DrWho

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About DrWho

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  1. Talking to engineer friends....the reason that responsible automotive companies (read not Tesla) are progressing in this manner is because it's a safe development strategy. It's much less about "readying the consumer." Many of these companies are installing cell networks in the cars so they can data mine the sensors. As the data comes in, they simulate how their new designs and algorithms will behave under real scenarios. More data = more refined. There is a lot of speculation from outsiders about the readiness of technology, its safety, etc, etc.... but then there are those operating with the real data. The list of corner cases to be handled continues to grow. Some of this information leaks out into the general public (like the mylar/foil balloons), but most of it stays in the realm of trade secrets. Gotta protect that intellectual property.... I have no doubt it will eventually happen, but it's not going to look the way it's being described here or in the marketing. It'll probably start down that path - lots of people will die - and then eventually the proper infrastructure will get put into place. It's a very straightforward design challenge with the proper infrastructure, but we simply don't have that infrastructure yet - and trying to put all the smarts into the car is the wrong solution. It may be a necessary development step, but I long for a day when we actually come together as a community and decide we want to do something well. All the misinformation and sensationalism does not push us in that direction - and that's really frustrating for me. But even more frustrating is the rogue irresponsible behavior from Tesla to inflate their stock prices.
  2. Btw, here are some STC for Plexiglas: http://www.eplastics.com/Plastic/plastics_library/Plexiglass-Noise-Reduction But note the caveat below the charts: For free-standing plexi, you can subtract a good 10 to 20dB from these charts. 4" may have been an exaggeration, but the point is you can't get it thick enough to pull off numbers equivalent to a good cardoid without other implications.
  3. The PZM pattern is the same as an omni-directional with the bottom half cut off - just a single hemisphere. In fact, the construction of a PZM is really just an omni-directional capsule placed as close to the boundary as possible. This means comb-filtering above the 1/4 wavelength distance - and that's where the special sauce happens. Your rejection pattern (the side opposite the hemisphere) is limited by the attenuation properties of your boundary. A thin piece of plexi doesn't offer consistent attenuation and gets less as you go lower in frequency. And diffraction is the reason the panel needs to be large - again, something that gets worse as you go lower in frequency. Guess what - polar control of the speaker gets worse as you go lower in frequency too. And cardoid mics don't sound awful. When are you going to start recommending magic microphone cables and special power cords?
  4. The polar pattern tells you the relative pickup of the microphone in different directions. Directly in front is normalized to 0dB. As you rotate around the polar pattern, the sound pressure turned into voltage decreases as the polar bubble moves towards the center. The steps in these charts are generally 5dB. When you place a microphone in a system, the GBF benefit of the microphone is the on-axis response subtracted from the off-axis response. The on-axis response is defined by the angle of the mic relative to the desired source (the person talking). The off-axis response is defined by the angle of the mic relative to the undesired source (the speakers or reflection points in the room). If you want to be hardcore, then you need to sum the energy from all of the undesired sources. We don't need to run real numbers though - just point the rejection lobe of the microphone towards the source of feedback. Here are pictures illustrating the concept. The PZM on-axis and off-axis response is the same - so the GBF benefit is 0. With a cardoid, you could be as high as 20dB, but in practice it's usually more like 12dB (because there is usually more than one feedback path). Using plexiglas, there is a lot of complex frequency dependent behavior happening - even if you position the PZM so that the 180 degree pickup is pointed away from the speaker. It's because the plexiglas isn't infinitely large. The plexiglas also lets sound travel through it, as well as vibrates at certain frequencies (it becomes it's own reradiator). At the end of the day, the cardoid is already providing a 180 degree pickup in the forward lobe, and provides better attenuation in the rearward lobe. Now if you wanted to use some 4" thick plexi in a 4x4 ft sheet, then maybe you'd get enough blocking - but why stop there? Why not put everyone inside a fishbowl?
