Going to the movies is often described as an ‘immersive’ experience, like you are there in the movie, completely involved. This is because commercial cinemas are constructed with speakers that surround the listener, each of which are responsible for a different type of audio such as dialogue, soundtrack and ambient noise.
For decades, enjoying this experience has meant a trip to the local cinema. However, with surround sound systems and widescreen televisions becoming increasingly affordable and accessible, more and more of us are putting on our DIY caps and constructing full-fledged cinemas in home basements and living rooms.
Even a top-of-the-line setup can sound just plain bad if room acoustics are ignored.
This is because every room sounds unique. Without considering room acoustics, having multiple audio channels becomes pointless.
Take the following simple steps to tweak the acoustics in your DIY cinema and you too can skip overpriced popcorn and movie queues by recreating the cinema experience in the comfort of your own living room.
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For this guide, we will focus on the most common home cinema configuration – a 5.1 channel system using five speakers and a subwoofer.
As is often the case with the murky world of acoustics, there is a degree of give and take with everything suggested in this guide. While there are generally accepted best practices for speaker arrangement and acoustic treatment, if the set up just doesn’t sound right, experiment.
How To Properly Configure Your Speakers.
A common configuration of 5.1 surround sound system (Source: digitaltrends)
The centre channel reproduces speech from actors in films. Place the centre channel in line with your listening position and on centre with the screen. This allows the on-screen actors to be lined up with the audio channel, giving you the impression that the dialogue is in fact coming from the mouth of the actor/s.
Try and position the centre channel as close to ear height as your setup allows.
Left and Right Surround Sound Channels.
These two speakers are responsible for film scores and on-screen sound effects. They are best placed 30 degrees to the left and right of the centre channel.
Make sure the left, right and centre channel speakers are equal distance from the listener position for proper front channel blending.
Dual Rear Speakers.
These speakers emit ambient background noise. Position the speakers 110 degrees to the left and right of listening position. Some audiophiles suggest positioning the speaker drivers 30-60 cm higher than ear level to create a sense of spaciousness.
Subwoofer placement isn’t an exact science and requires trial and error. There are a few considerations to take to achieve good subwoofer bass:
Manufacturers often recommend placing your sub in the corner to amplify the bass energy. While this may be beneficial for subs with small drivers, for all other subs it may also amplify the bass too much, resulting in excessive bass energy. If the bass sounds too powerful in the corner, consider another location.
Try different locations in the front third of the room. Keeping the subwoofer in the front third of the room allows it to blend with low frequency sound from the main speakers and minimises localisation effects.
Do the sub crawl. Replace your listening location with the sub. Play some bass heavy music and crawl around on all fours, listening at ear height to the bass response at each point in the room. When you hear the most accurate and balanced sounding bass – that’s a good place for the sub.
If you place the sub against a wall, make sure that it is at least 8 inches into the room from the wall to allow airflow from the port.
Rules Of Speaker Placement
Speakers should be placed at least 60 centre metres away from boundaries, such as large pieces of furniture, walls and especially corners. This will help achieve a good mid-range and mid-bass response
Avoid placing front speakers directly next to your television. Doing so can harm the richness of playback.
Remember that speakers are generally designed so that they sound best when they are level with your ears when you are listening to them.
Mastering Home Cinema Acoustics.
Just as proper speaker configuration is essential to a home theatre – so is acoustic treatment for your room. Most rooms have little or no absorptive materials, meaning echoes from speakers make noise bounce off walls and causing the audio channels to blend together – muddying the soundscape.
Similarly, you don’t want the room to have too much absorption, or the auditory experience will be dull and flat.
As with all listening environments, the key to acoustic tuning is to find the ‘comfort zone’.
Having the right acoustic material is essential to target the frequencies generated by echoes. Acoustic glasswool with a density of 50kg/m3 is perfect for home cinemas. Making your own acoustic panels that are ideal for home cinemas isn’t too difficult. Here are some useful online tutorials:
Mid to high frequency soundwaves from loudspeakers don’t arrive at your ear as one uniform source as you would imagine.
Instead, multidirectional soundwaves reflect off your room’s walls, floor, windows and ceiling. These reflections arrive slightly after direct sound from the speakers. Even a delay of 20 milliseconds can compromise the auditory experience.
How To Identify Reflection Points With A Mirror
A common method to tackle these reflections involves using a mirror, a friend, and a little bit of patience. Sitting in your viewing position, with your speakers in their correct position, have a friend move a mirror parallel along the side wall. Every time you see the reflected image of a loudspeaker, mark this spot. Repeat this process for all five speakers on all four walls marketing every reflection points.
Side walls, Rear Wall and Ceiling.
Treat the reflection points identified on your ceiling, side walls and rear wall with your homemade acoustic panels.
If your room has hardwood floors, or any other hard floor surface, a thick rug padded rug will tackle any floor reflections. Be sure to cover all main reflection points located on the floor.
If any reflection points are located on windows, these can be treated using drapery. Acousticsfreq recommends using highly porous drapery with a 32 ounce fabric weight.
Loudspeakers emit sound omnidirectionally – not only from the front of the speaker – but from the sides and back.
High frequencies tend to be projected only outward from the driver position. Lower frequencies, however, are emitted from all around the speaker. Being responsible for music and sound effects, the front left and right speakers reproduce a wide range of frequencies.
Therefore, a better solution for front walls is at least 100mm of acoustic glasswool to tackle a wider frequency spectrum.
It is common to treat the whole surface of the front wall, however, if this isn’t possible, then a series of thick absorbers placed at speaker height covering the majority of the wall length is sufficient.
Rules of Absorption:
1. It’s a good idea to treat a larger area of the wall or ceiling than precise reflection point that you have identified. This means you can alter your listening position while still enjoying the benefits of minimal reflections.
2. Once you have completed all the previous steps, stand near a speaker and clap. You should not hear a strong echo. Some natural reverberation is desireable.
3. Aim to have absorption on around 60% of your room’s surfaces.
Trap That Bass!
All rooms have a high pressure build up or node in its corners. These nodes often cause distortion at 300 HZ and below, causing unnatural bass.
By placing bass traps in your home cinema you can control the build up of low frequency energy in corners. Place traps of at least 6-inch thickness, at the intersection of floor and wall and ceiling and wall.