Øyvind Kvålsvoll 20.08.2015 Download PDF
How to set up a Home Theater Sound System
26.03.2014: Minor updates for Room acoustics.
06.01.2015: Info for new immersive sound formats Dolby Atmos, Auro3D, DTS:X.
20.08.2015: Revised version. Updated content, more images, more on acoustics.
You need a source, to play your movies from, an AV-Receiver to process and amplify the sound, and speakers. Of course you also need some sort of screen for the picture as well, but that is not covered here.
The speakers are the most important and demanding to choose. Also, there are so many of them in a full 7.1 system, budget-wise they will be the most expensive parts by far.
You can choose to start with a minimum system, and add more speakers later. Center speaker and surround back speakers are good candidates for future upgrades.
|Item||What it does||How it affects sound|
|Source: Some sort of player or computer||Play movies and other content||Only important for functionality|
|AV-Receiver||Processes the sound and drives the speakers||Set-up options and amplifier quality will affect sound|
|Option: Power amplifiers||Drives the speakers||Amplifier quality will affect sound|
|Loudspeakers||Makes the sound||The most important for sound quality|
|Loudspeakers in the home theater|
|L||Front left main speaker.|
|R||Front right main speaker.|
|SL||Surround left speaker.|
|SR||Surround right speaker.|
|SBL||Surround back left speaker.|
|SBR||Surround back right speaker.|
|LTF, RTF, LTR, RTR||Ceiling speakers left top front, right top front, left top rear, right top rear.|
|Sub||Subwoofer, one or several, plays the lowest frequencies that gives the sound weight and impact.|
The simplest and least expensive is to use a Blu-ray player to play your movies. The Blu-ray player will not affect the sound quality, it only reads the disc and sends an untouched digital data stream to the AV-receiver.
If you have some knowledge of computers, you can build a HTPC - a media-player computer, capable of playing Blu-ray discs, movies from a hard drive and also stream content from the net. The HTPC also has no influence on sound quality, as it just passes the digital signal to the AV-receiver, provided the sound-card has HD-audio pass-through on HDMI.
The HTPC is superior to the Blu-ray player when it comes to versatility and functionality.
Regardless what you choose, the source should not have any significance for the sound. Provided your player or computer works properly.
Other sources, such as media streamers and old-fashioned tv can also be connected to the AV-Receiver, so you can watch tv-shows and other content with high quality sound. Some streaming services now provide HD-quality.
As the format of media content is constantly changing, this is a part of your system that is likely to need replacement later on.
The AV-Receiver controls the sound. It takes the signal from the source player and processes it to finally put out sound in all individual speakers.
A typical AV-Receiver has 5 or 7 power amplifiers for speakers, processing capability for all digital sound formats including Dolby True-HD and DTS HD-MA, and automatic calibration and room correction setup.
Receivers from the major brands all get you a lot for the money, and even the smaller budget-models will give good performance. The differences among brands and price range will be functionality, amplifier power, room correction capability, and connections.
Budget AVR for media room:
- 5.1 will be sufficient for 5.1 speaker system
- Typical 50-100W/channel
- HDMI input and output
- Look for ability to set individual crossover for front and surround
AVR for the enthusiast:
- 7.1 (or more), consider new formats like Dolby Atmos, Auro3D, DTS:X
- Typical 100-150W/channel
- HDMI input and output
- Look for ability to set individual crossover for front and surround
- Pre-out connectors for option to add external amplifiers
In 2014 new sound formats promising a more immersive sound experience were announced form several vendors. Receivers with Dolby Atmos decoding was released late 2014, Auro3D is available, and DTS has announced their DTS:X.
The introduction of ceiling speakers and new sound formats capable of rendering the sound for the actual speaker layout in use during playback promises a better and more immersive sound experience, where sound sources appear to have exact position not only left-right, fore-aft, but also in height.
This requires adding speakers in the ceiling, and even though you choose not to mount them right now, it could be wise to prepare for later installation by having wires threaded and mounting brackets in place.
Object-based audio means that individual sound objects can be placed anywhere in a 3-dimensional space surrounding the listeners. This makes it much faster and easier to create sound effects that fly around from different locations, the sound designer need only to consider the location of the sound, independent of sound channels and speaker layout.
When the object-based soundtrack is finished, it is then rendered according to the actual speaker configuration, such as 7.1. Many soundtracks for later film releases are made this way, and they often excel with a rich, immersive surround sound-field.
