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Author Topic: high pass protection filters for bass reflex speakers - surrounds? Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-11-2018 08:08 AM                     As I've stated, my surround failures have been minimal. Probably our biggest failure was the JBL 8330A midrange. They would die pretty easily, as it turns out so don't over drive that speaker. I'm sure the original 8330 would be similar. The voice coil former will bubble up as it overheats. By and large, we've had few woofers give it up (except for foam surrounds that simply deteriorate, e.g. Boston Acoustics that Regal put up in the late '90s). In the optical sound says, the surround was a 100Hz - 7KHz speaker (yeah it went beyond that but not much more) and the level was notably reduced so they didn't have a hard life. When digital hit film, that changed and, depending on digital system, they were asked to run 20-20KHz. I'm not saying the movies had all of that information but they could be asked to do that. That is, all but DTS who put Subwoofer on the Ls/Rs channels and used simple filtering to high pass the surrounds and low pass the subs. That said, how many heard when the the Empirical Test disc was run and it got to the subwoofer channel ALL of their poor surround speakers start flapping their woofers when the signal was that low? I don't know what filter they were using but it wasn't all THAT steep for that much information to make it to the surrounds. For DCinema the spec on surrounds is 102dBc/channel (should have been 105dBc) and 20-20KHz is the bandwidth. So, compared to optical surrounds, yeah, they are being asked a bit. To answer Carsten's question (which I'm sure he already knows but is trying to make a point!), no the CP750 does not have HPF for the various channels. I did qualify my statement with the word "most." And that leaves also MOST cinema amplifiers have a means of a HPF too. This is true not only for the surrounds but the stage channels. The subwoofers should also be HPF right. I can't think of any system I've put in where all of them weren't HPFed. We use QSC amps, pretty much exclusively, and they have the filters for the stage and surrounds built in, so it is free. I've worked with the likes of the Crown DSi (originally installed by "others") stuff and that has a little DSP on board so you can get a HPF there. I, personally, think that when the tech is tuning the room, you should be able to see, pretty clearly, where a speaker can and cannot reproduce sound and use available filtering to restrict the bandwidth to what it can handle. I am a bit surprised that it has become the problem that is being presented here. Lastly, as to reconing, no so much on surrounds. As a rule of thumb, if the driver is some sort of stamped out frame, you don't recone it, you replace it. Reconing, due to the labor involved, is something that is done to more expensive drivers and for perhaps obsolete drivers where one really wants to keep the original sound. So LF drives of a stage speaker but primarily, subwoofers is what we have seen the most of. I can recone a JBL 2241 for much less than half of a new one and it will perform identical (same parts!). But for a surround speaker, pitch the driver and replace it. If no longer available, consider changing the surround array and use the remaining speakers as parts for remaining auditoriums that still use it. We have one site were we are finding replacement "equivalent" woofers. Mind you those speakers are over 20-years old and owe nobody anything!  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 01-11-2018 10:33 AM                     Many modern soundtracks are pretty heavy on the surrounds and those surround channels are full-range channels so I guess it's almost mandatory to filter them properly, to avoid prematurely damaging the voice coils. quote: Carsten Kurz Does a CP750 allow to setup a protection HP for every speaker channel? The CP750 doesn't have HP protection on each channel, unless it was recently included in a firmware update. I'm pretty sure the hardware would be able to do it. As you've noticed yourself, the AP20 does have HP/LP filters though and we're using them on any channel. We're actually using filters on both the AP20 and the amplifiers for each of our channels. Since those DSPs are just software-driven, it cannot hurt to have a little extra protection, to avoid a bug creeping in. It's indeed not always trivial to spot a burnt-out surround speaker immediately, unless it's actually making unwanted noises. Still, if you run a continuous loop of pink noise on low volume and you just walk by, you can usually easily spot them. If they're mounted too high and too close to others, a microphone on a pole will help.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 01-12-2018 05:50 AM                     I see many cinemas over here use standard pa amps, which lack the necessary or optional input filter modules. Then who is using active crossovers on their surrounds that could cater for a LF protection HP? If in his daily work Steve mostly sees the auditoriums he spec'd himself, he will probably hardly ever see blown speakers ;-) - Carsten  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 01-12-2018 07:35 AM                     I guess you're right, many locations don't properly filter their surrounds. I know a few places around that have blown essentially all their JBL 8330(A)s. There is close to no surround left in those rooms. At least one of them is using proper QSC amps, not some no-name generic PA amps, but they simply never bothered to set the filters. I guess you can blame that on the installer either not knowing about the potential issue or not caring.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-12-2018 08:05 AM                     I do mostly see systems that I speced! Even on the ones I didn't, rarely do I see ordinary PA amps. I was going to say that the VERY popular DCM series of monitor/crossovers also has HPF on surrounds but I guess if you are using the DCM, then you probably had DCA or ISA amps that have HPF built in. So you all are saying that the intersection of cinema processors without HPF for the surrounds and amplifiers that also lack this is so large that this is a problem? Or are people just not designing systems worth a flip or possibly tuning rooms beyond the equipment capabilities? Now I do see under-speced systems, all of the time. Not just sound but in projectors too (too little light). The latest scam is putting in high-gain screens to offset the inadequate light (it isn't a new scam as it was done in film quite some time ago). The screen doesn't "make light." The screen just concentrates the light you have. If the customer doesn't want to "pay" for the proper system that can handle what they are playing, then they can pay to fix it indefinitely. For the most part, I don't have a problem talking people into a properly fitting system (light or sound) but I do come with hard numbers to back up my statements.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 01-13-2018 10:45 AM                     quote: Steve Guttag So you all are saying that the intersection of cinema processors without HPF for the surrounds and amplifiers that also lack this is so large that this is a problem? Yes! quote: Steve Guttag Or are people just not designing systems worth a flip Yes! quote: Steve Guttag or possibly tuning rooms beyond the equipment capabilities? Yes! Or the less annoying form: All of the above. I have to agree with Carsten, you'll find a lot of standard PA amps in quite some cinemas around here. The CP750 is also by far the most common cinema processor. And while even some of those standard PA amps will have limiters, they're not always configured correctly or at all. I also agree with Carsten that a lot of the damage to the surround speakers will also go unnoticed at many sites, especially those who aren't so good in their upkeep, which unfortunately, are plenty. So, many of those surround speakers will probably remain silent, until someone decides it's time to refurbish the room or until someone halfway competent is hired to recalibrate the room.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 01-13-2018 11:38 AM                     Yup. I also wanted to stress that, with clearly more low frequency content in surround signals turning up, installers should not ignore protection filters for surrounds in new installations or replacements - using a suitable CP and/OR amps with suitable protection filters should be on the checklist. I also follow other cinema forums closely, and I never saw that being mentioned. What I see is quite a few reports of complete arrays of surround speakers being blown. Some cinema operators, at least if these speakers are older, seem to reflect on this with pride - 'Oh yes, we do play it loud!', and replace them - but maybe it's time to think and talk to their installers. As Steve explained, this is not only to protect the speakers from early defects, but also to make the most from the existing installation - if you force speakers into reproducing frequencies they can not reproduce properly, they underperform in other areas of the spectrum. So, setting the right filters does not only protect the speakers, but will also improve the sound of an existing installation. - Carsten  |  IP: Logged

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