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[John Roberts {JR}]

Jerry Turnbow wrote on Thu, 10 April 2008 17:06

Hey folks - I got called in to help resolve some issues for a small church that has had their own DIY sound guy for quite a few years, who's put together a system using off-the-shelf MI grade gear - certainly not the stuff I was used to 16+ years ago when I worked for a system contractor. The good news is that most of the "fixes" are fairly straightforward - changing the way his monitors are set up, a little acoustic treatment, splitting up some circuits, etc. Their biggest problem is no GBF for the pastor's wireless mic.  After going through several purchases prior to calling me, they've actually got a pretty decent mic and wireless (EV, formerly known as "Telex" with RE97TX mic), but for monitors they're using the dreaded Galaxy Hot Spots (three of them) pointed at the dais where the pastor and several singers are. The singers all use Shure wireless mics with the tried and true SM58 capsules, and are not a problem, but the boom mic on the pastor, being omni, combined with the peakiness of the response of the Hot Spots, is unusuable. As I said, their main problem is simple - the Hot Spots right now are on the same amplifier and EQ as the mains, so they don't even have the option of pulling the Pastor down, or even out of the monitors.  I'll be correcting that with a separate amp, driven by a monitor buss, etc. for the monitors, and am wavering between putting a decent graph in or going with something like a Driverack or Sabine Navigator to try to go in and "tweak" the system to try to flatten out the response of the Hot Spots to the maximum extent possible. Galaxy doesn't publish any frequency or polar plots for these things (and before you suggest it, replacing them is not an option at this time), so all the "tweaking" and measuring will need to be done on site, using Smaart or perhaps a simple RTA.   Has anyone had any experience with taming these units down, and or done any response curve plots that I could use as a baseline to save time (and the client's money) in the field?

I get a sense that the hotspots are designed to be located pretty close to a fixed mic. Based on my unified theory of feedback that proximity would result in less feedback modes, that start higher, and are spaced further apart.  

Try moving the monitors closer. While I am not an expert on how the hot spots work, if their two drivers are combing with each other, that combing may be useful to counter the combing that occurs during feedback. I suspect there is a magic sweet spot (mic distance) for those monitors. Using them at other than that sweet spot may give much worse performance. Note: in addition to distance, polarity would make a difference for the notches to cancel rather than get worse.

Perhaps the reason they are not properly located now is the use of a common amp channel. Moving them closer would cause level problems without their own channel. At a minimum you need a separate amp channel and then 1/3rd oct EQ if you still have problems. It seems the power will be modest if you move these close in.

They should have application notes describing this. I haven't done a search because I don't have the problem.

JR

[Tom Young]

The problem with HotSpots is twofold:

1) the irratic frequency/phase response makes them less than the norm as far as feedback stability.

2) the irratic frequency/phase response makes their perceived response at the users ear change drastically with normal head movement.

I think the worst experience I have had with these is with a singing  pianist with a boom mounted mic. When they move their head the response changes. When they move the mic the feedback characteristics change.

This driver configuration is not employed anywhere else (other than as part of a 2-way system). It would be less of an issue if more drivers were used in either both axis (ala Bose 802) or in just that one axis (aka: column loudspeaker). With just the 2 drivers the phase interaction is more pronounced whereas with more drivers the net/overall response is more dense and is smeared in a (comparatively) beneficial manner.

If that makes sense.

[John Roberts {JR}]

Tom Young wrote on Sun, 13 April 2008 16:01

The problem with HotSpots is twofold: 1) the irratic frequency/phase response makes them less than the norm as far as feedback stability. 2) the irratic frequency/phase response makes their perceived response at the users ear change drastically with normal head movement. I think the worst experience I have had with these is with a singing  pianist with a boom mounted mic. When they move their head the response changes. When they move the mic the feedback characteristics change. This driver configuration is not employed anywhere else (other than as part of a 2-way system). It would be less of an issue if more drivers were used in either both axis (ala Bose 802) or in just that one axis (aka: column loudspeaker). With just the 2 drivers the phase interaction is more pronounced whereas with more drivers the net/overall response is more dense and is smeared in a (comparatively) beneficial manner. If that makes sense.

