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Gswitz

Sub w/ crossover == low shelf ??

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It occurs to me that a sub with a crossover is really a low shelf with a fixed q, adjustable frequency and gain.

Am I thinking of this correctly?

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I think it would be a hi shelf. You want the low freg. to pass through. Crossovers are to my understanding not adjustable unless they are electronic.

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Low Pass would be closer to the truth not Low Shelf.   A Low Shelf filter would actually reduce the amount of low end going to the sub. 

The EQ term shelf is when you reduce  all frequencies below or above a set threshold to a certain amount.  They are still there to what ever amount you have chosen but sitting  on a "shelf"  at a set level.  See diagram for a clear explaination.   

Pass filters   Low or Hi Pass which will totaly cut off with a sharp curve all frequancies below or above the threshold.  They are what would be used in a Crossover system And for a sub they would be lo passed  meaning only the low frequencies would PASS the crossover all hi frequencies are filtered out.  The Main speakers would then have the High Passed fequencies. 

It's very important if you work with audio and plan on using EQ to get this sorted out... 

 

 

 

The term Crossover is what is correct to use.   

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Edited by Cactus Music
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The shelf can raise or lower the gain of the shelf. I thought the word shelf just mean that it was a flat line that ran from some point to the end of the audible spectrum (or that carried by the hardware).

My sub does have a knob for adjusting the cross-over point and another for the gain. The speaker cables leave the sub and go to the speaks after the cross-over.

I'm not sure why a sub would be a HPF. Perhaps what is sent to the speaks is high-passed and what is carried by the sub is low-passed, but to me this is the definition of using a cross-over.

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What they are trying to say is that when you design a crossover for a Sub you wouldn't want to use a shelf filter. A Pass filter does a much better job in regard to what you want, which is to create a crossover frequency. If that crossover frequency is adjustable/slectable, it is still more than likely a pass filter.

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Ah yes. That's true. I understand. I wasn't designing the cross-over. More I was realizing that the cross-over frequency is like a shelf frequency and the cross-over gain like a shelf-gain for listening purposes. In other words, the sub with the volume knob and sweepable frequency for the cross-over ends up behaving like a shelf for the listener. Obviously the sound comes from a different place.

I remember being taught that it didn't matter where the sub is because you can't tell where bass comes from. That is entirely wrong imho. Why do people say that?

Edited by Gswitz

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According to Stereo Review years ago, a true sub is non-directional, i.e. using it to amplify frequencies below 40 Hz, such as sound effects in movies. Such as explosive blasts, stampeding elephant herds, earthquake rumble, passing locomotives, etc.

But if using it musically with a high crossover, say 200 Hz, it would be directional.

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The best reference I've seen for subwoofer placement is a book called "Sound Reproduction" by Floyd Toole.

Dr. Toole is either retired or deceased (or both), but was chief scientist for Harman and widely-recognized as one of the preeminent authorities on speaker design. I like this book because it treats the room and speakers as two halves of a single system, and thus deals more with acoustics than with speaker and speaker enclosure design,

Low frequencies are indeed relatively omnidirectional, that's not a myth. Why, then, do mastering studios often have two subwoofers? It has to do with their long wavelengths, which means most things in a room are physically too small to interfere with them (including the speaker enclosure itself.) If, for example, you were to take SPL readings around a subwoofer that was sitting outside in an open field and elevated off the ground, the readings you'd get for 360 degrees around the speaker would be surprisingly consistent.

Of course, nobody listens to music out in an open field. The point is that only very big things can get in the way of low frequencies. In a room, there are things big enough to interact with low frequencies, namely the floor, walls and ceiling. They are referred to as "boundaries" because they impact all frequencies. And they are the reason it matters where you place your subwoofer. It matters a lot, but it has nothing to do with directionality.

Boundaries reflect low frequencies and send them colliding back into themselves. If they happen to collide in phase, that's constructive interference that will result in a large volume increase at that point in space. Conversely, if they hit 180 degrees out of phase, cancellation will occur and that frequency will essentially disappear at that specific point in space. Finding the best spot for a sub is very important, and that spot will probably not be where you think it ought to be.

 

Oh, and crossovers are technically either low-pass or high-pass filters, depending on whether it's feeding a woofer or a tweeter.  Most subs have both.

Conventional passive filters are single-pole, meaning 6 dB per octave. Active filters can be designed as steep as you like, although most will be 6, 12 or 24 dB/octave. 

Crossover frequencies are typically between 70 and 150 Hz. However, that value will be determined by the characteristics of both the sub and your main speakers. There'd be no point in setting the crossover to 60 Hz if your mains only go down to 80 Hz. Also keep in mind that these filters have a gentle slope, so if your mains are 6 dB down at 60 Hz then 60 Hz would be too low a crossover frequency. You want to choose a frequency that's well above the point where your mains wimp out.

 

Edited by bitflipper
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 Dave wonderful post. One thing though is there is sometimes a transient that can have a very high frequency component. This can and does give a directionality to some  low frequency sounds.  A bass drum hit for example. 

 

Also the bouncing  back of low frequencies is what produces standing waves.  If you find one in your room, and you should be able to, you will find when the phase is right it gets louder with low frequencies.  In the good old days standard practice was placing speakers in corners so one could use the three surfaces to increase the bass response.  Because  back then most tweeters were highly directional many would point the speakers inward a little.  

 

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