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Bill Fitzmaurice

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Posts posted by Bill Fitzmaurice

  1. 31 minutes ago, jay42 said:

     in my mind if I went for 4 Ohm cabinets I could utilise full power from the amp

    Divest that notion. It's the rare cab that can make use of more than half its rated thermal input power before running out of the mechanical ability to use it, so the oft mentioned quest of 'getting all the watts out of my amp' is right up there with the search for the Holy Grail or an honest politician of things not worth the effort.

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  2. 4 hours ago, BassmanPaul said:

    It's how I would have wired it but I would have performed a battery test to verify which terminal is the positive on the driver.

    That wouldn't tell you which wire was which. If anything the white wire is probably chassis ground, assuming there is a chassis ground. Being an older amp it most likely has a chassis ground, but that's not the case with many micro amps. 

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    If you only use the combo on its own i.e. without an extension cab then it doesn't matter which way round you connect it.

    Quite right.

     

     

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  3. It's easier to hear the sound of the cab up close more like what's heard at a distance when there's no short ceiling or side walls on the stage. Those can cause boundary sourced low frequency null zones close to the cab, killing the lows if you're standing in one of those zones. Those zones go away the further you are away from the cab, explaining why the lows can sound much louder out in the audience than on the stage. This phenomenon lead to the myth of wave propagation, the notion that a wave can't be fully heard until one is a certain portion of a wavelength away from the source. Like most myths it was a seemingly plausible explanation for an observed result arrived at because the true explanation was unknown.

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  4. There's a big difference between the 3dB sensitivity increase you said was realized from each boundary and the 6dB that's actually realized. You're not going to get any increase by putting a cab on the floor compared to your chart because your chart already reflects the result with floor placement. I didn't mention flush mounting because it's not pertinent to this discussion. Placement some distance in front of a wall does not lead to comb filtering. It does result in an Allison Effect response notch, which I could have put on that chart, but that would be making things more complicated than necessary for this particular thread. What matters as far as boundary loading is concerned is that it will not turn a well executed ported cab into a boom machine, nor will turn a thin sounding sealed cab into a low end monster. Most of the inherent characteristics of both will be maintained, but they'll be louder, which is seldom a bad thing. Just by being louder they'll seem to go lower, as that's part and parcel of how our hearing works.

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  5. 16 hours ago, Phil Starr said:

    OK now move into a gig. Put the cab on the floor and the bottom end gets a 3db boost, back against the wall another 3db.

    Your chart is half-space, so it already shows response on the floor. Moving it close to the wall puts it in quarter-space, which adds as much as 6dB of axial sensitivity. It does so over a broad pass band, not just in the lows. Most speaker modeling software can't model the difference between different space loadings, but HornResp can. This shows the difference between quarter-space on the darker upper trace, and half-space in the lighter lower trace. You don't get just extra bass with wall loading, you get extra everything.

    Quarter%20space%20half%20space.jpg

     

    The effect is the same with ported and sealed cabs, so wall loading doesn't make a sealed cab work as well in the lows as a ported cab. It just makes both work better closer to the wall than away from it.

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  6. Placing subs close to walls can realize as much as 6dB higher output, from spatial loading, compared to well away from walls. It also prevents low frequency cancellation. When the distance from the sub to the wall is 1/4 wavelength the reflected wave meets the original wave 180 degrees out of phase, causing a response dip as much as 24dB deep. Another problem with subs under mains is the creation of a power alley with dual stacks, where the sub outputs alternately reinforce and cancel each other across the sound field. The only way to prevent that is to either place them together or to separate them by two wavelengths. At 40Hz that's almost 18 meters.

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  7. One can't overlook the fact that this particular arrangement was invented by Bose, a company that has always placed form ahead of function. The rest followed suit because it's easier, and far more profitable, to give buyers what they want rather than to educate them to the benefits of better engineered alternatives.

  8. How do you know? The knob position doesn't reveal the amp power output, nor does anything else. With a low voltage input signal the knobs could all be wide open and yet the output -10dB or more from rated output. With a high voltage input the knobs could all be at 9:00 and yet the output +6dB over rated output. I once blew a 200 watt EVM-15B with a 50 watt amp when I hooked up a pink noise generator for testing. I didn't realize that the generator was cranked, so when I turned it on the voice coil blew in less than a second.

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  9. The early bass cabs that were open backed might not have benefited from being sealed, as they used generic musical instrument drivers which were guitar oriented anyway. Besides, they could give a good tone, they just couldn't go loud. Play just about any Beach Boys recording and you're probably hearing Carol Kaye playing through an open back Fender Super Reverb.

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  10. 9 hours ago, itu said:

    Neither is superior, both can be designed well - and not so well. They are a bit different, but both are complicated to design.

    Sealed designs are simple compared to ported. The advantage to sealed is that they're hard to mess up, so long as the drivers used have the right specs for sealed. It just so happens that the right specs for sealed are high Qts, which are what's usually found with inexpensive drivers, so if you're going to use an inexpensive driver just stick it in a sealed box and you're good to go. Ported cabs work best with lower Qts drivers, which also tend to be more expensive, so if you have a more expensive drive it probably needs to be in a well designed ported box for best results. Ported goes lower than sealed when done right. What you don't want to do is to put an inexpensive high Qts driver in a badly designed ported box, as the result will usually be a boom box. When you hear people complain about ported speakers being boomy it's usually because they had a cheap driver in a bad box.

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    A generalisation is that sealed enclosures have a more contolled/attenuated low end response. It's a characteristic that can mean sealed cabs are a good partner with amps with a lower damping factor, such as valve amps.

    Like most generalizations it's not true. The Ampeg SVT, for instance, was made sealed because that's what gave the best results with the high Qts drivers that they used. They could have made it ported and probably would have if they were able to find low Qts 32 ohm drivers, but none were to be had in 1969.

    Damping factor is a non-factor other than in extreme cases that are very rarely seen.

    http://www.cartchunk.org/audiotopics/DampingFactor.pdf

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  11. You're not wrong, but also not entirely correct. Most of what's heard form electric bass is midrange harmonics, not low frequency fundamentals, exaggerated as the ear is most sensitive in the midrange. Therefore the comb filtering is happening where it's most audible. Then there's the matter of horizontal dispersion, which is halved when drivers are so placed compared to vertically aligned, and here again that halved dispersion occurs primarily in the midrange that's critical to intelligibility.

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  12. 9 hours ago, Chienmortbb said:

    If we assume that both amplifier amd speaker are substantially linear, then the correalation between watts and SPL is also linear.

    The problem is that assumption is incorrect, in a few ways, the most obvious being that speaker impedance, and as a result current and power, isn't a constant with respect to frequency. That's why neither amperes nor watts are used in SPL computations. Volts are, being unaffected by the load impedance, and being linear with respect to cone excursion.

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    Yes but amplifiers supply WATTS to an impedance. 

    See paragraph above.

  13. 1 hour ago, Stub Mandrel said:

     

    Interested to know more! The linear array attaches to the top of the woofers by built in sockets., Similarly on the other PA you can stick a rod in the top of the bass bin and balance the other speaker on top, if space is limited.

    For best results the mains must be out front, lest they feed back into the mics. The subs should be close to a wall, for boundary loading and to prevent boundary reflection sourced cancellations. If you have two or more subs they should be placed either together or at least 16 meters apart, to prevent phase sourced cancellations. This also isn't new by any means, but it is almost universally unknown by consumers, and therefore ignored by consumers and manufacturers alike.

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