Achieving 'Sensurround' LFE in HT aka. remember the earthquake. - Page 4 - Home Theater Forum and Systems -

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post #31 of 59 Old 01-19-09, 05:24 PM
andy summers
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

It’s possible the version I saw Earthquake was in 70mm six-track magnetic as the cinema is equipped with Phillips DP70 dual 35/70 projector.
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post #32 of 59 Old 01-19-09, 05:28 PM
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

GPM wrote: View Post
True, but the original set-up was for large theaters with the rumble track fed to twelve huge folded horns that were lag bolted to the concrete floor in front of the stage. With a coupled mouth area a full size 16 Hz (4 x 48 ft in half space), the 17 Hz lower limit seems reasonable to me from both a technical and personal experience POV, making me wonder if the DVD has all of the original soundtrack's intensity on it. The corner horns were good to 32 Hz, so basically there was enough acoustic power available to do some damage to both the cinema and the audience's hearing, ergo no doubt that amp levels were set just high enough to get the desired result. Smaller installs would have had fewer cabs to couple, ergo higher cut-offs, so I assume boost EQ was used to fill in the bottom end a bit.

Agree thatís why Iíd like to see a waterfall graph of the region 1 DVD which has two soundtrack versions the region 1 only has one soundtrack versions six-track Dolby digital.

I believe a few people out there in the world have several of these horn type speaker systems.
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post #33 of 59 Old 01-20-09, 07:25 AM
andy summers
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

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post #34 of 59 Old 01-20-09, 10:36 AM
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

That can't be with all the bass information? There's gotta be a channel missing? The 65Hz will hit you in the chest but won't rattle a building
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post #35 of 59 Old 01-21-09, 12:47 PM
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

There was allegedly infra-sonic content generated by the "pseudo random noise generator circuit" which fed the amplifiers and horns in the cinemas and this is probably the bit that you felt in your gut during the earthquake sequence of Earthquake.

The original Cerwin Vega Sensurround horn cabinets used 18 inch model 189E drivers. The cabinets were pretty ugly black/grey painted plywood and they were not capable of response down to 15 Hz as stand-alone units. Many were put together side-by-side with plywood mouth extension boards to increase their effective mouth area across the front of the cinema to create a huge wavefront emanating into the theatre space. As well as these, some were coupled to the rear walls and used the theatre corners is a kind of extension of the horn mouth.

This horn array was powered by a Cerwin Vega "Earth shaker" amplifier. It has about 350 W RMS into 8 ohms and something over 500 W into 4.

If you want this type of uber SPL in your HT going the DIY route, Build an array of highly sensitive folded horn subs like the Lab12, PI, Bill FitzMaurice Tubas/Titans. Or go the commerical route and buy a Tapped Horn from Danley or a compact folded horn from JTR (Growler).

FWIW. I'm planning this spring to build a Tuba30 to compliment my 14cu foot LLT.
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post #36 of 59 Old 04-11-09, 11:34 PM
Ian M
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

Hello guys. This will be my 2nd post (have to post 5 times before restrictions are lifted apparently). Sensurround is a pet interest of mine.

Unless using those stupid vibrating blade fans, wouldn't you need an electricity substation in your garden plus drivers and amplifiers that haven't been invented yet to reproduce 5 Hz at 120 dB? And who wants a giant fan in their home theatre (not to mention the necessary passage for air from another room) especially considering the trouble some people go to to reduce mere cooling fan noise from those cheap high powered "professional" amps?

Apparently the human body has a vertical resonant frequency around 5 Hz (tested on a vibrating platform), but my guess is that the resonant frequency of the torso to horizontal stimulus like propogating sound waves from speakers in front would be somewhere around 30 to 60 Hz - i.e. right in the middle of the Sensurround earthquake frequencies and right where many bass lines are present in rock music etc. I saw all of the Sensurround movies here in Sydney in the 1970s and recollect the feeling across my torso as it was getting pounded by the shockwaves! It was fantastic and exactly what the designers of the system were aiming for.

I have built the original earthquake noise generator circuit which I use as a "test rumble" feature in a home-built subwoofer processor (after correcting the several errors in the US patent circuit diagram which was derived from a pink noise circuit designed by D.B. Keele Jr. and published in the Audio Engineering Society Journals back in the early 70's) and have built replicas (of sorts) of the Sensurround folded horn subwoofers and use 1970's Cerwin Vega power amps to power them at home.

There seems to be a lot of myth about the frequencies involved. The circuit generates random amplitude frequencies predominantly from around 20 Hz up to about 80 Hz. It peaks out at around 40-50 Hz. Roll-off under 20 Hz is sharp. I don't know how those waterfall spectrums were generated (can someone please tell me what the software is called?) but the LFE ones look as though they came right off of my circuit board which suggests that the DVD was mixed with a rumble generated by the same circuit. Content around 5 Hz is insignificant to non-existent at more than -40 dB c.f. the level at 30-40 Hz.

