Listening to Light

The ear can sometimes hear what the eye can’t see

The eye has a persistence of .02 seconds meaning any light operating at 50hz or above is perceived to be continuously on.  The ear, on the other hand, is much better at hearing frequency change than the eye is at seeing it.

The human ear can detect frequencies from about 50Hz to around 18 KHz on average.  Therefore, many forms of light that can and cannot be discerned by the human eye can be heard by the human ear.

At this point you may be wondering how can you LISTEN to light?  It comes down to building a rather simple circuit that includes a photo-transistor for detecting the light source, an RC circuit to refine the input voltage, and an audio amplifier to boost the audio signal for listening through headphones or a speaker.

This device is sometimes known as a “Light Listener” or a “Photoacoustic Converter” and is the subject of many engineer and physics science projects.

The Light Listener

As with anything new, listening to light might sound like just a bunch of noise, but over time a trained ear could distinguish the difference between hundreds if not thousands of sources. Eventually, you could detect the subtle differences between a TV and fluorescent room lights, or between a flashlight and car headlights.

Some light sources are completely invisible to the human eye (infrared and ultraviolet) and can be converted easily to sound.  Infrared remote controls give off coded light signals that make easily discernable tones for each different function of the remote. Everything from LEDs to fireflies make a sound with the light that they emit.  By building this simple circuit you can listen to the light that you encounter on a day to day basis. This circuit can produce “loud” sounds, so don’t place the speaker close to your ears.

Listening to Natural Lights

Lightning flashes produce sharp clicks and pops. These sounds are picked up from the flashes of light and have no direct relationship with the sound of thunder that is heard much later after the lightning strike. Nighttime is the best time to listen to sounds produced by the light from lightening. It’s also possible that you will be able to hear sounds from lightning flashes that can not be seen by the human eye. Obviously, being outside in a lightning storm isn’t the safest course of action. If you intend to “listen” the lightning flashes, do it from inside a house or car.

Flames produce many different sounds. When the air is still, a soft rushing sound is heard. When the flame is disturbed by moving air, crackles and pops are heard. These sounds are seperate from the sounds emitted by a the fuel such as wood or wick that is reacting as it is burned.

Sounds can also be detected from winged insects that are flying in sunlight. When their wings reflect light to the detector, a buzz or hum will be heard. At dusk, a nearby firefly will produce a soft click for each flash. These sounds are independent from other physical sounds created by insects such as the hum of their wings moving air.

Sunlight can also produce sounds that can be detected by the listener as the sunlight is filtered through leaves, reflected off of windows, and passed through the gaps in a picket fence; resulting in a pop, pop, pop sound.

Listening to Artificial Lights

Flashlight beams, when swept across the light listener at different speeds, will create different sounds ranging from a soft swoosh to sharp pops. Tap the flashlight with a pencil and a ringing sound will be heard as the filament vibrates.

The headlights of cars, trucks, and motorcycles will produce a distinctive ringing sound when the vehicle is moving on a rough or bumpy road. The various rings can sometimes create what some describe as singing.

Electronic Displays are usually powered by rapid pulses of current. The flashes are merged into continuous light by the slow response of the eye. They can, however, be heard clearly as a buzz or a hum with the listener.

The displays of older televisions and computer monitors are formed by sweeping an electron beam across a phosphor coated screen. The light listener will transform that pulsating phosphor into a recognizable pulsating buzzing sound.

With a light listener circuit and a little time on your hands, you can listen to the sounds of the visible, and sometimes invisible, world around you as broadcast by light.

Further Reading:

- Engineer’s Mini-Notebook by Forrest M. Mims III
- Listening to Light Teacher’s Notes (PDF)
- The Physics Teacher -- September 2000—Volume 38, Issue 6, pp. 356-358

Discuss this Story

Display Name:

Submit the word you see below:

Previous Comments

1. you can replace the photo transistor with a solar cell and it works just as well.

   Posted by by the way  on  07/28/09  at  12:53 PM

2. Your first paragraph is misleading.  You are confusing wave frequency with the repitition frequency of a blink.  To try to explain, TV for example displays a picture at a certain frequency (60 Hz for standard def NTSC, 50 Hz PAL), the light within the picture however operates at a completely different frequency, each color having a unique frequency.

   Visible light has an extreemly high frequency, at between 430 Terrahertz for red to about 750 terrahertz for blue.  This is an electromagnetic wave, and cannot be heard by the human ear. 

   The phototransistor in the circuit doesn’t detect the frequency of the light (ie the color), but the frequency of the repitition of light (the on/off), therefore saying you are hearing light is misleading, because you are only hearing a representation of the repitition

   Posted by RT  on  09/22/06  at  11:57 PM

3. I liked your article. I found here very useful information about human senses. Good work! Waiting for more.

   Posted by Florist UK  on  09/15/06  at  12:35 PM

4. This seems believably true. I am sometimes in my room and someone opens the door behind be that’s not closed all the way and attempts to scare me for example. But I can hear it; it’s not the door, it’s not the person; it’s just a weird “can’t-be-described” sound. It’s like I can hear the air of the door come into the room or something.

   Posted by Alex  on  09/12/06  at  03:06 PM