Feedback

CurioCity
23 January 2012

Pop Quiz: What is feedback?

a) The input you received last semesters research report, which you may as well just torched and started over on!

b) What mother birds do for their babies when they are hungry- mmm!

c) A not-so-harsh name for the lecture you get from dad about the curfew you missed last Saturday night.

d) The re-amplification of a signal through a continuous acoustic-electronic loop.

Well if you answered (d), you're right!...at least for where this article is concerned.

Everyone has heard feedback, or more appropriately, that annoying high-pitched whine put out by a loud speaker system. And when it sounds, you likely just want to cover your ears and shout at the person causing the shock to your poor ears! Feedback can come from many sources. For example, you may have heard it:

1. Over the school PA system during morning announcements

2. When giving grandma a hug, and you got too close to her hearing aid

3. On those commercials where they want to emphasize the fact that they are making an announcement through a microphone

4. At rock concerts where the guitar creates anything from a smooth "hum" to a loud shriek

Outdoor rock concerts go hand in hand with summer and if your fave band is coming to town, you're likely planning on checking them out.

Well if you've gone to concerts before, and gotten there really early to catch the sound check, you may have seen one of the crew test out a guitar and get just a little too close to the amp, creating a high-pitched squeal. Or maybe you've actually heard feedback from a guitar during the concert, making you wonder if the band really knows what they're doing on stage.

So what's the deal? What causes those high-pitched sounds to begin with?!? Let's use our definition from the pop quiz to find out what feedback is all about.

Simply put, feedback, or more appropriately positive feedback, is the result of a particular sound that has been made louder and louder by running through the same system over and over. In other words, there is a loop that the sound travels through, and a key part of that loop includes an amplifier.

 

Did You Know?
Audio feedback is also known as the Larsen Effect

You've played a rubber-band guitar right? When you pluck that rubber band, you can see it vibrate back and fourth, and you also know that the vibration is what creates sound.

With an electric guitar, the concept is pretty much the same, except that the sound has to be converted into electricity at some point. So you simply strum the guitar, the vibrations of the strings are picked up by the pick-ups (named by a genius no doubt), and they convert the sound into electricity.

That electric signal can now be sent through a cable and into the amplifier. In its most basic role, the amplifier simply adds power to the signal, making it louder. Finally, the amplified signal is played out through the speaker within the guitar amp.

 

Did You Know?
Feedback can be avoided using automatic anti-feedback filters aka "feedback destroyers" or "feedback eliminators"

Now, the only thing that is missing from our feedback project is the loop. Somehow, we need to get the sound from the amplifier speaker, back to the guitar strings themselves. There are two ways to do this: Either get the face of the guitar and amplifier fairly close to each other, or turn up the output of the amp so that the signal is able to reach the guitar.

The reason that this completes the loop is because, like a loud car stereo vibrating car windows, the sound coming from the amplifier will vibrate a set of guitar strings. So the vibration of the guitar strings caused by the output of the amplifier just goes back through (or is fed-back through) the system, only to be made louder! Until the loop is broken, the feedback will continue to become amplified and sent back to it's source: the guitar.

 

Did You Know?
Music legend Jimi Hendrix and current bands like Audioslave have used feedback in a controlled manner to create inventive hip hop solos

So here's another question to ponder: Can you get feedback from an acoustic guitar? If so, how? If not, then why? Here's a word of caution to ponder as well: loud enough sounds including music and guitar feedback, either brief or sustained, can and will cause damage to your hearing. Do not subject yourself to sounds loud enough to cause ringing in your ears- that's your body's request to turn the sound down!

Mike is a cochlear implant audiologist for the Cleveland Clinic in Cleveland, Ohio. A cochlear implant audiologist specializes in working with people who have a significant hearing loss, providing them with the ability to hear sound again through a special electronic device. He is quite the handyman around the home and is also a great chef!

CurioCity

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