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» Film-Tech Forum   » Operations   » Digital Cinema Forum   » high pass protection filters for bass reflex speakers - surrounds? (Page 1)

 
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Author Topic: high pass protection filters for bass reflex speakers - surrounds?
Carsten Kurz
Film God

Posts: 3774
From: Cologne, NRW, Germany
Registered: Aug 2009


 - posted 01-09-2018 07:23 AM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
It is common knowledge that the drivers of bass reflex speakers as they are commonly used for cinema front systems and LFE need to be protected with high-pass filters that keep signals around the resonant frequency away.

Now, I guess most surround speakers are also of the bass reflex/vented type. What do you think about the need to protect your surround speakers the same way?

I am asking this because, since the digital rollout, I notice that in many cinemas the low frequency drivers of surrounds are dying like flies if the auditoriums are operated at close-to-spec levels.
I would think that in pre-digital days, critical signals never made it to the surrounds due to limitations in both the analog production (e.g. audio transformers, coupling capacitors, signal sources), as well as film sound systems (even SRD).

That clearly has changed. Digital production and presentation allows to put any spectral content at levels up to Fs into the surround channels now - and it does happen.

Common input filters for amps work in a frequency range targeted at larger 15-18" drivers that is not suited to typical surround speakers. The AP20 can employ suitable filters per output channel, but what about other processors? Do any of you installers/techs cater for that issue, or do you prefer to sell and install new surrounds every now and then?

- Carsten

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Harold Hallikainen
Jedi Master Film Handler

Posts: 763
From: Denver, CO, USA
Registered: Aug 2009


 - posted 01-09-2018 10:36 AM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
The JSD-60 has user configurable filters on each channel. See the "Filters Tab" at http://ftp.uslinc.com/ftp/Products/JSD-60/Documents/Manual/JSD-60%20User%20Manual%20160406.pdf#page=46

By the way, the SMPTE 2096-1:2017 section 7.3.4 calibration procedure suggests doing a sine wave sweep of screen speakers at -3 dB FS and surround speakers at -6 dB FS while looking for any harmonics higher than 20 dB below the fundamental. It is also suggested that -10 dB FS could be used if there are concerns about speaker damage. What do you think about putting continuous sine wave into speakers at these levels? Would -20 dB FS be safe?

Harold

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12206
From: Annapolis, MD
Registered: Dec 1999


 - posted 01-09-2018 07:37 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
I have not noted an uptick in driver failure. We do design our systems with the proper amount of power handling.

That said, I also think it is good practice to always high pass speakers so they are only fed what they can handle. What is the point of feeding 20Hz information and having to amplify it if the speaker can't reproduce it?

It is/was common for audio processors to offer a 50Hz HPF for the surrounds. QSC amplifiers typically have a selectable filter of 30Hz (stage) and 50Hz (surrounds).

If DSP or like processing is involved, then I let the amplifiers operate without filters and perform the High Pass filtering based on the actual response I'm seeing. This is true for all channels, including the subwoofer. I'd wager a lot of subs are playing with 30Hz HPF using the factory preset from QSC. We also generally use the SF-3 module and set both HPF and LFP on it for subwoofers, in addition to any B-6 alignment that a subwoofer may need for a flat/extended response. Again, if DSP is used, we'll handle the B6 HPF/LPF in the DSP.

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Marco Giustini
Film God

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From: Reading, UK
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 - posted 01-10-2018 04:27 AM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
I always set HP on my systems. If I have a capable DSP I choose the best frequency for the speaker otherwise I rely on the amplifiers - if an HP is available. Even subwoofers need a 20Hz filter as a minimum, higher if the system is underspec'd - I install what the customer pays for, Steve, I don't always have the luxury of installing 'properly spec'd systems'.
As Carsten says, soundtracks sometimes contain very very low frequencies on all channels and it's just adding distortion on speakers.
The AP20 is one of those processors with advanced DSP capabilities. I always set HP filters there, precisely tailored to the speakers/auditorium. Either in the Dirac or in the crossover setting.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12206
From: Annapolis, MD
Registered: Dec 1999


 - posted 01-10-2018 06:40 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Properly speced systems isn't a luxury. It is a responsibility. Do you find that you come up against this sort of thing a lot? It is quite rare for me. It isn't like we always have to put in top-shelf stuff either (in fact, far from). Putting a system in that doesn't blow up when run at spec, to me, would seem a pretty low threshold. If a customer chooses to under purchase on a system, I let the know the consequences of their choice, with hard numbers so, at least, they are informed on their decision. Normally, that is enough for them to find the money or, put in a less ambitious, but reliable system.

