As we discussed last month, a standing wave occurs when its wavelength equals the distance between the two walls. It resembles two people swinging a jump rope, except where their hands should be is a wall. The problem with a standing wave is that its amplitude (and, therefore, it's loudness) depends on where you are in a room. Near the center of a room it would be very loud, but at the walls nearly silent. This obviously would not be good for a recording studio.
Standing waves also occur when the distance between two walls is a multiple of the wavelength. In the illustration at the top of the page, you can see this for one, two, and three multiples. For example, last month we calculated the wavelength for low C, which has a frequency of 130.81, by dividing the speed of sound (1100 feet per second) by 130.81 to get approximately 8.4 feet. However, at one octave higher you would have a frequency of 261.62 and a wavelength of 4.2 feet. In fact, each time you create a standing wave for a given frequency, you also have a standing wave for all the harmonics of that same note.
So now, instead of having loud sounds in the center of the room and minimal sound at the walls, there can be several places where the sound diminishes. The points of minimal sound are called nodes or nulls. The point of maximum loudness are called antinodes or peaks. Obviously that complicates the recording process.
Standing waves not only occur between parallel walls, but any two surfaces. The different ways that sound waves bounce around a room is called room modes or sometimes eigentones. When the walls are parallel to one another, it is called axial mode. When the sound waves first bounce off a surface, it is called tangential mode And finally, when the waves bounce two or more times, it is oblique mode. This further complicates the standing wave problem.
Now that we know the problem, how do we avoid it. First we have to recognize that the problem occurs mainly within the bass range. As the frequency increase, the wavelength becomes shorter and eventually there are so many nodes within a room that they are no longer noticeable. So we are mostly concerned with the problem of recording bass instruments.
One way to avoid the problem is to use direct input (DI) with a bass guitar. By using DI the signal goes directly into the console or DAW, and standing waves are never created during the recording process, (but not necessarily so when it is played back—something to keep in mind.) Close miking is another technique to avoid the problem. By putting a microphone close to the bass guitar speaker cabinet or to a kick drum, any standing waves that occur will be of no concern. Another solution is to place the bass cabinet in an isolation booth. While this works well for a bass guitar, it is not easy to accomplish with a kick drum.
If you want to record a bass guitar cabinet, a kick drum, or a standup bass with a little distance to give it a more realistic sound, then you have to take another approach. Unfortunately, absorptive materials placed on the walls are not that effective with bass notes. The long wavelengths simply pass through the material and bounce off the walls. That is why you frequently see bass traps in studios. They are those large foam blocks, placed in the corners of the room. They are thick enough to reduce low-frequency sounds.
Another approach is to design a room with the proper ratios of width to length to height. Called room ratios, much science (and possibly voodoo) has gone into designing such a "perfect" room. Unfortunately, there is no agreement as to what that should be.
One answer is the so-called golden ratio. First discovered by the Greeks, the golden ratio (designated by the Greek letter φ, was considered to be very pleasing to the eye. It has been admired and used by artists, architects, scientists, and mathematicians. It is approximately equal to 1.618... In acoustics a room with a ratio of height to width to length of 1:1.6:2.6 (or the ratio of 0.62:1:1.62 as used by sound engineers) is considered to have "perfect" acoustics. It is claimed that no matter where you stand in such a room, the sound will be balanced and natural, with little interference from standing waves and no ringing. Unfortunately, to my knowledge, no one has ever built such a room.
And on that note, we will see you again next month.
• Many people know that Waylon Jennings gave up his seat to Buddy Holly on the plane that crashed shortly after take off in the early morning hours of February 3, 1959, killing Holly, Ritchie Valens, and J. P. "The Big Bopper" Richardson, but few know that Dion DiMucci of Dion & the Belmonts turned down a seat on the fateful flight because he couldn't afford the price of a ticket.
• James McCartney played piano and trumpet for a combo known as Jim Mac's Jazz Band in the 1950s. His son James Paul, Jr., whom we all know as Paul McCartney, once said that he hoped to be as good as his dad someday.
•In February and March of 1964, 60% of all the records sold in the US were by the Beatles.
Attending Music School?
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