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

So - would 2 subs - one for the front channels and one for the rear (or surround channel) be better for movies?

http://www.audioc.com/library1/subprimer.htm

A single sub provides a major and cost-effective improvement for most people in most rooms with most music. However, there are a number of reasons to consider more than one sub.

-Stereo subwoofers, are more important if the main speakers need a rather high crossover frequency. Some very careful listeners may want stereo subwoofers with crossover as low as 60Hz while many others might be more than happy with a single subwoofer crossed over above 100Hz. Many of the popular three-piece satellite-subwoofer systems in the $500-$1000 range have crossovers above 160Hz! Some listeners hear the problems immediately while others never even notice them.

-Multiple subs can increase output levels and lower distortion. People with large rooms and those who need higher than average output levels might want to use more than one sub. Two smaller medium cabinets might fit your space better than one large box. For large spaces, and or very high volume requirements, any number of subs can be added to a system.

-Multiple subs can yield smoother in-room response than a single subwoofer. Loading the bass into the room from two or more points can give a better balanced bass throughout the room. However: if the crossover frequency is low, (usually below 80Hz or even lower), the best bass may be with the multiple subs placed next to each other.

-A third sub may be used for the rear surround channels. Many films now have significant low bass in the rear channels. Some processing modes require extra subs for the ultimate experience. Some film lovers feel that having a subwoofer for the rear channel improves impact and realism with such films.

Pretty soon, the sound will be perfect but there will be no place left to sit.

Hi cvictorg'

Definitely not, if you use two subs they should be feed with the audio signal from the LFE dedicated channel which (when well configured) will also include all the lower requencies for all the channels (including front and rear). Having one sub for the fronts and one for the surrounds (rear and/or sides) could easily result at not only missing good bass information but at having a single sub working at a time (rarely in unisson).

Given that frequencies below ~80 Hz are non directional, there is no benefit to a rear channel sub. I'm not even sure how you would redirect bass from a surround channel specifically to one sub.

I think it might be interesting if you could setup a midbass unit for the rear channels. You could set the crossover in your AVR to 40hz and then use a sub-woofer with an internal high level crossover to have a second crossing for the rear channels at say 80hz. This way you wouldn't be missing out on any lower frequency surround effects that your average rear channel speaker is unable to reproduce.

No.

No.

Oh oh, somebody did a no no.

The -3 db point of the QS8 is 65 Hz. You can't localize bass below ~80 Hz (probably a little higher in most rooms). Unless you have a surround speaker with a -3db point of 100 Hz or higher, I just don't see the point of a sub or mid bass unit for surround duties.

If your surrounds are not that good, spend the $500 on better surrounds rather than an MBM.

Frankly, I don't totally believe the often quoted 80hz non-directional figure. This is why I said it might be interesting. I honestly don't know of any "surround" speaker that can even approach 40-50hz at meaningful levels of pressure. I suppose if I had deep pockets and 2 pair of M80s or a second subwoofer then I could try it. However, I don't have these things. Just because it's "taboo" doesn't mean it wouldn't be fun to try.

cvictorg, and all,

Whoever wrote that "sub primer" is deeply misguided and does not understand how movies are mixed. The following is simply wrong:

"A third sub may be used for the rear surround channels. Many films now have significant low bass in the rear channels. Some processing modes require extra subs for the ultimate experience. Some film lovers feel that having a subwoofer for the rear channel improves impact and realism with such films."

The whole point of the " .1" Low-frequency effects channel used in all cinema systems (mainly Dolby, dts and some Sony) and movie/TV show mixes is for deep bass frequencies below 100 Hz.

Regards,
Alan

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.

Yeah, however THX also recommends that you sit 30" from a 55" screen...

for a true theater like field of vision

80hz was also chosen as it was guesstimated that most home systems could easily handle this through the speakers.