Raspberry Pi Case

[Puckdropper]

I've got a Raspberry Pi 3 and the official Raspberry Pi Touchscreen that I'm working on building a case for. I've come across a design decision I'd like a little input on.

I have an amplifier and speaker arrangement to give me basic sound. I'm using 2 small speakers, about 1" by 2". The speakers are in their own plastic enclosure with a provision to mount the speaker using a sufficiently small screw, probably 2-56 maybe 1-72. FWIW, the case will probably be pine.

It makes sense to make some form of grill or hole in the wood to make sure the sound can pass unobstructed. I could possibly cut 1/8" wide slots with an endmill or drill a series of holes. Which would be better, or does it matter?

Also, I'd like to keep little bits of debris from sticking to the speaker cone. Would a piece of cotton cloth or something similar work as "speaker fabric" without affecting the sound too much?

Thanks,

Puckdropper

[tdacon]

Simplest might be to just bore a hole a little smaller than the diameter of the speaker cone, and glue a very light piece of cloth across the opening between the wood and the speaker. Shouldn't have much of an effect on the sound, if any. Maybe your wife or girlfriend has a pair of black nylons she's about ready to throw away ;-)

Tom

[Unquestionably Confused]

For that application (all things considered) he could probably glue a piece of cheese cloth over the opening and never know the difference other than a VERY slight degradation in volume. In short, most anything would work. Nylons would work but I'd have some concerns about longevity.

[OFWW]

Cheesecloth is too pores (SP) too soft/stretchy. Nylon's would be better and have a sexy sound. (g) but the best would be a stiffer cloth.

If the speaker is going in a box the harder the wood the more it reflects sound. Also drill a small hole for the back side of the speaker to allow the diaphragm to fluctuate freely, you can tune the speaker by the size of the hole.

[Unquestionably Confused]

All valid points but in this instance he refers to 1/8" slots covering the speaker opening. I wouldn't be worried too much about the "stretchiness" of the cloth nor, for that matter, the porosity (read: Acoustic Transparency) of that material. Likewise, if we go away from

1/8" slots, we may find that SWMBO's used panty hose will not be sufficiently strong to withstand repeated finger pokes after x number of months of exposure to UV light, etc.

[John McCoy]

OFWW wrote in news: [e-mail]:

This sort of depends on how concerned you are with the sound quality. A soft, porous cloth will attenuate the sound, and affect the tone. If you want good acoustics, use a stiff cloth. If you just want to keep debris out, use whatever's handy.

This is important - not the tuning, but having some sort of vent on the backside of the speaker. A speaker makes sound by pushing air forwards and back, and if there's no way for air to move behind the speaker, it's not going to move much air in front either. Result - not much sound. (this is assuming the box is smallish - a big box with small speakers has enough volume inside it won't need a vent).

Also, since you have two speakers, make sure they are phased correctly. If you connect them out of phase, they will cancel each other, and again you get not much sound. This is more of a problem with big bass speakers, but even with small ones it's worth checking the plug is right way round.

John

[Puckdropper]

Unquestionably Confused wrote in news:56fa75f5$0$27650$c3e8da3$ [e-mail]:

I'm thinking about just gluing the fabric to the backside of the speaker grill, which should take care of things nicely. I'm not worried about fingers or stuff like that getting in, just the annoying little bits of debris that these things tend to collect and can sometimes be a pain to remove.

The speaker has its own enclosure, and the enclosure will be mounted to the case.

Puckdropper

[Leon]

FWIW I have not ever witnessed this. Maybe a little if the cloth is denser/thicker than the length of the sound waves and that is more if a concern for higher notes where the sound is more directional. But I used drapery material in from of my L?R and center speakers and you can not tell if the material/door is open or closed.

That is not true either. Long ago many speakers were built to be air tight. Case in point many subwolfers use a driver speaker and a slave speaker inside the same enclosure. The whole speaker assembly is dependent on being air tight for the slave speaker to function and produce sound.

