There is a lot of talk at the moment about ‘3D’ audio for cinema. At first you’d be forgiven for thinking we already have 3D audio as we have speakers all around us. Although this is true, there is actually no component of height.
The question is, do we really need 3D sound? 3D picture hasn’t really worked, for instance Sky is discontinuing all its support of 3D sports programming. So does 3D audio actually offer something worth having?
Dolby are making a lot of noise about 3D audio, though there are 3 competing formats, Dolby Atmos , Auro 3D and THX, they all propose height speakers. Dolby Atmos require 2 or more speakers (actually they’d like quite a number more) in the ceiling and Auro 3D which features its additional speaker in the center of the ceiling – known as the ‘Voice of God’.
I have been to a demo of the Auro 3D system, in an excellent environment of a large dedicated home theatre, and was most surprised by what I heard.
But first let’s discuss a little bit about our ears. They are a funny shape, and as a child I did wonder what they were for. The science teacher had explained that your eardrum is way inside your head, safely out of harm’s way, but no explanation was given to our ears beyond making it easier to discern a sound from in front and behind.
What I only learnt later, when studying a degree in acoustics, is that the shape of our ears play a key role in pinpointing where sound comes from. It’s fairly obvious that they aid knowing if a sound comes from in-front or behind, but what’s less well known is their role in identifying the height of a sound. They add a small reflection (distortion) of sound that is characteristically different according to the height of where the sound came from. Each persons ear is a different shape, and this doesn’t really matter, what is key is that the ears shape will add a tiny variation to a sound that alters according to where the sound comes from. This allows our brain to know exactly where a sound comes from, left right, front back, and up or down.
Back to the demo; we sat down in preparation and I was wondering how on earth adding yet more speakers is going to enhance the pleasure of movie going (especially if more speakers means cheaper speakers and hence a worse sound to start with).
The demo started with the sound of birds twittering from all around, and a plane flying overhead, I began to think very well, adds a bit of localisation, a little bit of extra drama, but nothing to really get excited about.
Then there was a demo of a street scene in Amsterdam with a tram going by. First in traditional 5.1 and then with all the height speakers switched on.
This was the great ‘a-ha!’ moment as the sense of reality was extraordinary.
I’ve been to Amsterdam many times (for the big ISE show) and I am very familiar with the sound of the trams and the squeal of metal wheels on the curved rails. With the normal 5.1 Surround set up the sound was familiar and we were seeing the image of the trams, so it all felt recognizable. But, when the height dimension was added, the sound came through with a sense of reality that transported me immediately to Amsterdam, and no mistaking it with a tram from Croydon this time!
With the height speakers we were suddenly receiving the sound reflected off the buildings around and being conveyed the full acoustic environment.
It is another example of just how complex and incredible our hearing is. Everywhere we go, our ears are listening to the direct sounds and then adding the signature of the reverberations.
I think the potential for 3D audio is really huge.
With 3D audio, the movie will be able to fully convey the acoustic character of the space you are viewing; be it inside a car, a café, standing on a street corner in New York, or at the edge of the Grand Canyon. The acoustic contrast of these spaces will really add to the belief that you are in the movie.
I have never been a fan of 3D video, and it has failed because of 2 main problems –
1) You have to wear the special glasses.
2) Headaches and eye-stain from the fake depth of field – with 3D visuals you are fooled into thinking there are objects closer and further away, and so your eyes are focusing closer and further away, but in actual fact the focal plan is always at the same position – at the screen.
3D audio does not have these drawbacks. Your ears can already detect the height, and there is nothing ‘fake’ about the concept.
I don’t believe its just movies that will benefit, consider sporting events for a moment. These have been broadcast in 5.1 for some time now, to capture the audience. Now with 3D audio you will be placed fully within the crowd, and get a real sense of the scale of the arena.
I think the audio engineers will very quickly get to work on this effect, and it will be this that really contributes to the ‘immersion’. Not the tweeting of the birds, nice as it may be.
The next question is how do you go about achieving this at home?
The three main issues are:
1) Speaker positioning :-
The rear speakers now really must be at ear level, and not high up. Then you need the height speakers carefully positioned. To do this correctly you will need more speakers.
2) Acoustics of the room :-
In your dedicated home theatre, the importance of acoustics (particularly ceiling acoustics) will become even more key. The imperative is to reduce the sound of the room itself.
A common mistake is to employ an acoustician to look after the room sound isolation, and expect the cinema to sound terrific. Isolation is only part of the story, as it does not address the internal acoustics at all, it is concerned simply with preventing sound entering or leaving the cinema room.
Attention needs to be paid to speech intelligibility and controlling the bass.
In the ‘Atmos’ cinema room one must go much further, the room must acoustically “disappear”; this is analogous to turning out the lights to watch the movie.
The approach must be well balanced across the full frequency range. Our ears can hear across full 10 octaves of sound. From wavelengths 17m long to less than an inch. This requires many separate analyses and a carefully crafted balance of acoustic design.
To put this into perspective our eyes only see across one octave of light.
3) Don’t do it on the cheap :-
Dolby seem keen to promote the speakers that aim up and reflect the sound off the ceiling. This will never give a satisfactory result. It is also important (as always) to avoid speakers that sound nasty.
If you have a small budget then get a good stereo pair of speakers set up either side of your TV, don’t bother with surround sound at all!
Let me explain further about the Dolby upward firing speakers. If you read the Dolby marketing literature, the idea is that the speaker fires the sound up so that the sound rays hit the ceiling and then radiate down to the listener, appearing to come from above.
I have no doubt that this idea did not emanate from the technical laboratory, but came instead from the marketing dept. keen to make Dolby Atmos more domestically attractive.
The way our ears hear is complex and a 2 stage process. We hear the direct sound, where the sound comes from, then for a further 50-80 milliseconds our brain sums the reflected (reverberated) sound, and considers it part of the same sound. Beyond 80 milliseconds further reverberations of the original sound start to create an unpleasant smearing to the sound and start to be discerned as a discrete echo.
What is most interesting is the fact that the direct sound can be many times weaker than the reverberated sound, yet our brains still perceive the origin of the sound correctly. This is known as the Haas effect.
The upfiring drive unit will be heard as from the speaker not from the ceiling due to the Haas effect.
Take for instance a concert hall, you can still pinpoint the soloists even though 80-90% of the volume of sound your ears hear is actually the mixed reverberations from the room reflections.
Now back to the Dolby Atmos speakers, the sound from a normal speaker mostly radiates in a sphere from the drive unit. It doesn’t actually travel in ‘rays’. Stand to the side of your speaker and you will find that you can hear the music absolutely fine, just a little crispness is lost.
This means you will hear the sound coming directly from the up-firing speaker before you hear its reflection from the ceiling. The way your clever little ears work is that they detect the initial sound and pin-point its origin, then within the 50millis second time window they treat the reflections as cues to the room acoustics, and these reflections will simply be added to the sound, so the ceiling reflection will not convince you of the sound coming from above, but emanating from the side.
So, if you are designing a space dedicated to film & TV, make sure you get the acoustics right and wire for 3D audio!