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Marco Giustini
Film God

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From: Reading, UK
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 - posted 01-10-2018 07:01 AM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
when you say 'run at spec' do you mean the speaker should be within its rated power when running at 7? Then yes, it happens all the time mainly on LFE and surrounds.
I have found myself dialling a higher HP filter on subs when the subs (er... THE sub) is heavily underspec'd for the auditorium. At least it doesn't even try to play very low frequencies. Nothing terrible, I'm talking about a 30Hz filter.
That being said, to go back IT, even if the system is properly spec'd I always set an HP filter as a stage speaker will never be able to play 20Hz anyway and there is nothing preventing the soundtrack from carrying those frequencies.

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Carsten Kurz
Film God

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From: Cologne, NRW, Germany
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 - posted 01-10-2018 07:19 AM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
Running vented speakers with signals around the resonant frequency without a HP is not 'to spec'. It's a technical setup fault. It was probably rarely an issue in analog/film days since the surrounds never received those signals. That changed.

- Carsten

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Harold Hallikainen
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 - posted 01-10-2018 11:23 AM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Repeating a question I asked before...

By the way, the SMPTE 2096-1:2017 section 7.3.4 calibration procedure suggests doing a sine wave sweep of screen speakers at -3 dB FS and surround speakers at -6 dB FS while looking for any harmonics higher than 20 dB below the fundamental. It is also suggested that -10 dB FS could be used if there are concerns about speaker damage. What do you think about putting continuous sine wave into speakers at these levels? Would -20 dB FS be safe?

Ideas?

Thanks!

Harold

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Jay Wyatt
Film Handler

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From: Los Angeles, CA
Registered: Mar 2015


 - posted 01-10-2018 02:51 PM      Profile for Jay Wyatt     Send New Private Message       Edit/Delete Post 
Just a theory, but perhaps more LF content is getting sent to surrounds because of immersive soundtracks. A re-recording mixer doing a native immersive mix first, potentially could sent more LF to the surrounds -- operating under the assumption of bass management (or higher spec'd surrounds). Maybe during the 5.1 and 7.1 translation all the LF material is left in and not re-accounted for. End result being lots of >50Hz spectral content hitting surrounds in non-immersive rooms.

To Harold's question: Is the SMPTE doc referring to -3dbFS peak or RMS? Either way, -20dbFS seems 'too-safe'. Shouldn't a stress test actually stress the SUT? I think it'd make more sense for a sine sweep to at least be in the neighborhood of the peak ranges of the 'reference' noise signals (somewhere between -12 and -7).

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Harold Hallikainen
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From: Denver, CO, USA
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 - posted 01-10-2018 06:25 PM      Profile for Harold Hallikainen   Author's Homepage   Email Harold Hallikainen   Send New Private Message       Edit/Delete Post 
Thanks Jay! As I recall, the SMPTE standard pink noise has a crest factor of 12 dB or thereabouts. For a sine wave, I believe dB FS is the same whether RMS or peak. A -3 dB FS sine wave has a peak code value of 70.7% of full scale. The SMPTE test procedure puts a continuous sine sweep at -3 dB FS into the screen speakers, -6 dB FS into surrounds, or -10 dB FS if you are afraid of damaging the speakers. If we assume a 12 dB crest factor for program or pink noise, and our reference level is -20 dB FS, the peaks of program and noise are at -8 dB FS. With program or noise, therefore, we can run a maximum of -12 dB FS RMS (which puts peaks at 0 dB FS). We MIGHT assume this is the maximum continuous signal a speaker needs to handle as far as thermal considerations are concerned. The SMPTE test procedure will make the tones REAL LOUD! I'm thinking of running a series of tones, one per octave, starting at 50 Hz, and measuring the THD. I've heard "rattling" in a speaker in a theater when it's driven with -20 dB FS pink noise. Ideally this test would catch that. The concern is making this a destructive test...

