Calculation of SPL

[are...@googlemail.com]

Hi everybody,

I have a Problem in understanding.

I'm unsing a iMM-6 with a calibration file.

The APP Shows the file Name of the calibration file.

I set up in the APP

1/6 Octave / A Weight / Medium

When I watch the coumns of the frequencies db they are all below "28" and the Shows 38 dB(A) SPL

Even if I'm inside a closed measurement room mesured with 19 dB(A) the APP Shows a much higher Level.

What's wron with

- my Settings

- my expectation (SPL Value show at least a similar value as the professional measurement device)

I'm sure, the Problem is on my side as the APP is great.

Can anybody help me?

Best regards from Munich

Alf

[IanH]

    

   Having calibrated your mic + phone combination for a flat response you can then apply an offset to match a known SPL  

   If you have the luxury of access to a 19dB(A) measurement room this sounds a very good place to start.

[Julian Bunn]

Hi Alf,

Yes, as Ian says, there is a Global Offset as part of the calibration (via the Menu) which you can adjust so that the measured SPL in your 19dB(A) room shows 19dB with no sound source.

Does that help?

Julian

On Tue, Apr 17, 2018 at 9:34 AM, IanH <howl...@gmail.com> wrote:

    

   Having calibrated your mic + phone combination for a flat response you can then apply an offset to match a known SPL  

   If you have the luxury of access to a 19dB(A) measurement room this sounds a very good place to start.

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[are...@googlemail.com]

Hi Julian and Ian,

I found the "set cal Offset" but the proprtion between the shown SPL value and the db Level of the Octave columns does not fit.

For testing I made a offfset to 0 ab(A) and the coumns Show a Level of approx. - 10 db

Is there any relative calculation between the displayed dB(A)  and the db Level of each Octave column?

May be the frequency with the highest db Level is the source for the dB(A) Level .... would make sense.

In the "Chart" window it Looks to be like this .....

BR

Alf

Am Dienstag, 17. April 2018 18:43:12 UTC+2 schrieb Julian:

Hi Alf,

Yes, as Ian says, there is a Global Offset as part of the calibration (via the Menu) which you can adjust so that the measured SPL in your 19dB(A) room shows 19dB with no sound source.

Does that help?

Julian

On Tue, Apr 17, 2018 at 9:34 AM, IanH <howl...@gmail.com> wrote:

    

   Having calibrated your mic + phone combination for a flat response you can then apply an offset to match a known SPL  

   If you have the luxury of access to a 19dB(A) measurement room this sounds a very good place to start.

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[Julian Bunn]

Hi Alf,

If the columns are all showing 10dB less than they should, then you should set the global offset to 10dB, which will have the effect of adding 10dB to all the levels.

In general, if you look at the overall SPL level shown by AudioTool's SPL mode, and compare that with what you know the ambient sound level is (e.g. using an external meter or in your quiet chamber with no sound), then the difference is what you should set the global offset to.

One problem I see is that if you are in a very low noise chamber, then the noise floor of the electronics in your phone may be above the noise level of the chamber ... 

Julian

On Tue, Apr 17, 2018 at 11:02 PM, arempka via AudioTool Discussion Group <audiotool-dis...@googlegroups.com> wrote:

Hi Julian and Ian,

I found the "set cal Offset" but the proprtion between the shown SPL value and the db Level of the Octave columns does not fit.

For testing I made a offfset to 0 ab(A) and the coumns Show a Level of approx. - 10 db

Is there any relative calculation between the displayed dB(A)  and the db Level of each Octave column?

May be the frequency with the highest db Level is the source for the dB(A) Level .... would make sense.

In the "Chart" window it Looks to be like this .....

BR

Alf

Am Dienstag, 17. April 2018 18:43:12 UTC+2 schrieb Julian:

Hi Alf,

Yes, as Ian says, there is a Global Offset as part of the calibration (via the Menu) which you can adjust so that the measured SPL in your 19dB(A) room shows 19dB with no sound source.

Does that help?

Julian

On Tue, Apr 17, 2018 at 9:34 AM, IanH <howl...@gmail.com> wrote:

    

   Having calibrated your mic + phone combination for a flat response you can then apply an offset to match a known SPL  

   If you have the luxury of access to a 19dB(A) measurement room this sounds a very good place to start.

