Feel the vibes.


I haven't been adding much to my IB blog recently. My wife and I are still enjoying a couple of new (hired) films each weekend. The IB subwoofer continues to surprise and delight when the film asks for it. The system is also switched on for the odd, favourite TV series. Though TV soundtracks are never remotely on the scale of action films. Why should they bother when the average viewer is listening  to the TV's own speakers?

All our TV is sourced from our digital satellite receiver and two dishes. Terrestrial TV has never been successful locally due to our location in a hollow in the landscape surrounded by low hills. The Danish government chooses to code the Danish TV channels on satellite. This despite the highest TV license fee imaginable. 2260DKK = £264 GBP or $418 US. Is an exclamation mark really necessary?

Without a smart card one cannot watch Danish TV by satellite. The companies which provide Commercial TV channel "packs" just add in the Danish channels as an afterthought. Without  paying for a TV pack one cannot get a free smart card just for the Danish channels.

We can receive about 2000 different TV channels and probably almost as many radio channels . I've lost count of how many of these are free of coding. Literally hundreds and hundreds including HD channels. But no Danish channels at all.

For receiving Danish terrestrial TV we would need to take a chance on buying a digital tuner, antenna and possibly a mast to receive the digital Danish terrestrial channels. So we never see Danish TV. Nor listen to Danish radio. Not ever.

The irony is that the majority of immigrants to Denmark have satellite dishes to watch their home programmes. The government complains that they do not integrate fully into Danish society. (if at all) I wonder why? Could it be that the only Danish culture they ever see is in the checkout queue at the local supermarket?

This wasn't the real reason I started scribbling this post. We were watching a hired DVD of "X-men First Class" this evening. I went down to the kitchen to make a cup of coffee without pausing the film. The infrasonics in the kitchen and bathroom were quite remarkable. These rooms lie partly under the IB enclosure. So it is hardly surprising that one can hear the bass through the wooden floor. Which is boarded both above and below. Thus offering little attenuation or damping.

These infrasonics seem reduced upstairs. Or they are masked by higher frequencies. There is no shortage of weird pressure effects and the air being "shredded". (as I usually like to describe it) What seems to be missing are the very slow beating effects. Where each pressure cycle can be clearly sensed. Giving the illusion of almost being able to count the beats. Though one would need to be incredibly fleet of tongue to do so aloud. They are certainly present on classical organ music but not so often on films. Perhaps it is simply a case of being able to focus on the infrasonics downstairs in the absence of other frequencies?

It occurs to me to try a few REW sweeps with the microphone downstairs. Just to see what actually gets through the floor. My wife has often mentioned the kitchen ceiling rippling as she prepared a meal. Usually during one of my classical organ music, listening sessions. Or when she nipped downstairs to check a meal's progress during a film. Not quite the nausea that the array used to cause in the bathroom. But still worth trying an REW sweep or two:

NB. All doors between rooms were closed during testing. Peak frequency levels were lifted in REW to match the listening position. A Galaxy 140 SPL meter was used as the test microphone with matching REW .cal file. Meter set to low range, C-weighting on a 10m twin coax cable to Creative Live! external sound card: Only the subwoofer was in operation.

Too much complexity despite peak levels being matched?

Listening position ^

Some 9' from the manifold opening in the left wall.
Measured with the SPL meter placed on a cushion on the seat of my chair.
At normal, seated, ear level, with the SPL meter on a tripod the lower bass dominates more than this. 

Middle of IB enclosure ^

Only 3 feet from the 6' tall, 8 x 15" driver manifold. The enclosure is about 600cuft minus open storage. It is still very leaky to the outdoors at the very edge of the roof. Plans are afoot to finish the last row of insulation between the final pair of rafters before winter. The 45 degree sloping ceiling will then be finally boarded over.

Bottom of open stairs ^

There seems to be a distinct tuned port effect from the open stairwell. 20-25Hz is only borderline audible using sinewaves. My own audible barrier is at 25Hz as a very soft, fluttering effect.

Downstairs hall ^

A small space with a boarded ceiling below the IB enclosure. The hall doors to bathroom, kitchen and living room were closed. A double resonance.

Bathroom ^

Thinly boarded ceiling hiding the joists. It lies directly below the IB enclosure. A distinctly narrow plateau from 15-20Hz. Hardly surprising the vibes can be felt rather than heard!

