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This topic comprises 2 pages: 1 2
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Author
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Topic: high pass protection filters for bass reflex speakers - surrounds?
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 01-09-2018 07:23 AM
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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Steve Guttag
We forgot the crackers Gromit!!!
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Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 01-10-2018 06:40 AM
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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Harold Hallikainen
Jedi Master Film Handler
Posts: 906
From: Denver, CO, USA
Registered: Aug 2009
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posted 01-10-2018 06:25 PM
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
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 01-10-2018 06:44 PM
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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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 01-11-2018 04:13 AM
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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