Getting in a flap!

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You all know that Lionsgate film intro with all the shiny gear wheels? :-)



Well, anyway, Lionsgate nearly broke the house! I had started the hired DVD disk and turned the volume somewhere around the usual setting.  There had a been a couple of wimpy trailers...when all of a sudden the Lionsgate intro started. The entire house shook like a leaf! The walls and ceiling creaked and vibrated incredibly violently! Of course I quickly turned it down...

We were about to watch "Abduction" on an ordinary, hired DVD. I chose Dolby 5.1 rather than DTS. It wasn't long before we realised that the film makers  took their bass rather seriously. Every so often there would be a loud roar and the curtains across the double doors to the IB enclosure would waft 6" in and out! I'd never seen this happen before! It became a habit with frequent excursions of the full length, heavy velvet. Great fun! The first motorbike scene sounded as if the bike was right in the room!

The film was panned on IMDB with a 4.3 score. It's true that there were weak moments. Not least Sigourny Weaver's wooden acting!

The scenes between the two young stars were much too "teenage movie/Moonlight" and completely superfluous. (except for teenagers perhaps?) We all had the human biology lesson. We do get the picture!

There were moments in the story where the suspension of disbelief was hanging by a burning thread.

The action plot still managed to hold the excitement for us. We would happily have given the film an overall mid-5s... except for the incredibly weak ending. Which limply burst the rapidly deflating balloon.

Bass-wise this film has to be a firm 8 or even a 9!

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Weird vibes!

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Hardly worth a post on its own but I have noticed a couple, or three, audio oddities this week.

I was down at the snow-covered beach photographing the sea ice for another of my blogs. Out across the sea it sounded as if several large manufacturing workshops were working away. Probably an inversion layer but what could be making so much "machinery" noise out of doors to carry for so many miles across the sea? The nearest land is about 10 miles away. With no guarantee that several hundred people are beavering away out of doors in sub zero conditions with heavy machinery. Absolutely no sign of any ships or boats. Very odd.

Later, I was descending the stairs in a four story office block. Halfway down I was subjected to a VLF vibration of immense proportions. A very uncomfortable experience indeed! As I exited the building I spotted a large defence helicopter some way off. Could this have excited the incredibly heavy air vibrations I experienced? I can't think of anything else which would have enough acoustic energy. There was no sign of any building work in the neighbourhood. The sound was clearly located in just one section of the middle run of stairs. Above and below it was quite inaudible.

Case three is ongoing. A neighbour, who lives two hundred yards away, brings his lorry/truck home with him. The house shakes to the sound of the diesel motor even at this considerable distance. He used to start the engine at 4am and leave it running for 20 minutes to warm up. Lately he has been leaving at more sociable hours. Though the effect is always the same. Even in the closed bathroom the engine sounds incredibly loud. Or rather the frequency with which the room vibrates in sympathy is very loud. It doesn't actually sound like an engine. More of a heavy beating in the upper infrasonics. More felt than heard.

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Effect of flat surfaces facing IB drivers.

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It has often been recommended that IB drivers should not face flat surfaces. Particularly close walls.

The subject was raised on AVForums with regards to an IB project. (DIY Subwoofer Build forum)

It was desired to have a pair of well spaced manifolds but this brought the 18" driver face to about 18" from a solid wall. There was some discussion (and disagreement) of the likely effect of their relatively close proximity to each other.

It seemed worthwhile to try and prove, using REW, whether an IB is really affected by flat surfaces. Reflected pressure wavefronts returning from the wall could theoretically affect the large driver cones. Or the wall might increase the pressure seen by the cone. Not a desirable effect in an IB if it affects the TS parameters.

I decided to place cardboard boxes on the floor to provide nominally flat surfaces facing my 15" drivers. The boxes reached almost to the top of the lower four drivers but were open at the top and sides.

The flat surfaces were placed about 6" from the faces of the lowest four drivers in my 8 x 15" driver manifold. Due to the access door to the IB enclosure I could not place the box on that side any further away. Only the bottom four drivers of the IB manifold were driven.

My Galaxy 140 SPL meter was placed on top of a pile of cushions. About 6" from the mouth of the manifold and about a foot above the bottom of the manifold.

The lower 4 x 15" IB was swept 3 times using the longest 1M REW frequency sweep both with and without the boxes in place. Each half of the manifold is physically separated by a shelf midway up the 6' high box.

A clear and repeatable difference of about 1.5dB (maximum) existed between each set of graphs. The individual traces were identical within each set above 8Hz. The set with the boxes in place consistently reproduced the sweep at the higher level.

Only at 12Hz did the sets match: Below 8Hz variations are commonplace even using the longest REW sweep. This could be due to random air movements or draughts.

It seems rather surprising that such a clear result was possible from such a simple test.

There was a difference between the waterfall graphs too. Mostly at infrasonic frequencies:

 With boards

No boards in place.

Now I ought to repeat the test using more massive boards of much greater size to see if this affects the outcome. Also to examine the effect of distance of the flat surfaces from the cones.

There is also the question of whether a diver would be similarly affected if the magnet faced the wall. Rather than the driver cone. 

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BTW; I am still suffering from intermittent switch problems on the cheap CX2310 Behringer active crossover. Perhaps some specialist electronics switch cleaner would help? I have never felt the use for any until now.