Kitchen ^

Boarded ceiling between exposed joists partially under the IB enclosure.The peak is at 20hz with strong extension to even lower frequencies.

Other end of listening room ^

A triangular opening, about the size of a door, leads from one half of the attic listening room to the other. The room is split almost exactly in two by the chimney. A broad peak with an emphasis on the very low bass. 9-15Hz is all infrasonic.

Middle of downstairs living room ^

Open to the listening room via the open stairwell. Actually a simple rectangular opening in the floor/ceiling. There may be a bandpass effect at work here. A 9Hz peak!

As expected, the lower frequencies dominate. With an emphasis on the infrasonics. This is hardly surprising given the ready willingness of boarded surfaces to resonate and retransmit the acoustic (cyclic pressure variation) signal which excited them in the first place.

From memory, the levels downstairs were 10-15dB below the listening room levels. Though I made no real effort to record them. REW has recorded all calibration levels as the same. So I had better get busy and record actual levels relative to the listening position. The information might be useful in that it will give some idea of bass "leakage" between rooms with wooden floors.


I have discussed contractor's, plate vibrators here before. Today I came across another. Outside a supermarket I was visiting was a very deep trench. A contractor was working at consolidating the gravel backfill. The noise he was making was all low frequency. My chest was resonating strongly which usually means somewhere up around 140hz. Though the clearly audible sound reminded me more of my high level tests with sinewaves at 40hz. Very unpleasant it was too! It was actually far more comfortable to go much closer than to stand 20 yards away loading my shopping. I don't know why this should be.

As promised I have checked the noise levels leaking downstairs and elsewhere:

I opened REW and chose Preferences, Subwoofer, Check Levels. Then I switched on the audio system. I set the level at 85dB(C) at the listening position using the system's volume control and the Galaxy140 SPL  meter. This was set to the 80-100dB range C-weighting  - Slow. I shut all the doors as I moved from room to room with the meter held aloft like an Olympic torch.

The levels were roughly 8-10dB down in the other half of the room. 10-15dB down in the downstairs living room, bathroom and hall. And, all of 20dB down in the kitchen. Turning off the speaker amp and repeating the exercise made no real difference. There was some variation (about 3-5dB) as I moved around in each space. There were measurably higher sound pressure levels against solid room boundaries.

The ceiling vibrations downstairs must be quite narrow band. On film LFE the IB easily hits peaks of 110dB+ as read directly on the SPL meter. This would easily equate to 90dB+ levels even in the kitchen.

I had no desire to set the system to a steady state SPL much above 85dB(C). It is not at all pleasant to subject oneself to loud, pink noise. Nor would playing high levels of sinewaves be any more comfortable.

No doubt one could eventually reproduce the resonant frequency of the downstairs ceilings. Provided, of course, one wore ear defenders while in the listening room. An aid downstairs to report any resonant effects, as frequency was adjusted up and down via the mouse, would speed things up. I would imagine these resonances lie deep in the infrasonic region rather than the audible.


I tried something I haven't watched for ages. Lord of The Rings: Return of The King. I laid the Galaxy 140 SPL meter down on a cushion about 9 feet from the IB manifold. It was left on Max Hold, C-weighting, 80-130dB range. By the time the film was finished it had hit 119.7dB(C). My wife was outside gardening for the first hour. She said the bass was very audible and rather repetitive. Boom-boom, rattle. Boom-boom, rattle! :-)

A small victory was gained against a continuing, slight problem of mould on the inner back panel of the manifold. I simply stapled a black, closed-cell foam, camping mattress onto the outer back panel of the manifold. (from top to bottom) The extra insulation, provided by the foam, seems to have cured the problem of continuous dampness. This only occurred on the lower back panel inside the manifold. The drivers and side panels seemed unaffected.

Only thin, office-type staples were used to fix the foam. To allow easy removal if desired. This panel is invisible from the audio room but I used black foam to match the manifold anyway. Checking with a torch shows that the inside back panel remains perfectly dry now.

I really must finish insulating the last bay of the roof above the IB enclosure. This requires working from above. Weather conditions never seemed suitable this year. With constant winds. Having fallen off the roof before I am not allowed up there if there is the slightest risk.

Click on any image for an enlargement.