  5. Hah! Nice!
  6. The difference between a choir and a theatrical performance is the number of people speaking (singing) at the same time. The distance from the acoustic source to the microphone is offset by the number of acoustic sources. A single person talking would need about 10dB more gain versus 8 people covered by the same mic. Ever heard a solo from the middle of a choir that wasn't using a separate mic for the solo? The other problem is the amplitude variation as the talent moves around. That effect gets masked a bit when multi-mic'ing a choir. It's much more noticeable with a single voice. Perhaps it's because I'm a professional, but I simply won't "attempt" something that won't confidently meet a minimum quality standard. It's also been my experience that inconsistent sound quality is a lot more frustrating than consistently bad. It's also been my experience that a consistent bad scenario is the best way to drum up the resources to actually fix the problem correctly. When you go in trying to make a less than ideal setup work, then you waste all this time distinguishing between user error and the actual demerits of the approach. But hey, you armchair quarterbacks won't be on the receiving end of the scowls generated by these crazy ideas. It has been my experience that doing something poorly is much worse than doing nothing. When doing nothing sounds bad, then you can say "hey, here's the proper way to solve this problem". If every parent of these kids donated $200 like this one parent wants to, then the camp is going to be set for a very long time with a proper solution. This burden shouldn't be carried by the one dad with enough experience to know that it's a solvable problem.
  7. This isn't my opinion - physics tells us that it's not ideal for GBF. You are very keen to point out that you've had success in the past - I won't argue with that perception at all (it would be insane actually). You're an honest dude and I fully believe your experience. You had enough GBF in your application. However.... You had enough GBF despite your approach - not because of it. The people in your example were were simply projecting enough to not require much gain from the system. The thing about GBF is either you have enough or you don't. If you have enough, then it doesn't matter if one approach gives you more - once you don't have feedback, then it's not like you can have less than no feedback. This doesn't mean your approach is the best approach - it just means it worked for your application. And that point is important because your approach doesn't work for the other 90%. If you were giving advice, would you recommend something that worked 10% of the time, or 90% of the time? And if you saw someone offering up crap advice, then wouldn't you speak up?
  8. GBF = Gain Before Feedback.
  9. I forgot to mention one thing....with any microphone solution, you're going to need to EQ the feedback frequencies. These frequencies are unique frequencies that wrap around the speaker or microphone due to their construction. There are often 3 to 6 of these frequencies that exist - while all the others tend to be well behaved. Be careful though because these frequencies slide around with position. EQ'ing out these frequencies is basically flattening the off-axis response of the system, which lets you turn the microphone channel up higher. There are feedback reducers that do this automatically for you, but they don't work better than an educated operator. All you need is a high Q parametric EQ. Don't get crazy with this though - notch the first few frequencies and then don't try to add more. You'll hear when it's enough because the feedback will move from one tone to a series of multiple tones. This is why hyper-cardoid mics are bad....they have big spikes in the off-axis response. The goal is to have smooth rejection patterns. For both the speaker and the mic. The thing is, the good microphones and speakers that do this aren't cheap.
  10. It's more of a liability to be honest. You don't get a rejection pattern with a PZM - this reduces your GBF. The Heresy absolutely would not work. That small cabinet with a single 12" driver has no control over the polar response....that translates to lots of bleed back onto the stage, which also reduces GBF. Most of the Klipsch pro line suffers from the same problem. You need to get into the bigger horns before the pattern control starts to get controlled adequately. There is some Klipsch pro that accomplishes this. Line arrays control the vertical polars extremely well - but they often don't control the horizontal very well. Whether or not that matters depends on a lot of factors. The amount of horizontal control varies quite a bit too. The thing about these devices is that they are tools. Anyone getting on a philosophical pillar about which method works best doesn't understand the intricacies of the tools. Just as a carpenter may have multiple saws and hammers, so does the sound engineer. The factors for sound quality in the live sound world are totally different than any audiophile pursuits that might be discussed here. If you're looking for a good pro audio resource, then check out this website: http://www.prosoundweb.com/ They have forums too.