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[are...@googlemail.com]

Hi Julian,

I did a Offset calibration with our calibater (74 db / 94 db / 114 db) - so the SPL Level Shows these values after Offset.

But I would try the global Offset .....

Where can I find it?

I only see the "Set Cal. Offset" in the menue .... but the global seems to be somewhere different

Alf

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[Julian Bunn]

Hi Alf,

It's the same thing :-) Here is the Manual link:

https://sites.google.com/site/bofinit/audiotool#TOC-Global-Offset

Julian

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[are...@googlemail.com]

Hi Julian,
if the is no difference between the offset's the offset itself doesn't help.
There is still a big difference between the Octave columns and the SPL.

Do you have the calculation formula for the single Oktav Peak level on the SPL?
I'm sure it's correct as it is but I would like to understand it.

Alf

[Julian Bunn]

I'm not sure I understand the problem. Can you show a screenshot of what your octave display looks like when measuring an environment at a known SPL?

The Wikipedia entry on sound pressure https://en.wikipedia.org/wiki/Sound_pressure shows the formula for summing SPL levels (in the section called "Sound Pressure Level").

Julian

[are...@googlemail.com]

Hi Julian,

enclosed the screenshot.

You see SPL Shows 20 dB(A) and the top Level of the columns is at about 5 dB

I think, the 5 dB are 5 dB(A) as the raw dB Level should be much higher ....

So for my understanding the SPL should be similar to the highest dB Level of the columns .....

Am Mittwoch, 18. April 2018 21:45:32 UTC+2 schrieb Julian:

I'm not sure I understand the problem. Can you show a screenshot of what your octave display looks like when measuring an environment at a known SPL?

The Wikipedia entry on sound pressure https://en.wikipedia.org/wiki/Sound_pressure shows the formula for summing SPL levels (in the section called "Sound Pressure Level").

Julian

On Wed, Apr 18, 2018 at 11:30 AM, arempka via AudioTool Discussion Group <audiotool-dis...@googlegroups.com> wrote:

Hi Julian,
if the is no difference between the offset's the offset itself doesn't help.
There is still a big difference between the Octave columns and the SPL.

Do you have the calculation formula for the single Oktav Peak level on the SPL?
I'm sure it's correct as it is but I would like to understand it.

Alf

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[IanH]

Are you perhaps confusing peak levels at spot frequencies with the overall averaged SPL. I've just checked on a calibrated phone, picking up ambient (mostly traffic) noise, & my A-weighted SPL is roughly 10dB higher than the individual 1/3rd-octave peaks, which sounds like what you are seeing on your phone.

[Julian Bunn]

Hi Alf,

Let's do the calculation using the formula on Wikipedia. There are 60 bins in your spectrum most at around 5dB plus or minus a few dB. The broadband SPL is thus:

SPL = 10*Log10(60*10^0.5) = 23 dB 

So this looks correct to me. 

Having said that, these levels you are measuring are very, very low! I don't think that 5dB SPL is physically possible to measure in practice, and certainly not with a phone :-)

Hope this helps.

Julian

On Wed, Apr 18, 2018 at 11:56 PM, arempka via AudioTool Discussion Group <audiotool-dis...@googlegroups.com> wrote:

Hi Julian,

enclosed the screenshot.

You see SPL Shows 20 dB(A) and the top Level of the columns is at about 5 dB

I think, the 5 dB are 5 dB(A) as the raw dB Level should be much higher ....

So for my understanding the SPL should be similar to the highest dB Level of the columns .....

Am Mittwoch, 18. April 2018 21:45:32 UTC+2 schrieb Julian:

I'm not sure I understand the problem. Can you show a screenshot of what your octave display looks like when measuring an environment at a known SPL?

The Wikipedia entry on sound pressure https://en.wikipedia.org/wiki/Sound_pressure shows the formula for summing SPL levels (in the section called "Sound Pressure Level").

Julian

On Wed, Apr 18, 2018 at 11:30 AM, arempka via AudioTool Discussion Group <audiotool-dis...@googlegroups.com> wrote:

Hi Julian,
if the is no difference between the offset's the offset itself doesn't help.
There is still a big difference between the Octave columns and the SPL.

Do you have the calculation formula for the single Oktav Peak level on the SPL?
I'm sure it's correct as it is but I would like to understand it.

Alf

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