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Re: Whats the low-frequency slope of BASSMAXX One's?

Posted by tomservo (M) on August 24, 2003 at 09:41:57

In Reply to: Whats the low-frequency slope of BASSMAXX One's? posted by freddyi on August 22, 2003 at 22:03:46:

Hi Freddy

The Bassmaxx boxes are an another example (like our Unity horns) of a horn who’s driver is not coupled at the throat but rather some where “upstream”.
The advantage in this case is that the horn path can be longer than the drivers you have would allow you to make it.
In other words, as one moves up the horn, the appropriate driver (for that point in the horn) becomes lighter and with less motor (per area) than at the apex.
Automatically the hf limit (or where you have a big notch in the hf response) is set by the distance up stream being less than ¼ wl from the blocked throat.
One approach to the math shows that for a real low frequency horn, the proper driver has been and is only now possible to construct (with a voice coil motor).
The LAB 12 being an example of near the edge so far as what can be built at reasonable cost with a VC motor.
Anyway, the point is by moving up the horn, it becomes possible to use a really beefy Pro sound driver which are “lighter /weaker” by area.

As the frequency falls the mobility of any horn driver’s radiator becomes governed more by the compliance or “spring” and less by the acoustic radiation.
The “best” response possible results when the compliance provides the best possible trade off with the decreasing radiation resistance (makes the shape and position of the roll off slope have the flattest /lowest cutoff).
That compliance is made of the driver suspension AND the sealed air volume in the back chamber (the two compliance’s acting in parallel).
Should one have the right compliance in the driver, it would require a back volume of infinity so the alternative is no back volume.
Acoustically this part is the same as a “Scoop” type bass horn except for appearances (because the driver faces the other way).

In the most recent Live-sound” there was an article by the designer, a fellow I met at a trade show and who seems like a nice guy, any way, I don’t buy several of his arguments.
I also thought odd there were no measurements.

He says there is distortion produced loading the drivers “the other way”.
In reality, I found that the reduction in the measured distortion with the “upstream” mounting was mostly due to the acoustical “low pass” filter the ¼ w and above cancellation produced.
The basic distortion is mostly from the driver’s motor / mechanical system.
Here, a sealed box, especially if one can driver the driver FS down, the air spring which now dominates the total compliance is usually far more linear than a driver suspension.
I am sure he was somewhat puzzled at the Michigan shoot out that the LAB’s went a little lower, went a little louder AND had even less distortion than the BM’s, something that “shouldn’t have been” and disturbing enough to have a new driver tooled up.

We will see how that one measures as there is another shoot out in NY next week, finally they are getting in some of the more well known (and dismally performing) products from some of the “big names” in Pro sound. Mark S. is going too with a couple of our new smaller horns (42X42X18 inches) called a Bdeap, intended for use in corners etc. He has sold these into some big home theater and also night clubs that have live music.
They weren’t suppose to be used this way (when I designed it) but people are using them in pairs standing upright, out in the open air, (1/2 space). This configuration makes the horn couple into ¼ space for 42 inches before reaching half space, some but not what I intended so far as boundary loading..

This shootout should be interesting, as I have been measuring Bdeap’s outdoors here this last week and even in this “wrong” configuration in half space, the response is dead flat from 48 to 180Hz. (-3@44Hz).
When measured at a more fair 3.16 meters (-10dB from 1 meter and removing some of the box size problem at 1 meter) one also finds a broad bump of +4dB at 110 Hz and sensitivity of 110.7 dB 1w re:1m in this region. With a power capacity of about 3200Watts (for the two boxes), it should allow for some significant bass although not as low down as when used "properly" with a corner or wall.
Stacking two on two (because of increased directivity) shows a 3.16 meter sensitivity of 103.5 dB for 1 Watt system input equaling 113.5 dB 1W1M with a power capacity of 6400Watts.
I think Mark will have 4.

The other parts of having the back radiation are also worth a thought or two.
With the driver’s radiation arriving something like 12ms sooner than the horns output, what effect does this have on an impulsive signal or one who’s waveshape one wanted to preserve?
There are also “lobes” in the system’s radiation pattern due to having two sources with two “times”or phases..
Given that the measurements of the unit show a distinctly “non flat” frequency response, one wonders why this is desirable.
Cheers,

Tom

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