The new thing about new formats like Dolby Atmos is that the rendering from sound object to speaker channels is done during playback, according to the exact speaker configuration in use, making it much more adaptable and flexible.
Those new formats can give improved surround sound-field presentation, due to the added speakers and more information in the soundtrack. But still it is the overall performance of the sound system that is important for the sound experience, and a properly calibrated and set-up system with proper room acoustics and good speakers is the foundation for good sound. A good 7.1 system can already provide a very immersive sound experience.
External power amplifiers for main and surround speakers are only necessary if your AV-receiver has insufficient power. If requirements estimate that more than around 50W on main front channels are needed, it may be worth looking for external amplifiers, to make sure there is enough headroom. Reliability may also be an issue - the typical receiver will quickly run very hot if pushed to the limit, which will increase risk of failure significantly.
Many higher performance subwoofers does not have the amplifier built-in. Typical power requirements are high, 500W-2000W, which means a professional style PA-amplifier is what to get. They are often noisy due to fans used for cooling, so it might be a good idea to place those amplifiers in a different room.
Professional amplifiers is a good alternative for main speakers as well, if more power than around 100W is needed.
The main speakers are the most important for good sound in the theater. The midrange is especially important, for the best reproduction of dialogue.
Dynamics must meet sound pressure requirements, while bass extension below 60 or 80Hz is not needed, because the lower bass is handled by the subwoofers.
Woofer size is a very good indicator for sound pressure capability of a speaker. This is because the output will be displacement limited at the lowest frequencies, 60 - 100Hz, it does not matter what sensitivity and power handling a speaker has if there is not enough capacity to move air, the sound will be compressed and distorted.
Rule-of-thumb for woofer size and output requirements:
|Woofer size||Recommended use|
|6"||6" or smaller are really out of fashion..|
|8"||Small theater >80Hz|
|12", 2x8"||Small theater >60Hz and medium theater >80Hz|
|15", 2x12", or even more..||Medium theater >60Hz and small theater with high fun-factor|
Directivity - how the speaker radiates sound in the 3-dimensional space - should cover all seats, so that all listeners get the same sound, but at the same time limit sound radiated into walls and other boundaries as much as possible.
Reasonably high efficiency is required to reproduce the full dynamics of a movie soundtrack at reference levels. Sensitivity is specified as dB/1m/2.83V, or dB/1m/1W. Higher value for higher efficiency, typical hi-fi speakers are in the 84-88dB range, and can never play loud even with unlimited power put into them because they will reach power compression or simply burn out. Typical usable speakers have sensitivity from 90dB and up, some full-range horn loaded systems more than 100dB.
A sensitivity difference of 6dB means four times power difference - a 90dB speaker requires 400W to play equally loud as a 96dB sensitive speaker with 100W.
Recommendation for main loudspeakers specification:
|Media room or small dedicated cinema main front speaker requirements|
|Woofer size||Minimum 2x 8"/ 1x 12"|
|Directivity||60 degrees (+-30) degrees coverage with linear frequency response|
Speakers can be compared by looking at frequency response and polar charts, and sensitivity. A smooth frequency response in the required range 80Hz - 20KHz is desired. Polar plots show off-axis response, this should be smooth and not fall off at high frequencies inside the angle of the seating area. If the manufacturer can not provide this information, then go elsewhere.
For larger and medium sized rooms it is recommended to add dedicated mid-bass speakers to get more headroom in the important mid-bass and upper bass range, this is where much of the impact and energy will be in both music and movies. Such speakers are typically bass reflex enclosures equipped with one or two 15" or 18" bass drivers.
As already mentioned, subwoofers are very important for the theater sound experience. Frequency response should extend to below 20Hz, and output level must match requirements for chosen listening level in your room.
You need to ensure there is enough output capacity, and have a reasonably flat frequency response.
Practical incarnations of subwoofers tend to have different sound characteristics, due to different priorities in each design, here is an attempt to describe briefly the typical consequences of different approaches:
|Small sealed low-cost||Cheap.||Boomy, no impact and punch. Best to turn it off, as it will not contribute in any positive way to the overall sound experience.|
|Sealed with long-excursion driver||The best for very low frequency extension, small size, easy to design.||Lacking in dynamics and impact, often sounds kind of soft and rounded.|
|Ported with large motor driver||More output capacity, more impact and punch.||Port compression limits low frequency output, larger than sealed.|
|Horn||Great impact and punch, dry and powerful bass, high output capacity.||Limited usable bandwidth, very large, can be affected by resonances.|
|Compact Horn Subwoofer||Great impact and punch, dry and powerful bass, small compared to performance.||Difficult and advanced to design, expensive to build.|
Location of subwoofers significantly affects the response. By placing several subwoofers at different locations it is usually possible to get a reasonably smooth frequency response, across most of the room. Most subwoofers also employ some sort of DSP with equalizer, with manual or in some cases automatic setup.
