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Home Theater Handbook

Selecting, interconnecting, placing and adjusting home theater speaker questions

Part 2

See also: all questions, part 1

How do we begin the adjustment of the speaker levels?

We begin the adjustment by placing the decibel meter initially at the center of the room, at the height of our ears. We start reproducing the pink noise and increase the master volume until we reach 80 - 85 dB. We then measure each channel individually. If there are variations, we adjust the level for the specific channel to match the other channels.

What is considered normal audio level variation following channel adjustments?

We aim for a perfect match on the reproduced audio levels, however a small difference of 0.5 to 1 dB between the channels can be considered normal, especially due to the acoustics of the room.

Why is the adjustment of the subwoofer a little different?

The adjustment of the subwoofer level is performed somewhat differently. The test tone for the subwoofer adjustment is an alteration of high and low frequencies of pink noise. The test tone is reproduced by one of the front main speakers and the subwoofer. We set the volume level so that the tone is reproduced at a normal intensity. We measure the level reproduced by the subwoofer and the main speaker and adjust the level of the subwoofer until we get identical readings. We must always remember that bass can have unpredictable behavior in our room. If we have standing waves, there will be spots in the room where the intensity is much louder or lower. In this case we must experiment with the position of the subwoofer until we find a location which minimizes standing waves and produces even bass across the room.

What happens when two speakers are out of phase?

After we connect our speakers, we are ready to check that the polarity of the connections is correct. If we can not manually check each cable to make sure that the connection is correct, we use an input signal of white noise to be reproduced by the speakers. If the polarity is wrong the reproduced sound will not be focused between the speakers, there will be a distinct hole in the image and a lack of bass. We then need to change the polarity on one of the speakers so that the speakers are in phase.

How do we check the phase of the subwoofer?

Checking the phase of the subwoofer requires a different procedure, as low frequency wavelengths are very long and the acoustics of the room impact on the phase. So even though the cables may be connected correctly, the subwoofer may be out of phase with the main speakers. Most active subwoofers have a phase inversion switch. When the subwoofer is in phase the bass is full and tonally rich. If the bass is not tonally balanced with the main speakers then changing the setting of the phase switch should correct the problem.

What is the desired audio image of a multichannel audio system?

The quality of the audio image of a multichannel system is judged by the reproduction of pink noise that moves steadily between the speakers of our system, The desired soundstage is a compact "ball" without any holes or lapses.

What is usually the problem with the audio image?

Usually the audio image created by the main and center speakers is unified and without problems. In contrast the surround image often exhibits a hole in the center or discontinuity, depending on the height of the speakers and their in between distance. In general most problems are detected in the sound movement between the front and rear surround speakers. This is primarily a result of speaker placement and the performance of the multichannel sound formats (Dolby/DTS).

Which is one of the most parameters of the quality of a multichannel system?

One of the most critical parameters of the quality of a multi-channel system is the matching of the center speaker to the other front main speakers, so that they have identical audio characteristics. In the ideal case all three speakers are the same, this is not always the case and adjustments are needed to minimize any differences.

How do we evaluate the center speaker matching with our main speakers?

If we are in the process of choosing a center speaker or of adjusting one to match our main speakers, we must compare its audio response and sound using a pink noise signal. The first step is to match the sound level of the center speaker to one of the two main speakers so that they are identical. It is also best if we place the center and main speakers next to each other. The pink noise reproduced alternatively by the center and main speakers will give an indication of how closely they are matched tonally.

How can we achieve a closer match?

If there are tonal differences we must adjust the equalization of the center and main channels so that we achieve a closer match. Of course there are limits to the correction we can achieve. If the differences are too great we may want to consider replacing the center speaker with one better matched to our main loudspeakers.

Why is the acoustical behavior of a room important?

The acoustical behavior of the Home Theater room is the last link of the reproduction chain between recorded music and our ears. Acoustics is a complicated subject explained by complicated science. The placement of loudspeakers in a room must follow certain rules that take into account the room's acoustics. The use of special material that modifies the acoustical behavior should only be performed by professionals, who have the tools to identify the problems and the experience to solve them.

What is an acoustically "live" room?

Some rooms are acoustically Ā«live"; they exhibit intense reverberation and emphasize high frequencies. Other rooms are acoustically "dead"; they exhibit lifeless sound with no reverberation and reduced high frequencies. Rooms with hard surfaces, such as marble floors, cement walls and large glass surfaces are usually acoustically "live" rooms. Rooms with carpeted floors, drywall construction and porous surfaces are usually acoustically "dead" rooms.

How can we temper the acoustic behavior of a room?

A listening room void of carpets, rugs, drapes and other furniture will behave like a live room. We will experience intense reverberation, with unclear bass and harsh and tiring high frequencies. A listening room full of sound absorbing materials will result in lifeless sound without any of the feeling of live music that is generated by reverberation. Depending therefore on the natural tendency of a room to be acoustically either "dead" or "live", we can temper its behavior by adding or removing furniture, rugs, etc.

How is bass affected by a wall and a corner?

Walls act like mirrors that create reflections of sound. These reflections radiate at lower sound level than the direct sound. They are also out of phase with the direct sound. Reflections are desired for surround sound since they produce a diffused soundstage, but are disastrous to middle and low frequencies due to cancellations in the 200-500 Hz range. On the other hand bass frequencies below 150 Hz are amplified by 6 dB. Bass is also increased near corners by an additional 12 dB. Placing a subwoofer in a corner may increase bass by a total of 18 dB.

When are two speakers in phase?

  • Two speakers that have the same input signal are in phase when their cones move in or out simultaneously. When they are out of phase the cone of one speaker moves in the opposite direction from the other.
  • When two speakers are out of phase, they tend to cancel each other's sound. The stereo image has a hole and the bass is weak.

What is the ideal placement of surround speakers?

  • In contrast to the main and center speakers, the surround speakers aim to generate a diffused soundfield. Their ideal placement is 60 cm to 90 cm above a seated listener and at the same line (or slightly behind) with his ears.
  • Do not aim the surround speakers towards the listeners so as to minimize sound directivity. Consider using dipole speakers, which by nature have a very diffuse sound.

Should speaker cables have the same length?

The length of the speaker influences its electrical characteristics of impedance and inductance and affects the damping of the amplifier. We must therefore try to have identical lengths of speaker cable for each channel, however small differences should not matter.

What cables do you recommend?

It is a fact that if we do not use good quality cables we will not get the best sound out of our system. We suggest that you use the best cable you can afford, such as the products of Monster Cable that we have found to meet or exceed the highest quality standards.

Do speakers require a break in period?

Every new speaker requires a break in period, before it starts to perform at its best. Depending on the speaker this should last between 50 and 150 hours.

What is the relationship of listener distance to the speakers and Sound Pressure Level?

The sound pressure level generated by a loudspeaker at a specific distance is reduced by 6dB when the distance is doubled and increased by 6 dB when the distance is halved.

Why is sensitivity related to the generated Sound Pressure Level?

Sensitivity is measured in dBs of sound generated at a distance of 1 meter from the speaker, when 1 Watt (or 2,83 V) is delivered by the amplifier to the speaker. A more sensitive speaker generates a greater Sound Pressure Level for a given amplifier power output. A speaker with +3 dB sensitivity requires half the amplifier power to produce the same sound level!

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