Spatial Sound, Surround Sound, and Basic Audio Terms Explained

If you have seen labels such as Spatial, Surround, HRTF, or Headphone EQ Profile and wondered what they change, this guide is for you. It explains the audio terms you will encounter in Equalizer APO - 3D SoundFx, from sound positioning to tonal adjustments.

1. Mono and Stereo: How Many Channels Are There?

An audio channel is an individual stream of sound within a recording or mix.

• Mono has one channel. Playing the same signal through both sides of your headphones usually creates a centered sound.
• Stereo has two channels: left and right. They can carry different content, allowing instruments to spread across the listening space.

For example, a stereo track might place a guitar on the left, a piano on the right, and vocals in the center. That does not mean the track contains separate rear or height channels.

2. Surround Sound: Audio Around the Listener

Surround sound uses multiple channels arranged around the listener, extending beyond left and right.

5.1 usually includes five main channels—front left, front right, center, surround left, and surround right—plus an LFE channel for low-frequency effects. 7.1 includes seven main channels and one LFE channel. The “.1” refers to the LFE channel, rather than the number of physical subwoofers you must own.

In a movie, dialogue might play through the center channel while the sound of a passing car moves between other channels. With physical speakers, each channel plays from its corresponding speaker position.

Virtual surround simulates that arrangement. Software can create virtual speaker positions and render them for the two sides of your headphones. The headphone output still has two channels; processing adds cues that help you perceive where the sources are located. Microsoft explains virtual surround over headphones.

3. Spatial Sound: How It Relates to Surround

Spatial sound, also called spatial audio, is a broad term for sound perceived at locations in space: left, right, front, behind, and, in some systems, above or below.

Surround typically describes the channel arrangement around the listener. Spatial audio describes the sense of location and the techniques used to create it. The concepts overlap: a surround mix can be spatially rendered for headphone listening.

For example, footsteps may seem to come from slightly in front of you or behind you, rather than simply from the left. How clearly you perceive the position depends on the content, headphones, processing, and listener. Microsoft Research describes the cues used to locate sounds.

In Equalizer APO - 3D SoundFx, 3D Audio uses HRTF processing to create a virtual speaker stage over headphones. The Stereo input mode prepares two-channel content for a virtual surround stage. The Surround input mode preserves a compatible incoming multichannel mix for processing. Selecting Surround does not turn a stereo recording into an original multichannel recording.

4. Binaural, HRTF, and SOFA: Creating a Sense of 3D

Binaural audio uses the cues received by each ear to reproduce a sense of position. It can come from a binaural recording or be synthesized through processing.

HRTF stands for Head-Related Transfer Function. It describes how your head, outer ears, and body change a sound before it reaches each ear. A source on your left typically reaches your left ear earlier, with differences in level and frequency content compared with your right ear.

A processor uses these characteristics to simulate a source at a particular position. Because ear shapes differ, the same HRTF can sound natural to one listener and be harder for another to localize.

SOFA, or Spatially Oriented Format for Acoustics, is a format for storing acoustic data, including HRTFs. The app’s SOFA profiles offer different sets of spatial cues to try.

A profile name such as “Personal Fit” does not mean the app has scanned your ears. You choose a profile by listening and comparing how it works for you.

5. 3D Blend, Music Focus, and Loudness Matching

These controls help you adjust and compare the spatial experience:

• 3D Blend mixes the ordinary stereo downmix with the binaural result. At 0%, you hear the dry stereo path; at 100%, you hear the full HRTF result. It controls the effect mix, rather than playback volume.
• Music Focus works with Stereo input. It keeps bass and central elements more focused by reducing their contribution to the virtual surround sources. It does not extract vocals into a separate recording.
• Loudness Matching gradually compensates for loudness differences introduced by spatial processing, making comparisons fairer. Louder audio can make a stronger first impression even when its tonal balance is not your preference.

Music Focus is available only in Stereo mode. Switching to Surround disables it; switching back to Stereo does not automatically enable it again.

6. EQ, Frequency, Hz, and dB: Shaping the Tone

An EQ, or equalizer, boosts or reduces levels in different frequency regions. It mainly changes tonal balance, such as making bass fuller or vocals more prominent.

