3 Live Sound Myths for the New Year

3 Live Sound Myths for the New Year

Originally posted on January 1st 2019.

In the tradition of the incomparable Ethan Winer, here are three audio myths pertaining to live sound that have cropped up around the live sound forums this year. May your new year be peaceful, happy, productive, and factually accurate.

  1. Myth: Turning a subwoofer away from you reverses the polarity.

FACT: The rear radiation of a subwoofer is not out of polarity compared to the front.

Coverage patterns from a single box in the subwoofer range are omnidirectional or pretty darn close. They do sound louder in the front, mostly because our ears are far more sensitive to the frequencies above the subwoofer range in which the coverage pattern is directional indeed. A MAPP prediction of a single subwoofer viewed from above confirms that there’s nothing unique about the radiation behind the cabinet.

In fact, there’s barely enough directionality to tell which way is forward. It is absolutely true that the acoustic radiation behind a loudspeaker cone is out of polarity with the front radiation, but the rear of the loudspeaker is not open to the world – except in the case of an open-back guitar amp, in which case there really is some out of polarity energy in the mix. That looks like this.

The two lobes are out of polarity, and thus create a cancellation midway between them, where they meet at equal level in this example. This is the same principle behind a Figure 8 coverage pattern on a mic. A dual channel analyzer would confirm that the two lobes are out of polarity with respect to each other.

With the ported/vented loudspeakers that are the norm in live sound, the rear radiation from the driver is released at the front of the box via a porting system. Around the port’s tuning frequency, the polarity inversion combines with a 180° phase delay to sum with the direct output. I’m not a loudspeaker design expert, so maybe someone can further elaborate on that. An audio analyzer can confirm that the energy behind the sub is in polarity with the energy out front – it’s just later, because it needs time to wrap around the box to get there. Merlijn’s article linked above provides a great demonstration of this.

Special models of subwoofers designed to produce single-box cardioid pattern use multiple drivers (or in rare cases a resonant rear membrane on the box) to create out-of-polarity (or out-of-phase) energy in the rear, but these are specialty cases.

In the general case, since pointing a sub the other way doesn’t do much to steer its coverage, we have to resort to black magic math and science to create directional arrays.

EDIT: This also applies to full-range loudspeakers. You will sometimes encounter the statement that monitor wedges on stage should be run polarity-inverted because they’re pointed away from the mains. Same thing: the rearward radiation (primarily LF) is late but not out of polarity.

2. Myth: The flat tops of a square wave or clipped signal indicate the presence of DC.

FACT: Clipped signals / square waves do not necessarily contain DC.

People see the flat wave tops and assume that this indicates the presence of DC. It does not, as a spectrum analyzer will reveal. Mathematically, a square wave consists of a fundamental plus odd harmonics to infinity. This is enough to create a flat-top response, no DC required.

(Nerd clarification: In a band-limited system such as a digital audio system, the missing harmonics above the system’s HF cutoff (Nyquist frequency) can cause the wave to look a little lumpy. This isn’t caused by “ringing imparted by a low-pass filter” but rather simply that the wave is missing the HF components necessary to flatten the tops completely. It’s called the Gibbs Phenomenon and is wonderfully illustrated in this video.)

There’s a common objection here, and one that convinced me for years: “adding a high-pass filter to the square wave removes the DC component and causes the flat tops of the wave to slant.” This explanation shows up in many lectures and textbooks, and the only problem with it is that it’s not true. We can create a square wave just by stacking up odd harmonics, so there’s no DC to begin with. DC content would produce an offset that would shift the entire waveform vertically. The real reason that the HPF causes slanting is that the filter’s phase response shifts the relationship between the frequency components so they no longer sum together in a way that creates a flat top.

This Desmos calculator applet shows the first 11 components of a square wave. Moving the “b” slider will shift the fundamental with respect to the fundamentals. You can see the flat tops tilting based on the phase offset. (A high pass filter would also reduce the magnitude of lower components. You can simulate this by moving the “a” slider, which reduces the magnitude of the fundamental.)

This is well explained in this excellent article by Charlie Hughes. In it, Charlie proves that the cause is the filter’s phase response, rather than the magnitude response, by using an all-pass filter which leaves all the LF components intact and just shifts them, causing an identical effect. This was a tough pill for me to swallow, but after going through it with the brain trust over at SynAudCon, I see the error of my ways.

My clipping article applies the same concept to signal clipping and has a hilarious title, if I do say so myself.

3. Myth: The tech rider is up to date.

FACT: The tech rider is not up to date.

I know they said it’s the most current one, but that doesn’t mean it’s current. You know all that backline they requested? Yeah, that’s from the previous tour. They’re carrying their own this time. And you can strike those wedges. They’re all on IEMs now.

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