Hi all,
Most mics have self noise specifications around 15dB to 30dB equivalent SPL but this is weighted with a curve. There are
DIN IEC 651
CCIR 468-3
A-weighted
This makes it difficult to compare with noise level that I usally calculate from the trace on my scope (Vnoise rms = 6 x Vnpp on scope usually works pretty good to check opamp noise specs).
Does anyone know how to comvert A-weighted noise to 'flat'-weighted noise? Is it possible, just in the right ballpark?
Is A-weighted noise something like 20dB lower that 'flat' weighted noise?
Kind regards,
Thijs
Most mics have self noise specifications around 15dB to 30dB equivalent SPL but this is weighted with a curve. There are
DIN IEC 651
CCIR 468-3
A-weighted
This makes it difficult to compare with noise level that I usally calculate from the trace on my scope (Vnoise rms = 6 x Vnpp on scope usually works pretty good to check opamp noise specs).
Does anyone know how to comvert A-weighted noise to 'flat'-weighted noise? Is it possible, just in the right ballpark?
Is A-weighted noise something like 20dB lower that 'flat' weighted noise?
Kind regards,
Thijs
If you are interested in 20 - 20 000 Hz the difference is only a couple of dB's.
It's hard to make a judgement with an oscilloscope 100 MHz bandwidth and peak values only.
It's hard to make a judgement with an oscilloscope 100 MHz bandwidth and peak values only.
Thanks Peranders.
I actually find it very easy to use a osciloscope. But you made a good reamark. In my case the bandwidth of the scope doesn't matter as long as the scope noise is (much) less the the device under test and the bandwidth of the device is know. I allways use Line-trigger mode to differentiate between humm and noise.
I still have a question about the various weighting curves.
The difference between
DIN IEC 651
and
CCIR 468-3
is about 10dB.
How do they compare with A-weighted?
Reagrds,
Thijs
I actually find it very easy to use a osciloscope. But you made a good reamark. In my case the bandwidth of the scope doesn't matter as long as the scope noise is (much) less the the device under test and the bandwidth of the device is know. I allways use Line-trigger mode to differentiate between humm and noise.
I still have a question about the various weighting curves.
The difference between
DIN IEC 651
and
CCIR 468-3
is about 10dB.
How do they compare with A-weighted?
Reagrds,
Thijs
The incoming bandwidth matters. 20-20kHz or 20-100 kHz or even more will matter compared to A-weighted values.
Anecdotally it often seems that unweighted band-limited noise (20-20kHz) is generally about 3-6dB higher than A-weighted, but this is at best a poor rule of thumb. Peranders and I discussed this a little recently and I gave some examples where you could see big differences with A-weighted versus unweighted (but band-limited).
I believe Peranders is cautioning you that at 100MHz you are integrating noise over a much broader bandwidth, so the numbers will not be comparable in any way to A-weighted numbers. Even if the noise is the same, at 100MHz the total value will be much higher than at 20kHz.
Something like a Tek 7A22 comes in handy for this kind of work.
I believe Peranders is cautioning you that at 100MHz you are integrating noise over a much broader bandwidth, so the numbers will not be comparable in any way to A-weighted numbers. Even if the noise is the same, at 100MHz the total value will be much higher than at 20kHz.
Something like a Tek 7A22 comes in handy for this kind of work.
Thanks for all the replies.
Let's not talk about the scope anymore. I realy do have that under controll.
This all concerns a Sennheiser MKH-70 microphone that seems to be off-specifications and needs to be recallibrated....
Let's talk about A-weighting and the other norms.
3dB to 6dB is a something to start with. I have the A-weighting curve in front of me and I can see the attenuation at the low and very high frequencies. I also understand why this is done and why it is therefore only suiteble for low hearing level application like background noise.
As I understand now, the A-weigthing curve is limmiting the bandwidth to 1/2 or even 1/4 of the unweighted bandwidth?
And the 1/f noise that might be will no longer be of any significance when A-weighting is applied.
But I can't seem te find information about those other two norms:
DIN IEC 651
CCIR 468-3
Anyone got some eperince or info about those....
Google turn up with all those german pages that I have trouble reading with,
Thanks,
Thijs
Let's not talk about the scope anymore. I realy do have that under controll.
This all concerns a Sennheiser MKH-70 microphone that seems to be off-specifications and needs to be recallibrated....
Let's talk about A-weighting and the other norms.
3dB to 6dB is a something to start with. I have the A-weighting curve in front of me and I can see the attenuation at the low and very high frequencies. I also understand why this is done and why it is therefore only suiteble for low hearing level application like background noise.
As I understand now, the A-weigthing curve is limmiting the bandwidth to 1/2 or even 1/4 of the unweighted bandwidth?
And the 1/f noise that might be will no longer be of any significance when A-weighting is applied.
But I can't seem te find information about those other two norms:
DIN IEC 651
CCIR 468-3
Anyone got some eperince or info about those....
Google turn up with all those german pages that I have trouble reading with,
Thanks,
Thijs
tiroth said:
Duhhh, an oscilloscope is not integrating like a moving coil meter. It just shows you the momentary value.
With noise it is not easy to compare pp values as seen on an oscilloscope to RMS values measured with a meter. Lots depend on the “crest” factor.
BTW if you Google on noise+measurement+weighting there are tons of information.
Cheers 😎
Hi Thijs,
Didn’t realise it, but you can also do it the other way around and use an A-weighting filter for your measurements:
http://sound.westhost.com/project17.htm
Cheers
Didn’t realise it, but you can also do it the other way around and use an A-weighting filter for your measurements:
http://sound.westhost.com/project17.htm
Cheers
The difference in dB between the results of different noise measuring filters depend on the character of the noise. For instance, if your noise is completely white, then there is a difference between A-weighting and flat (20Hz-20kHz bandlimited) of approximately 3dB (dBA is 3 dB better than flat). CCIR gives far worse results.
