Axiom Home Page Forums General Discussion Home Theater Separate sub for the rear channel

Alan. I was wondering exactly what the sound techs do as a general rule. How low do you really need your surrounds to go.

What comes out of your surround channel is really interesting, as I found out when I was first setting up my system and managed to send surround to both my mains and surrounds.

Something that just occured to me is that SACD or DVD-A or some other 5.1 mixed 'music' only format might have deeper bass directed to the rear channels. I am still interested to hear what the standard or recommended practice is for audio engineers.

I wonder if Axiom has ever experimented with a "QS12" that used a pair the 6.5" woofers.

Hi Fred,

Sorry for the delay; busy with the newsletter. If we're speaking of movie soundtrack mixes and TV 5.1 mixes, the bass is mixed to the ".1" subwoofer channel, or at least encoded in that channel, and that includes the soundtracks for live opera broadcasts from the Metropolitan Opera as well as DVD/Blu-ray mixes of music concerts. During a live mix, or a live-to-digital storage mix, the mix guys at the board usually don't have a subwoofer in a mobile van or control room, but the bass will be encoded in the sub channel, which is only for bass below 100 Hz.

Cheers,
Alan

Hi prototype3a,

When we talk about DVD-Audio and SACD and the mixing of bass, we have a different kettle of fish. In my limited experience with these commercially moribund formats, mixing practices seemed to vary.

Sometimes, they used full-range speakers for all the channels; the two "rears" were at diagonals to your seating location and you were supposed to sit in the center of the room. That was the arrangement for a Sony SACD demo I attended. I don't recall a subwoofer in that demo, and some SACD and DVD-Audio mixes do not have a subwoofer channel.

It was totally impractical for home setups and didn't work well in demos. If you shifted your seat the slightest bit away from the sweet spot, the imaging defaulted to the rears if you were closer to the rears than the front left and right or to one side or the other. It was for this and numerous other reasons that these formats failed to catch on.

You may not believe that deep bass below 80 Hz is non-directional but it's true. When I was just an amateur enthusiast, I thought deep bass was directional too, and had left and right stereo subwoofers. Then I did some experiments, met Dr. Floyd Toole and some other acoustical scientists, and came to learn that the "directional" qualities of deep bass sounds are provided by the midrange and upper bass--the "crack" of a mallet against a bass drum, for instance, is what tells your ears and brain the direction and location of the bass drum. The lingering very deep reverberations, around 30 Hz are totally non-directional. As they linger and bounce around a concert hall, you have no idea where they are coming from.

Regards,
Alan

Thanks Alan! Is there a physics reason as to why we are unable to locate the origin of these frequencies? I suspect it has something to do with the long wavelengths, the relatively close spacing of our ears and the ability of the ear/hearing centers to do the necessary triangulation calculations based on the rough data it is likely receiving.

I do believe that extreme low frequency sounds are difficult or impossible for us to localize. The thing I question is the often quoted cutoff frequency for this. I suspect it is lower than 80Hz but I am fairly certain that it is not lower than 50Hz. Either way, I think we have all learned some interesting new things today.

80hz is a generalization, some people can find this point localized while others not so much. I also think this point is room dependent as the room will emphasize certain frequencies more than others contributing to this localization.

Bingo.

From what I understand we can triangulate the direction of sound based on the different arrival times of the waves from 1 ear to the other. Under 100Hz the length of the sound wave gets too long for our ears to tell apart. Grade 12 Physics was a looong time ago for me, but a principle of imaging is that you cannot see an object which is smaller than the wavelength of the wave with which you view it.

In other words, if the wavelength of the sound is greater than the size of the... driver?, you can't image where it is.

I think.

snazzed


EDIT: The above is wrong... it just doesn't compute. But it does have something to do with the length of sound waves under 100Hz. I guess it was just *too* long ago.

I think the imaging idea only applies to photons. Hence why blurei players use a blue laser. It has a shorter wavelength and thus can "see" smaller bumps on the discs. However, in the case of sound I suspect it has more to do with the idea that the wavelengths are very long relative to the distance between our ears. So much so that there is not sufficient difference in the signal received by each ear in order to localize it.

Under 150-200Hz your ears are often not nearly as important as your body in perceiving the directionality of the “sound.” The reason is that your body can feel the direction of the low frequency waves even though the wavelength is long enough that your ears can’t readily determine the direction. The reason is that as the room grows smaller and the frequency lower that directionality of bass becomes harder to distinguish is that standing waves tend to override the directional waves from the source. Given sufficient acoustical energy low bass directionality in the open is distinguishable at great distances, 20+ miles in some cases. In the old days, when I "was" a grunt, “sound and flash ranging” was a common technique in locating enemy artillery batteries for providing counter battery fire.

In a room the directionality becomes ambiguous because the reflected waves tend to blend with the directional ones confusing ones perception of their directionality. The brain then uses other cues (e.g. higher frequency waves) to determine the directionality of the source of the lower frequencies, an important reason to properly calibrate ones system both for distance and volume.

As Jay says 80Hz is a generalization, and as such is not correct for everyone in every situation, in determining the best crossover frequency. The larger the room, the closer the bass source to the listener, the louder the bass source (relative to the rest of the system), the location of the subwoofer relative to the other bass sources (mostly the mains), the crossover frequency, and ones individual perception (often born of real world experience) all combine to lower the frequency at which the directionality of bass in an enclosed space is perceivable. There is a reason that companies making mid-bass speakers suggest that they be placed near-field it has something to do with Fred’s signature “Physics, It works Bitches!”

Ooooo! Deaner the keaner gets the gold star again.

I am willing to bet that the THX standard is at 80 Hz because that is the point at which sound becomes non directional in the vast majority of rooms: say 95-99%. It wouldn't do to have a standard that didn't work for some people.

I have seen the same numbers as Dean. Bass can become non-directional as high as 200 Hz.