  11. For $200 you're simply not going to accomplish your goals. Floor mic'ing is not a viable solution to this problem. I could explain why, but my experience tells me you're going to try it anyway. So to get the most out of floor mic'ing, the first step is to get the right polar pattern. Don't use highly directional mics (no hyper-cardoid). Use a standard small diaphragm condenser microphone with a cardoid pickup pattern. Position the mics in such a way that the rejection null of the microphone is pointed towards the source of your feedback (the speakers). And if you have control over the speaker position, then work hard to minimize the amount of sound from the speaker that ends up on the stage. Your goal here is to maximize the "gain before feedback". This is a boring physical reality that can be quite readily calculated. There are ton of articles on the subject, but it all amounts to minimizing how much sound from the speaker ends up in the microphone. The reality is you will be limited by the gain before feedback. The other thing that limits the gain before feedback is the number of open channels. You can buy yourself another 2 to 4 dB by riding the faders and only having one mic turned up all the way at a time. The unused mics don't need to be attenuated by more than 10dB for this to work. There are automixers that do this for you (Shure SCM810 or SCM820), and some digital consoles have this functionality built-in. (Yamaha TF and QL series). I'm a little biased towards the SCM820 since that was one of my projects at Shure. ======================== That said, if you really want to solve this problem as dictated by the laws of physics, then you need to individually mic each kid with a lapel or headset mic. Even then you're going to be limited by the gain before feedback - especially if they're not projecting. In the professional world, this is solved by using several wireless microphones. Ya, I work for a wireless mic company, but that is not biasing my recommendation here. You might be able to rent enough wireless gear for $500 to $1000 for this musical depending on how many channels you need. Figure $50 to $100 per mic per day.
  12. Yikes, that's a horrible idea....but I think it'd require a lot of battery engineering experience to explain why. The Model S is a crap car though, so maybe the exploding batteries might do the world a favor by getting rid of these obnoxious cars and their owners, hah. I'm a big fan of swapping batteries, but standardizing on Lego pieces doesn't work. The real solutions for batteries won't be ready until 2025'ish based on the battery conferences I attend. Btw, autonomous doesn't require electric.
  13. 20log(Sd/St) Where Sd = driver area St = throat area Conceptually, the driver moves one unit volume of air for a given excursion. When that volume of air moves through a smaller opening, then the velocity increases proportionally (because the same total volume gets displaced). The job of the horn flare that comes afterwards is to maintain as much of that higher velocity all the way to the mouth - where the surface area is even greater, and you're displacing even more air. In order for this to work, the horn needs to be long enough to allow the pressure to build up into a proper wave. If the horn flares too fast, or is too short, then the air "rushes around the corner" of the mouth / horn wall and yields a large pressure change. That pressure change creates an opposite polarity wave that travels in all directions, including back into the horn. That's why the quarter wavelength horn length is such a magic number. This is a horrible explanation for the concept of impedance matching, but I think it's helpful from an empirical perspective. The compression ratio basically sets your starting point.
  14. Hornresp is fairly accurate if you maintain the assumptions behind its equations. I use it as a best case first order sanity check. It's usually not too hard to calculate by hand when you're deviating from the ideal assumptions - I'll often manipulate the inputs to the tool to see the results of those deviations. You kind of have to put the full story together in your head after the fact....or just build it and measure. Hornresp is modelling the performance of a circular cross-section with the defined area expansion. Once you deviate from that, then hornresp is totally invalid. It's also assuming the wavefront follows the shape assumed by the exponential/conical/tractrix equations, which isn't reality either - even for circular horns. So from that perspective it's probably better to say that it's inaccurate for most systems. FEA (or BEM) is the way to go, but you're not going to get a nice interface like winISD or anything like that....and you can really muck things up if you don't get your meshes right. That or it will take weeks to solve the model.
  15. There is a big difference between comparing designs and comparing a working circuit against a circuit that was damaged or built improperly. Circuits behave the way they're built, not how they're designed.