If your noise is not white, the difference might become much more, e.g. if you have 50Hz/60Hz hum in your noise the flat value will change a lot, but the A weighted value far less because the A-weighting takes these low frequencies not so much into account. A-weighting is also less critical about the noise above 5kHz.
If you have a circuit that uses FETs or MOSFETs you may have quite an amount of 1/f noise, typical corner frequencies between 100Hz (good FETs) and even 10kHz (MOSFETs). In these cases the difference between A-weighting and 20Hz-20kHz flat will be more than 3dB.
These kind of differences you also get with digital audio circuits. Multibit ADCs and DACs generate mostly white noise (quantisation noise), but bitstream (sigma-delta) type of converters generate more high frequency noise because of the applied noise shaping. Bitstream converters are almost allways measured with A-weighting of the results, then you get (much) better values.
Steven
If your noise is not white, the difference might become much more, e.g. if you have 50Hz/60Hz hum in your noise the flat value will change a lot, but the A weighted value far less because the A-weighting takes these low frequencies not so much into account. A-weighting is also less critical about the noise above 5kHz.
If you have a circuit that uses FETs or MOSFETs you may have quite an amount of 1/f noise, typical corner frequencies between 100Hz (good FETs) and even 10kHz (MOSFETs). In these cases the difference between A-weighting and 20Hz-20kHz flat will be more than 3dB.
These kind of differences you also get with digital audio circuits. Multibit ADCs and DACs generate mostly white noise (quantisation noise), but bitstream (sigma-delta) type of converters generate more high frequency noise because of the applied noise shaping. Bitstream converters are almost allways measured with A-weighting of the results, then you get (much) better values.
Steven
Hi Thijs,
Here is a site that gives you the pole and zero locations of the various weighting filters:
http://www.ptpart.co.uk/noise.htm
If you are familiar with the basics of filter theory you can build them. BTW searching around the net I found several times that the difference between flat and A-weighting filters for white noise is 10 dB when integrated over the 20 – 20 kHz bandwidth. Is there a mathematician out there to check this?
Also this is informative reading:
http://www.dwelle.de/rtc/infotheque/qual_parameter/qualpar_05.html
Cheers
Here is a site that gives you the pole and zero locations of the various weighting filters:
http://www.ptpart.co.uk/noise.htm
If you are familiar with the basics of filter theory you can build them. BTW searching around the net I found several times that the difference between flat and A-weighting filters for white noise is 10 dB when integrated over the 20 – 20 kHz bandwidth. Is there a mathematician out there to check this?
Also this is informative reading:
http://www.dwelle.de/rtc/infotheque/qual_parameter/qualpar_05.html
Cheers
These kind of differences you also get with digital audio circuits. Multibit ADCs and DACs generate mostly white noise (quantisation noise), but bitstream (sigma-delta) type of converters generate more high frequency noise because of the applied noise shaping. Bitstream converters are almost allways measured with A-weighting of the results, then you get (much) better values.
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The specs can cheat; although this is supposedly based on human response to sine waves! I always look at broadband noise first. The A weighting can hide a lot of circuit probelms.
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The specs can cheat; although this is supposedly based on human response to sine waves! I always look at broadband noise first. The A weighting can hide a lot of circuit probelms.
Hi Thijs,
Have a look at http://www.rane.com/par-w.html where you can find the subjects A-weighting and C-weighting. It appears that CCIR (now ITU-R) has its source in telephony switching.
Hi Pjotr,
Also here a difference of 10dB between flat and A-weighted figures is mentioned by Dennis Bohn, but more as a warning that the difference can be as big as 10dB in case of 'nasty low-frequency hum components'. Normally for white noise it is my experience (no mathematics) that the difference is some 3dB. In the past, I found this numerous times when measuring noise with a Sennheiser UPM550 and switching between flat 20Hz-20kHz and A-weighted.
Steven
Have a look at http://www.rane.com/par-w.html where you can find the subjects A-weighting and C-weighting. It appears that CCIR (now ITU-R) has its source in telephony switching.
Hi Pjotr,
Also here a difference of 10dB between flat and A-weighted figures is mentioned by Dennis Bohn, but more as a warning that the difference can be as big as 10dB in case of 'nasty low-frequency hum components'. Normally for white noise it is my experience (no mathematics) that the difference is some 3dB. In the past, I found this numerous times when measuring noise with a Sennheiser UPM550 and switching between flat 20Hz-20kHz and A-weighted.
Steven
Hi Steven,
Maybe you are right that the 3 dB figure is more close. Most of the att. is at the lower frequencies for an A-weighting filter and most of the energy of white noise is in the upper frequencies above 1 kHz where the filter att. is much lesser. Also the method of measuring makes sense, is it quasi peak or is it true RMS? For true RMS you can calculate mathematically the difference by integrating the noise content along the filter slope. But I am too lazy to blow the dust from my old schoolbooks 😀
Cheers
Maybe you are right that the 3 dB figure is more close. Most of the att. is at the lower frequencies for an A-weighting filter and most of the energy of white noise is in the upper frequencies above 1 kHz where the filter att. is much lesser. Also the method of measuring makes sense, is it quasi peak or is it true RMS? For true RMS you can calculate mathematically the difference by integrating the noise content along the filter slope. But I am too lazy to blow the dust from my old schoolbooks 😀
Cheers
I once numerically calculated the noise bandwidth of an A filter. Theoretically, with white noise at the input, the RMS noise at the output of an A filter with unity gain at 1kHz should be identical to the noise at the output of a brick-wall filter with unity pass band gain and a 13471Hz bandwidth. That's 1.712dB less than in a 19980Hz bandwidth.
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