Can a Speaker Enclosure Sometimes be Too Rigid and/or Too Air Tight?

[tehachapifan]

Especially if you mount the enclosure on the diaphragm side of the speaker? I certainly didn't think so, until I recently installed a sound decoder that had a particularly deep diesel rumble per the sample sound file, but it just wasn't reproducing it very well at all using the small 8x12 8Ohm speaker needed for this install (although the bell and horn sounded good). I'm aware that these small speakers can't be expected to produce bass sounds very well and was pretty much figuring this was as good as it's was going to get, when I saw elsewhere someone mentioning the need to drill a tiny hole in the enclosure so that the diaphragm would be able to move without too much air restriction. I tried this, drilling a #80 hole in the side of custom styrene enclosure (so as not to poke thru and hit the diaphragm) and I believe the sound has improved a little. I believe I can make out some more of the individual bass sounds now where before it was just more of a garbled (almost static or motorboat-like) noise. I believe the volume may have gone up a little too. What's a little bit of a head scratcher now is, in previous installs, I would chase and seal any tiny air leaks until the sound quality and volume improved, so I'm guessing there's a fine line here somewhere regarding hole size, etc., if this is indeed a factor.

Pardon me if this has all been discussed before. I did I brief search and did not see anything obvious but have vague recall of this possibly being discussed here.

[nickelplate759]

There is such a thing as a ported speaker enclosure.  It is airtight except for a carefully designed hole, usually with a tube.   It is intended to increase efficiency at certain lower frequencies, but there are variables involving hole dimensions,  tube length, enclosure volume, etc. at play here that I don't have a handle on.

It is used to boost woofers.  Here's a link to an article:

http://www.subwoofer-builder.com/vents.htm

[jdcolombo]

Especially if you mount the enclosure on the diaphragm side of the speaker? I certainly didn't think so, until I recently installed a sound decoder that had a particularly deep diesel rumble per the sample sound file, but it just wasn't reproducing it very well at all using the small 8x12 8Ohm speaker needed for this install (although the bell and horn sounded good). I'm aware that these small speakers can't be expected to produce bass sounds very well and was pretty much figuring this was as good as it's was going to get, when I saw elsewhere someone mentioning the need to drill a tiny hole in the enclosure so that the diaphragm would be able to move without too much air restriction. I tried this, drilling a #80 hole in the side of custom styrene enclosure (so as not to poke thru and hit the diaphragm) and I believe the sound has improved a little. I believe I can make out some more of the individual bass sounds now where before it was just more of a garbled (almost static or motorboat-like) noise. I believe the volume may have gone up a little too. What's a little bit of a head scratcher now is, in previous installs, I would chase and seal any tiny air leaks until the sound quality and volume improved, so I'm guessing there's a fine line here somewhere regarding hole size, etc., if this is indeed a factor.

Pardon me if this has all been discussed before. I did I brief search and did not see anything obvious but have vague recall of this possibly being discussed here.

Hi Russ.  An enclosure can't be too air-tight, but it CAN be too small.  The speaker manufacturers generally test their speakers with a 1 cubic centimeter enclosure - or 1000 cubic mm.  If you take a Soberton 8x12 and mount it inside a rectangular box that is 6mm high, the interior volume of that box is 6x8x12=576 cubic mm.  By the time you subtract the volume you lose with the speaker frame and diagphram, you're at about 500 cubic mm, and I've found that this is the absolute minimum necessary to get decent performance from these speakers.  More is better, at least up to that magical 1000 cubic mm that the manufacturers use for test purposes.  After that, the benefits fall off, and at some point you can actually have an enclosure that is too large.

As George points out, what you have done with your enclosure is make it into a sort of bass-reflex (ported) design.  Many home hi-fi speakers use ports to extend bass response in a smaller cabinet; the downside of ported designs is that bass response falls off very rapidly past the resonant frequency, whereas with a sealed box, bass response declines slowly until you reach the speaker's physical limits.  With a sealed box, you can sometimes compensate for a thin low-end by using equalization; with a ported design, once you reach the resonant frequency, that's it.  But above the resonant frequency, ported designs are more efficient, and permit smaller enclosures.

So I suspect what is happening is that your enclosure is just a tad too small for a good sealed box design, and by poking a hole in it, you made a bass-reflex enclosure.  The problem is that bass-reflex enclosures have to be VERY carefully designed to provide smooth frequency response.  What probably happened is that your hole is emphasizing one small range of frequencies at the expense of others, but it sounds better to you because these frequencies happen to be the ones you are looking for.
That's not necessarily bad; if it sounds better to you, that's all that matters.  Speaker sound is inherently subjective; in fact, tests done at the Canadian national sound laboratory show that people actually pretty much hate truly flat speaker response.  In general, we prefer speakers that roll off the high frequencies some and emphasize the bass frequencies some.  But everyone has their own preferences about this, which is why there are so many speaker designs and so many companies making speakers for the home audio market. 

If the "hole in the wall" speaker sounds better to you, go for it and don't worry about it.

John C.

[C855B]

...the interior volume of that box is 6x8x12=576 cubic mm... resonant frequenc[ies]...

Hi, John. I don't mean to be a crank about this , but we're talking resonant frequencies in the 10K-25KHz range. I'm baffled that bass response - audio "bass" generally defined as 16-256 Hz - is even remotely involved in this discussion.

OTOH, I'm surprised every time I accidentally play a music clip on my bare smart phone. That they get any recognizable sound besides beeps and bloops out of those microscopic speakers has me cursing Dr. Wicher, my acoustics prof, over how much we're suspending the laws of physics these days.

[tehachapifan]

Thanks for the replies, guys!

