Spatial Audio Frequently Asked Questions

These FAQs are part of a growing resource of information we are compiling to help you understand what immersive audio, especially surround sound, is all about and how you should be able to get the best experience from it.

Surround Sound Over Headphones

1) What is binaural audio?

Binaural is a two channel audio format (as is stereo) that is designed to be listened to using headphones. It is intended to provide a more immersive audio experience than traditional stereo recordings. The aim with binaural audio is to provide an experience where sounds are easy to locate and sound externalised (like they are coming from outside your head and not from your headphones).

2) Does it matter which headphones I use?

Headphones significantly affect audio signals in different ways. Ideally the binaural audio should compensate for the effects of the headphones used. However there are so many different types and makes of headphone that an average compensation is often used. The short answer here is just to use the headphones that you have available.

3) Does it only work with headphones?

The immersive effect will only work using headphones, although the sound can still be played through speakers.

Timing and geometry- how we find sound.

4) How does it work?

We only have two ears. When we hear a sound we can work out where it's coming from using only two signals, one from the left ear and the other from the right. Take the example of a sound coming from the left, it will arrive at the left ear slightly before the right. It will also be slightly louder in the left ear than the right. These time and level differences are two of the cues we use to localise a sound source. However, things are not that simple. The shapes of our ears, head and torso have a significant effect on the frequency spectrum of sound as arrives at our ears. All of these factors allow us to perceive the direction from which the sound is coming.

5) What is a Head Related Transfer Function (HRTF)?

It is possible to measure all of the factors mentioned in the answer to question 4. The data from these measurements are called Head Related Transfer Functions. Everyone's head and ears are different shapes, so everyone's HRTFs are different. This is one of the biggest challenges when creating universally effective binaural content.

6) How is a binaural recording made?

There are two main ways of making a binaural recording. The simplest method is to use a dummy head (or put microphones in someone's ears) and place the dummy head in an appropriate location. This method produces results that work well for some people, but poorly for others. This is because a dummy head is designed to be representative of a typical human head shape, so these recordings work best for people with a similar head shape to the dummy. Another way to make a binaural recording is to process the sounds using HRTF measurements (see question 3). The advantage of this method is that the recording can be made once, and rendered multiple times using the HRTF data of different individuals. This can potentially give an accurate sound image for more people.

7) How is the signal processed?

HRTFs are measured in anechoic conditions (free of reverberation), but most people never actually experience anechoic listening conditions. So using anechoic HRTF data to create binaural audio often results in poor localisation and externalisation. Most of the time that we listen to sound in a normal room, we hear a mixture of the direct sound (the sound from the source), and reflected sound (sounds from the source that have bounced off the floor, walls and other surfaces). These reflections help us to perceive the location of a sound source, so binaural audio is more effective when this room response is included in the signal. The final consideration is the headphones. In addition to having differently shaped heads and ears, people use a wide variety of headphones. These different headphones all have a different response, which affects the sound differently, and ideally this should be compensated for too.

Processing of HRTF audio

Speaker based or home Cinema Surround Sound

1) How should I arrange my loudspeakers?

The ideal set up for home cinema is described in a standard called Download ITU-R BS.775. For a 5.1 signal, it is recommended to use 5 speakers arranged around the listening position as shown in the figure below, plus an optional subwoofer to reinforce loud bass effects such as explosions or a passing train.

Surround Sound recommended speaker geometry

Note two things about these speaker positions. Firstly that the rear or surround speakers are ideally positioned almost either side of the listener and more widely separated than the front Left & Right. Secondly you may have noticed that the drawing doesn’t indicate a position for the LFE speaker. This is because the human ears cannot easily detect the direction of sound at low frequencies, such as those produced by a subwoofer. . For more information about the “.1” LFE channel and subwoofers, please refer to Click BBC R&D White Paper 203.

2) What other surround sound formats are there?

As well as the most common 5.1 layout discussed in (1) other layouts are also in use including 7.1 & 9.1 in some more advanced home cinema systems. BBC R&D are working with Japanese broadcaster NHK on their Download 22.2 audio system. Some of these larger systems position some of the speakers above and below the listener as well as at ear height in order to give an even more immersive experience. BBC Audio and Music are also experimenting with 4.0 (which is a kin to the quadraphonic systems of the 1970s). This offers many of the advantages of 5.1 surround sound out but with a lower complexity and cost in both production and reproduction.

3) What should I hear from the front left and front right loudspeakers?

These speakers can operate on their own as a traditional (2 channel) stereo system but are usually used to add the motion and positioning sounds in surround sound. They can also carry some of the dialogue, so that speech becomes spread across the front 3 speakers, helping to create a more enveloping experience when you sit close to the front speakers (and the TV screen) as well as increasing the size of the listening area where good surround sound can be heard (known as the "sweet spot").

4) What should I hear from the rear left and rear right loudspeakers?

Usually the rear speakers are used to create a sense of atmosphere or ambiance, a sense of being in a particular space. Sometimes they are used for positioning sounds or creating movement across the listening position for dramatic effect. They may also be used for voices or action coming from behind but such dramatic uses require the right circumstances and need to be skilfully executed.

5) What should I hear from the Centre loudspeaker?

This speaker is usually used for dialogue in films and TV. It anchors the image in the middle and supports clear speech (especially when it replaces the rearward or downward facing speakers of many modern flat panel TVs). It can also help to create a larger listening position or sweet spot.

6) What should I hear from the subwoofer?

The LFE (or Low Frequency Effects) channel only carries sounds below 120 Hz and should be used only for creating the dramatic effects like explosions and collisions and is often used for dramatic effect in Hollywood films. However some systems use it to help support bass reproduction. Signals in the LFE channel, as well as low frequency content from the other channels, will often be sent to a subwoofer. How much sound comes from the subwoofer depends both on the content of the programme and the configuration and capabilities of your loudspeaker system. For more information about the LFE channel and subwoofers, please refer to Click BBC R&D White Paper 203.

7) What is Bass management?

Many low-cost home cinema systems have small loudspeakers for front left, front right, centre, rear left and rear right speaker positions. Although this small size can make it easier to arrange them in the living room it also means that they don’t reproduce low frequencies very well. To overcome this it is not uncommon for the subwoofer to be used a bass speaker for the other 5 speakers. While this does improve the low frequency response (bass) the sound that would normally come from the other 5 speakers now comes from the subwoofer, which is, of course, in a different position. For more information about the LFE channel and subwoofers, please refer to Click BBC R&D White Paper 203.

8) What does Dolby Digital do?

Click Dolby Digital and other technologies from Dolby are used to code, control and deliver surround sound signals so they can be presented in the home so it can sound good without using excessive data bandwidth. Dolby’s technology is used on DVDs, in cinemas and by broadcasters (especially with HDTV) and is built into virtually all DVD and home cinema systems. There are other technologies that can achieve this too, for example DTS (Digital Theatre Systems), AAC (Advanced Audio coding) and MPEG Surround.

9) What is MPEG Surround?

This is the technology that was used for the recent surround sound experiments by Click Radio 3 and Click Radio 4. It is a bandwidth efficient technique for delivering surround sound developed by Fraunhofer. It has the important advantage of being compatible with standard stereo players (to produce stereo) without the need for modification or upgrading.

More information about surround sound and its challenges can be found on the Click Radio blog.