And the speakers do not move enough to be hampered by air that easily compressed or expands from the movement of the speaker. I have a totally enclosed 12" subwolfer with no issues.

Swingman would be the one to consult here, he has built, owned and operated recording studios. He told me that the material would not matter when covering my entertainment center doors with drapery material, and you cannot tell the difference. My only concern was that the weave was not so dense that IR light would not pass through. I simply held the material up to a light to see if the light penetrated.

[Puckdropper]

Leon wrote in news:d-ednSYZkf_9JmfLnZ2dnUU7- [e-mail]:

*snip & trim*

What would the slave speaker tend to look like? Basically like a speaker without a magnet? I picked up a speaker from Walmart that had one real speaker and one that kinda looked like a decoy or fake speaker.

*snip & trim*

Puckdropper

[John McCoy]

Puckdropper wrote in news:56fac3f2$0$8270$c3e8da3$ [e-mail]:

Slave speaker is a special case - in that case the slave is moving outward when the driven speaker is moving inward, and vice-versa, so the volume of air inside the enclosure doesn't change.

With two driven speakers, if the enclosure is airtight you're compressing (or decompressing) the air in the enclosure, which is generating a force opposing the way the speaker is being driven. How much force depends on how big the speaker is, and how much volume is in the enclosure. If your enclosure is small, it can have an appreciable effect.

John

[Leon]

That would be it! If you dare thump the driven speaker the slave should move a bit.

[Leon]

The air volume does not change in a sealed speaker either. The pressure does change but not the volume.

Not so if the speakers are air tight in their own compartments inside the speaker housing, which apparently is how my center speaker with 2,

5" speakers is set up.

How much force depends on how big the speaker is, and

[krw]

That depends on the speaker. "Acoustic suspension" speakers aren't ported. They rely on the back pressure to return the speaker to its midpoint.

[krw]

The volume of air obviously changes as the volume increases. ;-) The quantity of air is constant, the pressure is changing, so the volume must change (inversely). Remember, the speaker cone is moving.

[OFWW]

Yeah, you are right about that, I was not even thinking of the fact that it is a plastic material.

[OFWW]

Yet if there is no vent the volume of air does not change, only the pressure changes. So the sound is dampened in that the speaker is not free to fully travel, unless it is a metal coned speaker. I believe that is what Leon was referring to.

[OFWW]

I'm curious, is your Pi for a special purpose or experimentation? How did you get one so fast?

[Puckdropper]

OFWW wrote in news: [e-mail]:

It's mainly for playing with right now. I've got a Pi 2 that I'm hooking up my Digitrax PR3 programmer to, and may do so with the Pi 3.

I got lucky, I guess. I ordered the official touch screen from MCM and preordered a Pi 3 about 3 weeks before it was available. They delayed shipping the touch screen until the Pi 3 was available.

The touch screen is a nice bit of hardware, I've played with it for several hours already and the only problems I have are basically Fitt's Law related... Kinda hard to hit a 5-pixel by 10-pixel target with your huge

50x50 pixel fingers.

Puckdropper

[John McCoy]

OFWW wrote in news: [e-mail]:

Eh? You guys are trying to re-write the laws of physics here. Boyle's gas law applies, p1v1 = p2v2, just as krw said.

This is not complicated - assume there is a speaker in a sealed box. The speaker cone moves as it's being driven with the audio signal. As the cone moves outward, the volume inside the box increases, and the pressure decreases. As it moves inwards, the volume in the box decreases and the pressure increases.

As noted (several times) whether these changes are significant depends on the size of the box relative to the speaker.

John

[Leon]

The volume remains the same, where does it go when it is in a sealed container? Would volume and quantity be the same measurement? Now if the speaker has an opening the volume does change. Speaker movement in a sealed cabinet would simply increase and decrease pressure. We are measuring air, not displacement. Displacement would certainly change.