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Live Sound 101: What’s the Frequency Kenneth?

If you don’t know the reference in the headline, get yourself amused HERE.

It seemed apropos because we are going to be talking about waves here and waves are defined by two criteria, frequency and amplitude. The reason for discussing waves—in this case sound waves—is a review we recently did on the Mackie DLM series loudspeakers. There was a bit of confusion even among our own team about the “format” of theses speakers and how it all worked. and we figured if there was confusion in-house, there is likely confusion out there as well.

Let’s start with a basic look at what sound is and how it moves. There is a great video from 1950 that looks at the basic science of sound. It is WELL worth the 10 mins to watch it.

Now, the examples in that video are all of sound emanating from a single source, like a tuning fork or an instrument. But take a look at a typical P.A. loudspeaker cabinet.

Even the most basic speaker cabinet has at least two and often more sound sources. We call each of these “drivers” and they are usually referred to as LF (low frequency), MF (mid frequency) and HF (high frequency depending on which part of the sonic spectrum they are supposed to recreate.

Now, take a look at this video of some waves of a different kind.

Pay close attention to how the waves interact with each other. See how when some waves meet, the water gets very rough and when others meet it becomes almost completely flat? This is a great visual representation of what physicists call an “interference pattern.”

It works like this. When two wave slam into each other from different sources, the result depends on where each wave is in its development when they collide. If they are both at the top or “crest” of the wave they combine energy and make a larger wave (more amplitude, think of it as volume). If one is at the crest and the other at the “trough” or bottom, they cancel each other out. Anywhere in-between those ups and downs and there is some degree of cancellation. That is the “smeary” stuff you see in the waves video.

And a wave is a wave is a wave. (Except when it is a particle, or a “wave-icle” but that is way more quantum than we want to get here.) They react the same regardless of if the energy is sound or light or the motion of water. Waves of the same frequency from two sources react to each other in definable and predictable ways.

But, the drivers in a speaker cabinet do not put out the same frequencies, right? So one might think they would not interfere with each other. Here is where it gets a little complicated. Take a look at the image below…

This is the combined response of a subwoofer that goes from about 20 Hz up to 80 Hz and a mid/high box that handles from 80 Hz and up. Neither driver starts or stops “clean” at 80 Hz. They roll off at the lower and upper ends of their ranges. And both drivers are actually creating sound between about 50 and almost 200 Hz. This area is called the “crossover region” and this is where interference matters.

Speaker designers go to huge lengths to minimize the size of this region and the effects of interference. If they did not, you would hear lack of clarity, distortion and a muddy or “smeared” sound. The entire idea of modern line-array P.A.s is designing cabinets and “wave guides” (which do exactly what they sound like they would do) to create single “wave front” not only within a cabinet but between cabinets in an array.

So that brings us back to those Mackie speakers we are reviewing. They are what are known as “co-axial.” This means that the low frequency and high-frequency drivers are not next to each other. They are literally on top of each other. There is no center dust cap in a co-axial LF driver because the HF driver is behind the LF and firing right through the center of the LF driver. In the case of the DLMs, Mackie (with some help from the geniuses at sister company EAW) has figured out a way for the two to share a common magnet structure.

Something kinda sorta like this.

Because the LF and HF sound content is all emanating from a single “point source”, interference and the resulting distortion and lack of clarity are minimal.

There is one other thing the DLMs have which is very unusual for “musician grade” loudspeakers. That would be digital processing that allow you to set delay time between speakers. What’s that about?

Interference, again . Usually, if bands need more volume they tend to add more speakers right next to each other. Now you not only have interference between the drivers in a cabinet, you also get interference between the two cabinets. The result is something called “comb filtering” because if you look at the result on an oscilloscope, it looks like the teeth of a comb. And all of the places between the teeth are frequencies that are being “nulled out” and not heard.

If you were to need more volume that a single DLM can supply and you put another one next to the first one, you are—to a great degree—“nullifying” the positive effects of the coaxial design by introducing interference between the cabinets. So, the answer is to put another DRM IN FRONT OF the first one. In effect, you are bolstering the wavefront of the first speaker when it starts to peter out.

But there is a problem with this. Light travels a lot faster than sound. And electricity is basically light. and the signal that drives your speakers is electricity. So if you put one speaker in front of another with enough distance between them you will actually hear a kind of “echo.” You will hear the sound of the first speaker right away followed a fraction of a second later by the sound from the speaker that is further away. The intensity of this effect depends on the relative volume of each speaker and the distance between them.

The answer is to delay the speaker out front so that it produces sound at the same time that the sound from the speaker closer to the stage reaches that point in the room. I know, it sounds like a lot of math. But the DLMs make it simple. You just measure the physical distance between the two cabinets and enter that number into the speaker furthest from the stage and it calculates the amount of delay needed to line things up. Easy peasy.

So, there you have it. As Jesse on Breaking Bad would have said, “Science, bitches.” Yep, sorry to break the news but there is more to music and audio than sex and drugs and rock n roll. You might wanna pay a little more attention in physics class. It WILL come in handy if you stay in this music/audio game long term.

 

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Bill Evans

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