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.
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?
Thanks . So are you saying that standing waves will be produced by 2x the length of the wall say for the sound coming out of an 8 foot organ pipe C (about 64 hz)?
So we are planning to put in an organ in an auditorium that is 100 feet in lenght from the organ Case with 32′ pipes in it (16hz). and with of the building is 60 feet with a height of 70 feet. The organ will stand on a balcony that is 12 feet above the floor.
We have been told that putting in a 32′ Bourdon rank (that is a humming flute kind of sound) or a 32 Gedackt in the Organ we are planning might be wasted money because of standing waves especially low flute sounds. . Some people can hear this low frequency but others either do not or just feel the sound waves hitting them.
So according to what you say —would we have a problem with the Bourdon or Gedackt sounds or not? (you can hear these sounds in a good room with K—speakers used for Allan Electronic Keyboard pseudo-organs (an Organ is not an organ unless it has pipes sounded by compressed air sitting on an air chest controlled by an console (that is where they keyboards and stop knobs are,
There’s a lot of math that goes into giving you an answer to your specific situation. If I was in your shoes, I’d hire a company that works with acoustics and has organ experience.