“Your Attenion Please!”
Although this page has been up for a long time, I recently learned that the old “Alien” THX widescreen laserdisc has an “experimental” Sensurround soundtrack on it! So I dug out my copy and WOW!!!! I made a video and it will soon be embedded at the bottom of the page (after I finish the editing) …
In 1974 I was just a kid and saw Earthquake at the Forum Cinema in Haymarket, Sydney. It was incredible and remains as one of my most exciting childhood memories – not for the “acting” or “plot”, but for the “startling multi-dimension” of Sensurround ! 😆
The Sensurround rumble effect could not be recorded conventionally on the film tracks, as the equipment of the day didn’t have the necessary frequency response or dynamic range. Instead a circuit board in each cinema generated the quake sound effect and control tones on the film were used to trigger it on and apply one of three gain settings to banks of high-powered amplifiers and sub-woofers.
The body-shaking impact from the sub-woofers was more intense than anything I had experienced before it or have experienced since (but read on). The whole cinema space was engulfed with an intense vibration that actually shook right through your torso to physically move your body in the seat. You had to be there. My words here cannot do it justice. There were banks of horn subwoofers under the screen and at the back corners of the cinema like these:
Reproducing the same effect as all of those sub-woofers would be a tall order, but I wanted to at least try and achieve something approaching it in the limited space of my rumpus room to experience “everything” in Sensurround! 🙂 I “kind of” jest, but giving a “Sensurround treatment” to Blu-Ray disks with modern Dolby TrueHD audio tracks and the like could be quite a feat.
So I started by building a pair of “rear” subwoofers each with a Cerwin-Vega 189E 18-inch driver identical to the units used in the cinemas in the 70’s:
They don’t make ’em like that any more! Heavy rough-cast, brut force transducers. No finesse required. 🙂
Sealed into tight (28 litre or 1 cubic ft) 32mm MDF enclosures:
Which face into this throat:
The sound “bends” around the sides:
And out via two paths each like this:
These are based loosely on the original “Model M” folded horns. Even more so than in the original theatre horns the drivers had to be in tight sealed enclosures to deliver real punch in a small home cinema space at the very low frequencies desired of the system. Bass reflex subs were employed in recent Sensurround revivals in the USA and Europe to the dismay of those who remembered the original screenings, so I dismissed them.
They were set up along the walls at the back corners of the room for maximum coupling to the room space:
And driven by one of these monsters:
Cerwin-Vega “A-3000Is”. Brutal! The “3000” version of these amps (without the VU meters) were used in some Sensurround installations in the 70’s to drive four 189E drivers – two per channel – presenting a nominal 4 Ohm load at which they delivered 500 Watts and were therefore described as “1000 Watt amplifiers” at the time (1975!!!).
The subs worked pretty well – causing things to rattle including a cupboard door that just wouldn’t stay shut and while initially testing them there was a “Henny Penny” moment: A ridge-capping tile shook loose from the roof and tiling cement fell to the deck outside! 🙂 Responseat around 50 Hz was good and thumpy, but it rolled off quickly below that and they were annoying because above about 60 Hz the level was excessive. The horn mouths/exits are not large enough to naturally extend as low as the large banks of horns in the cinemas due to the long wavelengths involved, so some kind of EQ was needed beyond what could be achieved with the settings of even the most elaborate consumer surround processors.
I then found “ESP Project 48” (“Subwoofer Controller”) and built one into a box and it worked wonders -reducing the unwanted upper frequency SPL to comparatively “boost” the low frequencies. This dramatically reduced the overall SPL, but the bass was all still coming from the back of the room and it didn’t quite hit you in the chest like the “real thing”.
Something bigger and better was needed up at the front to face directly into the seat, so I built a new cabinet based on the original “Model W” folded horn which had two 18 inch 189E drivers stacked vertically:
Too tall for a “plasma stand”. Mine could only be roughly half height and so could accommodate just one driver. I experimented with a Cerwin-Vega 189JE Mk II 18 inch driver which has a bigger voice coil and magnet. This was bolted to the throat at the back with a sealed enclosure behind it.
The performance was disappointing. 🙁 It really needed two drivers, but how?
Then I stumbled across a cheap pair of unused JBL 2241H air-cooled 18 inch 8 Ohm drivers still in their boxes and came up with an idea to fit them in the available vertical space.
I removed the original Cerwin-Vega driver, extended the horn entry throat and replaced the back sealed box with this crazy contraption:
Each sealed enclosure is 100 litres. The box is of 32mm MDF:
The drivers were individually connected to a respective channel of one of those Cerwin-Vega power amps (365W times 2 = 730 Watts continuously into the sub-woofer).
The sub-woofer weighs over 250 Kg and is too bulky to be moved out of the house in one piece!
