Forum Home » Sound Reinforcement » Installed Sound/Contracting » "Cooling" (or at least taming) the "Hot Spots"
| "Cooling" (or at least taming) the "Hot Spots" [message #311160] |
Thu, 10 April 2008 17:06  |
 Jerry Turnbow
Messages: 145
Registered: April 2005
Location: Saint Louis, MO |
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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?
- Jerry Turnbow (aka 'Mako')
Owner - Sound on Site Audio Services
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #311214 is a reply to message #311160 ] |
Thu, 10 April 2008 19:38  |
Tom Young
Messages: 1694
Registered: April 2004
Location: Oxford CT |
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I have rather strong opinions about HotSpots, the class of mini spot monitors that employ more than one driver covering the same frequency range.
From a loudspeaker engineering standpoint this configuration is just plain "wrong". The manner in which the two drivers interact with one another is the textbook definition of destructive interference. Perhaps the greatest flaw, or operational roadblock, is that the response changes considerably as you move about the coverage of this loudspeaker. So even if you equalize one of these to have smooth response at one position, a few degrees in any direction from that measurement point will exhibit completely different response. In your case (with a "moving target" mic'd pastor) ....... all bets are off.
Here's a graph showing my measurements of the frequency response of a brand new Galaxy HotSpot, on axis and at 10-degree increments. Despite the poor picture quality you should see that the blue trace is the on axis measurement.
Note the 8kHz, 1/2-octave wide peak. This is probably what your pastor's mic is interacting with, along with its own presence peak.
The reason 2 drivers are deemed necessary is to increase acoustic output and especially at lower frequencies. In many 2-way fullrange loudspeakers with dual cones plus a HF device, no effort is made to compensate for the dual driver interaction. But in this type of 2-way design the destructive interference occurs over a narrow band of mid-frequencies because the HF driver takes over at some point.
In some very well-engineered loudspeakers of this type, they actually roll-off one of the two drivers (above the frequencies where they sum fairly well and add bass) so that the mid-frequency combfiltering is almost completely eliminated. Cool.
Not to completely rain on your parade: it will be worthwhile to equalize these as best you can. But when possible, torch them.
[Updated on: Fri, 11 April 2008 20:08]
Tom Young
Electroacoustic Design Services
Oxford CT
Tel: 203.888.6217
Email: dbspl@earthlink.net
www.dbspl.com
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #311355 is a reply to message #311160 ] |
Fri, 11 April 2008 06:54 |
Brad Weber
Messages: 1158
Registered: December 2005
Location: Marietta, GA |
Has No Life |
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If the pastor has the mic properly positioned then the increased gain allowed by it being close to his mouth should offset the cardioid pattern of the SM58's, especially if they are not well versed in keeping the null directed at the monitors. I think that getting the monitors onto their own mix will definitely help, as will having their own processing.
The comments that they have three Hot Spots to cover the pastor and several singers and that these are fed the house signal indicates that they might be misapplied. The Galaxy Hot Spot is intended to be a personal monitor located near the performer, typically on a mic stand. As Tom noted, the Hot Spots have an interesting polar response, here are polars for the current model - http://www.galaxyaudio.com/pdf/HotSpotPolar.pdf. The -6dB horizontal pattern is apparently around 30 degrees at 250Hz, 90 degrees at 500Hz, 60 degrees at 1,000Hz, 40 degrees at 2,000Hz and 20 degrees at 5,000Hz, so the pattern varies pretty significantly and they have have a very narrow horizontal coverage within which the response will not differ greatly even if you did EQ them on-axis. The Hot Spots are also intended to be primarily a vocal monitor and if they are being fed full range instruments or music then the low end below 150-200Hz might need to be EQ'd out.
So while you might get things much better by breaking out the monitor send and being able to EQ the signal, the Hot Spots are not intended to be full range monitors or to serve several people, in fact the Hot Spot manual actually suggests there should be one per performer located within arm's length of the performer they serve. It sounds like this is perhaps a misapplication of the product and that this would not change, which would likely limit the potential results.
Brad Weber
muse Audio Video
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #311503 is a reply to message #311355 ] |
Fri, 11 April 2008 14:34 |
Jerry Turnbow
Messages: 145
Registered: April 2005
Location: Saint Louis, MO |
Has No Life |
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Thanks, Brad!
I appreciate the polar plot on the Hot Spots.
I'm aware that they're intended for use as vocal monitors, which was the reason for considering the use of a Drive-rack or Sabine Navigator unit instead of a garden-variety 31-band graph. That would allow me to tightly bandpass just the vocal range where the things want to operate, with true shelving filters, etc. and some parametrics to smooth out any peaks in the boxes, instead of trying to "fix" the problem by just pulling down a bunch of 1/3 octave filters, and maybe adjusting a LPF on the graph. And getting them on a true monitor buss would allow me to route only the vocals to them.
Do you know if there's a frequency response plot on that site as well? I poked around, but got an error message when simply trying to access Galaxy's .pdf page.
The units are mounted on mic stands, and while perhaps not as close as recommended, they are pointing toward the null points for the handheld wireless mics. They work surprisingly well, considering they're getting the same signal as the house, so I'm fairly confident that the situation can be made tolerable by splitting them up on their own buss/processor/amplifier chain.
