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RE: 100 hz or higher

Posted by Dave_K on December 5, 2014 at 07:52:23

In Reply to: RE: 100 hz or higher posted by genungo on December 5, 2014 at 02:29:16:

Back in the days when vinyl recordings were the name of the game for most people, bass was summed to mono at 100 hz. So, it used to be more of an OK thing to use a subwoofer crossover point of 100 hz, because doing so would not necessarily degrade stereo imaging and soundstaging.
Now flash forward to the digital age where the cutoff point has been lowered to 80 hz, which corresponds to the point at which bass begins to become omni-directional...

If you listen to modern recordings with a crossover point from mains to a subwoofer higher than 80 hz, you risk shutting out some of the stereo information encoded in your recordings (unless you use two subwoofers that are, preferably, in very close proximity to the associated main speakers).

I agree. I am a big proponent of using multiple subwoofers in a stereo configuration, even with lower crossover frequencies. I currently use two in a stereo configuration in both of my systems, but in the past I've used up to 4 in the system (Geddes approach) with all 4 mono, or in two stereo pairs. I will never go back to using a single sub or a multiple mono configuration unless the crossover frequency is below 50Hz.

You bring up a good point about recordings because a lot of people say you should configure two subs as dual mono rather than stereo, to get more of the benefit of mode cancellation. But as you mentioned, a lot of bass content is already mono, either because it was mixed down to mono or just recorded mono and then panned dead center. For that bass content, there's no difference between dual mono and stereo sub configurations. But as you noted, there is also stereo content in the bass, and you need a stereo configuration to reproduce that.

You say that the placement of subwoofers very close to the main speakers is usually "not recommended" but that would mean that the majority of classically designed tower speakers are extremely compromised designs (from the standpoint of acoustics), since most of them have immovable subwoofers in very close proximity to the other drivers.

Sure they are compromised. There are tradeoffs with every approach. Unless you have a large, acoustically optimum room (either by luck or by design ala Mike Lavigne), full range speakers will almost always have more issues with peaks and nulls in the bass than a properly set up combination of small speakers + subs. But the tradeoff is that the designer of the full range speaker has presumably selected bass drivers that are a good sonic match, and a crossover that ensures a relatively seemless blend. Whereas it can be quite difficult to select subwoofers that are a good sonic match with small standmount speakers, and then optimize their placement, and possibly try different crossover slopes and EQ to blend them properly. I can't say that either design approach is better than the other because it depends on the room, and the speakers, and the experience and acoustics knowledge of the user.

Attempting to keep each of two subwoofers in close proximity to the two main speakers in a stereo system is certainly recommended and is always worth trying before other placement options are investigated, when crossover points approach 80 hz or higher.

I disagree. While placing the subs next to the main speakers may turn out to be the best placement for some systems & rooms, you really need to go through the effort of exploring all possible placements and finding the one that offers the most even response and optimizing the subwoofer settings and even consider eq on the subs. In most cases, especially in small to medium sized rooms, the location of the main speakers is usually not the best place for the subs. In my experience, side wall placement or front wall placement is usually better, but there are no absolutes.

When using only one subwoofer I try to place it about the same distance away from the listening seat as the main speakers, whenever possible.

I agree, especially if the crossover frequency is above 50 Hz. Below that it doesn't matter as the wavelengths are too long. Some would say it doesn't matter up to 80 Hz, but my experience indicates 50 Hz is the threshold for me. Also, if you can do time delay correction, then this isn't required.

Omni-directional sound has a unique sound signature. The ears can sense the shift from omni to uni within the context of a stereo soundstage. 100 hz should represent *the extreme upper limit* for a subwoofer crossover point if you like to listen to modern stereo recordings. A crossover point of 150 hz or higher, as recommended by you, can of course be used but this practice should be viewed as being less than optimal. I don't think you'll find too many manufacturers designing systems with a subwoofer crossover point higher than 100 hz these days, and for good reason. No one wants to be FORCED to buy two subwoofers for their stereo system when one might possibly do.

I don't *recommend* crossover points of 150 Hz or higher. I don't know where you're getting that. What I was saying is that using a crossover point of 150 Hz or higher is a valid design choice that can work if you follow certain design guidelines. And if properly implemented, it can have advantages (e.g. mounting woofers in an infinite baffle can eliminate the front wall 1/4 wavelength null). It's not common in domestic listening rooms because it's way harder to set up and requires knowledge of acoustics and measurement tools, but it is common in recording studios.

BTW, I wasn't referring to your drawing when I noted that your listening room is square. You have described your listening room as being, basically, an acoustically compromised square in your system profile page. I therefore assumed that this was the room that contained the old "configuration" of yours, mentioned above. Frankly speaking, if your listening room is as problematic as you say it is it wouldn't surprise me that the kinds of sonic nuances I've been talking about here would not be easily noticed.

The room is roughly 13x16', but it's not a simple shape which is why I didn't attempt to draw it. The primary acoustic problem caused by the shape is that the first order axial modes associated with the length and width of the room are close together at 35 Hz and 42 Hz. These modes are strong and close enough to interact. Without equalization, bass notes in that range boom. Since I prefer not to use equalization on the main speakers, full range speakers are out. And I need at least one band of PEQ on the subs.

However, like I said in my previous message, the issues I was raising relative to sub placement are not related to the shape of the room. They are only dependent on the location of the subs relative to room boundaries.

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