Filters Quick Tip

“Without a Filter, man is just chaos walking.” – Patrick Ness

EEG is no different than the quote above…our recordings would be chaos without them.

In simple terms:

a filter is a device or process that removes some unwanted components or features from a signal.

Mental Note:  30%

Thats the % to remember when asked about filters…

When a low-frequency filter encounters a sine wave that happens to be exactly at its cutoff frequency, it cuts down the amplitude of that wave by approximately 30%.

So since for EEG our LFF (low frequency filter) is 1 Hz – the amplitude of a 1 Hz will be reduced by 30%.

Key Point to Note:

Waves at frequencies below the LFF are reduced by somewhat more than 30%—Also, waves at frequencies somewhat above the LFF are also reduced in size, but by less than 30%.

So its more like a roll off than a cut-off.

See graph below:

Graph shows a LFF of 5 Hz

The same is true on the other end of the range…when talking about the HFF (high frequency filter):

Since our HFF is 70 Hz – the amplitude of a 70 Hz will be reduced by 30%.

But this time:

Waves at frequencies higher than the HFF are reduced by somewhat more than 30%—and waves at frequencies somewhat slower than the cutoff frequency are also reduced in size by the filter, but by less than 30%.

Understanding filters is key to understanding what you are looking at on the tracing…it is also important to understand how your use of filters can alter the display.

Need additional help preparing for your boards? Check out the Resource Library on this site….or just email me at

Lets Stay Curious and Grow What We Know

North Texas Neurodiagnostic Academy

Enrolling Now for January 2022 Start Date

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