Two or more subwoofers should be used, and the largest improvements can be seen going from one up to around four - adding even more does not necessarily give improvements comparable to the extra effort.
Relevant subwoofer specifications can be difficult to find, as most hi-fi and home-theater subwoofers are unspecified - it is not possible to see the output capability from the data provided. However, there is often a good correlation between subwoofer size, design type, and output:
|Subwoofer typical performance|
|Type and size||Typical max output at 20Hz (1m/2π)||Suitable for|
|Sealed with 12" driver||100 dB||2 for up to small room with reduced maximum spl (-10dB)|
|Sealed with 18" driver||110 dB||2 for up to small sized room|
|Reflex box with 12" driver||103 dB||2 for up to small sized room|
|Large (200l) reflex box with 18" driver||116 dB||2 for up to medium sized room, 4 of these will be great|
|Horn 500l||124+ dB||1 or 2 for any room, 2 of these will be great|
Requirements for surround speakers are less strict:
- Lower spl requirements (usually located closer to listeners)
- Hearing is less sensitive to faults in frequency response balance for sources from behind
- Mostly ambient and some sound effects comes from the surround channels
In a typical room the surround speakers tend to be located very close to the listeners, and this causes some problems with speaker localization cues - instead of hearing birds in the wood you hear a bird in the speaker right next to your head. The solution to this is to have speakers with a more diffuse sound radiation pattern- wider dispersion, bipole or dipole speakers:
- Normal speaker: Speaker localization worst, less immersive sound image
- Wide: Speaker with wider than usual dispersion, more immersive sound image
- Bipole: Speaker radiating sound forward and backwards, very wide dispersion, diffuse sound image
- Dipole: Radiates sound forward and backward in opposite phase, little or no sound radiates directly to the listener, very diffuse sound with little localization cues
Many surround speakers have selectable configuration for bipole or dipole.
When deciding between a 7.1 or a 5.1 setup, consider placement limitations due to room design.
Recommendation for surround loudspeaker specification:
|Media room or small dedicated cinema surround speaker requirements|
|Woofer size||2x 6.5" or 1x 8"|
The considerations for main speakers are mostly valid for the center as well, though there are some differences:
- For placement under or above screen/TV size limitations can have serious impact on sound
- Wider horizontal dispersion coverage needed to cover wide seating arrangement
- Optionally leave it out and run phantom center
The center speaker will play most of the dialogue on movies and give the impression that the voices comes from the center of the screen, even for those viewers seated to one of the sides.
Phantom center means to leave out the center speaker and route the center channel sound to left and right main speakers. This works very well if you sit in the sweet-spot right in front of the screen, in the middle between the speakers. Obviously this is not possible if more than one person is watching. When seated off-center towards one of the speakers, the center image tends to collapse to the nearest speaker, so that the dialogue seems to come from that speaker only.
Improved phantom image off-axis can be achieved using main speakers with constant directivity - speakers with waveguide or horns. The theory behind this is time - level trading, our hearing uses both sound intensity and time to determine location. The closer speaker will have lower level due to the reduced sound slightly off-axis, causing the farther speaker to be louder, and this compensates for the distance being closer to the nearest speaker. However, this will not create a center image as good as a center speaker. Speakers of reasonable size and construction does not have directivity control across the whole frequency range, and in the lower midrange the time - level trading does not work that well, because the ears use phase and time information to determine where the sound comes from in the midrange.
Many center speakers with horizontal arrangement of the drivers are designed with severely non-linear horizontal off-axis response. Such designs should be avoided. To ensure the center is creating a seamless sound image across all seats, the horizontal dispersion should be reasonably linear, and this can be determined from off-axis frequency response charts, or polar plots.
A center channel with carefully designed radiation pattern: Off-axis and in-room listening position measurements show smooth and very similar response across whole listening area, while sound drops fast further off-axis for reduced interference from early reflections
Use the same spl requirements as for other surround speakers, wide radiation pattern is desired.