<p>I suggest you take the instrument that you used to make recordings in the wild, and make recordings in a quiet room, ideally one padded with foam - like those used for bat echolocation research - with the lights out (as lights have a hum that can contaminate your recordings). Then run your TOL analysis on this data, and plot whatever percentile of this atop the TOL percentiles of your ambient data. In the figure you attached, we can see that above ~30 kHz, the ambient noise recordings are limited by the self noise of the instrument. This means that above this frequency, your recording chain is not able to capture how quiet the environment is.</p>

CC BY-SA 4.0
2022-06-30T22:14:40.960Z
Strictly speaking, I understand that you are measuring the maximum of the ambient noise of the laboratory room and of the microphone+preamp selfnoise. For instance, in my lab soundproof chamber, the ambient lowfrequency level is very high (building fan system) and definetely higher than the selfnoise of some of my low-noise microphones.
Noil
CC BY-SA 4.0
2022-07-01T07:27:56.247Z
Agree @Noil, but I fail to see where I could readily obtain quieter ambient noise measurements
Chloe
CC BY-SA 4.0
2022-06-30T15:19:57.007Z
2022-06-30T15:19:57.007Z
Chloe
<p>I would remove the micro/hydrophone and replace it with a small resistors. what you then measure is the thermal noise of the resistor.</p>
<p>If you change the resistor value, you can measure in addition to the electronic noise also the gain of the acquisition chain</p>
<p>The impact of the resistor R on the measured voltage V_out is</p>
<p>V_out^2=(e_n^2+i_n^2 * R^2 + 4 * k_B T * R) * G^2</p>
<p>where</p>
<p>e_n input noise voltage;
i_n input noise current;
G gain</p>
<p>For R=0 (short) we have
V_0^2=(e_n^2 ) * G^2</p>
<p>which gives us an expression for Gain G</p>
<p>In the end we have a set of linear equations for e_n^2, i_n^2</p>
<p>e_n^2 ((V_out^2)/(V_0^2 )-1)-i_n^2 * R^2 = 4k_B T *R</p>
<p>At 20 C (293 K) we have :</p>
<p>4k_B T=0.016173 (nV^2)/(Ω*Hz)</p>
<p>Using at least 2 measurements with R&gt;0 one obtains values for input noise voltage, input noise current, and gain</p>
<p>Edit: this gives self noise for whole acquisition chain.</p>
<p>Edit 2: The expression for 4k_B T is relative 1 Hz. What I typically do is not to measure the broadband noise voltage but the spectral density so may self noise estimates ate also relative to 1 Hz.</p>
<p>Edit 3: In practice, I record data having sensor (hydrophone in my case) replaced with different resistors and use MATLAB's pwelch function to get the PSD.</p>
<p>ceterum censeo: we need latex style math formula!</p>

CC BY-SA 4.0
2022-06-30T22:21:48.777Z
Could you confirm that this is for measuring the whole aquisition chain selfoise minus the microphone selfnoise?
Noil
CC BY-SA 4.0
2022-07-01T05:23:01.180Z
Yes, this is self noise of the whole analog chain, excluding microphone. I edited answer
WMXZ
CC BY-SA 4.0
2022-07-01T07:29:43.533Z
Thanks for this useful answer and walking me through it, @WMXZ. I should have specified that the instrument is a SoundTrap (Ocean Instruments), and so I cannot disassemble the recording chain.
Chloe
CC BY-SA 4.0
2022-07-01T15:25:38.067Z
For such systems you have to find an silent place. As most self noise is flat at higher frequencies, you only should avoid strong 50/60 Hz lines, that are picked up by hydrophone electronics and connection.
WMXZ
CC BY-SA 4.0
2022-07-04T12:43:02.213Z
Agreed, that is what I tried to do.
Chloe
CC BY-SA 4.0
2022-06-30T18:40:24.883Z
2022-07-01T05:35:08.210Z
2022-07-01T05:35:08.210Z
WMXZ
<p>I have some ambient noise recordings and am uncertain if the lower SPLs represent true low noise levels in the environment, or are perhaps due to the noise floor of the instrument itself.</p>
<p>I have plotted the third octave levels of an ambient noise recording, but am noticing that the minimum sound pressure level values increase at high frequencies where I don't expect them to. I suspect that this is a reflection of the sensitivity of my instrument, rather than a true representation of ambient noise in the environment.</p>
<p>I have seen some examples in the literature (<a href="https://marinebioacoustics.files.wordpress.com/2020/02/malinka-et-al-2020.pdf" rel="nofollow noreferrer">see attached</a>) of plotting the &quot;self noise&quot; of an instrument on top of this, however I am uncertain how to get these values across the frequency range of my instrument. How do I determine what this self noise (or 'noise floor') is, so I can plot it atop mine too? I want to get this across the spectrum of the recording range of an acoustic monitoring instrument.</p>
<p><a href="https://i.sstatic.net/WyhZJ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WyhZJ.png" alt="enter image description here" /></a></p>
<p>EDIT: Device is a <a href="http://www.oceaninstruments.co.nz/soundtrap-300/" rel="nofollow noreferrer">SoundTrap</a>, so I cannot disassemble the recording chain.</p>

CC BY-SA 4.0
2022-06-30T22:01:27.953Z
what does TOL stand for in the Y axis?
Noil
CC BY-SA 4.0
2022-07-01T07:26:42.593Z
Apologies, TOL = third octave level
Chloe
<noise><instrumentation>
How do you know when minimum noise measurements are due to the instrument's noise floor or due to the environment?
# How do you know when minimum noise measurements are due to the instrument's noise floor or due to the environment?

