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Article doesn't mention one of the more interesting (to me) aspects which was how feedback was avoided. The solution is elegant: each vocal microphone is double

Article doesn't mention one of the more interesting (to me) aspects which was how feedback was avoided. The solution is elegant: each vocal microphone is doubled, meaning there are two at each position. The phase is inverted on one of them, the singer sings into only one, and both are sent to the speakers via their channel's amp. The effect of that setup is that only the difference between the two microphones is amplified; common signal in both (i.e. the sound coming out of the speakers) is nulled out, but the difference signal (the voice) makes it through. It apparently wasn't quite perfect but was absolutely a lot better than wailing feedback. The thing that made it sound so good was that any given speaker only reproduces a single source, but the article touches on that. The mic arrangement I described is simply what makes it possible.

itishappy 2y ago
> common signal in both (i.e. the sound coming out of the speakers) is nulled out, but the difference signal (the voice) makes it through. What drives this? Singers and speakers are both localized sources, so I'd expect the mics to pick up similar phases for each. I bet it's distance! Falloff depends on distance to source, so there should be a larger difference in volume for closer sources.
- llamaimperative 2y ago
  Yeah one microphone was behind the other, though I was under the impression it was half a wavelength behind and thus created something quite akin to modern active noise-cancellation? Edit: Apparently this is not the case!
  - itishappy 2y ago
    A very literal phased array!
  - bregma 2y ago
    When I saw them (back in 83, it's been a long strange trip) it looked more like one mic was on top of the other with about a 6 inch separation.
  - dekhn 2y ago
    See https://archive.org/post/256492/the-betty-question-answered https://archive.org/post/256492/the-betty-question-answered for more details both on WoS and the microphone setup.
  - dylan604 2y ago
    Half wave length in what frequency would be the first thing that would give me pause to this. I'm reading this after your edit, but even before I got to the edit my brain was already heading towards nope
    - llamaimperative 2y ago
      Yeah that asterisk popping up in my head was why I did some more digging as well :) thought tbf, the Dead never seemed to care much about vocal quality so it wouldn’t have been crazy for this to work well only for a narrow band. I don’t have any intuition for exactly how narrow that band would be and how that’d sound in practice though.
- deleted 2y ago
  [deleted]
- scrumper 2y ago
  Yep, you sing into one but not the other, so there's a big difference in the vocal signal, whereas spill from the speakers is going to hit both mics pretty well evenly.
- deleted 2y ago
  [deleted]
plussed_reader 2y ago
Since balanced cables predate the GD, this strikes me as an acoustic implementation of an EE concept. Neat!
- insaneirish 2y ago
  Here's a fun one. I'm involved in maintaining the audio system for an auditorium used by a non-profit. After a flood and remodel, including replacing some audio components (like microphones), it was observed that the microphone on the main podium always had a 60 Hz hum. The hum depended on where the microphone was facing. Sometimes it was there, sometimes it was not. Being a non-profit facility, there are no fancy DSPs to notch out the hum or anything like that, so more creative solutions were investigated. It was determined through dumb experimentation that orienting an identical microphone 180 degrees to the one with the hum and setting the gain similarly would nearly eliminate the hum. Eventually, the working theory became that a relatively new large pad transformer installed across the street was being picked up by the microphones. Orienting one microphone 180 degrees from the other caused the hum to be picked up out of phase from the main, and thus could be mixed in to cancel out the main mic hum. Ultimately the real solution was simply buying better microphones, but there was a period of some months while a microphone sat off stage, pointed backwards.
  - ChainOfFools 2y ago
    It's been quite a few years since I last worked in live show production, but on any show at a venue where we couldn't be sure of access to clean power, humbuckers (not the guitar pickups, a nickname for what I believe was just a dumb 60hz notch filter or ground loop isolator inlined on house power taps) were a standard pack out in the road kit. I would have expected that this decades-old and well established component of power infrastructure would have been commoditized by now and integrated into any dedicated AV performance/production space such as an auditorium.
    - bombcar 2y ago
      Nonprofit and church auditoriums and halls are often about fifty years out of date and are somewhat around “barely working”.
TylerE 2y ago
While it's true that they did that and why, I'd ultimately chalk it up as more of a flaw than a feature. Vocals never sounded great on wall of sound shows because they could never sing perfectly into one mic. This can be confirmed by listentng to soundboard tapes of the shows, and comparing them with ones a year or two either side - the full on Wall was only used for about a year. While the WoS laid much of the groundwork for how modern PAs are designed and operated, it was more of a white elephant than anything, and many of it's actual ideas were discarded. It was totally impractical to tour with and they lost money doing so. The only real technical legacy it has is of using coherent phased line arrays. Really it's whole reason for existence (getting a coherent, in phase, non-canceled signal at an extended distance from the stage) isn't even relevant, as these days secondary speaker arrays with delay lines (to sync them perfectly with the mains) is almost childs play. Literally plug and play. Modern PAs can self-tune the whole system just from playing a short burst of white noise through the system, and listening for the response.
- dekhn 2y ago
  (I assume you're aware, but for the larger audience)... the grateful released an album "Two From the Vault" which was a soundboard recording... but the original soundboard had huge phase cancellation errors due to microphone placement. To recover it, some 20+ years later, with digital tech, the sound engineers could recover the original signal using some clever FFT and phasing very similar to what you describe modern secondary arrays use to self-tune.
  - TylerE 2y ago
    Ironically i haven't really listened to most of the official live albums much. I tend to just go straight to the board tapes, which often sound better due to having a few decades of technological advancement - many were transferred in the 90s or 2000s. Of course they didn't have then what we have now, but even consumers by then had access to software for things like mastering that would have made any 70's engineer drool - certain kinds of repairs are much more easily done digitally - back in the day cutting out a spot of stactic or a mic pop involved literal tape and razor blades.
upheaval7276 2y ago
Here is a much more detailed article that covers the mics and so much more: https://www.vice.com/en/article/wnnayb/the-wall-of-sound https://www.vice.com/en/article/wnnayb/the-wall-of-sound
neckro23 2y ago
This is very similar to how noise cancellation works on cell phones. The secondary mic is typically on the back of the phone and picks up the ambient noise to be subtracted from the primary mic’s signal.

itishappy 2y ago

> common signal in both (i.e. the sound coming out of the speakers) is nulled out, but the difference signal (the voice) makes it through. What drives this? Singers and speakers are both localized sources, so I'd expect the mics to pick up similar phases for each. I bet it's distance! Falloff depends on distance to source, so there should be a larger difference in volume for closer sources.

llamaimperative 2y ago
Yeah one microphone was behind the other, though I was under the impression it was half a wavelength behind and thus created something quite akin to modern active noise-cancellation? Edit: Apparently this is not the case!
- itishappy 2y ago
  A very literal phased array!
- bregma 2y ago
  When I saw them (back in 83, it's been a long strange trip) it looked more like one mic was on top of the other with about a 6 inch separation.
- dekhn 2y ago
  See https://archive.org/post/256492/the-betty-question-answered https://archive.org/post/256492/the-betty-question-answered for more details both on WoS and the microphone setup.
- dylan604 2y ago
  Half wave length in what frequency would be the first thing that would give me pause to this. I'm reading this after your edit, but even before I got to the edit my brain was already heading towards nope
  - llamaimperative 2y ago
    Yeah that asterisk popping up in my head was why I did some more digging as well :) thought tbf, the Dead never seemed to care much about vocal quality so it wouldn’t have been crazy for this to work well only for a narrow band. I don’t have any intuition for exactly how narrow that band would be and how that’d sound in practice though.
deleted 2y ago
[deleted]
scrumper 2y ago
Yep, you sing into one but not the other, so there's a big difference in the vocal signal, whereas spill from the speakers is going to hit both mics pretty well evenly.
deleted 2y ago
[deleted]

plussed_reader 2y ago

Since balanced cables predate the GD, this strikes me as an acoustic implementation of an EE concept. Neat!

insaneirish 2y ago
Here's a fun one. I'm involved in maintaining the audio system for an auditorium used by a non-profit. After a flood and remodel, including replacing some audio components (like microphones), it was observed that the microphone on the main podium always had a 60 Hz hum. The hum depended on where the microphone was facing. Sometimes it was there, sometimes it was not. Being a non-profit facility, there are no fancy DSPs to notch out the hum or anything like that, so more creative solutions were investigated. It was determined through dumb experimentation that orienting an identical microphone 180 degrees to the one with the hum and setting the gain similarly would nearly eliminate the hum. Eventually, the working theory became that a relatively new large pad transformer installed across the street was being picked up by the microphones. Orienting one microphone 180 degrees from the other caused the hum to be picked up out of phase from the main, and thus could be mixed in to cancel out the main mic hum. Ultimately the real solution was simply buying better microphones, but there was a period of some months while a microphone sat off stage, pointed backwards.
- ChainOfFools 2y ago
  It's been quite a few years since I last worked in live show production, but on any show at a venue where we couldn't be sure of access to clean power, humbuckers (not the guitar pickups, a nickname for what I believe was just a dumb 60hz notch filter or ground loop isolator inlined on house power taps) were a standard pack out in the road kit. I would have expected that this decades-old and well established component of power infrastructure would have been commoditized by now and integrated into any dedicated AV performance/production space such as an auditorium.
  - bombcar 2y ago
    Nonprofit and church auditoriums and halls are often about fifty years out of date and are somewhat around “barely working”.

TylerE 2y ago

While it's true that they did that and why, I'd ultimately chalk it up as more of a flaw than a feature. Vocals never sounded great on wall of sound shows because they could never sing perfectly into one mic. This can be confirmed by listentng to soundboard tapes of the shows, and comparing them with ones a year or two either side - the full on Wall was only used for about a year. While the WoS laid much of the groundwork for how modern PAs are designed and operated, it was more of a white elephant than anything, and many of it's actual ideas were discarded. It was totally impractical to tour with and they lost money doing so. The only real technical legacy it has is of using coherent phased line arrays. Really it's whole reason for existence (getting a coherent, in phase, non-canceled signal at an extended distance from the stage) isn't even relevant, as these days secondary speaker arrays with delay lines (to sync them perfectly with the mains) is almost childs play. Literally plug and play. Modern PAs can self-tune the whole system just from playing a short burst of white noise through the system, and listening for the response.

dekhn 2y ago
(I assume you're aware, but for the larger audience)... the grateful released an album "Two From the Vault" which was a soundboard recording... but the original soundboard had huge phase cancellation errors due to microphone placement. To recover it, some 20+ years later, with digital tech, the sound engineers could recover the original signal using some clever FFT and phasing very similar to what you describe modern secondary arrays use to self-tune.
- TylerE 2y ago
  Ironically i haven't really listened to most of the official live albums much. I tend to just go straight to the board tapes, which often sound better due to having a few decades of technological advancement - many were transferred in the 90s or 2000s. Of course they didn't have then what we have now, but even consumers by then had access to software for things like mastering that would have made any 70's engineer drool - certain kinds of repairs are much more easily done digitally - back in the day cutting out a spot of stactic or a mic pop involved literal tape and razor blades.

upheaval7276 2y ago

Here is a much more detailed article that covers the mics and so much more: https://www.vice.com/en/article/wnnayb/the-wall-of-sound https://www.vice.com/en/article/wnnayb/the-wall-of-sound

neckro23 2y ago

This is very similar to how noise cancellation works on cell phones. The secondary mic is typically on the back of the phone and picks up the ambient noise to be subtracted from the primary mic’s signal.