# I want to remove AC noise **URL:** \ **Category:** Support & Discussions\ **Tags:** eeg\ **Created:** [September 9, 2022, 11:25am UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561 "2022-09-09T11:25:15Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![OnoreCoffee](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/onorecoffee/32/1887_2.png) [@OnoreCoffee](https://mne.discourse.group/u/OnoreCoffee)\ **Post date:** [September 9, 2022, 11:25am UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/1 "2022-09-09T11:25:15Z")
MNE version: 1.0.3 OS: macOS Monterey Hello. I am trying to measure event-related potentials. Preliminary experiments have shown that there is too much AC noise, so I am thinking of removing it. What code should I write to remove it? Thank you in advance for your help. ---
**Author:** ![agramfort](https://avatars.discourse-cdn.com/v4/letter/a/7c8e57/32.png) [@agramfort](https://mne.discourse.group/u/agramfort)\ **Post date:** [September 9, 2022, 12:17pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/2 "2022-09-09T12:17:31Z")
what is AC noise ? you mean line noise? can you show the PSD of your data? Alex ---
**Author:** ![OnoreCoffee](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/onorecoffee/32/1887_2.png) [@OnoreCoffee](https://mne.discourse.group/u/OnoreCoffee)\ **Post date:** [September 9, 2022, 2:59pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/3 "2022-09-09T14:59:52Z")
Thank you for your reply. It is about the line noise. I wrote AC noise because I think it is caused by AC power supply. I am wondering how to write the code to remove this in MNEPython. Sorry for being a beginner. What do you mean by PSD? Sorry for the lack of explanation. Thank you in advance. ---
**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\ **Post date:** [September 9, 2022, 4:24pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/4 "2022-09-09T16:24:10Z")
PSD = Power Spectral Density. Let’s start by confirming that your data is contaminated by powerline noise, 50 Hz in Europe, 60 Hz in the US (and I don’t know elsewhere). Could you load your data into a `Raw` object (see the `Loading Data` section of the first tutorial: [Overview of MEG/EEG analysis with MNE-Python — MNE 1.1.1 documentation](https://mne.tools/stable/auto_tutorials/intro/10_overview.html#loading-data)) and then use the [`plot_psd`](https://mne.tools/stable/generated/mne.io.Raw.html#mne.io.Raw.plot_psd) method to display the PSD? Link: [mne.io.Raw — MNE 1.1.1 documentation](https://mne.tools/stable/generated/mne.io.Raw.html#mne.io.Raw.plot_psd) Code snippet: ```python from mne.io import read_raw raw = read_raw(fname, preload=True) # where fname is the path to your compatible file raw.plot_psd() ``` ---
**Author:** ![OnoreCoffee](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/onorecoffee/32/1887_2.png) [@OnoreCoffee](https://mne.discourse.group/u/OnoreCoffee)\ **Post date:** [September 11, 2022, 6:33am UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/5 "2022-09-11T06:33:33Z")
Thank you for your reply. Sorry for the delay in responding. In my area the power supply noise is considered 50Hz. I ran it on the raw data and the following figure was output. ![2022-09-11 15.31.26](https://global.discourse-cdn.com/free1/uploads/mne/original/2X/e/e495ccb7815c5b6e6ed31c9b0c559acd116d2208.png) ---
**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\ **Post date:** [September 11, 2022, 10:05am UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/6 "2022-09-11T10:05:06Z")
As you can see, there is no powerline noise. You do not have a peak at 50 Hz or 60 Hz. Also, the dataset seems to be bandpass filtered between 1 and 20 Hz. The peak at 20 Hz looks weird to me. Could you do the same plot on the `raw` data **before** BP filter? ---
**Author:** ![OnoreCoffee](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/onorecoffee/32/1887_2.png) [@OnoreCoffee](https://mne.discourse.group/u/OnoreCoffee)\ **Post date:** [September 11, 2022, 10:30am UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/7 "2022-09-11T10:30:31Z")
Thank you for your reply. As you can see, the data was after filtering. Here are the results plotted with the data before filtering. ![2022-09-11 19.25.00](https://global.discourse-cdn.com/free1/uploads/mne/original/2X/e/e6e2d777cbf064541f3a480346af525965174bd5.png) ---
**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\ **Post date:** [September 11, 2022, 10:52am UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/8 "2022-09-11T10:52:34Z")
I have no idea what can cause noise at 20 Hz. If someone know, I’m very curious. Anyway, you can apply a notch for 20 Hz and the harmonics. This way, you will also be able to increase your BP upper limit to more than 20 Hz if you want: ```python import numpy as np raw = [...] # load data raw.notch_filter(np.arange(20, 120, 20)) ``` ---
**Author:** ![OnoreCoffee](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/onorecoffee/32/1887_2.png) [@OnoreCoffee](https://mne.discourse.group/u/OnoreCoffee)\ **Post date:** [September 11, 2022, 12:09pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/9 "2022-09-11T12:09:15Z")
Thank you for your reply. Filtering at every 20 Hz resulted in the figure. I think I have lowered it more than necessary, but is it OK? ![2022-09-11 21.00.13](https://global.discourse-cdn.com/free1/uploads/mne/original/2X/6/6edc7e48a1d82709b464af39dca603be28fa7764.png) ---
**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\ **Post date:** [September 11, 2022, 1:28pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/10 "2022-09-11T13:28:50Z")
That’s what a filter does, it looks OK. I prefer to avoid using `notch` filters, but I don’t see another solution in your case. Alternatively, you should figure out where this noise is coming from, remove the source, and record clean data. ---
**Author:** ![OnoreCoffee](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/onorecoffee/32/1887_2.png) [@OnoreCoffee](https://mne.discourse.group/u/OnoreCoffee)\ **Post date:** [September 11, 2022, 2:27pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/11 "2022-09-11T14:27:33Z")
I understand. Thank you for your kind . ---
**Author:** ![drammock](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/drammock/32/4_2.png) [@drammock](https://mne.discourse.group/u/drammock)\ **Post date:** [September 14, 2022, 7:40pm UTC](https://mne.discourse.group/t/i-want-to-remove-ac-noise/5561/12 "2022-09-14T19:40:00Z")
> [@mscheltienne](#): > > I have no idea what can cause noise at 20 Hz. If someone know, I’m very curious. One thing to check is if the sampling frequency was somehow mis-specified, so that interference at 50 Hz was showing up as if it were 20 Hz. I don’t know _how_ this would have happened though so it’s a wild guess. Another possibility is that it’s a coming from equipment in an adjacent lab in the same building. I’ve seen this once, it was coming through on an audio recording in a sound-attenuated booth. It was a timing signal for some custom physics equipment, and it was exactly 1kHz. (at 20 Hz this is almost surely not a timing signal 🙂 but may still be caused by nearby equipment)