are nulls normal?

[v1rtu0s1ty]

hi folks, as you know, I still haven't started my ht room yet. Anyways, I was playing different LFE demo scenes this afternoon and I noticed that when I am in the center but close to the screen about 8 ft from it(sub is on my right about 9 ft diagonal from me), I hear/feel the bass just a little as if there was no sub. However, once I go to other positions, it is totally insane.

Am I correct that the affected area is called the NULL area? Can they be fixed?

Thanks!

[ThomasW]

home.comcast.net/~thomasw-2/SubwooferSetupandEQpage2.html

[v1rtu0s1ty]

Thanks for the link. It was very informative. I'm glad that the area where there is so much NULL is not the listening area. It's actually close to the screen. Anyways, I read somewhere that NULLS on subwoofers can be caused by main speakers. If I remember correctly, the poster mentioned that it's cancelling the sound coming from the sub. Is the person correct when he said that?

[carvernut]

v1rtu0s1ty said:

If I remember correctly, the poster mentioned that it's cancelling the sound coming from the sub. Is the person correct when he said that?

yes, if they are not in phase then they will begin to cancel each other out, i had that problem with my old setup where my sub was behind me and half the distance to me as the tower speaker and i ended up with a huge null from 30-70hz -20dB's, it was really anoying... but after reading that article on nulls and placement i started lugging the sub around the room and finally ended up putting it in-between the main and then was like, WOW

[ThomasW]

v1rtu0s1ty said:

Anyways, I read somewhere that NULLS on subwoofers can be caused by main speakers. If I remember correctly, the poster mentioned that it's cancelling the sound coming from the sub. Is the person correct when he said that?

That would only occur if the mains and the sub were playing the same frequencies. This is an unlikely situation given that overlapping frequencies are all but eliminated by the operation of the crossover.

[carvernut]

admin said:

v1rtu0s1ty said:

Anyways, I read somewhere that NULLS on subwoofers can be caused by main speakers. If I remember correctly, the poster mentioned that it's cancelling the sound coming from the sub. Is the person correct when he said that?

That would only occur if the mains and the sub were playing the same frequencies. This is an unlikely situation given that overlapping frequencies are all but eliminated by the operation of the crossover.

yeah i was running my mains down to 30hz and the sub up to 80hz at that time, hence the null DUH

;D

[v1rtu0s1ty]

My brain was visualizing the sounds as if the sound coming out from the mains is colliding with the sound coming out from the sub irregardless of what freq they are. This is just my guess. It's because, from what was mentioned in the link, air is fluid and sound travels via air.

So when a sound is created by a driver and played in a very open space like our backyard, is sound distributed equally like 360 degrees?

[Nordo]

v1rtu0s1ty said:

So when a sound is created by a driver and played in a very open space like our backyard, is sound distributed equally like 360 degrees?

Depends.
The higher the frequency, the more directional is the emitted sound wave. If you play a high frequency through your mains, then stand on axis, with your ears on axis, then you will hear the correct intensity of the high frequency. If you move off line, the intensity will drop off. If you try the same experiment with lower frequencies, the drop-off won't be as strong. Once you get below 80Hz, you can more completely off axis with losing intensity. HOWEVER, you WILL lose intensity with signals below 80Hz if your in a room because of nodes that get created in the room by the low frequency signal. And as you've found, the location of nulls and peaks, has nothing to do with being on or off axis.

There is heaps to read on this in the Cult and around the web. There are simple programs that can calculate where you should expect your nodes to be. They are really only accurate for simple "shoebox" rooms, but they still give a good indication, and can help decide on the best listening area.

[mojomike]

admin said:

That would only occur if the mains and the sub were playing the same frequencies. This is an unlikely situation given that overlapping frequencies are all but eliminated by the operation of the crossover.

Actually, Thomas, I would beg to differ on that point. At and around the crossover frequency, the mains and the subs are very much overlapping. At the crossover point, the mains and the subs would ideally be equally contributing to the total output. That's why getting the subs and mains in phase is so important to prevent cancellation. The high pass and low pass doesn't apruptly stop frequencies below or above the filter frequency. They roll them off. The rate of rolloff would, of course, affect how wide of a band of significant overlap exists.

[ThomasW]

mojomike said:

Actually, Thomas, I would beg to differ on that point. At and around the crossover frequency, the mains and the subs are very much overlapping. At the crossover point, the mains and the subs would ideally be equally contributing to the total output. That's why getting the subs and mains in phase is so important to prevent cancellation. The high pass and low pass doesn't apruptly stop frequencies below or above the filter frequency. They roll them off. The rate of rolloff would, of course, affect how wide of a band of significant overlap exists.

Mike,

I've been building speakers for more than 45yrs, so I have a reasonable understanding of how crossovers function.

One of my most recent builds was a variant of the schematic seen below.

[mojomike]

Thomas, there is no doubt in my mind that you have a thorough comprehension of crossover operation, but I stiill don't understand your statement, "...overlapping frequencies are all but eliminated by the operation of the crossover." Is there something I'm missing about the context?