In the cinemas there were two rumble levels (high and low). The effects pre-amp gain was controlled by a combination of control tones on a sound track on the film. A 20 Hz control tone said "rumble on" and a 30 Hz tone on top of that said "louder now". The silly Earthquake DVD has these tones present on the .1 channel when you select "3.1 Sensurround" instead of "Dolby Digital 5.1" on the set-up menu. The DVD of Midway is stupid too. That film used a second generation of the Sensurround process which did not rely on a rumble generator circuit in each cinema as the sounds were recorded on an audio track, but the 20 and 30 Hz control tones were superimposed at reduces dB over the audio track and used to steer the sound to either the front or rear effects speakers or both. It would be utterly impossible to steer 5 Hz around even a huge cinema space as the wavelength is about 68 metres (bigger than the theatre space). Rumbles around 20 to 30 Hz (around 11 to 17 m wavelengths) were steered around larger cinemas in the 70s. The very lowest (barely infrasonic) frequencies were modulated between the back and front subwoofers in Battlestar Galactica to simulate viper launch "wind" as those things went down the long launch tunnels. It was truly amazing. The DVD of Midway clearly has the control tones mixed in with the rumbles and bomb effects which are mastered onto the DVD at a stupidly low level. Turning the sub channel up to get the effects brings the control tones into your face! At the beginning of each battle sequence a monotone drone can be heard through the subs. It needs to be fixed if ever they decide to remaster it. The Galactica DVD is just terrible with a very low rumble present somewhere but no dynamics whatsoever and of course no "steering". I would LOVE to see a "7.2" system with two independent subwoofer channels. That would be brilliant.

I doubt whether the Sensurround effect could be reproduced well with ported speaker enclosures which have higher group delay or with glossy high-excursion 12 inch drivers having fancy names and a big "Thank you. You must be brilliant for buying me" message in their owner's booklets as a diversion from the incredibly LOW (84 dB) efficiency quietly quoted in the small print on the back page somewhere! What kind of crazy power amps would you need to get reasonable levels out of those things? Anyway 12 inchers cannot reproduce Sensurround.

Multiple high efficiency 18 inch drivers with high electrical power handling, properly tuned sealed enclosures, horn loading as well as very powerful amplifiers and a very big room would be a good start.

I use a 12 dB/octave filter/bottom end booster (ESP project 48) to compensate for the natural roll-off of the sealed drivers below box resonance (about 70Hz in my case). Single channels of vintage Cerwin Vega A-3000I amplifiers (365W RMS into 8 Ohm - cool running without a cooling fan) power each driver and it still doesn't come close to the cinema experience as the room it too small and many more subs would be needed (and I have four horn loaded 18 inch drivers already).

I have pictures, videos (oscilloscope) and a Clio graph of the rumble circuit output which really ties in with the LFE .1 waterfall graph above to upload later.

Incidentally, the BGW 750 amplifiers used for Sensurround in the 70's were about 250 Watts (maybe 280 - can't remember exactly) per channel into 8 Ohms and I believe they were driving pairs - ie one stereo amp driving 4 woofers so each channel saw a 4 Ohm nominal load. They would be no good for home cinema use without a fan upgrade as their output transistors are transverse fan-cooled by a noisy old fan at one side. Anyway, they are too old and tend to rust out anyway. I guess they were used for the model M horns which had insufficient power handling for the bigger CV 3000 amps due to their over-sized sealed chambers which could not properly control the cones of the 189E drivers which had a very low Vas and very limited Xmax (about 5mm). Having built horn replicas with original 189E drivers, I have a fair idea why they had a reputation for tearing their cones. You can do a lot better with modern drivers (tuned to a bigger sealed enclosure). My latest attempt used a pair of JBL 2241H 18 inch drivers (still old but very efficient at 98 dB 1W/1m) and is much better (about 120 dB to around 25 Hz on low power with EQ and with the drivers mutually coupled for 6dB gain like in the original Sensurround W horns - two drivers into a single horn throat).

Only thing needed now is a bigger room with more subs and amps.

Addendum. I thought some of this thread looked familiar. One passage comprises my words (including typos) grabbed off another forum! I don't mind although if quoting someone its appropriate to acknowledge the source.

Another addendum. The Region 2 DVD release of Rollercoaster has a very good Sensurround rumble in a .1 track. The Region 1 release does not have it.

Last edited by Ian M; 04-17-09 at 06:21 AM. Reason: improve content and fix typos
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post #37 of 59 Old 04-16-09, 12:28 AM
Ian M
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

Averaged output from replica Sensurround earthquake generator card:

Just for fun, here is what it looks like with 12 dB/octave boost on the scope:

Last edited by Ian M; 04-18-09 at 10:08 PM. Reason: Add video link
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post #38 of 59 Old 04-29-09, 04:34 PM
Jeff A
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

avaserfi wrote: View Post
There is one driver of which I am aware that would be capable of producing massive SPL down to 5Hz. The Phoenix Gold Cyclone which is no longer in production. It is very likely a pair of these rotary woofers would be capable of producing 115-120dB at 5Hz.

If you are interested in getting them look on ebay the going price is around $450 a pop. Phoenix still refurbishes broken models for about $150 as well.

Now, the pragmatic side of this recommendation: It is unlikely you will actually find source material to make use of these woofers at such frequencies, but you could do it to say you have the power .

Check out this website for a different take on an 'air pump' type of sub. It's an interesting read for low-freq fans. I remember the PG Cyclone cool idea. ;-)
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post #39 of 59 Old 04-29-09, 06:17 PM
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

As mentioned by Ian, the original system on Earthquake used 18" horn loaded CV woofers and tones of 25Hz and 35Hz for the effect. The bandwidth of the film system was limited at the time so they used external signals, with some of the mains added in. Later the bandwidth was widened but never below about 16Hz, IIRC. I don't know where the 5 Hz number came from, other than some harmonic that might have been present in relatively small amounts. The movie was mastered with a normal .1 track for the bass but did not attempt to reproduce the original effect, as it would be more likely to cause damage in most home systems than to be efective.

Looking for me, just google my username. I have used the same one for most sites for many years.
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post #40 of 59 Old 05-02-09, 07:03 PM
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Re: Achieving 'Sensurround' LFE in HT aka. remember the earthquake.

tones to trigger the rumble generator...which had a much broader spectrum than 25-35 Hz...right?
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