The following is simulated SPL of the two sealed drivers in their new enclosures in half space (on the floor) without horn loading. Due to their mutual proximity, the bass drivers acoustically couple for 6dB gain. This graph is based on the full power output of the Cerwin-Vega amplifier and includes 2nd order Butterworth low pass filtering to simulate the effect of the “subwoofer controller” circuit used:
With just a couple of hundred Watts input, the measured room response (with the horn-loading and in something more like “quarter space”) is around 120 dB across the range.
The following shows the simulated SPL curve without the ESP controller’s filtering:
The unwanted output over 50 Hz and extending above resonance (75 Hz) would be deafening cone-tearing distortion adding nothing “good” to be felt in the body.
Anyway as set-up it sounded dynamic, punchy and very deep and sent mild shock waves into your body in the seating position about 6 metres away in the 30 – 50 Hz range – still not quite the real thing, but pretty good.
It was working well but the amplifier was 35 years old and “blew-up” occasionally and needed regular repairing and a few vital parts (like the obsolete driver board metal can opamps) are no longer available should they fail, so I decided to convert the subwoofer to an active one using an 800 Watt (4 Ohm) Class-D plate amp – a reasonably close power match to the old amp for which the driver enclosures were designed.
The stupid plate amp whilst apparently competent electronically seemed like it was designed by first year Industrial Design students and leaked like a sieve through its input sockets and switches etc., so I had to seal it at the back like this:
Being 230 Volt-rated for Europe and out of an abundance of caution I connected it to a 220V 2kVA step-down transformer rather than running it straight from the 240/250V mains. But it didn’t have the same reserves of continuous energy available as the old Cerwin-Vega amps despite its higher quoted power figure. It otherwise worked efficiently and reliably. I have since connected it directly to the mains (my house voltage measures 250V) and the output increased significantly and the sound became a lot more dynamic – now out-performing the old CV amps. The plate amp has a pair of bridged 400W Class-D modules in it so the power output is proportional to the square of double the DC supply rail voltage which I figure increased from about 54V to about 61V (still below the protection level of 68V).
The next upgrade was to add thick glass doors to the front. They kind of disguise the thing as a cabinet, but open out to give the horn mouth a bit of extra width or “corner-loading”:
The doors may slightly extend the bottom end and seem to concentrate the sound toward the seating position. Measured SPL (of the main subwoofer alone fed a bit more power and with all the other speakers receiving no signal) is over 120 dB (C scale) back at the seating position with the Earthquake rumble so it meets the original specification. Even so, I have to admit that on it’s own it l doesn’t really recreate large cinema-like Sensurround of the 70’s.
With the plate amp fed a monaural “.1” signal from DVDs via the subwoofer controller and the glass doors opened, the ceiling rattles and the plasterboard walls bang against their timber studs. In certain positions within the room, there are horrendous pressure fluctuations that render sitting down an urgency. It is actually a little frightening (which is good – I think). 😉 And what’s especially good is that the effect is achieved with cone excursion no higher than around 4mm even at 20 Hz.
The big front sub kind of rendered the rear two superfluous (but not any more). That is, the rear ones didn’t seem to add any additional impact when the front one was running. If anything they almost seemed to reduce the sound pressure at low frequencies which made no sense at first, but I put it down to standing waves and unequal distances between each sub and the seating position causing phase cancellations.
So when one of the Cerwin-Vega amps blew up (yet again) I had an idea and purchased a couple of these smaller 210W (8 Ohm) Class-D plate amps:
These came from a supplier right here in Australia with next-day interstate delivery and the cost was lower than buying directly from the manufacturer in the Netherlands! And they didn’t stuff me around either!
Why these? Well like the 800W one up front, they have an all-pass filter for adjusting the phase lag continuously from zero to 360º. It’s included to help integrate a subwoofer with main speakers at the upper/crossover frequency. I’m not interested in that at all, but what I hoped the feature might enable is a method of steering standing wave peaks around the room to add oomph to the “kick band” at the seating position!
And a standard (10 Ohm/100 nF) Zobel network attached across the driver to suppress the Class-D carrier frequency (and maybe flatten the impedance curve a little):
..and amps attached:
And it worked. The system now more convincingly replicates Sensurround. It now hits you in the belly almost like it did in the cinema in the 70’s!
With a broad spectrum low frequency rumble, adjustment of the phase lag to the rear subs did little or nothing to the SPL reading at the seat (it simply shifted the response around), but calibration with function-generated monotones at around 85dBC enabled a boost of around 5 dB at 25-30 Hz with a setting of around 40° and no addition power input. I found 25-30 Hz to be what “hits” best. 🙂
And the lounge chair bounces around with the Buttkicker turned OFF!
😆 😆 😆
Update (27 Oct 2013): Just watched the new Blu-Ray release of Battlestar Galactica (1978). That was intense!
Update 2 (20 July 2014): With a calibrated Earthworks microphone I measured this at-the-seat response at very moderate levels so as not to overload the software:
How about that eh? 😀