Question for Roland

[Jim D]

Glad to see you back. A while ago another fellow and I got in discussion about (truly) infinite baffles where the large size of the enclosure has no effect on the suspension of the woofer. The question asked was what is the F3 of the system. My reply was it would be quite near the loudspeakers FS, probably not more than the square root of 2 (x1.41) times the FS. As I remembered, from our "no enclosure" correspondence, you had some knowledge of this. Please respond. Jim

[rolandr]

Hello Jim glad to be back. A new baby sure has limited my free time! My point in my comments listed under the topic "THE BEST ENCLOSURE IS NONE" was to emphasize the forgotten benefits of
the true infinite baffle(IB). Early in speaker development a very large baffle was used to totally separate the speaker front wave from the rear wave and prevent interference and cancellation. It was noted that the speaker had
very good tight bass response. The bass was smooth and fairly flat all the way down to cone resonance (Fs) at which time a slight rise occurred followed by a drop of 12db per octave. So to answer your question F3 should be BELOW Fs approximately 1/4 octave below!! This bass response is actually unheard of in speakers in enclosures where the F3 is always reached way before Fs. Additionally the bass in infinite baffles is tight and not boomy or exagerated as is the case in bass reflex cabinets. In bass reflex enclosures the boomy bass occurs because the ported rear wave does not reinforce the front wave in precise phase at all the low frequencies. Bass reflex enclosures and later the Olson patented acoustic suspension enclosures came about only as a practical way to reduce speaker enclosure size but ALWAYS sacrificing sound quality!! Horn enclosures although very efficient need to be very long with huge mouth sizes to reproduce deep bass and lets not talk about horn induced colorations. Now all this would be a wasted theoretical discussion if it were not for the fact that infinite baffles abound if you look for them. An attic a basement
a closet.... a wall dividing a room are all places were the rear wave of a speaker could be dissipated in effect having the IB benefits. Imagine small enclosures barely bigger than the actual driver in the corners of the ceiling in your sound room connected and venting the rear wave into your attic. Bass would be smooth and fairly flat to cone resonance without bulky floor occupying cabinets, you even get the benefit of bass corner reinforcement!!
You don't have to worry about Thiele and Small parameters, a stiff enough cabinet etc etc. Only thing is IB drivers need to have stiff suspensions and moderate efficiency like most altec speakers. Drivers designed for acoustic suspension with high compliance suspensions shlould not be used. I hope I answered your question and provided some useful info for the rest.

[rolandr]

Jim I asked the same question of Fs in relation to F3 in infinite baffles at the IB site "cult of the infinitely baffled site". See above. My answer is totally wrong refer to that site for a detailed mathematical explanation. F3 will never be below Fs. Sorry

[Jim D]

Roland, saw your reference to the infinitely baffled site. I do not believe the answer you got is right.

If that was the case, you would get better bass out of a small box with a 515 in it than by using your attic. The person presumes that the "infinite space" has a box resonance just like a small box and this affects the cone behavior. A true infinite baffle will act as if the speaker is hanging on a string with equal pressure on both sides of the cone. I do not believe this can be predicted in any manner without more knowledge of the back space or measuring the room resonance.

I do believe that you probably will not get an F3 under Fs and perhaps somewhat above, but 150Hz F3 out of a 515 in a truly infinite baffle setup is just plain rediculous. You can achieve this with a 1 cubic foot sealed box. I think for all the mighty math out there, the only way to find this out is empirically. From my experience with 604s and 416s, we got great bass out of these installations. I'm doing one soon and will let you know.

Jim Dickinson

<font color="#FFFFFF" size="1">[ January 14, 2003 08:52 PM: Message edited by: Todd W. White ]</font>

[rolandr]

You may be right. T/S is not best applicable to IB situations since it was originally designed for vented boxes. I don't have access to the original IB studies in the earlier part of the century. Only have Radiotron' designer handbook references. But the response is supposed to be smooth till Fs where a slight peak occurs followed by a 12 Db/octave drop. Agree with you that a 515 in IB will give much better response than 150Hz. I have gotten down to 40s in a few cubic feet!!! Let me know what you get. The only weakness of the IB I can see is the increased cone motion of the driver as it approaches Fs. High cone excursion drivers deal with this better but at the expense of resolution and efficiency. But for home size rooms with our efficient drivers results are usually superior.

Roland

[jmarkwart]

Hi Guys,
I thought it might be interesting to calculate the closed box -3db points and box volumes for some of our favorite Altecs. Using the Altec "Unofficial" Site T/S parameters and calculating for Qtc=.707 and Fc=F3 produced the following F3/Vb combos:
411-A 39/8.8
416-8B 55/6.8
416-C 68/3.2
515-B 103/1.2
515-C 103/1.2
515-E 85/1.8
515-8G 97/2.1
604-E 91/1.5
604-8G 73/3.1
604-8H 73/3.4
604-8K 101/1.3
605A 86/2

I threw in the 411 as a sanity check.

Re-calculating for Qtc ~ Qts and Fc=Fs produced these:

411-A 49/1040
416-8B 70/840
416-8C 89/556
515-B 141/330
515-C 141/330
515-E 116/441
515-8G 127/332
604-E 123/327
604-8G 97/546
604-8H 96/528
604-8K 137/288
605A 113/355

This second group would be the response predicted in a large infinite baffle where the volume of air acting on the rear of the cone does not provide any additonal cone loading relative to the front, and the IB enclosure resonance is the same as Fs. Vb is in cubic feet.

Food for thought.

Jeff

[rolandr]

Jeff

That's a lot of food!! Significant descrepancy between T/S and the OLD definition of behavior in
IB. Laws of Physics are not suppose to lie.
Empirical vs Theoretical. Time will tell.

[rolandr]

Jim and Jeff and others:

Refer to the "Cult of the infinitely baffled" site where the question posed was:

"In your experience how accurate are the T/S parameters in IB use??

Let's see if we can eventually get answers for this "apparent" discrepancy of audible vs theoretical bass response in IB. I may need to pull the original T/S study from the AES files to see if they have data in "IB" settings

[jmarkwart]

Roland,
I read the Q&A at the "Cult" site and think there may be a partial explanation for perceived deeper bass in IB. An IB radiates into a half-space and as such does not suffer a diffraction spreading loss (DSL) like a free-standing cabinet. This loss can be from -3 to -6db depending on how far the free-standing cabinet is located from a rear wall. DSL onset starts at fHz=564/cab width(in ft) and effectively shelves all lower bass frequencies by 3 to 6 db.
Jeff

[rolandr]

Got it Jeff...The boundary effects are an important factor with as you know +3dB for half space, +6dB for quarter space and +9 dB for an eight space. This is in SPL gain which is perceived mostly as increased bass response.

I use IBs in small enclosures venting into the attic in the corner ceiling of a room. The +9Db reinforcement of the three walls (eight space)
certainly is audible (>3 Db).

But it seems to me that IBs are in the fringe of T/S theory and that the calculated bass response may not be as accurate as in the smaller sealed boxes or vented enclosures. I will eventually review the original T/S article to see if they have data for IB type settings and perhaps Jim will also add his experiences.