Frequency is measured in Hz. One kHz equals 1,000 Hz. As a rough guide:

• Bass / Low frequencies give drums and bass instruments their weight.
• Mid frequencies carry much of the information in voices and instruments.
• Treble / High frequencies contribute brightness and crispness.

dB expresses a ratio of signal levels. On an EQ control, a positive value boosts, a negative value cuts, and 0 dB applies no change at that control. The dB value on an EQ slider is not a measurement of the actual sound level at your ears.

Graphic EQ divides the frequency range into adjustable bands. The app offers 16- and 32-band views. The 16-band view provides simpler editing of the same underlying 32-band bank.

A preset is a ready-made EQ curve, such as Warm, Vocal, or Rock. It is a starting point to audition, rather than a requirement for a particular genre. In the app, Preset, Low/Mid/High tone controls, and Graphic EQ are alternative user-EQ modes.

7. Headphone Profiles and Correction: Tuning for Your Model

Headphone correction adjusts the sound based on the characteristics of a headphone model. A Headphone EQ Profile contains correction settings based on measured frequency-response data and a target curve. The app applies those settings to change the headphones’ tonal balance.

Think of a Warm preset as a tonal preference, while a headphone profile attempts to compensate for the characteristics of your headphone model. These are independent layers in Equalizer APO - 3D SoundFx.

Choose the correct model. Results also depend on fit and seal, differences between individual headphones, and personal preference. The Headphones EQ preset is a general tonal curve, distinct from model-specific headphone correction.

8. Crossfeed, Stereo Width, and Room Ambience

Crossfeed sends some of each channel to the opposite ear, often with filtering or delay. It partly simulates how both ears hear both speakers when listening to stereo through loudspeakers.

Stereo Width adjusts how wide or narrow the stereo image feels. Increasing width does not add surround channels or recover new positional information.

Room ambience adds a sense of reflections and room space. Small, Medium, and Hall suggest different spaces; they do not measure or correct your actual room. The Large Hall EQ preset is also different from the Room: Hall reflection effect.

In the app, Crossfeed, Stereo Width, and spatial processing are alternative imaging modes. Selecting one may disable a conflicting mode.

9. Dynamic EQ, Night Mode, Headroom, and Clipping

Dynamic EQ changes its action in individual frequency regions according to the incoming signal, rather than keeping every adjustment fixed like ordinary EQ.

Dynamic range is the difference between quiet and loud parts of the audio. Night Mode helps reduce that difference, making quiet dialogue and loud action scenes easier to follow at a lower listening volume.

Headroom is the margin remaining before a signal reaches its limit. Boosting several EQ bands at once can push the signal beyond that limit. Clipping occurs when peaks exceed the available range and are cut off, often causing distortion.

A limiter reduces large peaks to help control overload. Headroom management and limiting help manage signal levels, but they cannot guarantee clean playback from every device when volume or bass is pushed too far.

10. Virtual Audio Device and Target Speaker: Where the Sound Goes

A virtual audio device is a software audio device. Sound sent to the 3D SoundFx virtual device passes through the app’s processing before playing through a physical output.

Target speaker is the selected physical output, such as USB headphones or speakers. You can picture the path like this:

Source app → 3D SoundFx → EQ / spatial processing → selected headphones or speakers.

Only audio routed through this path receives the effects. An app playing directly to another physical device can bypass the processing.

The dashboard’s output volume controls the playback level of Equalizer APO - 3D SoundFx. The tray’s Speaker Volume sliders control the Windows volume of individual devices. Choosing an output device for each app is also different from having a separate EQ for each app.

Getting Started with Listening Comparisons

1. Choose Stereo or Surround input to match the content you are playing.
2. If you use headphone correction, select the correct headphone model and start with Flat user EQ.
3. Play a familiar passage, try different HRTF profiles, and adjust 3D Blend.
4. Change one setting at a time and compare at similar listening levels.
5. Add a preset or adjust EQ to taste once you understand what each layer changes.

There is no single profile or effect level that suits everyone. The aim is to find comfortable listening that works for your content. See the Equalizer APO - 3D SoundFx feature overview to compare the available controls.