Thanks for the comments!

Harold

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Carsten Kurz
Film God

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From: Cologne, NRW, Germany
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 - posted 01-10-2018 06:44 PM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
That's the question, what's the intention of such a stress test? Should SMPTE demand that systems are actually stress tested?

I haven't done real tests with our surrounds, but I am pretty sure that -6 to -10dB FS could be a real threat to 'real'/existing systems. Steve is certainly right that it's a fundamental mistake to underspec systems. But there is little benefit in demanding that all existing systems in the field to be retrofitted with an array of higher spec'd surrounds. Even if that has been done, do you stop there? Soon you'd be requested to replace a 'working' system with a properly spec'd system at 15.000 to 20.000...

My intention to post this was my observation that many surround setups seem to lack these protection filters. And the consequences seem to become more obvious since the digital rollout, fully digital sound production, and, as Jay correctly states, object based sound systems which are much more demanding in the low frequency range of surround systems.

Fundamentally, yes, people should probably shell out more money for sound systems that are properly spec'd as opposed to rule of thumb or 'we're mostly playing arthouse'.

- Carsten

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Steve Guttag
We forgot the crackers Gromit!!!

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From: Annapolis, MD
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 - posted 01-10-2018 06:55 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
Object based systems SHOULD be new systems since the demands on the individual surround speakers are clearly VASTLY different than arrayed surround speakers.

And yes, every speaker should be high-passed such that frequencies it cannot play are not fed to it. As I said before, not only are you protecting the speaker, you aren't wasting amplifier power making sound that nobody will hear (aside from the woofer flapping about prior to complete failure). Most cinema-specific processors provide for this. Many amplifiers provide for this and ALL DSP systems allow for this too so it would seem that one would have to try and not apply a suitable filter.

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Carsten Kurz
Film God

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From: Cologne, NRW, Germany
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 - posted 01-10-2018 07:10 PM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
Steve, of course, 'real' object based audio systems will usually be equipped with properly spec'd surrounds and have ample ressources to tune individual speaker response. What Jay was referring to is that, as more mixes are initially created for the more capable object based systems, and on speaker systems tailored for object based sound, more low frequency signals make it into 5.1 and 7.1 mixes derrived from these object based mixes or being mastered on these systems.

Does a CP750 allow to setup a protection HP for every speaker channel?

- Carsten

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Jay Wyatt
Film Handler

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From: Los Angeles, CA
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 - posted 01-10-2018 10:16 PM      Profile for Jay Wyatt     Send New Private Message       Edit/Delete Post 
That's only a theory I was throwing out there. Someone would need to analyze the LF content of features done natively in immersive formats and compare to channel based only movies.

Steve and Carsten: when you guys pull blown speakers off the wall, does it seem to be an age thing or is it speakers asked to reproduce frequencies they cannot? Or would one even be able to diagnose the cause?

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Carsten Kurz
Film God

Posts: 3774
From: Cologne, NRW, Germany
Registered: Aug 2009


 - posted 01-11-2018 04:13 AM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
I have seen both voice coils being burnt (bubbles) and diaphragms being torn. Some speakers use foam suspension which deteriorates over the years all by itself.

Steve recones speakers and knows more, although the typical surround speaker probably rarely experiences a reconing job.

The trouble is that in classic 5.1 and 7.1 setups, many speakers carry the same signal, so it may turn out that from 4 speakers assigned to e.g. back surround left, 3 LF speakers are gone, with the remaining one still masking the defect. Since they are usually paralleled, there is no quick test procedure to find that out, as there is for the screen speakers or in object based audio where every surround speaker has it's own signal/amp channel. Every now and then I think about a special wiring/switching solution that allows me to test individual surround speakers that use no dedicated amp channels.

When we went digital 5 years ago, we noticed when calibrating the sound system that most of our surround LFs were gone. Diaphragms and suspensions looked like new, but many coils had bubbles. They would stick and not move with low signals, but make very unpleasant scratchy sounds at high levels. But we only found this out when we supplied high level test signals to them. We did not have an idea for how long this was going on already. I was shocked this went unnoticed for an unknown time. To complete the ongoing installation, we configured 200Hz high pass filters in the AP20 for the surround channels to keep the LF speaker from being driven, until we replaced them.

- Carsten

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