This scratch-built enclosure is or is just about 6mm high (I will measure it again the next time I need to take the shell off) and is definitely bigger than some that are available for purchase. Again, I think that, because this particular sound file seems to mostly produce low diesel rumble tones as compared to some others I have, this makeshift bass-reflex hole as it's been described is actually helping in this case. Maybe I just happened to accidentally get it just right, but it doesn't seem to be negatively affecting the higher tones, like the bell sound, and everything seems to sound a little more robust overall. I'm pretty happy now as the diesel rumble actually sounds, well, more like a diesel rumble now. I went from trying to convince myself it sounded OK before to actually enjoying the sound now. Perhaps this is just something to maybe try if the sound is not up to par and all else fails, as a tiny hole is certainly easy enough to fill.

Thanks!

[jdcolombo]

Hi, John. I don't mean to be a crank about this , but we're talking resonant frequencies in the 10K-25KHz range. I'm baffled that bass response - audio "bass" generally defined as 16-256 Hz - is even remotely involved in this discussion.

OTOH, I'm surprised every time I accidentally play a music clip on my bare smart phone. That they get any recognizable sound besides beeps and bloops out of those microscopic speakers has me cursing Dr. Wicher, my acoustics prof, over how much we're suspending the laws of physics these days.

Well actually, the resonant frequency of the Soberton 8x12 is 500hz, not 10K (you might have meant 1K - 2.5K; 25,000 hz is above normal human hearing range . . .)

Nevertheless, I agree that we're not talking about any real "bass" here; we're really discussing lower midrange.  But the principle of porting is the same; you can use a properly-designed port to emphasize lower-midrange sound, too, which I'm sure is what Russ is hearing.

Experimentation is good; often we discover things that work purely by accident.  It may well be that the Soberton 8x12 in a 500 cubic mm enclosure sounds better with a tiny port than a fully sealed box, at least when mounted diaphragm-down.  As Russ points out, it's easy enough to "re-seal" a #80 hole if you need to.  I might start playing around with one of my own Soberton 8x12's to see if I can reproduce what Russ is hearing.

John

[C855B]

No, I did mean 10KHz. I ran those dimensions through a conventional speaker enclosure calculator, mostly just for grins, therefore the 10K for the "best" mode, to >25K for other modes. As I said, getting any resonance (or significant changes in resonance with changes in dimensions) just doesn't make sense with these tiny boxes relative to what I had learned and later applied in building 1:1 speaker enclosures.

Spooky physics. Yeah, that's the ticket.

[jdcolombo]

No, I did mean 10KHz. I ran those dimensions through a conventional speaker enclosure calculator, mostly just for grins, therefore the 10K for the "best" mode, to >25K for other modes. As I said, getting any resonance (or significant changes in resonance with changes in dimensions) just doesn't make sense with these tiny boxes relative to what I had learned and later applied in building 1:1 speaker enclosures.

Spooky physics. Yeah, that's the ticket.

Ah.  OK - you weren't talking about the manufacturer's free-air resonant frequency.

Yeah, I don't know how "big boy" speaker enclosure theory applies to these tiny things; I only know what works via experimentation.  I have found that an enclosure of about 800 cubic mm is a good "sweet spot" for the Soberton 8x12, but the problem is that there usually isn't enough room in a hood diesel to put an enclosure that big, unless you use some sort of "offset" design where the box is longer than 12mm, and you mount the speaker at one end, sealing off the other end with styrene or lead sheet.  Folks on the ESU Yahoo group swear by using very thin plywood for enclosures (in HO scale) because they think the wood imparts a warmer sound.  Maybe.  I'm still not exactly sure how my Magneplanar 1.6's make sound with a thin Mylar sheet stretched between magnets on either side.  But they do.  And it is good.

John C.

[C855B]

... they think the wood imparts a warmer sound. ...

My favorite audio quality expression. Not. Everybody likes to use it to describe their particular "improved sound" fiddly-bit - tube finals, vinyl, certain speaker construction materials, oxygen-free Litz cable, you name it. It seems to be the favorite crutch when they can't prove the improvement with specs, or, heaven forbid, before/after oscilloscope plots.

Now I'll give them that tube finals are "warmer", but it has nothing to do with the sound.

[jdcolombo]

My favorite audio quality expression. Not. Everybody likes to use it to describe their particular "improved sound" fiddly-bit - tube finals, vinyl, certain speaker construction materials, oxygen-free Litz cable, you name it. It seems to be the favorite crutch when they can't prove the improvement with specs, or, heaven forbid, before/after oscilloscope plots.

Now I'll give them that tube finals are "warmer", but it has nothing to do with the sound.

I'm also not a believer in audiophile tweaks.  But I guess it is possible that using thin plywood as an enclosure could create a resonance in the lower-midrange that some people would describe as a "warmer" sound.  Me, I'd make my enclosures out of lead sheet if I could (hard to do with the cell-phone speakers given where the connections for wires are); don't think it sounds "warmer" , but it adds weight, and THAT is certainly an important part of N-scale engines!

[tehachapifan]

Ok, as an experiment I tried adding a second #80 hole on the other side of the enclosure. I don't think it added anything of value and it may have actually cut down on the sound quality some, much like it was before the first hole was added. I plugged the second hole back up and the sound, in my opinion, returned to being more defined in the lower tones and more robust overall. Keep in mind my findings are not very scientific and are pretty subjective, as I couldn't create a side-by-side comparison and I don't have a decibel meter, but I believe the single #80 hole has definitely improved the sound. That is, with this particular sound file (LokSound EMD 6cyl 567A FT), this particular speaker and enclosure, and while being set to a particular volume.