Reprinted from the June 2008 issue of Church Production Magazine
Debunking Spec-ology, Part 3
Distortion, Speaker Cables and Damping Factor—The seamy underbelly of making sound
In the previous two installments of “Debunking Spec-ology”, I covered some very basic but very important topics including speaker and amplifier power ratings, Ohm’s Law, microphone characteristics and the basic idea that if specifications are missing or incomplete in audio equipment documentation, you should be worried about that particular product or supplier. In this current article, I will cover some additional considerations when connecting speakers to amplifiers. But first, let’s get one thing straight: Speakers = Distortion
Let’s face it – among the audio components we generally work with, loudspeakers introduce by far the largest amount of distortion. Unfortunately, this can not be seen by simply looking at the specifications for even the best professional loudspeaker systems. None of the speakers I looked at, including from Klipsch, JBL, Meyer Sound, etc. included a distortion measurement in their specifications. Part of this is because for an acoustic device, the operating characteristics are very heavily dependant on the acoustic environment—i.e. the room they are in.
But before we go much further, let’s define “distortion”. The American Heritage Dictionary, under entry #4, Electronics, states:
An undesired change in the waveform of a signal.
A consequence of such a change, especially a lack of fidelity in reception or reproduction.
This describes it very well, I think. And some types of distortion are inherent in certain design criteria—i.e. that smaller speakers, for instance, do not reproduce extreme low frequency information, and thus, by definition, show some distortion in the lows (mainly, a complete lack of low frequency extension). Then there is the less obvious type: that even within the speaker’s audio band, there are changes to the signal that we don’t want. Take a look at Figure 1 showing the distortion measurements for a high-quality loudspeaker (just the woofer in this case). Note that the overall THD (total harmonic distortion) plus noise is a seemingly mild .7% at 100 Hz. Amplifiers on the other hand, have distortion figures well below .1% and often below .01%!
A few decades ago, when amplifier power was expensive, speakers were made to be very efficient (see Spec-ology #2) so that they could maximize acoustic output for a given amount of power. Particularly, horn-loaded loudspeakers were chosen for this purpose. But horns introduce certain kinds of distortion related to the shape of the horn and the use of a compression driver. In the past 15 years, amplifier power has become much more affordable, and thus more and more efforts have been put in place to continue minimizing speaker distortion while still maintaining good efficiency. This has resulted in speakers that perform amazingly well for our purposes of sound reinforcement. In addition, more and more efforts have gone into processing the signals for those speakers via internal DSP and amplifier circuitry.
Power, Impedance and Damping Factor
But before we get all fancy, let’s talk about some of the basics. One of the major issues involving amplifiers and speakers is the ability for the amplifier to “control” the speaker. This is more of an issue for low frequencies where lots of power is being used, coupled with the fact that the moving parts of a woofer have quite a bit of mass. And along with any other case with a relationship of source to load, the issue of impedance plays a major role in how much power is developed. In the case where you have an additional factor in place, i.e., speaker wires, the wires themselves add their own resistance or impedance. So let’s do a little review about Ohm’s law as it applies here.
watts = volts*amps
amperes = volts/ohms
volts = amps*ohms
As was covered in Spec-ology Part 2, power is developed with a combination of source and load. Figure 2 shows an idealized source/load relationship.
Calculations of power are easy here because of the fixed resistance of the load and the ideal source (no impedance). This does not exist in the real world.
Figure 3 shows a more realistic source/load relationship, where the source has a certain impedance, the load has an impedance, and the connection between the two has some resistance as well. This is a lot like an amplifier/cable/speaker relationship found in the real world. You can see that the greater the impedance (resistance) of the speaker cables, the less power is developed. This effect is known as “insertion loss”.
For instance, let’s say that with an amplifier delivering 100 watts into a speaker with a nominal impedance of eight ohms. We now add a length of speaker wire: 100 feet of 16 AWG(American Wire Gauge) “lamp cord” between the amp and speaker. Table 1 shows the various gauges of wire and their characteristic impedance per 100 feet. Keep in mind this is per conductor (one way), and speaker cables have two conductors (round trip). So if 100 watts is developed into eight ohms (idealized relationship), then only about 90 watts are developed when the speaker wire is added (an additional 0.8 ohms total). By increasing the wire gauge to 12 AWG (0.3 ohms total), the loss is reduced, allowing about 96 watts to be developed.
