October 29, 2018

ATM should standardize cash-spooling sound

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ATM Cash-Spooling Sound

I wondered if it is possible to determine how much money someone removed from the ATM based on how long the ATM cash-spooling sound is.  The cash-spooling sound is the sound of bills rotating around rollers as your money is counted before it exists the ATM.

I assumed that this sound would be standardized. If you were taking out one bill, or 100 bills, the machine would produce a sound of roughly the same duration.  

Turns out, this is not true.

I did a quick experiment taking out the smallest amount an ATM would produce, as well as the largest amount.

The smallest amount (1 bill) produced noise for about 2 seconds, and the largest amount (40 bills) made noise for about 12 seconds.

Listen for yourself:

Why is this important?

In theory you could tell how much money someone removed from an ATM by listening, either behind them in line, or outside of an ATM machine room.

Problem? It does impact user privacy.

Problem? If a criminal was trying to select who to rob after they used an ATM, listening to the machine sound could influence this decision.

How can this be fixed?

The cash spooling sound originates from cash going through the rollers.

The ATM could have a compartment that holds placeholder/fake bills that is pulls from to make up for the difference in the number of bills requested compared to the standardized number that should be spooled.

The placeholder bills would return to the placeholder compartment afterwords to be re-used.


Extra wear and tear on the machine passing placeholder bills through to standardize the sound to the longest dispensing time.

How often do criminals actually listen to how long an ATM machine was operating when selecting their victim?

Criticism: some may suggest it is impossible to know how much money a machine is dispensing, because if it pulled from larger bill denominations, a shorter sound may in fact mean a higher total amount of cash. True. However, from my limited experience many machines only have one bill they primarily pull from. Knowing how long the machine made noise for would be a pretty good way to know roughly how much was taken out.

Note: TIME tried to suggest in a post back in 2011 that the sound is fake. But I suspect this is not true. Article Brad Tuttle June 23 2011.

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