Lowpass filter: What’s your 2 pence?

Sliced Bread

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Jul 28, 2010
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Following on from a previous thread on crossovers, what’s your opinion on the use of lowpass filers? Do they help or detract?

It seems that the lowpass filter for the lfe channel filters out and discard any material above the set frequency. So if you set it to 80hz you will lose anything in the recording sent to the LFE (.1 channel) above this frequency. As far as I can tell it is not redirected.
It is interesting as it only effects the LFE channel and not re-directed bass from satellites, so it implies that there is a valid use for this. But (big BUT), information above the cut off will be completely lost.

There seems to be two schools of thought:
1) Set it to 80hz (or other depending upon room acoustics). Sounds above this point are easy to localized in the room, so your sub may become a point source IF the sound mix contains info above 80hz. It is considered that if there is sound above this point then it is a mistake on behalf of the sound mixer and therefore should be discarded.

2) Set it to 120hz. If there is bass there above 80hz then it is intentional and there for a reason.

Does anyone know of any films that are good testers to hear the difference? If the first school of thought is correct then there shouldn’t be many as film shouldn’t be mixed like this to begin with.
What’s your take?

I like the idea of ensuring the sub is not easy to localize as well as reducing its load, but I don’t like the idea of throwing away information that was put their intentionally by the sound designer. On the other hand the fact that so many AV receivers have a lowpass filter option shows that many AV technicians believe it’s an important part of the setup.

I’d love your opinions, cos I’m going round in circles on this...it’ll be easier if I knew which films were mixed this way so I could do A-B comparisons.
 
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Anonymous

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I'd always set it to a little over the bass roll-off of the main speakers, regardless. If they are tiny little ones that can't reproduce below 100, then you need more careful positioning that's all. I don't think people mixing the soundtracks care 2 hoots. There isn't a sudden break point at which bass ends, it's gradual. The main advantage I see is that it reduces the load on the main speakers and improves clarity.
 

Sliced Bread

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snivilisationism said:
I'd always set it to a little over the bass roll-off of the main speakers, regardless. If they are tiny little ones that can't reproduce below 100, then you need more careful positioning that's all. I don't think people mixing the soundtracks care 2 hoots. There isn't a sudden break point at which bass ends, it's gradual. The main advantage I see is that it reduces the load on the main speakers and improves clarity.
Thanks snivilisationism, but that is the crossover. This refers to the "lowpass of the lfe". Some receivers do not have this, but it is on the Onkyos. Just as crossover cuts low frequency from the satelites and redirects bass to the sub, the "lowpass of the lfe" cuts high-frequency from the subwoofer, although it seems to just discard it rather than directing it to the sats. So they are different things...but similar.
 
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Anonymous

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Ah, Ok, I see, I jumped in there!

Well, I don't think my AV amp (RXV 667) has a selection for that, only bass-crossover. But I'd set it to 120 in that case.

I work out the wavelength to be a little under 3 metres at 120 Hz, which in an average room is probably not particularly directional.
 
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Anonymous

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Sliced Bread said:
That is what I thought too, but many people (including THX) suggest it should be set lower.

IMO THX is just a way to get people to spend more on the same thing. I think I'd rather get all the sound that's there. I think mine just does 20 to 120 by default... Fine by me :)
 

Andrew Everard

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Sliced Bread said:
Hey Andrew, what's your take on this? You use an 875 if I remember rightly. Thanks

Well, I'm not using THX certified speakers with it, and my fronts, centre and surrounds are run full-range. So long since I set the thing up I'm not sure what the sub roll-off is set to, but I suspect it's set to 80Hz (THX)
 

Sliced Bread

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Andrew Everard said:
Sliced Bread said:
Hey Andrew, what's your take on this? You use an 875 if I remember rightly. Thanks

Well, I'm not using THX certified speakers with it, and my fronts, centre and surrounds are run full-range. So long since I set the thing up I'm not sure what the sub roll-off is set to, but I suspect it's set to 80Hz (THX)
Thanks Andrew. With speakers set with a crossover in the 60hz - 70hz range, should the "low-pass filter of lfe" be at 120hz or reduced to the 80hz region? How do you tell if it is correct?
 

Andrew Everard

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Given that the LPF of LFE is really only there to stop any hum, shouldn't matter too much. And how do you know it's right? You listen - not a smart answer, but really the best way.
 

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