OK, I get that you don't like "Hotspots".  I am not pimping hotspots but just maybe there is some method to their madness. Keep in mind these are very application specific vocal monitors

But first I need to puncture the common (over simplified) understanding of feedback... namely making a monitor speaker or microphone flat will give you the most GBF.  If feedback was simply determined by, and closely followed, the frequency response of microphones and speakers, you would never be able to EQ feedback out with a handful of narrow band notch filters or even 1/3 oct EQ. SInce most speaker  bumps are broad, you would use up all of your notches just trying to kill the first peak.

Instead I offer a slightly more nuanced mechanism for feedback. When the microphone picks up sound from a speaker, it amplifies that and it almost instantaneously shows up again at the speaker. The next repeat takes the time to travel the path length between speaker and mic, and so on. This path length (wavelength), determines the lowest node and spacing between feedback nodes. Changing path polarity just swaps the peaks with notches in this comb filter variant (much more narrow).  Now that we have established  this series of nodes where feedback "can" occur, system frequency response determines which nodes will take off first, second, etc. If the path gain is less than unity, each repeat is at a lower level and will just tail away (like cheap reverb), if gain is more than unity it tries to make a mostly sinewave at that pitch. In a well rung out system when pushed to the edge of feedback you can multiple notes. Since these feedback nodes (combs) are very narrow, the corrective EQ can likewise be narrow, unless the mic (or speaker) is not fixed, in which case the varying path length smears the nodes around.

Getting back to the Hotspots, with their lumpy off-axis response, if those response dips are coordinated with the feedback nodes, presto we can get an improvement in GBF. Also the relatively short path length from keeping mic close, prevents lower frequency (longer path) nodes from even developing.  

This suggests to me a very specific geometry for mic and hotspot location to benefit from this mechanism.    

------

Nobody is saying hotspots are remotely hifi especially off axis, and I wouldn't generally try to use them with wireless mics (no mic stand to put them on)   If these monitors are more than several inches from the microphone (my guess in the OP's case), I see little benefit and lots of issues.

  JR

[Tom Young]

Thanks for your explanation of feedback. I will add it to what I already know. Feedback is one of those issues in live sound that I do not fully understand (despite dealing with it and studying for a very long time) and I enjoy alternate takes, which may help me to grok it.

I have (here in my office) a brand spanking new HotSpot I bought to measure a few years ago. I want to find the time to experiment with low-passing one of the 2 drivers to see how much it would be improved if done that way. The theory makes sense. Let's see what the reality is.

Like lots of equipment choices available in live sound / pro audio, when one feels strongly that a better choice can/should be made it is often a losing battle because of the cost difference. For example; there is a pretty cool looking new mini monitor from Mackie (SRM150) that is self powered, is not much bigger and employs a single 5.25" neodymium fullrange driver. I have one on its way to me to evaluate.  I will be very surprised if it isn't far superior in at least several ways and in particuar as far as its frequency/phase response. But unfortunately it costs 2.5-3 times as much as the HotSpot. So I know it isn't going to take the HotSpots's place in the market place. But it will allow some to get better results for that need. And I think it will make a cool little test loudspeaker for me, to use here and on job sites.

Hope I'm not beating a dead horse. And thanks again for this exchange.

[John Roberts {JR}]

Tom Young wrote on Sun, 13 April 2008 18:47

Thanks for your explanation of feedback. I will add it to what I already know. Feedback is one of those issues in live sound that I do not fully understand (despite dealing with it and studying for a very long time) and I enjoy alternate takes, which may help me to grok it. I have (here in my office) a brand spanking new HotSpot I bought to measure a few years ago. I want to find the time to experiment with low-passing one of the 2 drivers to see how much it would be improved if done that way. The theory makes sense. Let's see what the reality is. Like lots of equipment choices available in live sound / pro audio, when one feels strongly that a better choice can/should be made it is often a losing battle because of the cost difference. For example; there is a pretty cool looking new mini monitor from Mackie (SRM150) that is self powered, is not much bigger and employs a single 5.25" neodymium fullrange driver. I have one on its way to me to evaluate.  I will be very surprised if it isn't far superior in at least several ways and in particuar as far as its frequency/phase response. But unfortunately it costs 2.5-3 times as much as the HotSpot. So I know it isn't going to take the HotSpots's place in the market place. But it will allow some to get better results for that need. And I think it will make a cool little test loudspeaker for me, to use here and on job sites. Hope I'm not beating a dead horse. And thanks again for this exchange.

