The Importance of Professional design and planning for home theaters (part II)

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There are a couple key issues we're concerned about with room acoustics: (1) sound transmission in and out of the room; and, (2) sound reproduction quality in the room.

Sound Transmission.

From the homeowner's perspective, the desire is to eliminate the transmission of sound from the theater venue into the remainder of the residence. From the theater designer's perspective, the focus is preventing sounds from outside the theater from entering the room.

Neither of these objectives is mutually exclusive. The reality of the situation, however, is you cannot completely eliminate sound transmission without extraordinary efforts and, in most cases, significant cost. So it becomes a balancing act. Another point, which needs be made, is those things that affect sound transmission have little effect on room acoustics and vice versa. Just because you have succeeded in isolating the room, does not mean you have good acoustics in the room.

None-the-less, considerable strides can be made in room isolation if the theater designer becomes involved in the project before framing is started -- most certainly before sheet rock or dry wall is applied.

Common means to reduce sound transmission include staggered framing, double sheet rock and profuse use of insulation materials.

But these techniques alone will not totally solve the matter, and, indeed, could all be for naught if there is no attention to the details. Details such caulking, sealing electrical outlets and attention to things such as the HVAC system.

As a general concept, if the room is air tight, you can solve the problem of sound flanking, but this is not the total solution.

Some designers will suggest the use of a product called resilient channel. While this can provide a means of sound isolation, if used under the wrong circumstance, it will also kill bass response in the room. The use of resilient channel may help smooth out bass response; but, there’s no way to accurately predict its impact…and that impact can be positive or negative. More problematic is it’s affect on the room cannot be predicted (including the wall’s resonance frequency) and, if one guesses wrong, the only solution is to rebuild the wall. A poor choice at best. Even with the sheet rock, the devil is in the details.

Sheet rock must be "screwed and glued" to framing members. Why? Rattles!

In the end, there are far, far better ways to smooth out bass response in a predictable manner with materials and

constructs that will already be in your well designed room. Why are we concerned about noise from outside the theater? It has to do with the "noise floor". Imagine our swimming pool. If the pool is empty, you can be at the lowest level of the pool and still have plenty of air. As the pool begins to fill, you must position yourself higher and higher in the pool.

In a theater, as noise begins to 'fill' the room, you must turn up the volume more and more in order to hear the dialog. Further, low level sounds from outside the room can drown out, or hide, low volume sounds from the film, create a loss of audible clues and make the dialog difficult to understand.

See if you've ever found yourself in this situation. When someone in the movie begins to whisper, you turn up the volume to hear what is being said. Later, when the train wrecks, the tornado hits, or the T-Rex runs, you're running to turn the volume down. Been there? Done that?

That's a result of bad sound isolation, bad acoustics or both. In a properly calibrated theater, you should never need to "touch the dial".

Room Acoustics.

Here is where the real challenge begins. You can put some incredibly expensive, high-end speakers in any given room and they'll sound horrible. Conversely, you can install moderate priced speakers in a properly treated room, and have them sound delightful. The single most important component in the sound reproduction chain is the room itself! It is not the speakers, the amplifier, sound processor, or $10,000 exotic speaker cables that make the difference. It's the room.

Sadly, it is not uncommon to see someone put $16,000 speakers in a room and fail to spend $3,000 to treat the room acoustics.

Ever heard this line: “If you upgrade to these really awesome speakers, your room will sound better?”

Nonsense! By far the majority of sound you hear in your room doesn’t come directly from the speakers. Bad room, bad sound and new speakers or equipment won’t fix that.

The acoustic needs of a home theater are significantly different than the needs of a two speaker stereo system.

Indeed, the acoustic needs for any multi-channel playback (music or cinema) are significantly different than the requirements for two channel systems. The reverberation time (called RT60) must be significantly less.

Further, where you want reverberation and do not want reverberation is also different.

The primary objective in a home cinema is to reproduce the sound of the movie as the director wanted the sound heard by the audience. When the movie was made, and the sound mix completed, it was assembled based upon the acoustics typical of a large room or theater - one much larger than the typical home theater venue. The smaller the room, the more 'room modes' affect sound reproduction.

More than 90% of all movies are mixed in one of 12 standard, identical sounding, sound stages.

