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Home Theater Handbook

Selecting, interconnecting, placing and adjusting home theater speaker questions

Part 1


See also: all questions, part 2

Why selecting loudspeakers is a difficult task?

One of the main challenges in setting up a home theater is choosing loudspeakers. This task is complicated by the incredible variety of loudspeaker brands, models, designs and prices. If you think that spending more money guarantees a better loudspeaker, you are making a big mistake. Audio experts agree that only a small percentage of loudspeakers are worth buying and there is often little relationship between performance and price. Within a price range the majority of loudspeakers are underachievers, the rest are good performers and only a select few are star performers.

What criteria should be considered before selecting loudspeakers?

The decisions to be made before selecting loudspeakers are their price range and their physical and technical characteristics.

Loudspeakers must be matched to the sound levels desired and the size of the room they will play in. Use the Listen® software, to find appropriate loudspeakers based on price, listening preferences and room's dimensions.

Why are a speaker’s sensitivity and impedance important?

Loudspeakers must be matched to the A/V receiver or power amplifier they will be connected to. A powerful receiver may be able to drive less efficient loudspeakers; a less powerful must be connected to sensitive models. A speaker's impedance is also very important, as it affects the current demands on the amplifier. Low impedance loudspeakers draw more electrical current, which makes them difficult loads for amplifiers.

Why use reference program materials for speaker evaluation?

Once the appropriate size and type of loudspeaker is determined, it is time to choose the particular model that meets the criteria. A common place to start searching for loudspeakers is by reading the reviews of reputable home theater and hi-fi magazines. You may also consult with reputable stores to narrow down your list. Once you create a short list of likely candidates, select the reference audio material you will use to audition the loudspeakers. Never buy loudspeakers without listening to them first, no matter how rave their reviews are! For your convenience, we have included a list of suggested CDs, DVDs, SACDs and DVD-Audio reference program material at the end of the book, perfect for evaluating loudspeakers and home theater components.

Why is a decibel meter useful when auditioning loudspeakers?

Any reputable store will allow you all the time you need to audition loudspeakers, to play a variety of material and to answer any questions you may have. If they do not, chose another store. Audition all candidate loudspeakers first, before making your selection. If the speakers are in different stores, keep notes and compare with the same reference material. We suggest you carry with you a portable decibel meter, so that all of your comparisons are performed at identical sound levels. Auditioning loudspeakers at different sound levels may influence your judgment. Choose a loudspeaker with a balanced performance. Overwhelming bass and bright highs may be impressive in the short term, but will fatigue any listener after a while. Remember, don't rush your decision!

How many speaker outputs do typical A/V amplifiers have?

Speakers are connected via speaker cable with the speaker output connectors found at the back of amplifiers. AV amplifiers have at least five outputs for the main front, center and surround speakers and a subwoofer output. New multichannel audio formats specify seven or more speaker outputs. In addition to speakers used for multichannel audio reproduction, many amplifiers have outputs for additional pairs of main speakers to be used in a multi-room installation.

How are speakers connected to an amplifier?

Connecting speakers to the amplifier is a fairly simple procedure as long as we pay attention to a few points. We must make sure that the speaker is connected to the appropriate amplifier speaker binding posts. For example we must make sure that the center speaker is connected to the amplifier binding posts for the center speaker and not some other speaker.

When is a speaker connected out of phase?

Each speaker is connected to two binding posts, one positive and one negative. Each speaker has input binding posts marked positive (+) and negative (-). If the speaker inputs are inversely connected, in other words the positive input is connected to the negative binding post of the amplifier and vice versa, the speaker will be out of phase.

When can we use bi-wiring to connect speakers?

Quality loudspeakers allow bi-wiring. They have separate binding posts for high and low frequencies. When single wiring is used, the two separate binding posts are connected with a high quality metal bridge. If the speakers support bi-wiring, it is recommended that we use special double wiring cable to connect it to the amplifier, as this will improve the performance.

What happens when the cable resistance becomes too high?

Speaker cables carry power signals. The resistance of the cable affects the signal. The resistance is proportional to the length of the cable. For very long lengths of cable with small diameter, the resistance can become excessive, which affects the system's performance. This is because excessive resistance on the speaker outputs of the amplifier results in loss of appropriate control of the speaker (electrical damping).

How is the resistance of a cable reduced?

Cable resistance is inversely proportional to its conductor's diameter. Thicker cables have thicker conductors and therefore have smaller resistance, for a given length of cable. It is therefore wise to use the largest diameter high quality speaker cable we can afford, to make our speaker connections.

What is the impedance of the cable?

Resistance however is only one of the characteristics of a cable which can influence its performance. The others are capacitance and inductance. The combination of these characteristics defines the specific impedance of a cable. The specific impedance of the cable influences any signal that travels through it and consequently influences the sound of any system. Speaker cable selection is therefore very important and should not be overlooked when selecting any entertainment system's accessories.

What is a factor which affects the overall multichannel system performance?

