Nondirectional & Inaudible Bass: What Frequencies?


Hello. A few quick technical questions: I am curious as to what frequency bass goes "nondirectional." I saw a reference in print to it being around 100-125Hz, but that doesn't seem right to me. Does the room have an effect or is it purely psychoacoustic?

Also, wondering at what frequency bass becomes inaudible. Thanks much!
socrates

Showing 6 responses by sean

Viridian hit it on the head. Moncrieff's research shows that bass becomes non-directional or "loads evenly" at appr 100 Hz. What allows us to precisely place the woofer or subwoofer at frequencies below that are either high levels of harmonic overtones or plain old distortion. This is one of the reasons why downloaded subs sound very different from front loaded subs. The harmonic overtones and distortion are masked to a great degree when downloading, making them harder to localize AND affecting their tonal balance or "attack" characteristics. Try listening to a sub that is downfiring and then turn it on the side so that the driver is now front firing. Other than the difference in the type of footers used, i think that you'll hear a MAJOR difference. Sean
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I agree Unsound, Viridian did bring up a VERY good point. The fact that the mass majority of powered subs that are out there are not properly calibrated in terms of crossover frequency does not help any either. Most all subs are actually crossing over at a higher frequency than what the controls state. Top that off with most of them having a less than "ultra-steep" crossover slope and you have a LOT of "blending" between the sub and the woofers. Too much in most cases. That is why so many people have a hard time trying to get a good blend between the mains and the subs.

As i mentioned earlier though, downloading will help to reduce the amount of apparent upper bass output. This has to do with the "tuning" of the space between the driver / baffle board and the flooring underneath it. This type of system has a narrow band of efficiency with everything else acoustically sloping off outside of that band. By playing with the distance of the driver from the floor, one can alter the loading / coupling of the driver via the pressure front and "fine tune" the bandpass that you want highlighted most. This also changes the "Q" of the speaker system, which will affect the low frequency extension, output and transient response of the sub. If you are looking for anything BUT the lowest bass though, downloaded designs do not have as much apparent "attack" and / or defintion due to the lack of upper harmonic structure. As such, they do not work well when being used as a woofer and are best suited for sub-woofer only use. Sean
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See, if Dunlavy would have just listened to me years ago, they might still be in business today : )

Honestly though, i don't know what you are refering to, but i'd love to see it if you can find a link to it. Sean
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Unsound: I had forgotton all about those models. Personally, i think that downloaded subs lack "attack". This probably has to do with the longer signal path to our ears as compared to a direct front radiating design and the damping of upper harmonics that typically takes place in such a design. I only find such designs suitable for use as SUB-woofers if you know what i mean. I don't think it works well for anything but the bottom octave or two at the very most. The downloaded subs in my HT system are actively crossed at 30 Hz and the downloaded subs ( as used with small two way monitors on stands ) in my bedroom system are actively crossed at 65 Hz.

As far as exciting the floor and the building structure itself, yes, you get a lot more of that. It is especially noticeable if you are on a suspended floor aka over a basement or on the second floor, etc...

Your observations about various loading conditions from installation to installation are also quite valid. There are two different approaches that one can take to this. One can allow adjustable loading via moving the box up and down via threaded spiked feet or cones. This allows one to adjust the amount of loading and vary the tuning to best accomodate individual rooms and tastes.

The other approach is to have a permanent "sound board" ( like a piano uses ) or "loading plate" built into the design. This approach maintains consistent loading characteristics due to the consistent space between the board and the driver regardless of what type of flooring ( carpet, tile, etc... ) you have underneath it.

Both are valid approaches and up to the designer and end user as to what they like best. Personally, i like the adaptability of adjustable feet but some people would prefer relying on the design expertise of a professional as they don't trust themselves to "get it right". Sean
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I don't doubt that Allison did something like that as he was always playing with / taking into account room loading and gain. There have been other designs that fire the woofer upwards into a "loading plate", which achieves the same effect without causing the spider ( part of a speakers' suspension ) to sag quite as much. Obviously, it is just another variation on a somewhat common design. As such, it has trade-offs of its' own to deal with. Sean
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You might be right. I think that the first commercially available "loaded" woofer system was the Janus made by Jan Marovskis. That is, as far as i can remember.

Design Acoustics, a division of Audio Technica, also did this on some of their models. These were probably the most commercially succesful down-firing or "loaded" speakers that i recall. I personally think that DA took this approach primarily for space saving reasons, but they really weren't bad sounding speakers for the price and era. Sean
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