, 1:15 am, "Kari Pesonen" <Kari.Peso...@rem.welho.com> wrote:
"DigitalSignal" <digitalsignal...@yahoo.com> wrote in message
news:0bfac652-4d36-4f00-a388-5dba63c76cd0@r37g2000prr.googlegroups.com...
Realistically, how quiet a sound chamber can be made? Ever heard of
anything below 0dB SPL?
James
SPLs achievable depend on frequency. Usually level at (very) low frequencies
are higher than levels at medium and higher frequencies.
Levels near 0 dB and bellow are difficult to measure because of electrical
noise produced by microphones and instruments. Very low levels can
be reached in deep mines (when ventilation and other noise sources shut
down).
I know one underground acoustical laboratory where background noise level
is ca. -10 dB(A) (when ventilation and other noise sources are shut down).
Kari Pesonen
Kari, It is good to know. Thanks.
James
www.go-ci.com
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Note also that in the above you are considering SPL across a relatively
wide frequency range. Within a narrow band (e.g. a single auditory
filter) of course the ambient SPL can be much lower and many people can
perceive tones below 0 dB SPL.
Paul |
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