The popularity of Home Theaters brought speakers, their roles and their placement to center stage. Multichannel systems require at least five speakers and a subwoofer to operate to their full potential and each speaker's character and placement affects the overall system performance.

What are the basic rules about speaker placement?

Ideally the five speakers are equidistant from the listener and generate identical sound levels when fed an identical signal. In real life the speakers may not be equidistant; time delays must be employed, ensuring that sound from all speakers arrives to the listener's ears at the same time. The sound field must be homogeneous and controlled. It is desirable that no secondary reflections reach the listener in the initial 15 to 20 msec after the primary sound. Speakers should be isolated from the floor with elastic couplings or spikes, to minimize sound wave transmission through the frame of the building.

What helps minimize the room's standing waves?

Speaker installation begins by placing the main speakers at the desired locations, remembering that the speakers must be symmetrically placed with reference to the axis that connects the center speaker with the listening position. An excellent rule of thumb for main speaker placement is the rule of the odd numbers, which helps to minimize standing waves. We divide the length of the room by an odd number (three, five, seven, etc.) and we place the front speakers at a distance from the back wall equal to the result of the division. We do the same for the width.

Should the main speakers be placed close to the walls?

Slightly angling the main speakers towards the listener usually improves audio imaging and sound focus. Placing the speakers close to the side walls will increase bass which may be either good or bad, depending on the speaker's bass response and the acoustics of the room. Since main speakers usually have adequate bass response it is better to avoid placement close to the side walls to minimize boundary effects on the sound. Reflections for the main speakers are unwanted, in contrast to surround speakers.

How far between them should the main speakers be?

Place the main speakers slightly in front of the TV or video display screen, at an angle toward the listener. The distance between the main speakers is important for the creation of the soundstage. If the speakers are too far apart, the soundstage may not be cohesive. If the speakers are too close together the audio image will not be wide. As a rule the main speakers should form an angle of 60° with the listener. If the speakers are not floor standing, they should be on stands, so that the tweeters are at the ear level of a seated listener.

Why is the location of the center speaker critical?

Center speakers are ideally placed just above or just below our video display. If we are projecting on a video screen which is acoustically transparent, we can place the center speaker behind the screen just like the movie theaters. The location of the center speaker is critical for the localization of dialogs, which must be anchored to the screen for better realism. As the center speaker is usually placed in a shelf, make sure that it is flush mount to the front, to avoid unwanted diffractions. The center speaker's facade should also be aligned with the TV screen, to avoid any diffractions. For cohesive and seamless frontal sound stage, avoid a height difference greater than 75 cm between the center and main speakers. Tilting the center speaker slightly to point to the listener will improve its sound as the ears will be on axis with the tweeter.

What is the ideal subwoofer placement position?

The ideal placement position of the subwoofer is very difficult to locate, as bass interacts with the room and is susceptible to standing wave generation. Placing the subwoofer is a trial and error matter, but is worth the effort. A properly placed subwoofer gives clean, tight bass, without excessive boom. Bass sounds should not be localized as coming from the subwoofer. In general the subwoofer should never be placed at a corner or the center of the room, as these locations are prone to room standing wave creation. Also avoid placing the subwoofer equidistant from any two walls as this also creates standing waves in the room. Placing the subwoofer close to the listener transfers more of the punch of the sound effects. If your room is large enough use more than one subwoofers.

Where the surround speakers should be placed?

The surround speaker placement depends on the audio format to be reproduced and the types of speakers to be used. Dipoles create a diffuse sound ideal for surround effects and are preferred for THX certified systems. Dipoles should be placed directly to the sides of the listeners. Regular loudspeakers should be placed so that the sound bounces off the walls before reaching the audience, to mimic the array of speakers found in movie theaters. For Dolby Digital and DTS, surround speakers should be placed 60 cm to 90 cm above the seated listener and to the side or slightly to the rear (up to 30°). For Dolby Digital EX and DTS ES the rear surround speakers should be placed directly behind the audience and close to each other.

How the speaker level is best adjusted?

Multichannel audio formats specify at least five audio channels for sound production. In any such system the loudspeaker placement position and the particular loudspeaker sensitivity has an effect on the sound level it produces. In a properly set up system the sound level produced by any loudspeaker for a given signal, should be equal. This creates the need to adjust the level of each channel separately, so that the speakers reproducing pink noise play equally loud. The adjustment is best performed with a decibel meter. If we do not have a decibel meter, we must depend on our hearing, which can be fairly accurate if we train it a little. We will also need a pink noise source provided either by an integrated tone generator in the AV amplifier or from a home theater setup DVD.

How does the intensity of pink noise change as the frequency rises?

Pink noise contains all frequencies from 20 Hz to 20 kHz with a random distribution of strength and a level that is reduced 3dB per octave. In other words the intensity of pink noise at 2 kHz is 3 dB below 1 kHz and at 4 kHz 6 dB below 1 kHz. We use pink noise for speaker level adjustment because the distribution of signal intensity is similar to music, where the information of the lower frequencies is greater than that of higher frequencies.




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