Low-End Sound Considerations
Specific decisions regarding subwoofer choices can significantly impact the overall room design and surround sound design. This page provides background information gathered and used to make decisions, considering budget constraints and how to use and configure the subwoofers. This is an area that may have been somewhat "under-designed" when compared to other home theaters. (BTW, low-end, in this case, refers to frequency response, not product quality.)
Is a Subwoofer Needed?
Last Updated: 12/13/2024
Adding a subwoofer to this home theater was an easy decision. However, one could use full-range speakers or accept a limited low-frequency response. A subwoofer enhances the overall sound experience and may influence the selection of other speakers. So, it is interesting to go through the considerations and tradeoffs involved in using a subwoofer. Some of my thoughts on this are:
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Humans cannot reliably detect the direction of low-frequency sound (less than approximately 150Hz).​ Therefore, placement is not dependent on the direction of sound.
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The room itself (and the subwoofer placement) dramatically impacts the audio quality (especially sound uniformity).
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The frequency response of full-range speakers typically doesn't extend below ~30-50Hz; if it does, they tend to be more expensive.
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Full-range speaker placement is dictated by surround sound requirements, not the room, so this placement may be sub-optimal for bass frequencies
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Employing a subwoofer to cover frequencies below ~100Hz enables the use of bookshelf speakers, which typically don't have as deep a low-frequency extension as floor-standing speakers.
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Subwoofers, particularly high-performance ones (e.g., frequency response down to 20Hz at reference volume levels), can be physically very large.
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Based on the above considerations, it seemed rather obvious (to me) that using one or more subwoofers would benefit from a dedicated home theater. Subwoofers improve low-frequency response and enable a wider choice of regular speakers because they handle frequencies below ~80Hz.
Performance Goals
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Before using subwoofers, I needed to establish low-frequency reproduction goals, including the subwoofer type and performance requirements. Acknowledging self-imposed budget limitations and the small room size, I settled on two (OK, maybe three) primary goals.
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Subwoofer frequency performance minimum: 25 Hz;
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Employ a small subwoofer (or two) to minimize floor space;
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Third, I planned for at least two subwoofers for the below reasons.
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I realize that many home theater enthusiasts feel much more strongly about achieving very high performance, even for base frequencies below 20Hz, and there is a lot of merit to this. Still, I am trying to get good performance for this room while keeping the costs down.
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Ported or Sealed
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Another less critical decision was using a ported or a sealed subwoofer. The tradeoffs between them are:
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Ported subwoofers generally have a lower frequency response than the equivalent sealed subwoofer.
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Sealed subwoofers don't extend as low as ported ones, but their roll-off is much more gradual.
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Ported subwoofers typically are much larger boxes than sealed subwoofers.
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Ported subwoofers are usually more efficient and have a higher sound output than comparable sealed models.
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From a pure performance perspective, a ported subwoofer has the advantage. A sealed subwoofer's main advantages are mostly size and aesthetics. SVS Sound has a good write-up of ported vs. sealed subwoofers here: svssound.com ported vs sealed.​
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Figure 1. Example of an Actual Sealed Subwoofer that is a small 14-inch cube
Figure 3. Another example, this time a real ported Subwoofer (19x15x20 inches) (© Copyright 2024 SVS Sound Inc.)
Room size plays a role in subwoofer selection. Larger rooms require more powerful subwoofers, and the performance advantage of a ported subwoofer is more attractive in larger rooms. Audioholics.com has a detailed discussion of this. This room falls into the small side of a medium-sized room.​
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To summarize the decisions so far, a pair of medium-performance sealed subs should be sufficient for this small-medium room to provide performance down to at least 25Hz.
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In-Wall Subwoofers
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Given my goal of minimizing the footprint of the subwoofer(s), one might think that an in-wall subwoofer would be a viable option. However, I quickly dismissed this possibility. Most in-wall subwoofers tend to perform poorly. While some better models claim a frequency response of 35 Hz, this specification is likely exaggerated, considering the typical 8-inch driver size. Additionally, many reasonably priced options do not come with an amplifier. There are a few in-wall subwoofers that seem to perform well (for example, this recently introduced one), but they are significantly more expensive than stand-alone subwoofers. Another factor to consider is that achieving sound isolation with in-wall subwoofers would add considerable complexity to their installation. For these reasons, I did not seriously consider in-wall subwoofer options.
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Last Updated: 09/29/2024
​As discussed in the previous subwoofer section, having separate low-frequency speakers in the room makes sense to optimize the bass frequency response. Research has shown that base frequencies in a typical home theater room depend more on the room and subwoofer placement than the speakers. Harmon Audio has published research regarding subwoofer placement in a rectangular room to achieve an even bass response. (A few links to this information are at the end of this section.)
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The main takeaway from these papers is that the subwoofer configurations shown in Figure 3 provide the most consistent frequency response throughout the listening area. Two subs (top blue room) give a more even response, and four subs (green rooms) are a little better than two. The goal is to achieve an even response across the listening area, and then one can use equalization or room correction to get a flat response throughout the listening area.
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Based on this research and budget limitations, the best subwoofer configuration for this room is the two subwoofers at the midpoint of opposite walls, shown in the top room diagram on the right. Since our home theater is a little narrow, placing the subwoofer at the front and back of the room is less intrusive. Two more subs could be added for greater output and enhanced evenness across the listening area.
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Subwoofer Placement References​
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Audioholics.com—Optimum locations for subwoofers
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Audioholics.com—Multiple subwoofer setup.
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Harmon Research—Subwoofers: Optimum Number
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"Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms," Dr. Floyd Toole (Amazon Link)
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“How Many Subwoofers Are Enough,” Todd Welti, 112th Convention, Audio Eng. Soc. Preprint 5602.
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"In-Room Low Frequency Optimization, "Todd Welti, Allan Devantier, 115th Convention, Audio Eng. Soc. Preprint 5942
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Figure 3. Subwoofer Placements to Achieve Even Base Response