Improving Your Stereo’s Sound Quality: Understanding Lateral Sound Detection

By understanding how your ears detect the direction of high- and low-pitched sounds, you can optimize your speaker and subwoofer placement and acoustic treatments for the best imaging and soundstage.

Lateral (side-to-side) sound positioning for imaging and soundstage width are related concepts but the former addresses the specific placement of an instrument or singer on the horizontal plane between and outside the front main speakers while the latter refers to overall breadth of the farthest left and right sounds.

How We Detect Lateral Sound Direction

Our ears have a fascinating way of determining the lateral direction of sounds based on their frequency, and the size of your head plays a role in this process.

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From the Wikipedia Interaural Time Difference page:

The average head width affects how we detect sounds coming from the left or right sides due to the time delay between the sounds that reach each ear. An average head width of about 22-23cm equates to about a 1,500Hz sound wave.

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Regarding low frequencies (below about 1,500Hz), the sound waves are longer than the width of your head, resulting in your brain using the difference in time between when the sound hits each ear to figure out where it’s coming from. This is known as phase detection or “Interaural Time Delay” (ITD). Regarding high frequencies (above 1,500Hz), the sound waves are shorter than your head’s width, resulting in your brain relying on the difference in volume (i.e. loudness) perceived by each ear to determine the sound’s lateral direction. This is called “Interaural Level Difference” (ILD). There’s a transition zone between 800Hz and 1,500Hz where phase detection becomes less reliable and volume differences take over as the primary cue. Once the frequency drops below 80Hz, it becomes difficult for your ears to detect the sound’s lateral direction because the phase differences are too subtle.

Positioning Your Speakers for High Frequencies

If high-frequency sounds are primarily detected by the differences in volume heard by both ears, then placing your speakers’ tweeters and midrange drivers symmetrically in your listening room becomes crucial. First reflections (when the speaker’s sound hits only one room surface before reaching your ears) will be a little less loud than direct sounds (from speaker to ear) because sound reflections lose some of their energy after striking a surface and traveling a longer distance to your ears. Despite some attenuation and slight time delay, first reflections have a big impact on sound quality as they mix with the direct sound. First reflection timbres will also be influenced by the materials used in the room’s construction.

For example, if one sidewall is a brick fireplace while the opposite sidewall is constructed from drywall, there will likely be an audible timbral difference produced by the first reflection and the reflections of that reflection. Imaging and the soundstage will be pulled towards the louder first reflection surface. 

Now imagine a music listener’s room with a hard left-side wall (e.g. drywall), while the right side of the room is open to an adjoining room. In this situation, music from the left speaker will likely sound a bit louder than the right one due to stronger left sidewall reflections from both speakers. The ears detect the louder high frequency reflections from the left sidewall and localize it accordingly. In our example, an imbalance in lateral sidewall reflections will skew the soundstage width leftwards and deteriorate the imaging as if the singer was placed leftwards rather than centred between the speakers. To correct this imbalance, several tactics can be tried: (1) toeing in the left speaker more toward the listener, (2) using a thick (minimum 6″-thick) absorption panel on the left sidewall to mimic the acoustic properties of the open right side, (3) lowering the left speaker’s volume slightly using a balance control or digital signal processing (DSP), or, (4) setting up a temporary and movable wall on the right side first-reflection point to balance out the reflections. If your left wall is a large window, then it acts as a bass trap, in which case heavy curtains can be used as high-frequency absorbers to help restore symmetry.

Since our ears are more sensitive to lateral differences than to sounds coming from above or below, you might get more of an impact by focusing on sidewall symmetry before treating the ceiling.

A Quick Story: My Own Experience

In my listening room, I once noticed the vocals on a favourite song sounding skewed slightly to the right instead of being dead centre. After doing a frequency sweep and analyzing the reflections, I discovered that there was a strong reflection coming from the left speaker. This strong reflection was coming from the right sidewall, so by adjusting the angle of a reflective board on the right wall, I redirected the sound so it didn’t bounce directly to my listening position, but went past my head. After remeasuring and re-listening to the song, the issue disappeared, and the vocals returned to the centre.

Positioning Your Subwoofers for Low Frequencies

To the extent that your subwoofer produces sounds higher than 80Hz, lateral detection of its output relies on the time and phase differences between the sound waves hitting each ear. If you’re using just one subwoofer, placing it in the middle of the room, either between the speakers or behind your seating area, usually provides consistent lateral sound across the room.

If you’re using two subwoofers in a rectangular room, they should generally be placed along the sidewalls to minimize phase and time differences, ensuring consistent lateral sound detection. However, in asymmetrical rooms, your subwoofers may end up in very different room locations to get the best frequency response from each. In this situation, it’s crucial to time-align them to each other and to the main speakers, and to match the subwoofers’ volume levels to avoid any lateral imaging issues.

Another Quick Story: My Subwoofer Setup

In my room, I initially had one subwoofer closer to my listening position than the other and visitors could detect the closer subwoofer despite my best efforts to time-align them and match their volumes. Was the detection due to an ITD issue or something else? I can only surmise that the closer subwoofer’s sound waves were stronger, which made them more noticeable. After moving the left subwoofer backward to match the distance of the right subwoofer to my seat, and rerunning the time alignment, the issue disappeared.

Final Thoughts: Striving for Symmetry

Asymmetrical rooms can create unbalanced reflections that negatively impact stereo imaging and lateral sound detection. By aiming for symmetry in your stereo setup, you help ensure that your ears receive sound evenly from both sides. When you achieve this balance, the soundstage and imaging lock in, making your music sound more natural, dimensional, and, ultimately, enjoyable.

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Fielding Acoustic Consulting specializes in two channel stereo setup and optimization. Kevin Fielding is a musician and devoted audiophile with a long work history in data analytics.  He can be reached at kevinfielding@hotmail.com.