I should explain that my unified theory of feedback is "my personal" unified theory of feedback, but it seems IMO to fit the actual behavior of feedback much better than standard simple frequency response explanations.

Regarding tweaking the hotspot I guess it depends on whether making it a flatter speaker makes it a better close in monitor, or not.

I have never messed with one so don't know if their only voodoo is the close to mic spacing, or if they get some extra mojo from off axis combing. Please be sure to let us know what you find out.

JR

[Jerry Turnbow]

Thanks to everyone who responded!

I'm putting together my proposal for the client, which will include a system processor of some sort to give me a good "toolkit" for dealing with the problem, along with the obvious correction to the monitor system to give it it's own amp, buss, signal chain, etc.

FWIW, I think the geometry is such that these things might actually be made acceptable, as it's a very small dais, and there pretty much is a 1:1 relationship between monitors and individuals.  Although they are mounted on stands, the distance is probably a little farther than optimal, but the nature of the feedback I experienced on my survey seems to coincide with the 8K peak that was found on the graph Tom was kind enough to post.  That peak was there, even in the off-axis measurments, so dealing with it alone will have to make a huge improvement.

I'm considering temporarily placing my DBX Driverack 260  in there as a demo, as a "proof of design" before they make the investment.

I'll advise to all the results if they decide to go with my recommendations, and thanks again!

[Dick Rees]

I have some Hot Spots and some home-made 16 ohm compact monitors made with 2x4" 100W full range car audio speakers.  I'm no great carpenter or speaker designer, but these beat the Hot Spots hands down.  I still use the Hot Spots when the home-made look won't pass a style inspection, but for audio quality it seems to be quite easy to get a better sounding box.

[Jeff Babcock]

Tom Young wrote on Sun, 13 April 2008 19:47

there is a pretty cool looking new mini monitor from Mackie (SRM150) that is self powered, is not much bigger and employs a single 5.25" neodymium fullrange driver. I have one on its way to me to evaluate.

Hi Tom,
I have heard both the hotspot and the SRM150, and indeed the SRM150 sounds considerably better, and gets louder.  They still are nowhere close to a pro wedge, but for small low volume scenarios I think they would be much more usable than a hotspot.  Certainly they are less peaky, not great off axis but nothing as bad as what happens to the hotspot.  I'm sure Mackie will sell gobs of them so we'll all have to deal with them at some point.

Cheers
Jeff

[Jerry Turnbow]

Jeff Babcock wrote on Tue, 29 April 2008 15:17

Tom Young wrote on Sun, 13 April 2008 19:47 there is a pretty cool looking new mini monitor from Mackie (SRM150) that is self powered, is not much bigger and employs a single 5.25" neodymium fullrange driver. I have one on its way to me to evaluate.   Hi Tom, I have heard both the hotspot and the SRM150, and indeed the SRM150 sounds considerably better, and gets louder.  They still are nowhere close to a pro wedge, but for small low volume scenarios I think they would be much more usable than a hotspot.  Certainly they are less peaky, not great off axis but nothing as bad as what happens to the hotspot.  I'm sure Mackie will sell gobs of them so we'll all have to deal with them at some point. Cheers Jeff

Jeff -

I've actually tried one of the SRM150's for my personal use on another project, and you're right - they are way better than the hot spots, although they're a little lacking in the upper end.  

Unfortunately, with this project being on a tight budget (actually, no budget, probably), and with the obvious need to "normalize" the monitor setup with it's own buss, amplifier, processor, regardless of what type of monitors are used, I've proposed that they fix that part of the system first.  

At this point I'm waiting for them to get back with me on my proposal.

Thanks, all!