Let’s talk about length

OK, this is the first post, so there may be some terms that some readers are not familiar with. There will be more postings which will cover each term or concept in more detail. For the uninitiated, just wait until the subsequent posts. For everyone else, please don’t hesitate to ask a specific question, ask about a specific topic, or discuss or add your own comments.

Let’s talk about length

Most people who know even a little about speakers know that the wires should be as close to the same length as possible, right? Right, and it’s because if the lengths are different, you will get slight variations in the phase, right? Wrong! Here’s the correct reason:

To make the explanation simpler, let’s assume that you’re not setting up for 5.1 or more, and we’ll use the example of a simple stereo pair of speakers, left and right. And so there’s no confusion, what we’re talking about is non-powered speakers which are going to be fed a speaker-level signal, because when talking about powered speakers with a line-level input, none of the following applies. (more on all that stuff later.)

Speakers generally present very little electrical resistance, measured in units called ohms, and are more commonly specified using a similar unit of measurement called impedance, also measured in ohms, and typically range from 2 to 16 ohms. Because of many other factors, the amount of resistance measured may not be anything close to the actual amount of impedance, but in the case of non-powered speakers, these measurements are nearly identical. A speaker with 8 ohms impedance typically measures about 6.5 ohms in resistance. Are you still with me? OK, let’s use the common 8-ohm speaker in our example. Let’s also assume that you have already matched the speakers with an amplifier that is rated for 8-ohm speakers. This means that each amplifier output wants to “see” about 8 ohms across each pair of terminals, that is, 8 ohms across the left channel and 8 ohms across the right channel.

There are actually quite a few different variables involved, but for simplification, we can say this: As long as both left and right speakers are identical, and both left and right channels of the amp are identical, we can assume that each speaker will behave the same, and this will translate to them sounding the same (well, ok, provided you’re sending them each an identical mono signal instead of a two-channel stereo signal whose left and right channels will actually differ. With a true stereo signal, the left and right channels will have different program information and will sound different, but, nevertheless, you want to make sure that your monitor paths are identical, even if the program isn’t.)

Now, what would you guess would happen if you took one of those 8-ohm speakers away and replaced it with, say, a 10-ohm speaker? You don’t have to be a rocket scientist or even mediocre at math to be able to guess that they will sound different. And you probably wouldn’t recommend that anyone who wanted an accurate stereo monitoring environment do this. But there’s one important variable that we forgot to account for, which, when not taken into account, can have the same effect as replacing one of those correct speakers with a wrong one – the CABLE connecting the amp to the speakers.

See, everything has some resistance, including wire. In fact, resistance of speaker wire can be as high as 0.1 ohms per foot. So, let’s say you have 15 feet of speaker cable on the left side and 15 feet on the right. That totals 1.5 ohms of resistance just for the cable. And, since many 8-ohm speakers actually measure about 6.5 ohms, let’s add 6.5 (instead of 8) to 1.5 to get a total of 8 ohms resistance on each side of our speaker/cable path. That’s good – both sides of your monitor path have identical amp outputs, speaker cable length, and speakers. But what happens if you use only 5 feet of cable for one speaker and 25 feet for the other. You now have 7 ohms on one side and 9 ohms on the other, respectively. This difference may not seem like much, but because the numbers are so small to begin with, you’re talking about somewhere between 20 - 25% of difference in resistance between the two channels! That is a significant difference, and certainly enough to cause a drastic difference in sound between the left and right sides.

So now you’ve seen why even only 20 feet of difference in length can cause a significant difference in sound, or, to use the correct term, frequency response. Now here’s why even a difference of 200 feet or more will have absolutely NO NOTICEABLE EFFECT on phase:

Sound traveling through the air is very slow – only about 1000 feet per second. Sound traveling through wires is very fast. In fact, sound traveling through wires is actually electricity. Electricity travels very near the speed of light, or 186,000 miles per second, which translates to nearly a billion feet per second! Or, about a billionth of a second per foot of cable. So 20 feet of cable on one side and 220 feet of cable on the other side will result in sound arriving at only 200 billionths of a second later on the side with 220 feet of cable! For the mathematically-challenged, that’s less than a microsecond, which will not be audible as far as phase is concerned. But the resistance of the side with 200 feet of cable is now going to increase the overall resistance of that side to about 26.5 ohms (6.5 for the speaker and 20 for the cable.) That WILL result in a significant difference in the sound of the two speakers, but it has absolutely nothing to do with phase. So, in theory, there is a difference in phase, but you show me the “golden ears” that can actually hear that slight difference in phase, and I want some of whatever they are on!

Stay tuned for the next post, where we will discuss phase, or maybe resistance, or maybe Ohm’s Law. I don’t know yet…