how low should I go

[Don Hills]

Ben,
All I can suggest in way of proof is that you try it as Tom suggested: with a small, sealed subwoofer, measured outdoors and then in a "room" (car or house). Any difference in level that you measure between the open-air response and the in-room response, at frequencies below the lowest room resonant mode, is "room gain". If you still cannot accept its existence, all I can say to you is what Galileo said: "Nevertheless, it moves."

I suspect, though, that you do not deny the "room gain" effect, you just feel it is insignificant compared with other effects that alter the perceived level of bass in a normal listening room. Am I right ?

I also agree that frequencies below 32 Hz are rarely encountered in music, although I have several pieces with large drums that have significant levels down to 20 Hz or so. Corvus Corax are notorious for this, they also have a studio track with a sub-bass generator that has significant content down to 19 Hz. (Corvus Corax, "O Varium Fortune" - YouTube - Corvus Corax- Oh Varium Fortune ) (Also check out some of their live videos.)

I also have Bob Katz's Space Shuttle launch recording, which peaks as low as 5 Hz. Look for the 24 bit/96 KHz, 4 channel version. There's a video version with a stereo mix but it has a music overlay.

[weltersys]

Originally Posted by bentoronto

BTW, I don't think there is anybody on this forum who more consistently encourages (or has practiced) home-brew R&D and how easy it is to collect useful data, even with pretty simple tools. And I like amateur testing even though I worked at Bell Labs, a long time ago. But, not to seem hostile, I should thank you for your efforts to educate me.

OK! Now we are talking numbers! Now let's try some real numbers!

1. 1/2 wavelength in a pretty small room with longest dimension of 13 feet (about my room size... but I am some distance south of my usual room just at this moment, you betcha)?

2. Rate at which this "room gain" theoretically boosts the loudness of bass in the fraction-of-an-octave below that frequency?

Footnote: except for an exceptional recording of fireworks, etc. by Tom, I have few samples in my nearly 600 CD collection including numerous hifi goodies and railroads that goes below 32 Hz... so let's not get overwhelmed about the importance of boosting 20 Hz playback.

As I stated before, I have done the simple test, a small woofer outdoors vs in a small cabin, got around a 14 dB gain down low (probably around 35-40 Hz) in the Astro Van. More details would be in the thread on PSW, . Since I mostly do sound reinforcement outdoors or in rooms large enough that cabin gain has little or no effect, I have no reason to work out any specifics.

As far as below 30 Hz information on recordings, starting a bit after the CD era, LF has increased on recordings. I have a DEQ830 (1/3 octave EQ) inserted on my home stereo, it has a built in RTA which makes it easy to see the spectral content of the recorded material.

The lowest frequency on it is 25, nearly all the material not mastered for vinyl has some content down that low.
Lots of pop now has 25 Hz equal in recorded amplitude to upper bass frequencies.
It isn't just pipe organs that have 16 Hz tones anymore, just look at the spectrum of tracks from "Raising Sand" Robert Plant /Alison Krauss, a T Bone Burnett production.
Plenty of sub 30 horsepower on that disc, makes Led Zepplin sound rather thin by comparison, in fact.

Happened to play some sine waves through my stereo today, the subs go low, response is decent to 20 Hz, but not much more than 100 dBC at my listening position even at 40 Hz. Was surprised to see one of my doors moving on it's hinges at a level of around 90 dBC at 25 or 30 Hz.

[weltersys]

I was going to look up my post from PSW showing cabin gain, but Josh Ricci just did a much better graphic representation, I just had some dB meter readings at various positions and frequencies.

It is easy to see that cabin gain can far exceed what 1/8 space loading can do, making a drooping response flat down to 10 Hz, an over 30 dB increase in SPL.

His Jeep is smaller than my Astro van cabin, the cabin gain in the Jeep was greater.

Data-Bass

[bentoronto]

Flashing up two curves representing two undefined situations (and with certain other variables such as the possibility of commercial conflict of interest as in the previous reference from Event, that it would not be courteous of me to emphasize) is not my idea of a slam-dunk proof of anything. Which is what I said earlier when Tom suggested some quick woofer test in my car.

The upper curve has giant perturbations including one little one towards the low end - is that room gain???

The enhancement in the propane-cylinder-like room seems to be VASTLY greater than anybody thinks ought to be the case.... Just why should that be? Could it be that the test "room" is really only 3 feet across (unlikely) or maybe factors different than or apart from "room gain" have a part in that real big improvement.

Doing traces is very informative and advances the discussion... provided the source and the operations are understood. I can produce some really immense variations in curves simply by moving the mic around a little and that takes no talent on my part. But my point is that just showing a single chart is good but only a bare beginning.

And besides, took quite a while to dig up a chart with a bump indoors that doesn't show up out of doors. And nobody has been able to find "room gain" in the index to Beranek or Olson or Toole, eh. Still waiting.....

[weltersys]

Originally Posted by bentoronto

Flashing up two curves representing two undefined situations ... is not my idea of a slam-dunk proof of anything. Which is what I said earlier when Tom suggested some quick woofer test in my car.

The upper curve has giant perturbations including one little one towards the low end - is that room gain???

Doing traces is very informative and advances the discussion... provided the source and the operations are understood. I can produce some really immense variations in curves simply by moving the mic around a little and that takes no talent on my part. But my point is that just showing a single chart is good but only a bare beginning.

And besides, took quite a while to dig up a chart with a bump indoors that doesn't show up out of doors. And nobody has been able to find "room gain" in the index to Beranek or Olson or Toole, eh. Still waiting.....