Interestingly enough, the feedback that they get with the omni RE97TX on the pastor is extremely high-pitched. I'm guessing 3-5K, which I wouldn't have expected. Given the comments from other posts regarding these boxes, I'd have expected problems in the 800 Hz-1.2KHz range. That being said, if they decide they can't afford to put a full-fledged processor on the monitors, I might be able to get by with a 31-band graph, and then split the pastor's mic at the direct out and into a spare channel, where I could use a different EQ for his monitor.
Thanks, everyone for your feedback (okay, pun intended there.)
Tom - I'm still interested in your frequency plot if you have it, unless you're referring to the polar plot that Brad sent.
- Jerry Turnbow (aka 'Mako')
Owner - Sound on Site Audio Services
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #311653 is a reply to message #311537 ] |
Fri, 11 April 2008 21:58 |
Jerry Turnbow
Messages: 145
Registered: April 2005
Location: Saint Louis, MO |
Has No Life |
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| Don Boomer wrote on Fri, 11 April 2008 21:46 |
If you're gonna use a Navigator why not just let the FBX filters set up in auto mode. Then you can take a look at where the filters set ... maybe use the PEQs, maybe not.
You didn't say how you were using the Navigator in the system. Can you dedicate one Navigator channel to the Pastor's wireless mic (you'd also get comp/limiting in the deal). If money is a big deal maybe just use a single FBX in line with the wireless.
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Thanks for the idea, Don! I didn't know that Navigator would tell you what the filters "found" and locked on to. I currently used the Driverack 260 on my rig, but have looked into getting the NAV 480 for my own monitor setup, so I don't have any hands-on experience with it at this point.
The idea behind using the Navigator would be to use the model without front panel controls, put it in the signal path between the monitor buss and the monitor amp, then dial in the curve to band-limit and shape the response for optimal use with the (ugh) Hot Spots, then take away the computer so the user can't putz with the settings. I think there's a two-input version, where I could use the other channel on the house, or perhaps to shape the bandwidth and limit the feed to the rest of the church (cry room, vestibule, rectory, etc.)
I do own and have used the FBX solo units in the past - particularly to solve problems with acoustic guitars, but in my experience, they need to be "trained" each time for reliable use, and I'm not sure my client wants to go through that every Sunday, but they are great little problem solvers when I need them.
The other thing is, I'm not sure this client will spring the $$ for any system processor. Their mode of operation in the past has been to go to GC and buy something inexpensive off the rack, so I may be limited to using an MI-grade graphic EQ to get the monitors separated, but even that will be a major improvement over the current setup. I'd just like to propose to them a couple of options (good, better, best sort of thing.)
Thanks again for your input!
- Jerry Turnbow (aka 'Mako')
Owner - Sound on Site Audio Services
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #311971 is a reply to message #311160 ] |
Sun, 13 April 2008 04:18 |
Philip Roberts
Messages: 139
Registered: April 2004
Location: SW Michigan |
Has No Life |
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Here's a idea for you.
Install a high current relay that disconnects the hot spots during the when needed.
P&B now part of Tyco is a major manufacture, you'd be looking for a DPST relay a minimum.
Philip
Philip Roberts
Director of Audio
Pioneer Memorial Seventh Day Adventist Church
Berrien Springs MI
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #312018 is a reply to message #311160 ] |
Sun, 13 April 2008 10:40 |
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John Roberts {JR}
Messages: 6401
Registered: April 2004
Location: MS |
Has No Life |
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| 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?
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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
https://www.resotune.com/
"A bus in a console is spelled with one 's', but you can buss your girlfriend while riding in a bus."
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #312096 is a reply to message #312018 ] |
Sun, 13 April 2008 16:01 |
Tom Young
Messages: 1694
Registered: April 2004
Location: Oxford CT |
Has No Life |
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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.
Tom Young
Electroacoustic Design Services
Oxford CT
Tel: 203.888.6217
Email: dbspl@earthlink.net
www.dbspl.com
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #312124 is a reply to message #312096 ] |
Sun, 13 April 2008 17:46 |
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John Roberts {JR}
Messages: 6401
Registered: April 2004
Location: MS |
Has No Life |
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| 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.
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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.
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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
https://www.resotune.com/
"A bus in a console is spelled with one 's', but you can buss your girlfriend while riding in a bus."
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #312138 is a reply to message #312124 ] |
Sun, 13 April 2008 18:47 |
Tom Young
Messages: 1694
Registered: April 2004
Location: Oxford CT |
Has No Life |
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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.
Tom Young
Electroacoustic Design Services
Oxford CT
Tel: 203.888.6217
Email: dbspl@earthlink.net
www.dbspl.com
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #312217 is a reply to message #312138 ] |
Sun, 13 April 2008 22:33 |
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John Roberts {JR}
Messages: 6401
Registered: April 2004
Location: MS |
Has No Life |
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| 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.
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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
https://www.resotune.com/
"A bus in a console is spelled with one 's', but you can buss your girlfriend while riding in a bus."
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| Re: "Cooling" (or at least taming) the "Hot Spots" [message #312228 is a reply to message #312217 ] |
Sun, 13 April 2008 23:41 |
Jerry Turnbow
Messages: 145
Registered: April 2005
Location: Saint Louis, MO |
Has No Life |
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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!
- Jerry Turnbow (aka 'Mako')
Owner - Sound on Site Audio Services
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