Pat answers this question, “When does correct polarity matter?”. As with most things audio, the answer isn’t that simple.
All audio components that pass a signal should be properly polarized. The concept of polarity can be confusing. Like many principles, it is most easily understood by non-audio examples. When you change the batteries in a product, the + and – indicators must be observed. When you drill a hole the drill bit must rotate clockwise. A fan blows air out and a vacuum cleaner draws it in. These are all examples of the need to observe proper polarity. In each example there exist only two possible choices which are mutually exclusive – like flipping a coin for heads or tails. In fact, in most every case you can substitute “flip” or “reverse” for “invert” in polarity discussions without changing the meaning. Polarity must be considered when interconnecting audio components.
I will not take up the issue of whether absolute signal polarity matters – I will just assume that it does. As with all things audio, the answer is always “it depends” and it is always better to ere on the side of caution. It should suffice to know that improper polarity can be devastating to the response of a sound system under some conditions.
Balanced audio gear, by design, allows the polarity of the signal to be easily reversed. This is both good and bad. It allows easy correction of mis-wired components, but makes it more likely that those mis-wirings will occur in the first place. This article proceeds on the assumption that correct absolute polarity is desired, where an overpressure at the microphone element should produce an overpressure at the listener.
Absolute vs. Relative
Loudspeaker polarity can be absolute or relative. Absolute polarity means that the cone moves outward when a positive-going signal is applied to the “+” terminal relative to the “-” terminal. Relative polarity means that the loudspeaker is polarized the same as another loudspeaker(s), without regard as to whether either has correct absolute polarity. The significance of maintaining correct absolute polarity has been hotly debated for years. There appear to be cases when it matters and cases when it does not. The need for correct relative polarity is not debatable. Loudspeakers producing the same program material whose coverage patterns overlap should be polarized the same in all but a few special cases.
The human auditory system does not appear to be sensitive to absolute polarity. The ear drum responds to changes in the atmospheric pressure from an equilibrium condition. It’s the change that matters, not the direction of the change. While this is often debated, we can at least say that if there is a sensitivity to absolute polarity it is subtle.
It’s not Phase
Timing differences affect the phase response of a system, and phase should not be confused with polarity. Phase shifts are measured in degrees. Polarity is indicated by a sign. Contrary to popular belief, “reverse polarity” and “180-degrees out-of-phase” are not the same thing – at all. The first involves a sign change, the second a signal delay. Carefully contemplate the difference between the meaning of “shift” and “invert” and you will have it. Many references to phase in the literature and on products are actually referring to polarity. That said, the term “phase reversal” is widely accepted as being synonymous with inverting polarity, where the term “phase shift” is not.
“Hot” or “Cold”
The inputs and outputs on professional audio gear are balanced. A balanced input is sensitive to a voltage differential between two input terminals that have the same common-mode impedance (a different article!). A balanced output produces a voltage differential between two output terminals that have the same common-mode impedance (same different article). One input terminal is non-inverting, and the other is inverting. Both terminals can accept a signal, but one inverts the polarity and one does not. The non-inverting terminal is labeled with a “+” and the inverting terminal with a “-.” To further confuse things, the “+” terminal is sometimes referred to as “hot” and the minus as “cold.” The hot and cold nomenclature causes confusion because it implies that one is voltage sensitive and the other is not, when in fact they are equally so.
The electrical contacts on XL connectors are numbered 1 through 3. Pin 1 is always the connection for the cable shield, and the twisted-pair connects to pins 2 and 3. This sets the stage for the “Which pin is hot?” argument that has raged for years.
So, Which Pin is Hot?
First, let’s go with “+” or “-” rather than hot or cold. The meaning is more clear. This debate was settled many years ago so let’s put it to rest. AES26 states as a “recommended practice (their word for standard)” that pin 2 on the XL connector shall drive the non-inverting input (or “+”) and pin 3 shall drive the inverting input or “-.” This is no longer debatable. What is debatable is whether it matters, which I will address shortly.
Most audio products comply with AES26.
There are some exceptions:
- 1. Legacy gear that was designed prior to 1984. If it has been around that long and is still in production, its probably a pretty good design. Manufacturers are reluctant to pull such products from the market to redesign them.