Remember from Spec-ology #1 that cutting the power in half results in a loss of 3 dB decibels, and it takes a loss of nearly 10dB before the sound is considered “half as loud”. Thus these losses from the speaker cable are relatively minor. However, if you have a specific SPL to reach at a specific distance from your speakers or are looking for very even coverage in the system design, you must include these calculations for accurate results.
Another thing that happens when additional impedance is added to the system is that the damping factor is affected. Damping factor is defined as the ratio of load impedance to source impedance. In our case, it is the speaker’s load impedance to the amplifier’s source impedance. For instance, you might see that an amplifier claims a damping factor of 20 at eight ohms. This would mean that the amplifier presents a source impedance of 0.4 ohms (8/0.4=20). Generally, the lower the better; the minimum damping factor is considered to be five, and most prefer it to be at least 10 or more.
But this does not take into account the change brought into the source/load impedance ratio due to speaker cables—an additional factor of insertion loss. Let’s take the case of the amp with 0.4 ohm output impedance and a speaker with 8 ohm load impedance, giving us a damping factor of 20. Now let’s say we run 100 feet of 16 AWG between the speaker and the amp, as in the above example.
DF=ZL/(ZC+ZS) where ZL= impedance of the load, ZC is impedance of the cable, and ZS is impedance of the source.
In this case, DF=8/(0.8+0.4) or DF=8/1.2=6.6
Thus, the damping factor has dropped significantly due to the insertion loss of the cable. This situation could be improved by using a larger gauge wire, such as 12 AWG:
DF=8/(0.3+0.4) or DF=8/0.7=11.2
Or by shortening the run of speaker cables, to say, 25 feet but keeping the 16 gauge wire (putting the amp rack closer to the speakers):
DF=8/(0.2+0.4) or DF=8/.6=13.3
But how important is this? Mainly, it is important in how well the bass is controlled in your speakers. Higher damping factors generally yield better control over the bass, all other things being equal. Here’s the point, though.
First of all, the wild claims about exotic speaker wires having a dramatic effect on the sound of your speaker system are largely unproven. The largest factor by far contributing to the any changes to the sound introduced by speaker cables is due to the resistance of the wire. If the speaker wires are too long, or not thick enough (conductors, not jacket, mind you), this is where problems come up, such as the losses of damping factor and power as described above. Certainly, construction quality plays a role and it has been found that speaker wire sold in Lowes and Home Depot, for instance, tends to show corrosion of the conductors. But again, the largest factor in speaker cable quality is the wire gauge.
Lately, Things Have Improved
The revolution towards “self-powered” loudspeakers has made a huge difference in performance, by integrating the amplifiers and associated circuitry into the cabinet and to a large extent eliminating insertion loss of the speaker cables.
This trend was pioneered in many ways by Meyer sound for PA speakers, and Genelec for studio speakers. Now, most manufacturers incorporate such products into their lines. The benefit to you, the user, is that it is much less necessary to work out all the system components for the loudspeakers. However, it is still a very good idea to get familiar with how such speaker systems work and how each manufacturer approaches the inherent problems.
Not only do powered speakers have the advantage of eliminating insertion loss, they also can incorporate advanced processing using DSP (digital signal processing) to tame response dips and bumps, to more carefully control crossover performance, and to overcome some of the phase problems associated with multi-way speaker systems. The results are less distortion, more even coverage and just plain better sound. We all need to be thankful that many companies really do work to develop better technologies and thus push the envelope of what can be done within the constraints of the laws of physics. I suggest that you become familiar with these innovative products but at the same time, newer technology is not always better for every application. It is important to know which specifications are important in your facility, with your system. And it is important to work with manufacturers and suppliers that will be open and upfront with you about the tools they are suggesting for the job at hand.
If you start hearing marketing hyperbole and you don’t understand what is being said, ask to have the basics explained to you. If they can’t or won’t do that, then it’s probably best to look for different products or a different supplier. Today, there are dozens of great companies making great products and doing things the right way. But these well-designed, well-made products cost money. So, finally, just remember that you get what you pay for, and if it sounds too good to be true, it probably is.
Karl Winkler has worked in the professional audio industry for more than 15 years including as a touring mixer, a recording engineer and a technician. He is currently Director of Business Development for Lectrosonics, Inc. in Rio Rancho, NM.