Infrasonic Speaker - Need advice for undergrad thesis project

[weltersys]

Excerpt from my thesis:
With no particular orientation as to which side of the box their head is located, the listener will be laying down in the box so that the speakers are located on either side of his or her body.

Although it's not depicted in the figure above, there is a well-fitting lid that rests on an inner lip placed ¾ inch down from the top of the box. The lid has four handles screwed so that it can be easily lifted from the box. I lined the inner lip with open-celled foam weathering striping, which provides a tighter seal for the lid and improving the efficiency of the box.

Using a spreadsheet calculator provided by Don Hills, a senior member of the site diyaudio.com, I estimated that the maximum SPL of the enclosure would be 165.6 dB. This calculation was reached by inputting the volume of the box, the surface area of the speaker cone, the x(max) of the driver and the number of total drivers into the program. It is important to note that this calculation is based on the x(max), which is the limit of the extension of the speaker cone or the point at which any increase in power will result in the speaker being destroyed.

Even if I turned the speakers up to the point at which they are fully extended, I will not reach anything near the calculated limit for a few reasons: I am running these speakers far below the frequency range they are rated for and I am fighting the high pass filters in the equipment I am using. The more power I put into the speakers, the more these factors begin to limit the quality of sound.

1) Did you subtract the volume of an average body from the "casket"?
If not, an even higher volume might be achieved.

2) Xmax is the range of linear motion for a speaker, typically the excursion where distortion will reach 10 %.

Xlim (or Xmech) is the limit where some portion of the driver may be damaged if exceeded, typically torn surround or spider(s), or voice coil smashed on the top plate. Xlim is often nearly double Xmax, and depending on the suspension, may not be reached before reaching the thermal limit, too much power will burn the voice coil if exceeded long term.

3) High pass filters have a fixed slope, the slope will remain the same regardless of power input.

4) Unbraced 3/4" MDF of the size you have built is quite floppy, adding upper harmonics to the fundamental tones. Bracing the cabinet would reduce this distortion, and increase the efficiency.

5) Weather stripping (not striping ) and the weight of the lid will not be enough to keep it airtight, it will flop and fart at a relatively low SPL.

Did you do any in box SPL measurements?
Did you measure the cone excursion during playback?
Any general reactions from the volunteers for the experiment?

Art

 

[Don Hills]

You might need a couple of sandbags for the lid of the box. In the same way that you work out the SPL, you can work out the peak pressure in pascals, convert to pounds per square inch, multiply by the area of the lid, and see if the result exceeds the weight of the lid. Also, the sides of the box will breathe in and out and may rattle against the edges of the lid. More foam strips...

 

[just a guy]

4) Unbraced 3/4" MDF of the size you have built is quite floppy, adding upper harmonics to the fundamental tones. Bracing the cabinet would reduce this distortion, and increase the efficiency.

5) Weather stripping (not striping ) and the weight of the lid will not be enough to keep it airtight, it will flop and fart at a relatively low SPL.

Just to add a bit to this, mdf is a very poor choice for this, plywood would have been much better. Plywood is much stronger and stiffer, which is what you want. Especially since the panels have a very large unbraced section right in the middle (the driver holes), these large unbraced panels have almost no resistance against the driver's inertia and will flop around wildly if you turn it up. At the very least, these large panels should be extensively braced, either on the inside or the outside of the box.

I'd suggest a lid with hinges and a latch (to keep the thing sealed tight) that can be locked in from the inside (for safety purposes) as well as a light inside and a volume control inside as well (all for safety). Unless the volume is kept at a very low level the volume control should be accessible to the person inside the box since low frequencies can make the listener very uneasy and possibly very sick. I'd also include some kind of grill screen on the inside (for the safety of the drivers).

And as weltersys pointed out, you've confused xmax and xlim.

Having said that, it looks like an interesting project but there's no way I'd get in that thing without being able to control the volume myself. One small mistake with the volume knob or frequency control, or if someone trips over a patch cord and pulls it out - this thing could cause instant and permanent hearing damage, so be careful...