Whether you like to listen to music, movies, or videos, you'll want a speaker with a good soundstage. Soundstage is the 'stage' for your audio, and it gives you a sense of how you'll perceive the speaker's sound in the space around you. For example, let's say you're hosting a dinner party with friends. If you place the speaker in the center of the table, you'll want to make sure each friend experiences sound in the same way. If you place the speaker on the kitchen counter, though, a more front-facing sound is more than suitable since the back side of the speaker just faces a wall.
To get a sense of a speaker's soundstage, we look at two factors: its directivity and whether it can play stereo sound.
Most music is mixed as stereo content, with certain voices and instruments devoted to either the left or right channels, respectively. If you want to listen to this music as intended by those who mixed it, it's important to find a speaker that supports stereo sound. If you listen to more dialogue-focused audio like podcasts or the news, this isn't as important since this content can have a more two-dimensional sound without taking away from the overall listening experience. Meanwhile, a speaker's directivity performance is more or less relevant depending on how you plan to position the speaker in your space. Some models bring a consistent sound all around, while others are more suitable for sending audio in only one direction.
To evaluate a speaker's soundstage, we measure its directivity index and whether it's a stereo or mono speaker. These components indicate if the audio sounds consistent at different angles or if there's deviation, which could make it harder for you to hear your audio if the speaker is turned a certain way. It also shows whether the speaker can playback stereo sound.
We conduct our soundstage test at the same time as the frequency response and dynamics tests. We use a room that's 20' (L) x 16' (W) x 9.5' (H). There's one couch in the room. We use some acoustic treatment, namely, a few acoustic panels to reduce noise levels and echo in the space. The shape and size of a particular room can impact how you perceive a speaker's sound, so your experience may be a touch different depending on your space.
The speaker is placed one meter away from the tip of the central microphone. Using a laser measuring tool, we ensure that the microphone array, table, and speaker are all at equal distances from the side walls. We also check that the speaker is placed at an equal distance from all of the table's sides. Finally, we also run this test using the speaker's battery, if applicable, and pair the speaker to our test phone via Bluetooth or Wi-Fi.
Speakers come in many different shapes and sizes, which can impact their soundstage. There are two general designs: a front-facing speaker that plays audio in one general direction and an omnidirectional (or 360-degree) speaker, which plays audio in all directions.
The directivity index is a measurement in decibels that shows how a speaker performs at different angles. We place the speaker on a moveable table one meter away from our mic setup to record this measurement. We play an audio file to measure its frequency response and turn the table to seven angles (0, 30, 60, 90, 120, 150, and 180) while recording each pass. In the graph, the 0 line is the flattened frequency response at 0 degrees. Each thin line corresponds to a different degree and is the difference between the 0-degree frequency response and the corresponding angle.
The red line on the graph represents the directivity index, indicating the average difference between these measurements. In short, the smaller the directivity index or the flatter the graphed responses, the more the speaker projects sound consistently around it.
In addition to representing the average, the directivity graph also shows the measurements for each individual pass. It's useful for understanding how the speaker will perform in your space. For example, let's say you plan to place the speaker on a small table beside your bed. You'd want a consistent sound at 0 degrees (in front of the speaker) and 90 degrees (in your bed). How it performs at 180 degrees won't matter as much since that side only faces a wall.
More front-facing or directional speakers tend to result in graphs similar to that of the Sony SRS-XB01 shown above. You can see that the directivity index is low at low frequencies, but it increases at higher frequencies. This means there's more discrepancy in the sound in the treble range. If you're standing behind this Sony speaker, treble-heavy sounds like cymbals aren't as consistent as if you stand in front of the speaker.
If you like to listen to music from your speaker, you'll want to know whether your speaker is mono or stereo. This refers to whether it can play either monophonic (mono) or stereophonic (stereo) signals. Mono signals contain a single audio channel, while stereo signals contain two channels, left and right, which sound somewhat different from each other.
We test whether a speaker is mono or stereo by checking the user specifications or tech drawings. We also use a test signal to see whether the speaker can play the left channel and then use a different test signal to see if it can play the right channel.
Since the vast majority of music is stereo sound, it's important to most people to find a speaker that can play back this audio as intended by those who mixed it. There are some instances where it's not as useful, though. Let's say you want to plug in a microphone or a guitar to a speaker—since these instruments are only one channel, a mono speaker would work fine.
A good soundstage can make all the difference when listening to audio. While the perception of a speaker's soundstage can vary depending on your room, our tests are a good start to understanding your speaker's sound. Directivity matters most to those who want a 360-degree experience with a consistent sound around the speaker. The speaker's ability to playback stereo sound is especially useful for music lovers who want a solid distinction between the left and right drivers in a song. Overall, a good soundstage lets you enjoy your favorite tunes from all around.