Standing Waves

Today in class we experimented in room 11 was the concept of standing waves. A standing wave is a sound wave that reflects off a surface back in the same path as it came from and continues this pattern back and forth over and over again. This can cause the certain frequencies to be emphasized or can cause them to be cancelled if the wave ever went out of phase. There are 3 different types of standing waves: Axial, tangential and oblique.
Axial standing waves are the reflections between 2 surfaces, tangential are the reflections between four surfaces and oblique are reflections between 6 surfaces.

In class today I was put into a group with Eoin and William and we were tasked with finding out the length of point B of room 11 as shown below.

We found the length of B to be 7.265m. To find out which frequencies standing waves would occur at we used the equation Hz= Speed of Sound/2 lambda (2 times the length of the room)

So the equation became Frequency= 344/14.53 which equaled 23.67 Hz as the fundamental frequency. Then we multiplied this fundamental frequency by different whole numbers to find out what overtones more standing waves would occur at and found this:

1- 23.67Hz
2- 47.35Hz
3- 71.02Hz
4- 94. 70Hz
5- 118.37Hz
6- 142.37Hz
7- 165.72Hz
8- 189.40Hz
9- 213.07Hz
10- 236. 75Hz

The frequencies we found were then generated on the computer and played through the speakers. We could just about hear the lower frequencies as they were like a low rumbling and I could feel some in my lower chest area. When the frequencies were played out we noticed that the railings and windows in the room started to strongly resonate with some of the frequencies especially when they were played louder. Overall you could definitely tell that these frequencies were being influenced by the room as moving the speakers positions had an affect on the reflections specifically they’re amplitude felt affected when this was done.

Standing waves can cause problems in the live environment as the emphasis and cancellation of frequencies brought from the standing waves can produce a very muddy sounding mix to the audience and negatively effect their listening experience.
In a studio situation these can be dealt with quite effectively through the use of diffusers, which try to reduce the amount of parallel surfaces in the room, and sound absorbers, which absorb frequencies for example bass traps soak up a considerable about of bass frequencies as best as possible. However live venues and live sound engineer generally don’t enjoy this type of control of the environment so to deal with standing waves the engineer will simply have to listen out for these waves by sweeping through the frequencies on the EQ and reducing or cutting these frequencies.