The sharpie just marks the exact length on the string. The distance that you are trying to trace. If you don't like sharpies, then you can also use a pen, pencil, nail polish, paint, or a quill dipped in ink...
Anything that leaves a clear mark on the string at the two points you need.
So, first lay out a long piece of string on the floor, then a couple of inches from one end, mark the string with the sharpie (or quill). That "couple of inches" is to give you enough extra string so you can tape it securely to the speaker. The the sharpie/quill mark is the spot that
just touches the front panel of the speaker.
But with the string still on the floor (before you pick it up to tape it to the speaker), measure from that first sharpie mark the distance that I specified above: that is, the distance from the tip of the mic to the acoustic center of the speaker (the "axis" in the model: just below the tweeter), PLUS the extra "bounce distance", which is 33 cm EXTRA in this case. Make another mark on the string at that exact spot (mic-speaker distance + 33 cm) , and that second sharpie mark is the spot that has to end up at the very tip of your mic. A couple of inches
beyond that second sharpie mark, you can cut the string. You now have a piece of string that is a few inches longer than the total bounce distance, with tow marks on it, and a but extra beyond each mark, for taping.
So, your string has two sharpie marks on it: one of those you position exactly at the tip of the mic and tape in place, and the other you position exactly at the acoustic center of the speaker and tape in place. The sharpie marks are just there so you can be accurate with the taping up of the string. 1ms is a very short delay, so you do need to be very accurate here, thus the sharpie marks: to give you accurate points for taping up the string.
OK; so now you have your string taped up, with one mark where the string meets the speaker, and the other mark where the string mics the mic, and the middle of the string is hanging down, but in a natural curved shape: Sound does not travel along curved lines: it travels in STRAIGHT lines, (at least for high frequencies in air....). So you need to use your finger to push down in the middle of that loop of slack, so that the string is stretched taught on either side of your finger into two straight segments. On one side of your finger the string now runs in a straight line to the speaker, and on the other side in a straight line to the mic.
As you run your finger along the string like that, at some point it will touch the desk surface: the point where it touches is the center of the specular reflection that is causing the problem. You might find that you can move the string left and right a bit, with your finger still pressing to keep it taught, and it still touches the desk surface. For example, you might find that there is one spot you can reach on the desk surface, and another on the "console tray" surface, and another on the vertical side of the angled rack section.
All of the spots that you can reach with your finger like that, keeping the string taught in both directions, are likely part of the reflection. Put small pieces of masking tape at every spot you can reach with your finger pressing on the string.
You might need help here: as you press down, the mic stand will want to topple over, or the tape might come loose at the speaker. So if you can find some helpful volunteers to hold those to spots steady, that would help!
In this first case, there's only enough extra loop that it can touch the desk surface: the loop isn't long enough to reach other surfaces. But for the OTHER ones that I mentioned, the bounce is quite a bit longer (several ms), so the string will be quite a bit longer too (roughly one foot longer for every ms), and could potentially reach many surfaces. For example, if the loop hangs down to the floor, you might find that you can also stretch it out sideways and touch the wall in a few spots, as well as stretching it upwards to touch the cloud in a few spots. If that happens, then mark ALL of those points with a bit of masking tape.
If you take a look at
Steve's thread, in the most recent photos you can see some colored markers on the front walls, floor, and soffit wings where he did this exact same process, and found several potential reflection points, which I am about to deal with. The different colored markers are for different string lengths = different flight times. That's the end result of this process: a bunch of markers on the desk and around the room that show where your strongest reflections are coming from.
Finally, not all of the points that you can reach with your finger really are reflection points: To be CERTAIN that it really is a reflection point, the "angle of incidence will be equal to the angle of reflection". So look closely at how the stretched-taught string touches the surface: the angle that it makes on one side of your finger (heading towards the speaker) between the string and the surface should be about the same as the angle of the string on the other side (heading towards the mic). If the angles are clearly very different, then that's probably not a refection point, but if they are similar, it probably is. In other words if the sound wave (string) is hitting the surface at an angle of 60°, but the angle from there to the mic is only 20°, then that's not a reflection point beause sound always bounces off a flat surface at the "same but opposite" angle: if it came in at 60°, then it will leave at 60° the other way. But if the second angle in this example (string to mic), is also about 60°, then it is a reflection point. The angles might be anything: both 20°, both 8°, both 41.297°, both 81.6°, or whatever), but they will be equal for all spots that really are causing that reflection to hit the mic (your head)
Not sure if that cleared it up a bit! Or maybe just confused you even more...
- Stuart -
Read it over again: I just edited a bit, to make it clearer... maybe!
