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Is noise audible above 20kHz
Is noise audible above 20kHz
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Old 14th November 2017, 05:49 PM   #1
flyingfishtw is offline flyingfishtw  Taiwan
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Default Is noise audible above 20kHz

When calculating noise, nV/hz * BW^1/2, bandwidth is contributing to noise. so for 2 amplifier, one with 1Mhz bandwidth, another limited to 200k hz, the noise generated by amp is in the chart.

for both configurations, noise at 20k hz is the same 10.7uV,
200khz noise is 34uV (200khz BW), 73uV (1Mhz BW),
as frequency progress, noise level is much higher on the 1Mhz BW amp.

Is the noise above 20khz audible ,
or the noise is only audible under Bandwidth?

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File Type: jpg Amp BW Noise.jpg (197.5 KB, 311 views)
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Old 14th November 2017, 05:57 PM   #2
lcsaszar is offline lcsaszar  Hungary
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Human hearing upper limit is around 18 kHz (young) or 12 kHz (elder).
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Old 14th November 2017, 07:32 PM   #3
Monte McGuire is offline Monte McGuire
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Is noise audible above 20kHz
For audio noise measurements, the tradition is to use a 20kHz measurement bandwidth. Of course, out of band noise, if large enough, can cause problems, so it doesn't always have to be ignored simply because it is not audible.

Imagine a shoddy PWM power amplifier that has no output filter. The amplifier will put out a lot of energy above the passband, and this will go directly to the tweeter, heating it up, reducing the safety margin for thermal failure, or causing it to fail when idling. So, sometimes it's important to consider above band noise.

Also, your graph shows the integrated noise magnitude vs. the integrating bandwidth, and it suggests that the 1MHz amplifier gets really nasty above 100kHz. When displaying noise vs frequency, the tradition is to graph the noise density per root Hz, not the integrated total noise from 0Hz to that frequency. Those two amplifiers will probably have overlapping noise density graphs, up to 200kHz, and it'd be easier to see what the amplifier is doing, rather than having to un-integrate the noise from 0Hz on up to the frequency of interest. It also makes displaying low frequency noise a lot easier, since it won't get auto-scaled away from the higher level of integrated HF noise.
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Old 14th November 2017, 08:26 PM   #4
flyingfishtw is offline flyingfishtw  Taiwan
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It's just a simulation chart, so it fits op amp's datasheet quite well with no real environment disturbance. You're spot on, It is much easier to see the effect of BW on noise with the RTO chart. Thank you for the good advice to help me understand the concept better.

What I don't quite understand is the relationship between noise uVrms & frequency. I forgot to tag flat region on the RTO chart which is 76nV/hz, for example when you measure 10khz, there will be 7.6uVrms noise masking 10khz signal, and on 20khz there will be 10.7uVrms noise masking 20khz signal?

or it's just 10.7uVrms over the whole audible bandwidth?
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Old 14th November 2017, 08:27 PM   #5
flyingfishtw is offline flyingfishtw  Taiwan
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Quote:
Originally Posted by lcsaszar View Post
Human hearing upper limit is around 18 kHz (young) or 12 kHz (elder).
Well, I listen to headphone, so I guess I'm in between
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Old 14th November 2017, 08:30 PM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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I am 60 and cant hear above about 9kHz.
Probably down to 40 years of playing electric guitar and being a mobile DJ.
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Old 14th November 2017, 09:31 PM   #7
flyingfishtw is offline flyingfishtw  Taiwan
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Do you test it with pure sine wave or you can identify the missing part in the music? Funny thing about brain is, it'll recall how music sounds like, and fill the details a bit for you. I don't know if that's myth or truth...
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Old 14th November 2017, 09:33 PM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Yes tested with signal generator and sine wave.
The music sounds fine to me.
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Old 14th November 2017, 09:38 PM   #9
scottjoplin is offline scottjoplin  Wales
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I have high frequency hearing loss, because it happens slowly over time you're not aware of it happening and you adapt. I recently got some digital hearing aids which compensate but they've taken a lot of getting used to due to everything sounding tinny. They help with TV and speech but are useless with music due to the delay at high frequencies which cause distortion. I listen to my hi-fi without them but turn the treble up.
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Old 14th November 2017, 09:43 PM   #10
Monte McGuire is offline Monte McGuire
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Is noise audible above 20kHz
Quote:
Originally Posted by flyingfishtw View Post
What I don't quite understand is the relationship between noise uVrms & frequency. I forgot to tag flat region on the RTO chart which is 76nV/hz, for example when you measure 10khz, there will be 7.6uVrms noise masking 10khz signal, and on 20khz there will be 10.7uVrms noise masking 20khz signal?

or it's just 10.7uVrms over the whole audible bandwidth?
The original chart shows the total noise vs frequency F. This is what you'd get if you integrated the noise from 0Hz to F. That magnitude is a voltage that would heat up a resistor just the same as a DC voltage of the same magnitude would - it's the total voltage at all frequencies from 0 to F.

A noise signal is a 'density', sort of like a gas, so when you zoom in to a frequency, using narrower and narrower bandwidth, the noise level continues to decrease. It's like driving in fog - at a distance, the fog is thick and you can't see but as you get closer, the total fog decreases and you can see. The density hasn't changed, you're just taking a smaller slice of it.

A non-random signal like a sine wave however does not behave this way - when you zoom in, analyzing a narrower and narrower bandwidth around the signal, the level stays the same. This has the result that an FFT of a random signal will show a level of noise in each bin according to the bandwidth of each FFT bin. So, a 16K point FFT will show a 'noise floor' very different from a 1024K point FFT, even though the signal is the same. Sine waves within that signal however will not change magnitude. This property is very useful when analyzing a signal, as it allows you to 'peel back' random signals and somewhat separate them from deterministic signals.
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