The affect of room modes can be controlled. In order to do so, attention must be paid to frequency absorption, reverberation (remember RT60?), dispersion, diffusion, speaker placement and seating placement. And, yes, you will require some form of equalization. The moment you hear "equalization is not needed", it's time to find a different designer. And, no, you cannot, absolutely cannot, equalize a room by ear.

But, there is need for caution here. You cannot approach your room design and seating placements from a room mode spreadsheet. These spreadsheets are interesting tools but present several serious problems.

First, a “good” seating position and a “bad” seating position can be separated by as little as 6”…not really practical in real life.

Second, the spreadsheets can determine where a room mode will occur, but cannot speak to its amplitude. You may be attempting to solve an inaudible problem!

Third, the spreadsheets assume a perfectly rectangular room with 100% reflective boundaries. Clearly, not reality.

Our rooms have soffits, bulkheads, prosceniums, columns, raised platforms and stages.

Fourth, it is our belief these programs focus undue attention on one of many, many factors affecting acoustic performance. Concerns including speaker boundary interference response (SBIR) and right tri-corner effects can be far more damaging to your room’s acoustics.

There are six areas that must be achieved with room acoustics: (1) dialog clarity; (2) front sound stage focus at all angles; (3) Diffuse surround sound field; (4) smooth and wide frequency response; (5) wide dynamic range; and (6), no bad seats.

Proper treatment of the room for acoustics does not mean the interior designer and theater designer will be at odds. It may mean compromise but you also don't need to have tall round columns of bass traps in the corners, funny looking beanbags in the corners and strange things hanging from your ceiling either.

In a real life example of compromise, George Lucas (yes, that George Lucas) wanted wood paneling below the chair rail in one of his screening rooms. These nice looking, very reflective surfaces would be exactly where you do not want reflective surfaces.

With a ranch full of acoustic experts, George got his wood panels, and the acoustics of the room were preserved. It was a compromise, but everyone was happy. If you were ever to look closely at the way the panels were installed, you'd see where the acoustic engineers had their way.

THX™ or not THX?

Before we answer that question, we must understand what THX is and what THX is not. The short answer? THX is tantamount to the Good Housekeeping Seal of Approval and Consumer's Reports Check Rated products.

It is indeed much more than that, but there's a quick, simple start.

THX is not a surround format, it is not an amplifier, or a product you can purchase. It is a set of specifications, including quality control requirements, designed to assure the consumer the product they're buying is suitable to the purpose intended.

The THX saga commenced when George Lucas tasked his engineers to establish a series of standards that would assure the movie going public that they would hear, feel and see a movie exactly as the director desired.

Thus, a director could feel confident the effect he wished to convey was indeed conveyed from theater to theater.

Later, in the evolutionary process, the challenge became how to convey that same experience in the smaller room's characteristic of a home theater. After considerable research, trial and error, a series of standards where developed for speakers, amplifiers, sound processors, and related equipment to achieve that goal. Lucas engineers didn't only focus on the minimal performance requirements, but as well on the ergonomics of certain components. In the case of receivers, surround processors, and pre-amplifiers, these specifications translated into a requirement to provide the ability to properly adjust equipment for phase, seating location, speaker size, and the like.

In the case of speakers, the THX specifications not only require proper frequency response and the capability to accurately reproduce the sound track (at appropriate volumes), but also addressed vertical dispersion (horizontal and vertical for professional speakers), lobing, tonal quality between speakers and off axis response.

Now this does not mean that non-THX equipment is of lesser quality or capability. There indeed are some wonderful speakers and electronics that are not THX on the market. But here's what it does mean. In short, it means the designer knows in advance exactly how speakers will perform and that the electronics contain the means to tailor the system to the environment. That facilitates placement and design. Without it, the design becomes more difficult to implement.

Should a non-THX speaker system be used, it means the designer must know exactly, on all planes, how a speaker will perform in order to properly place that speaker in the room. And, if you, the customer, are not an expert in electronics and speaker design, it is your assurance the equipment will reproduce the sound track accurately.

By the way, speakers don't have a clue that the sound is coming from a movie or a symphony. THX speakers are to accurately reproduce the sound and that holds true for music as well as film. So why the debate? It gets back to the room. The acoustic requirements of a room are entirely different for multi-channel sound (music and film) than for two channel stereo reproduction.

Proceed to Home theater design part III

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