Bentoronto,

The upper “giant perturbations” are response variations in the cabin.
The “situation” is well defined, if you bother to read how the test was performed.

As the text explains, the lower curve is outdoors, the other three curves are the same speaker at the same drive level in a 2004 Jeep WJ with three different microphone placements, which reflect the various problems in those locations in the small cabin.
The very low frequency response changes hardly at all with relatively large microphone position changes.

Whether previous audio giants have or have not noted cabin gain in their work does not change that it can be, and has been measured .

I doubt they were too concerned with very small room sound behavior. Not too many people were putting systems capable of 115 dB at 10 Hz with 100 watts in small rooms at the time the authors you mentioned were writing their works.

If you choose to ignore current valid evidence, and won’t bother with a simple test yourself which would corroborate it, that is your prerogative.

As they say, "ignorance is bliss", hope your bliss continues unabated by any facts that might get in the way.

Cheers,
Art

[18Hurts]

I don't understand why this is so hard to understand,

When I was a kid, I saved up for a year and bought the biggest boombox I could find. Even at the age of 11 I noted it played deeper bass in a car than it did outside. The bathroom had better bass than my bedroom but the garage did not sound as good--but it did have better bass than at the beach. Since my father got stuck explaining why (he was a mechanical engineer) he told me the smaller the space, the better the bass because of wavelength. This is why you get such good bass from the little speakers in headphones.

How a person that professes to be an engineer or involved in audio since the 50's never noticed the relationship between room size and bass response is beyond me.

Then again, he could be a troll. Don't feed the trolls!

[bentoronto]

Originally Posted by 18Hurts

I don't understand why this is so hard to understand,

When I was a kid, I saved up for a year and bought the biggest boombox I could find. Even at the age of 11 I noted it played deeper bass in a car than it did outside. The bathroom had better bass than my bedroom but the garage did not sound as good--but it did have better bass than at the beach. Since my father got stuck explaining why (he was a mechanical engineer) he told me the smaller the space, the better the bass because of wavelength. This is why you get such good bass from the little speakers in headphones.

How a person that professes to be an engineer or involved in audio since the 50's never noticed the relationship between room size and bass response is beyond me.

Then again, he could be a troll. Don't feed the trolls!

Am I the only person who feels that starting a post with "I don't understand why this is so hard to understand" is both demeaning to others in a discussion and patronizing? Not to mention the new-politics innuendo wrap-up hinting the other guy just might be a.... troll.

Then you go on to post thoughts which show you do not understand "cabin gain" by confusing together room size, open spaces, boom boxes, etc. Please read Tom's first two posts again.

OK, my education is improved. There IS something that happens to bass below half the lowest wave-length dimension in very well sealed containers. That phenomenon is not relevant to this thread (you will find the term "room" clear enough in the OP) or except in the creepyist of music rooms which are tiny, hardwalled, have well-sealed doors, and so on. As far as I can tell, "room gain" is interesting to learn about but it is not something significant for music rooms, only for car audio*.

The effect of "room gain" in even the worst music room I have ever seen is modest at best and just one small issue of many confounding factors reproducing music in rooms. If you play music in a tiny, cinder-block, college dorm room, you sure have acoustic factors of greater impact to fret about.

And, as is now clear as day, nobody in this thread has found "room gain" in any respected acoustic text on architectural acoustics (hint, hint)... no matter how often I repeat that challenge. I am sure the phenomenon is discussed somewhere authoritative. Keep on looking guys.

*The take-home lesson I learned: in solid small rooms, there might be some small amount of bass enhancement at pretty low frequencies - in many cases, likely to be a welcome boost down low.

[Robh3606]

how low should I go

Back to the OP. I go for an F3 of 32Hz in a box program. That gets me a solid low end below 25Hz for a pair of woofers in my room.

Rob

[18Hurts]

If Thomas Danley who builds sonic boom generators, tapped horns for the military and IMAX subwoofers is not an authority enough for you--I can't think of anyone better. From servo controlled subwoofers to Synergy horns, it would seem that he does understand the science of sound.

Back in my car audio days, we would actually test "cabin gain" to determine how the box needed to be tuned. Basically, grab a 15" subwoofer that was tuned to 25 Hz -3dB and would stick it in the vehicle. Ran a sweep and noted the huge rise in volume as the frequency dropped. We would then "tune" the sealed box to start to roll off as the cabin gain was about 6 dB above the 80 Hz point. (masks road rumble) The reason for that was to make the box as small as possible but still sound decent. Oddly enough, Mr. Danley mentioned doing about the same thing I did as a 20 something two decades ago--not high tech but I always preferred using the least amount of tech to get the desired result. It was a hobby for enjoyment, not my "day job".

I do have some tech info for you--Thomas Nousaine wrote about the very thing you mention. Very informative site and dispels a lot of snake oil that has showed up in audio, enjoy!



http://www.nousaine.com/pdfs/Cabin%20Gain.pdf

http://www.nousaine.com/pdfs/Tale%20of%202%20Rooms.pdf

[ßart West-VL.]

FWIW 2 D OP
or:
For what it's worth to the opening poster.
(sorry, had to do this to show I actually hate unofficial abbreviation)

My opinion on what the lowest frequency should be is from experience in a room only four meters, almost square with a bed in it and a lot of stuff laying around (school times )
back then I built a few sub woofers by the book and calculator, nothing too fancy but well thought about and I found that everything down to 35hz was very audible and quite fantastic but lower then that I thought it was more like movement, wind passing around the room, not that audible.
So, currently I think anything lower than 35hz is great for home theater but for music, down to 35hz is just right.