(Asked by: username_0 on 2022-06-30)

I have some ambient noise recordings and am uncertain if the lower SPLs represent true low noise levels in the environment, or are perhaps due to the noise floor of the instrument itself.

I have plotted the third octave levels of an ambient noise recording, but am noticing that the minimum sound pressure level values increase at high frequencies where I don't expect them to. I suspect that this is a reflection of the sensitivity of my instrument, rather than a true representation of ambient noise in the environment.

I have seen some examples in the literature ([see attached](https://marinebioacoustics.files.wordpress.com/2020/02/malinka-et-al-2020.pdf)) of plotting the "self noise" of an instrument on top of this, however I am uncertain how to get these values across the frequency range of my instrument. How do I determine what this self noise (or 'noise floor') is, so I can plot it atop mine too? I want to get this across the spectrum of the recording range of an acoustic monitoring instrument.

[![enter image description here](https://i.sstatic.net/WyhZJ.png)](https://i.sstatic.net/WyhZJ.png)

EDIT: Device is a [SoundTrap](http://www.oceaninstruments.co.nz/soundtrap-300/), so I cannot disassemble the recording chain.

**Tags:** noise, instrumentation

> **Comments:**
> **username_1**: what does TOL stand for in the Y axis?  
> **username_0**: Apologies, TOL = third octave level  

---

## 2 answers

### Answer by username_0 on 2022-06-30. Score: 4

I suggest you take the instrument that you used to make recordings in the wild, and make recordings in a quiet room, ideally one padded with foam - like those used for bat echolocation research - with the lights out (as lights have a hum that can contaminate your recordings). Then run your TOL analysis on this data, and plot whatever percentile of this atop the TOL percentiles of your ambient data. In the figure you attached, we can see that above ~30 kHz, the ambient noise recordings are limited by the self noise of the instrument. This means that above this frequency, your recording chain is not able to capture how quiet the environment is.

> **Comments:**
> **username_1**: Strictly speaking, I understand that you are measuring the maximum of the ambient noise of the laboratory room and of the microphone+preamp selfnoise. For instance, in my lab soundproof chamber, the ambient lowfrequency level is very high (building fan system) and definetely higher than the selfnoise of some of my low-noise microphones.  
> **username_0**: Agree @username_1, but I fail to see where I could readily obtain quieter ambient noise measurements  

---

### Answer by username_2 on 2022-06-30. Score: 2

I would remove the micro/hydrophone and replace it with a small resistors. what you then measure is the thermal noise of the resistor.

If you change the resistor value, you can measure in addition to the electronic noise also the gain of the acquisition chain

The impact of the resistor R on the measured voltage V\_out is

V\_out^2=(e\_n^2+i\_n^2 \* R^2 + 4 \* k\_B T \* R) \* G^2

where

e\_n input noise voltage;
i\_n input noise current;
G gain

For R=0 (short) we have
V\_0^2=(e\_n^2 ) \* G^2

which gives us an expression for Gain G

In the end we have a set of linear equations for e\_n^2, i\_n^2

e\_n^2 ((V\_out^2)/(V\_0^2 )-1)-i\_n^2 \* R^2 = 4k\_B T \*R

At 20 C (293 K) we have :

4k\_B T=0.016173 (nV^2)/(Ω\*Hz)

Using at least 2 measurements with R>0 one obtains values for input noise voltage, input noise current, and gain

Edit: this gives self noise for whole acquisition chain.

Edit 2: The expression for 4k\_B T is relative 1 Hz. What I typically do is not to measure the broadband noise voltage but the spectral density so may self noise estimates ate also relative to 1 Hz.

Edit 3: In practice, I record data having sensor (hydrophone in my case) replaced with different resistors and use MATLAB's pwelch function to get the PSD.

ceterum censeo: we need latex style math formula!

> **Comments:**
> **username_1**: Could you confirm that this is for measuring the whole aquisition chain selfoise minus the microphone selfnoise?  
> **username_2**: Yes, this is self noise of the whole analog chain, excluding microphone. I edited answer  
> **username_0**: Thanks for this useful answer and walking me through it, @username_2. I should have specified that the instrument is a SoundTrap (Ocean Instruments), and so I cannot disassemble the recording chain.  
> **username_2**: For such systems you have to find an silent place. As most self noise is flat at higher frequencies, you only should avoid strong 50/60 Hz lines, that are picked up by hydrophone electronics and connection.  
> **username_0**: Agreed, that is what I tried to do.  

---

