When listening to multi-channel content like movies, video games, and TV shows, it's important to hear the dialogue clearly to follow the action. Some soundbars come with a third channel, known as the center channel; this is dedicated to reproducing the dialogue found in your audio content. A discrete center channel can significantly improve the quality of dialogue, helping you hear voices clearly and accurately.
We test for several factors that influence the center channel performance, including localization, slope, standard error, max volume, and the total harmonic distortion performance.
If you want to hear the voices in your TV shows and movies clearly, look for a soundbar with a solid center channel performance. While many soundbars support a center channel, not all of them offer the same performance. Some soundbars lack a discrete center and have to use their left and right channels to simulate a phantom center, but this doesn't localize voices in the sound image as accurately as a discrete center. A discrete center channel also lets you adjust the volume of the dialogue independently from the rest of the mix. As with a stereo setup, the frequency response, volume level, and total harmonic distortion performances can also influence the way your audio sounds on the center channel. However, this is less important for music fans since most music is mixed into stereo and doesn't send a signal to the third channel.
To test the soundbar's center channel performance, we ensure that it only has a signal on the third channel. We measure its frequency response to get a better understanding of how well it can reproduce a variety of frequencies, from low-bass to high-treble. We also record how loud it gets, its total harmonic distortion at a normal listening volume of 80dB, and the THD at its max volume. These measurements can help show how clean and pure your audio reproduction is and whether your soundbar reproduces audio the way it was intended to be heard by the engineer who mixed it.
We perform our center channel tests with a similar setup to our stereo channel tests. We use a room that's 18' (L) x 16' (W) x 9' (H), with one couch and minimal sound treatment to represent a typical living room. We place the soundbar and the table it sits on 7.5 feet away from our microphone array. We use a laser measuring tool to ensure that the microphone array, table, and the soundbar are at the appropriate distance from the side walls, as well as the subwoofer and the satellites if they're included in the setup.
Most of the soundbars we've tested support an audio signal on the third, or center, channel. However, depending on the setup of the bar, they can reproduce this content in different ways. Localization shows how the center channel is supported by the soundbar and can make a difference when pinpointing voices in the sound image.
Many soundbars come with a dedicated or discrete center channel, which can more accurately localize voices to a pinpoint location in your sound image. However, other soundbars have phantom localization. This means that they use their left and right channels to simulate a phantom center. Unfortunately, this doesn't anchor the dialogue to the middle of the screen, so voices may seem like they're coming from a more general area.
During our testing, we measure the frequency response of the soundbar when it's playing content on the center channel. We play a sine wave tone at -6 dB FS between 20Hz and 20kHz and plot its output, in dB, on a graph. The frequency response graph helps to show how accurately the soundbar reproduces audio content at different frequencies, from the low-bass to the high-treble.
The slope of the frequency response graph helps demonstrate the overall tonal balance of the soundbar. For example, a negative slope means that the bar reproduces more bass than treble, so your audio may sound thumpy, punchy, or boomy. A positive slope shows more treble than bass, resulting in a bright, sparkling, or even piercing sound. A slope that's closer to zero means that the bar has a more balanced sound overall.
We don't score the slope of the soundbar's center channel. Depending on your personal preferences, you may prefer a bar with a different tonal balance. Some listeners like a more balanced, neutral sound since that's suitable for most types of audio content. Others may prefer a more bass-heavy sound, especially if they like to feel the deep thump and rumble in their audio.
Standard error is a measurement that shows the average deviation of the soundbar's in-room frequency response from the in-room target response. Essentially, it helps to show how accurately the soundbar can reproduce audio at different frequencies.
For our tests, we use the Harman in-room response curve as our target. It represents a frequency response found to be the most pleasing to the most amount of people, and it adds a little more emphasis in the bass range and a little less emphasis in the treble range. Ultimately, a lower amount of standard error is better since it tends to represent a cleaner, more even sound that's closer to the target curve.
You can usually determine which soundbars have higher amounts of standard error just by looking at their frequency response graph. A graph that deviates from the middle a lot will usually have more standard error than a graph that's flat and close to the middle. Depending on where the fluctuations are located, you can also see how it will influence the overall sound. For example, deviations in the mid-range can muddy voices or push them towards the back of the mix, and fluctuations in the treble can make voices sound dull or piercing.
Since the center channel is specifically for dialogue, it's most important to see how the frequencies are reproduced in the mid and treble ranges, as this is where most voices are located. As a result, it may not be very noticeable if a soundbar struggles to reproduce a lot of bass on its center channel.
If you love to turn up the volume when listening to your audio, you'll want a soundbar that can get loud on its center channel. Sound Pressure Level @ Max Volume is a measurement that shows how loud the soundbar can get. We record it in decibels (dB) using a microphone array.
We consider a good value to be above 92 dB SPL since this gives you more flexibility when you want to crank up the volume. That said, depending on your personal preferences, you may want a soundbar that gets even louder on the center channel. Some soundbars, like the Nakamichi Shockwafe Ultra 9.2Ch, can reach volumes of above 100 dB SPL on the center channel.
Total harmonic distortion, or THD, shows the number of unwanted frequencies that a soundbar reproduces alongside the intended frequencies. Essentially, it compares the input (the TV show, music, or movie you're playing) with the output (the audio played by the soundbar). THD is important for listeners who care about the fidelity of audio reproduction since a low amount of THD means that audio is reproduced the way it was intended to be heard by whoever mixed it.
For this test, we measure the amount of THD when the bar is set to a normal listening volume of 80dB. Then, we apply a spectrally pure sine wave and use the microphone array to measure the output. Our measurement is weighted, meaning that it adds a little more weight to higher frequencies since you're more likely to perceive distortion at those frequencies. We consider a good value to be below 0.7, but THD can be difficult to hear with real-life content.
Some listeners may like to crank up the volume on their soundbar, especially if they want their audio to fill up a large space. We measure the amount of total harmonic distortion when the bar is pushed to its max volume. Our THD measurement gives more weight to higher frequencies since you're more likely to perceive distortion in those frequencies.
Most of the soundbars we've tested see a jump in THD when the soundbar is pushed to its max volume. However, a soundbar should maintain clear and pure audio reproduction as long as the THD remains below one. THD can be very difficult to hear with real-life content, so this measurement is mostly for listeners who care about the fidelity of audio reproduction.
If you like to listen to multi-channel content like movies or video games, it can be important to find a soundbar with a good center channel performance. A discrete center channel can anchor voices in the middle of the screen, making it easier to pinpoint voices to an accurate location. We also evaluate how loud the center channel can get and measure its frequency response and total harmonic distortion performances. That said, the center channel performance is usually less important for fans of stereo content like music.