Quote:
Originally posted by Delicious2
"his room dimensions only easily supporting bass down to about 33Hz."
I never claimed or thought that a given room size could not support low bass. Lets leave that mistaken notion aside. |
While it is a mistaken notion, some have been lead down this path of reasoning, and I wanted to be sure the situation was understood. It looks like we've moved past this, so on we go...
Quote:
What I am starting to understand thanks to Mark S. and Iceman is that different size rooms have different frequencies below which they begin to go into a "pressure mode" . That is, they begin to behave like the inside of a closed box loudspeaker. |
This is an analogy I want to make and objection or ammendment to. This ideally closed room being discussed is only like a sealed box loudspeaker in that sound pressure on one side of the driver is contained within the space. UNLIKE a closed loudspeaker, it does not alter how the subwoofer operates, which would result in a change in the impedance curve of the subwoofer. This only occurs when the space in front of the woofer, in this case the entire room, approaches the internal volume of the subwoofer. In my own car this has a slight affect, since the cabin volume is probably less than 3 times the trunk's volume. For the record, if the front and rear volume are identical, the sub will behave as an infinite baffle where the air-spring no-longer dominates the alignment.
While "pressure mode" is a reasonably accurate term, we need to recognize what is going on, and why.
Quote:
And that frequencies that pressurize a room in this way are amplified not attenuated. So, as has already been said, it is actually easier to get really low sub-sonic bass in smaller rooms. |
True, yet we need to realize that the woofer is not producing any more acoustic power into this closed room. What we find is that in fact the woofer is producing the same acoustic power, but into a smaller space! Therefore, the INTENSITY is increased much in the same way a horn loudspeaker works to confine the acoustic output at higher frequencies. A horn lense works to confine the radiation of a drive unit to focus and confine it's output which thereby makes the drive unit produce a higher intensity (observed as higher SPL) within the space confined by the horn lense.
The same concept allows corner loading of a subwoofer to generally have 6dB more output above what would be measured outdoors, in a ground plane situation. A theoretical "anechoic" environment infers no boundaries to confine a loudspeaker's radiation. In mathmatical co-ordinate terms, this is considered 4Pi space, or full-space. A good explanation of this concept by John Murphy can be found
here at True Audio's website, where he also offers some good graphic aids to the explanation.
Now we get back to my point about sound being pressure variations over time, and not truely wavelengths, which only serve to help us predict how these pressure waves interact and react with boundaries and obstructions. I'm guessing someone is asking why intensity increases as frequency goes lower when the room size/volume is staying the same? How is the space getting smaller? Answer:
Acoustically, apparent size of a space or object varies with frequency and is directly related to wavelength (See it is of use after all

). What we find is that when 2 sound sources, or a boundary and a sound source are within 1/4 to 1/3rd of a wavelength apart they become "acoustically coupled." What this means is that the 2 sound sources behave as one, and the boundary no longer creates reflections, but rather confines the space the sound source is working in/on. If you start to really think about the implications of this phenomenon, you will begin to get a sense of why moving your speakers around a room have such an effect on the bass response. Furthermore, you will observe that at some low frequency, all 6 boundaries of a rectangular room will be "acoustically coupled" to a subwoofer, and will therefore confine the acoustic space, which results in the very low frequency gain observed in rooms.
Tom Nousaine's own measurements follow this very closely where his 7000 cu.ft. room begins to show gains below ~16Hz. While at 1/3rd to 1/4 wavelength we get full coupling, we find that this is not an immediate, all or nothing effect. In fact, we find that the gain can be approximated to start at 1/2 wavelength of the room's longest dimension. If you calculate the longest dimension of Tom Nousaine's 7000 cu.ft. space as a cube, you get ~33 ft. which corresponds to the 1/2 wavelength of ~17Hz. Considering I know T.N.'s room is not a cube, the diagonal would be a little longer, and perfectly explains the 16Hz hinge point for gain. His previous, smaller room also follows this prediction reasonably well. We do need to realize that this is not an exact prediction as it assumes true acoustic boundaries, and other than concrete walled basements, most rooms "leak" quite a bit at low frequencies. Just ask your neighboors!
Quote:
With that understood, what seems intuitive to me, however, is that the quality of that low bass in our small listening rooms is not the same as that in concert venues. I'm talking quality now, not quantity. I seems to me that you could reposition your subs wherever you want, carefully measure every bass frequency, even equalize them within +- 1db, but, the quality will never be the same as low bass in large concert hall environments because in that "pressure mode" you have to deal with the lack of fast transient response of the sub/room interaction. Low bass notes simply can't start and stop as fast in that pressure mode. |
Actually, in this "pressure mode" resonaces and modal problems don't have room to build up, so the only issue is how much energy is contained within the room, and how much leaks out. Once you measure this, and get a flat response within the space, you can get the same quality bass as is heard at an outdoor concert performance. In fact, it is above this frequency range where all the concern and headaches come in, as we now have to be concerned with what happens to the radiated energy as it reflects or is absorbed as it encounters different boundaries and obstructions in the room, and finally what percentage and WHEN the reflected energy gets back to the listener. Looking at the efforts taken by consultants like Russ Herschelman, Tony Grimani, and Keith Yates can make your head spin and eyes gloss over... all in the interest of controlling the room's affect on sound produced in the room.
Quote:
I wonder if this phenomenon is what led the Listener reviewer to critisize the HSU sub for "overhang". Lack of transient quickness is probably not a characteristic of the HSU relative to those other subs. It seems more likely to me that the HSU appeared to have this characteristic compared to the others because it interacted with the revewer's room differently in this "pressure mode". If my speculations have any truth, it leads to complications both in reviewing and comparing subs, and new questions about how to get the best low bass in a given room. How do we control for these sub/room interactions in the "pressure mode" beyond placement and equalization? How do we control for changes in quickness, responsiveness, transient speed in a given room? How can we make our small rooms behave or sound more like concert halls when it comes to low bass? |
Placement and equalization are in fact our best methods to improve the bass reproduction in most rooms.
The perception of "overhang" is most commonly caused from peaks in response from either a small room or much more commonly due to the excitation of modes causing peaks in the response. Low distortion at all output levels and reasonably flat response still appear to be key to getting accurate and articulate bass reproduction. Many small subwoofers which claim very low frequency extension employ limiters which limit deep bass output to set levels, and many other "musical" subwoofers simply don't have much deep bass output. These designs skate around the problem by not energizing a room at lower frequencies, or by limiting the engergy at low frequencies with their limiters. While this does greatly reduce the chance of causing modal peaks in the response, it also completely fails to reproduce the lower octaves, and has led to the great deal of mis-information and understandable, yet incorrect conclusions which are so prevalent in the industry.