cheap ported speakers for a demo?!?

[head_unit]

OK, this is not exactly DIY, but hoping for some sources:

I'd like to do a demo for a science class, and would need a ported speaker. I would:

1) Have the speaker out of the box, the port sealed. Talk about air moving and cancellation. "Listen-no bass!"

2) Put the speaker in the box, Presto! Low Frequencies!

3) Unplug the port-Presto! MORE low frequencies!

Sound quality is not important except that there must be an audible difference in the "bass" output-or more like midbass, in the case of cheap speakers.

- Could be fullrange. Probably should be, for a better demo. Single cone is OK.
- Cheap is good, since it comes out of my pocket
- Don't have time to build it
- Small is good
- Light weight is good. Actually, a flimsy enclosure might lead to another demo where one of the pair has been braced and coated and stuffed.
- Did I mention CHEAP

[Inductor]

Some factors come to mind:
You should talk or know about (Google)
Speed of sound (in air, other conditions), Helmholtz Resonator (also calculators),
EBP factor.
EBP--Ported or Sealed - Techtalk Speaker Building, Audio, Video, and Electronics Customer Discussion Forum From Parts-Express.com
Closed Subwoofer Box Equations Formulas Design Calculator EBP Efficiency Bandwidth Product
mh-audio.nl - Calculations
Your stand is valid for a driver that can work in both closed/sealed box and ported.
~The output curve is different in both cases, doesn't mean one is better than the other. The ideal volume/size of the enclosure might be different for each case (same driver).

[system7]

I know what you are hoping to demonstrate...it's something like this, if I've got it right:



I guess you'll want a signal generator like Sweepgen running from a PC to an amp. I'd think any small reflex design speaker will do, preferably front-ported.

My recall of reflex running on a tube, or one of those rubbery auxiliary bass radiator things like in the KEF Calinda pictured below, is that as you get down to a certain frequency, the cone almost stops moving and the reflex goes crazy! Below that, both cone and reflex do a lot but kinda cancel out. If you plug the hole, the bass cone is much more predictable, and I think there will be less bass. It's quite amusing to light a match in front of the reflex and extinguish it with a bass note.

Do you then conclude that reflex is better than closed box? Well, no actually. Thing is a reflex driver (Qts=0.38) is a different animal from a closed box driver (Qts=0.5) , and if you fit the RIGHT driver the closed box and the reflex do much the same bass:


Thiel_small_analysis

[sreten]

Hi,

With postage costs you should probably buy a pair, having
a spare is always a good idea with destructive types around.

Aurasound NS525-255-8A 5.25" Paper Cone 8 Ohm: Madisound Speaker Store

1) Out of box - no bass / not much mid bass
2) In a 10L internal box - bass and mid bass / limited bass < 100Hz
3) 38mm dia x 106mm port - 50Hz tuning - more lower bass > 40Hz.

Goldwood PT-F415 Port 1.5" x 4", Flared: Madisound Speaker Store

[As its tapered and flared you will need to shorten it to get 50Hz tuning.
You want the measured low bass impedance dip to be 50Hz in a foam lined box.
With this of vented box (big for driver 8L Vas), tuning has to be accurate.
If tuned too low you won't get the optimum bass boost 50Hz gives.]

1) to 2) will be be much more of a difference than 2) to 3).

How loud it goes without making horrible
noises @ 50Hz should be interesting .....

rgds, sreten.

If you can't twist the wood working teacher arm :
Knock-Down Birch 0.315 ft³ Mini Monitor Cabinet 300-7048

Or find a suitable (pair of) boxes, broken or not, dirt cheap used.

[weltersys]

Originally Posted by head_unit

Sound quality is not important except that there must be an audible difference in the "bass" output-or more like midbass, in the case of cheap speakers.

- Could be fullrange. Probably should be, for a better demo. Single cone is OK.
- Cheap is good, since it comes out of my pocket
- Don't have time to build it
- Small is good
- Light weight is good. Actually, a flimsy enclosure might lead to another demo where one of the pair has been braced and coated and stuffed.
- Did I mention CHEAP

Sounds like the usual speakers I see at Goodwill or Salvation Army stores.
Whimpy magnets, no bass with speaker outside the cabinet, little bass when port is sealed, boom boom bass with port.
Cost- $5-$20 per pair.

[Keriwena]

Originally Posted by system7

Do you then conclude that reflex is better than closed box? Well, no actually. Thing is a reflex driver (Qts=0.38) is a different animal from a closed box driver (Qts=0.5) , and if you fit the RIGHT driver the closed box and the reflex do much the same bass:


Thiel_small_analysis

head_unit, your demonstration would be called a 'hoax' if it were done by a speaker company as advertising.

You won't be demonstrating the difference in enclosure types but rather the importance of proper enclosure size.

[system7]

Originally Posted by Keriwena

head_unit, your demonstration would be called a 'hoax' if it were done by a speaker company as advertising.

You won't be demonstrating the difference in enclosure types but rather the importance of proper enclosure size.

Yup, I was thinking about that.

I think THIS is what is actually going on in such a demonstration:



Bit rough and ready...

[head_unit]

Wow. Cool stuff, but way past anything I'm discussing. The points of the demo are
- Putting the speaker in the box keeps the diffraction of the front and back waves from canceling each other.
- Unplugging the port makes more bass. Why? That is left for students to ponder until a bit later, when we briefly discuss Helmholtz resonators.

We will not talk about the frequency response or quality of the bass at all-it's a Physics demo, not a speaker or acoustics demo. (Although, I'll think about these posts which might make material for a quick sidebar in an AP Physics class).

[sreten]

Originally Posted by head_unit

- Unplugging the port makes more bass. Why?

Hi,

Or why it doesn't appreciably depending on what alignment is used .....

Just buy a pair of cheap single driver vented used speakers. Take the driver
out of one for one demo and plug/unplug the other for the other demos.
My local "cash converters" usually has something very suitable for ~ £5.

Have to agree with Weltersys here ..... though sealed may sound the best .....

rgds, sreten.

[head_unit]

I've done the first part of the demo with a cheap 4x6 speaker and a cardboard box with a 4x6 cutout. Even so crude and unsealed, there is a dramatic difference in "bass" (really midbass but the effect is still good). I want to extend that to the port, and so need a better enclosure than the cardboard box I think