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Dr Sound explains the echo phenomenon

Some of you might recognize me as Dr Sound but my real name is in fact John-Erik Eriksson and I work as an acoustic designer here at Konftel. In this blog post I will try to explain the phenomenon echo in a telecom context.

Most of us knows what echo sounds like – it is what we for example hear in a big room, when shouting “hello” the room will answer in a fading manner “hello… hello…” – but not that many knows what it really is, how it arises and how to avoid it.

When we speak, the sound of our voices travels from our mouths straight out in the air, but some also passes through our skull on the way to our ears. This is what we call side tone in telecom context. Even though our sensitivity to this sound is extremely low, it's really important for us, since we need to hear ourselves to be able to speak in a natural tone.

The sound of our voices also goes out into the room, bouncing against walls until returning to our ears. The sound will be slightly delayed, and this is something the human ear is very sensitive to, even at extremely low sound levels. The longer it takes for the sound to return, the more we'll notice it and the more disturbing it will be.

This is naturally something we want to avoid during conference calls, but when echoes occur, they can be in two different forms. One is line echo, which is caused by crosstalk in cabling or converters. It doesn't take much for this to be VERY disturbing. It's enough with less than one percent of the sound coming back to cause problems.

In a digital environment, this problem can be rather large since the sound passes through several converters. Each conversion takes time and consequently increases the risk for perceived echoes. When the sound later comes out in a room, there is additional echo. This is called acoustic, or spatial, echo.

The solution to this is echo cancelling. Both through line echo cancelling, to remove line echo, and through acoustic echo cancelling, which removes echoing caused by a room's configuration and general acoustics. The process of removing the acoustic echo represents the most difficult challenge, and it's here the focus must be placed in attaining optimal echo cancellation. From a purely practical standpoint, echo cancelling works by a device “learning” the echo and creating a counter-phase sound to override it.

Do you want to hear me talk about echo and listen to some examples? Check out this video that covers it all.