- 2. Modern gear that must remain compatible with legacy gear. A number of modular I/O products exist that must be compatible with products from the past, a case where the longevity of analog electronics may be working against us .
- 3. Modern gear that does not adhere to the standard. As I will show, some products don’t bother to comply with the standard, because they preserve the polarity of the applied signal.
When it Matters
There are several cases where the absolute polarity of a component is critical to maintaining the absolute polarity of a system. They include:
1. Any component that directly interfaces to the atmosphere. This includes both microphones and loudspeakers. A “pin 3 +” microphone will invert signal polarity. A reverse-wired loudspeaker will do the same.
2. Power amplifiers are interfaced directly to loudspeakers and the polarity of their output terminals are clearly labeled. This means that the amplifier’s input polarity is critical. A “pin 3 +” amplifier input will invert the signal polarity relative to a “pin 2 +” model.
3. Polarity conventions are also critical when interfacing balanced and unbalanced equipment. Since the unbalanced gear has only one “+” output terminal, it matters whether it drives the inverting or non-inverting terminal on a balanced input. The same is true when connecting a balanced output to an unbalanced input.
4. Some input modules of user-configurable utility amplifiers have balanced inputs and a fixed reference for the polarity of the output (i.e. the red or + terminal). This means that pin 2 or pin 3 must be designated as the “+” input. Since some of these products pre-date AES26, the issue arises as to weather it is better to redesign them to conform to AES26 or retain consistency between older and newer versions. The debate goes on.
In review, polarity is an absolute issue with regard to microphones, loudspeakers and the interfacing of balanced and unbalanced gear.
When it Doesn’t Matter
Some components in the signal chain simply pass a voltage waveform on to the next device after modifying it. They are driven from balanced sources and their outputs are connected to balanced inputs. Such devices must simply preserve the polarity of the waveform applied to them. In this case, it technically doesn’t matter if the device is “pin 2 +” or “pin 3 +” as long as it is the same on its inputs and outputs. Such devices are often labeled “non-inverting” on their specification sheets. These generally include line level signal processors such as equalizers, delays, crossover networks and compressor/limiters. Digital signal processors are also in this category, as are mixing consoles driven from balanced sources (i.e. microphones).
This is one reason why polarity can be a confusing topic. Sometimes the absolute polarity between input and output is important (i.e. microphones and loudspeakers) and other times the relative polarity between input and output is the important consideration. This is not meant as a justification for not complying with AES26, but it is important to know none-the-less. The common practice of swapping wires on the inputs and outputs of “pin 3 +” gear is unnecessary and causes extra work (and a lot of confusion).
The Worst Offenders
The “worse offenders” regarding accidental polarity reversals are microphones, loudspeakers and cables. These should always be checked by the installing technician. There are still many “pin 3 +” signal processor products in the marketplace, but as I have shown these do not necessarily invert signal polarity. The biggest danger is someone reverse-wiring an input on such a device to “correct” its polarity. This actually inverts the polarity unless it is done on both the inputs and outputs, which means that it shouldn’t be done at all – just interface to the product without any wiring modifications.
Lastly it must be remembered that sometimes the polarity of an audio component is intentionally inverted. The most common examples are some drum miking techniques that utilize two microphones and some loudspeaker configurations including Bessel arrays, cardioid subs, and multi-way loudspeaker systems that employ certain crossover topologies. In such cases, the polarity must be appropriate, and appropriate may mean reversed.
How to be Sure
The best way to avoid confusion is to always test the polarity at the output of all loudspeakers when installing a sound system. A variety of polarity test sets are commercially available and every audio tech should have one. Check the loudspeaker first, and then move the pulse generator back through the system one component at a time, leaving the receive unit in front of the loudspeaker. If a component inverts the signal, you will immediately know it.
Also, don’t forget to “test the tester” as was pointed out in our summer newsletter. Polarity testers can produce erroneous results when used with microphones other than the one that they are packaged with.
Don’t leave polarity to chance. It is one of the few system parameters that is more black and white than grey. The growing practice of unbalanced consumer gear finding its way into sound reinforcement systems assures that the “Which pin is hot?” question will remain relevant for the foreseeable future.
One mark of an audio professional is the ability to install properly polarized sound systems. pb