Evaluating Stereo Soundfields using Time Selective Response (100 Things #78)

Time selective response in SoundCheck can be used to evaluate a stereo sound system’s localization and envelopment characteristics. By calculating the interaural level differences using a head and torso simulator, SoundCheck can measure and display the sound localization. SoundCheck can measure both the interaural level difference, and interaural cross correlation, all in a single sequence.

Evaluating Stereo Soundfields using Time Selective Response

Check out our pre-written Stereo Soundfield Parameters sequence

Try time selective response for yourself! This complete sequence measures parameters such as Interaural level difference (ILD) and interaural cross-correlation (IACC) using a Head and Torso Simulator (HATS). The spectrum is measured at each ear and calculates interaural level differences (ILD), spectral balance, delay differences, and interaural cross-correlation (IACC). The measurements are made using different time windows, so that the direct, early, late or total parts are separated.

Video Script:

Time Selective Response (TSR) has been part of SoundCheck since 2001. It’s frequently used to make reflection-free measurements of loudspeakers in ordinary rooms.

But did you know it can also be used to evaluate localization and envelopment of stereo sound systems? Let’s listen to a full stereo music excerpt in which the locations of the instruments are easily heard. For the best listening, use either stereo headphones or a pair of speakers spaced apart.

(Audio: Full stereo example.) 

The sound of the band is well spread from left to right, and the soloist is clearly in the center. Next, listen to the same music, where the sound of the band is concentrated in the middle:

(Audio: Narrow stereo example.) 

Using TSR and simple post-processing steps, we can measure the Interaural Level Differences (ILDs), which are strongly related to stereo spread and localization.

Now listen to the same music as it might sound in a concert hall where you are enveloped by the reflections and reverberation of the hall:

(Audio: Full stereo with reverb example.) 

Using TSR and some advanced post-processing steps, we can measure the Interaural Cross Correlation (IACC), which is strongly related to envelopment or a sense or spaciousness.

The SoundCheck measurements start with a log sweep played first to the left channel, (Audio: TSR sweep L , under)

…then to the right channel, (Audio: TSR sweep R , under)

…and finally, to both channels. (Audio: TSR sweep L and R , under)

For each sweep, the frequency responses from the HATS ears are analyzed using Time Selective Response. These frequency responses are used to calculate the Interaural Level Differences, or ILDs.

The uncorrelated ILDs indicate the ability of the sound system to produce directional effects. ILD’s are mostly related to the perceived speaker locations.

The direct ILDs without reflections, shown in the dotted curves, are the most important. The large difference indicates very good localization. The solid lines, including all reflections, indicate that the room tends to reduce the direct localization effect a little. 

The frequency responses are again used to calculate the Interaural Cross Correlation (IACC).

IACC is most important above 500Hz.

The yellow curve is based on the direct sound when both speakers are playing the same signal. It is close to 1, indicating central location and little sense of envelopment.

The green curve is based on the direct responses, without reflections, when the speakers are playing different sounds. It is between 0 and 1, indicating some envelopment.

The blue curve is based on the total responses, including reflections. It is between 0 and 1, but a bit lower, indicating additional envelopment provided by the room.

Using TSR analysis, SoundCheck can measure Interaural Cross Correlation (IACC) and Interaural Level Difference (ILD). When interpreted together, they objectively describe many of the spatial characteristics of sound systems. The pre-written sequence “Stereo Soundfield Parameters,” available from Listen, puts all this and more together in an easy-to-use package. Check out the Listen website to learn more.