How to Stop Standing Waves

Have you ever had your hearing tested?  I'm talking about when you hear something and wonder WHAT IN THE WORLD IS THAT NOISE?  The band looks at you.  The people in the room look at you.  It's like nothing you've ever heard.  Welcome to a standing wave.

Working a recent event, this very phenomenon happened to me.  The drummer hit his toms and there, floating in the air, was a very very odd sound.  The sound was something akin to an echo but it wasn't.  In fact, it seemed the sound level should have degraded faster than it actually did.  Standing waves are weird!

What are Standing Waves?

A standing wave is the result of a sound wave that bounces between two or more surfaces and emphasizes one specific frequency that you hear as the waves reinforce each other.  

When the wave bounces off the surface, it changes phase.  In the case of waves that create a standing wave, the reflected wave is a mirror image of the original.  The waves then combine. If the amplitudes of the two waves have the same sign (both positive / both negative), they will add together to form a wave with a larger amplitude.  This adding together is called constructive interference.  This added wave doesn't appear to move, thus it's called a standing wave.

This image shows the blue and red waves moving back and forth with the resulting black standing wave being produced.  

(img source)

Standing waves are usually low frequency waves below 300 Hz.  Above 300hz, the waves tend not to reflect directly back and the sound is greatly influenced by the objects in the room and the composite of the room's walls, floor, and ceiling.  

Here's a chart to give you an idea of the size of wavelengths based on the frequency.  As you can see, the lower the frequency, the larger the wavelength such as a 20 foot distance from crest to crest.

What Causes Standing Waves?

Standing waves are created when the distance between the walls is a multiple of a sound's wavelength.  

Walls that are 20 feet apart, with a wavelength that is 19 feet long, won't produce the conditions for the wave reinforcement.  However, if the walls are 20 feet apart and the wavelength is 10 feet or 20 feet, then a standing wave will be produced because of the reinforcement.  

Standing waves can be caused when waves bounce between;

•     Opposite walls
•     Four sides of the room
•     All six sides of the room (given most rooms have four walls, a floor, and a ceiling!)

What Can I Do When I Hear a Standing Wave?

The solution to stopping a standing wave is cutting the offending frequency of the related instrument.  In the case of a digital mixing board which allows for surgical precision, cut a very narrow amount of the offending frequency.  

When I experienced this recently, I was running an analog mixer so I used the house EQ in the rack to cut the low end in the 80 Hz range.  This way, I could still use my channel EQ to get the right "tom" sound.

Summary

Low end sounds create standing waves when the reflected sound between walls is directly opposite in phase.  By cutting the frequency, you can eliminate the problem.

What's the oddest sound you've ever heard?