 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Home Theater Programming Sources
Off-Air TV
Basic Home Theater Acoustics
You can have all the latest home theater gear, with the best speakers and it can sound
pretty lackluster if you don’t pay attention to basic acoustics. This article will just touch upon
the basics, as entire texts have been written upon the subject of room acoustics. There are
two areas we are concerned with respect to acoustics in a home theater. The two areas are
interior acoustics, or how sound is affected inside the theater and noise transmission.
For a comprehensive guide to sound proofing to make sure the noise from your home
theater stays inside, and noise from outside doesn't ruin the show, The DIY Soundproofing
Guide is an excellent reference. It is comprehensive and can save you from spending
money on sound treatment methods that don't work very well, or worse, making a very
costly mistake.
How sound behaves from when it leaves the speaker to when it reaches your ear can have
a greater effect on the sound than the speaker itself. It is very important design the theater
to minimize negative room interaction with the audio. Arguably the two most important things
to consider are standing waves and what’s known as the “first reflection”.
Standing waves are caused by the relationship of a room dimension, be it height, width, or
length, and a certain frequency. These are known as axial modes because they coincide
with the main axis of the room. Standing waves occur mostly in the bass frequencies in most
normal sized listening rooms. A standing wave is caused by sound bouncing back & forth
between two parallel surfaces. If that frequency’s wavelength, ½ or ¼ wavelength
corresponds to one of the room’s dimensions, a standing wave will be created. This
manifests itself as a cancellation or accentuation of that frequency depending upon your
position in the room. Standing waves are a function of the room’s dimensions and cannot
be eliminated with equalization or room treatments.
Equalization can be used to take energy out of the room at certain frequencies to help
compensate for peaks, but areas of cancellation (nulls) cannot be fixed by adding energy.
Parametric EQs are the best for this application as they can be tuned to the specific
frequency where the problem lies. In addition, the Q, or width, of the frequency band being
adjusted can be altered to change only the specific frequency range desired.
The effects of standing waves can be minimized by careful selection of room dimensions,
speaker position and seating placement. Use room dimensions that are not even multiples
of each other. Obviously this is much easier if you are building the room from scratch. For
example, a room that was 10’ wide and 20’ long would cause more problems than a room
that was 13’ wide and 17’ long. A good ratio of height/width/length is 1:1.3:1.7 there are
many other good ratios. Another good one is 1.2:1.3:1.9. The worst ratio would be 1:1:1, a
perfect cubical room. The problem arises because standing waves can be created in any of
the three dimensions, and a cubical room would have standing wave problems in all 3
dimensions at the same frequency.
Seating is best placed in neither an area where many frequencies are being cancelled or
boosted, where the frequency response is the smoothest. Sitting in a null is a big no-no.
You will have a big dip in the frequency response at the seating location and you will be
unable to correct it.
Speaker placement to minimize standing waves mainly concerns the subwoofer(s) unless
you have full range main speakers. Placing the sub in a corner will energize the maximum
number of room modes and give the smoothest frequency response. It will also add the
most room gain and can over bass the room, making it sound boomy. If this is a problem,
simply move the sub along the wall farther into the room. In most cases avoid placing the
sub directly in the center of the wall, as this could place it in a null and eliminate certain
frequencies. As you try to compensate for this deficiency by increasing the level of the sub
it can become overdriven and distort.
First reflection refers to the sound that is reflected off an adjacent surface before reaching
the listener’s ear. The sound takes two paths to the listener’s ear, the direct path from the
speaker to the ear, and the reflected path from the speaker to the ear. Because the
reflected path is longer, the sound arrives at the listener’s ear a small fraction of a second
later than the direct sound. This causes, among other things, a loss in dialogue
intelligibility. The normal result to this loss is that most listeners will increase the volume.
The increase will exacerbate the problem.
Absorptive acoustic treatment on the wall surface area that creates the first reflection at the
listening area will reduce the first reflection. 1” thick panels will perform this function but 2”
thick panels will work better. A little trick for determining where to place the panels is as
follows. Sit at the listening position. Have someone move a small (4” x 6” or so) mirror along
the wall until the speaker is visible in the mirror. That is the location for the center of the
acoustic panel.
Noise transmission is also very important. You don’t want to disturb other members of the
household or condo complex with noise from explosions, sirens and car crashes. It is also
important to keep the noise floor inside the theater as low as possible. Keeping the noise
floor low will allow you to reproduce the greatest dynamic range.
For example, if the ambient noise inside the theater is reduced to say, 40b and the dynamic
range of a particular soundtrack is 70db you can hear the quietest sounds by setting the
volume to where the softest sounds are reproduced at a volume of just over 40db. If your
theater is capable, you can reproduce the loudest portions of the soundtrack and it will be
just over 110db (pretty loud). If the noise floor in the theater was 55db, you would need to
set the volume such that the softest portions of the soundtrack were louder than 55db. The
loudest parts would now reach 125db. That is extremely loud. