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The Cardioid Subwoofer
8 years ago
General
It was good to get back to Anime Expo this year after being away for two years. I was quite flattered at the responses I got... apparently I was missed! And not just because the previous year's director had a tendency to take cameras with no warning whatsoever...
One of the hired pros there (as opposed to us volunteers) who has been working there for many years is the second audio engineer (A2). Bruce is a guy I could spend all night in a pub with, just listening. Not only does he have an arsenal of stories, but the way he tells them is so energetic, you can't help but smile. Or laugh., It's absolutely infectious.
He's the guy that was wary of one of our volunteers taking an XLR cable from his box because people had already borrowed away cables from him. "I've got a 100-foot cable going ten feet 'cause people took all my 20-footers!" he wailed. But when he found out the volunteer was borrowing the XLR cable to teach another volunteer how to properly coil cables, his face immediately lit up. "Ohmygawd, I love you!" he exclaimed. And turned to me, "How great is this guy?!" And then he got up and gave us all a cable-coiling refresher himself. In his usual fashion. "Oh yeah, see that? That's an ornery cable right there. That one's gonna give you trouble."
One of the things I learned about back in 2014 and re-learned this year was the concept of the Cardioid Subwoofer. Most bass and sub-bass frequencies are non-directional. A sub-woofer may have a horn or a definitive output side, but the sound emanates in all directions. So what do you do when you have thousands of watts pounding out of subwoofers that are only ten feet from the monitor mixing console? How do you keep the bass from rattling the fillings out of the back stage crew?
Enter the Cardioid Subwoofer system. With this setup, bass is radiated in the familiar heart-shaped ("cardioid") pattern, with a prominent lobe out front, smaller lobes to the sides and not much to the rear. This allows the bass to travel forward into the audience where it's wanted and not so much backstage, where it just gets in the way.
The Cardioid Sub isn't a special type of speaker or cabinet. You can't go down to the local Pro A/V store and buy one. Instead, it's a system of speakers with delay timings. Although honestly, the delays are so short, it's more like a phase offset.
The sub cabinets are placed in a front-to-back line with space between them. The rearmost set has zero delay and moving forwards, just a few milliseconds of delay is added to the audio signal. The amount of delay added is enough so that in a forward direction, all the sound waves overlap perfectly and add to each other, reinforcing the sound. And going backwards? Well, as one web explanation put it, "they don't completely cancel each other out, but they definitely do not help each other." The net effect is that in front of the subs, the sound is very strong (the waves reinforce each other) while behind the sound is quieter (the waves are more mismatched).
Voila! The Cardioid Sub.
So what powers a setup like this? With two Cardioid Sub setups on each side of the stage, not to mention the arrays flown from the trusses? Bruce took me over to their amplifier racks... about seven amps per half-rack. All Crown I-Tech 12000s. And he looked at me and said, "So what d'you suppose the 12000 stands for?" I had an answer, but it seemed so ridiculous that I didn't dare to say it. But it was right: these are two channel amplifiers with SIX THOUSAND WATTS per channel. And it's Pro Audio gear so you know that's an honest 6000 watts, not what it can do peak for 30 milliseconds and then start clipping.
Even more interesting, the top of each rack held a 16-port Ethernet switch. Because that's how you control and configure and even monitor these amplifiers. They have very minimal front panel indications, instead piping all their information over the network in real time to monitoring software on a laptop. And each one has a full EQ for each channel, configurable through the software. Not that you would need to EQ that specifically, but the ability is there.
One of the hired pros there (as opposed to us volunteers) who has been working there for many years is the second audio engineer (A2). Bruce is a guy I could spend all night in a pub with, just listening. Not only does he have an arsenal of stories, but the way he tells them is so energetic, you can't help but smile. Or laugh., It's absolutely infectious.
He's the guy that was wary of one of our volunteers taking an XLR cable from his box because people had already borrowed away cables from him. "I've got a 100-foot cable going ten feet 'cause people took all my 20-footers!" he wailed. But when he found out the volunteer was borrowing the XLR cable to teach another volunteer how to properly coil cables, his face immediately lit up. "Ohmygawd, I love you!" he exclaimed. And turned to me, "How great is this guy?!" And then he got up and gave us all a cable-coiling refresher himself. In his usual fashion. "Oh yeah, see that? That's an ornery cable right there. That one's gonna give you trouble."
One of the things I learned about back in 2014 and re-learned this year was the concept of the Cardioid Subwoofer. Most bass and sub-bass frequencies are non-directional. A sub-woofer may have a horn or a definitive output side, but the sound emanates in all directions. So what do you do when you have thousands of watts pounding out of subwoofers that are only ten feet from the monitor mixing console? How do you keep the bass from rattling the fillings out of the back stage crew?
Enter the Cardioid Subwoofer system. With this setup, bass is radiated in the familiar heart-shaped ("cardioid") pattern, with a prominent lobe out front, smaller lobes to the sides and not much to the rear. This allows the bass to travel forward into the audience where it's wanted and not so much backstage, where it just gets in the way.
The Cardioid Sub isn't a special type of speaker or cabinet. You can't go down to the local Pro A/V store and buy one. Instead, it's a system of speakers with delay timings. Although honestly, the delays are so short, it's more like a phase offset.
The sub cabinets are placed in a front-to-back line with space between them. The rearmost set has zero delay and moving forwards, just a few milliseconds of delay is added to the audio signal. The amount of delay added is enough so that in a forward direction, all the sound waves overlap perfectly and add to each other, reinforcing the sound. And going backwards? Well, as one web explanation put it, "they don't completely cancel each other out, but they definitely do not help each other." The net effect is that in front of the subs, the sound is very strong (the waves reinforce each other) while behind the sound is quieter (the waves are more mismatched).
Voila! The Cardioid Sub.
So what powers a setup like this? With two Cardioid Sub setups on each side of the stage, not to mention the arrays flown from the trusses? Bruce took me over to their amplifier racks... about seven amps per half-rack. All Crown I-Tech 12000s. And he looked at me and said, "So what d'you suppose the 12000 stands for?" I had an answer, but it seemed so ridiculous that I didn't dare to say it. But it was right: these are two channel amplifiers with SIX THOUSAND WATTS per channel. And it's Pro Audio gear so you know that's an honest 6000 watts, not what it can do peak for 30 milliseconds and then start clipping.
Even more interesting, the top of each rack held a 16-port Ethernet switch. Because that's how you control and configure and even monitor these amplifiers. They have very minimal front panel indications, instead piping all their information over the network in real time to monitoring software on a laptop. And each one has a full EQ for each channel, configurable through the software. Not that you would need to EQ that specifically, but the ability is there.
I'm not sure why you would want to steer sound in that manner. And it wouldn't be perfect, because you're still going to get reflections scattering everywhere - you couldn't mute out sections of the crowd entirely, just make it quieter.
I've seen this exact design used before, though at much higher frequencies. There is one application where the ability to steer a beam of sound is very useful: Sonar. Higher-end sonar equipment (ie, military and research, not your consumer fish-finder) uses a phased transducer array.
That impressive stack of amplifiers probably would have scared me just a tad. All that power being channeled into pushing air, yikes! The power supply draw must have been massive once the whole thing gets cranking.
I'm glad you were able to get back to AX. We need to catch up sometime soon! Send me a note or a text and let me know how your schedule looks these days. Could always hop on for some Diablo as well! :D
My schedule has been relatively stable this month, but that's going to run out next week. I'll give you a shout, I may have some free time to catch up. Definitely been a while.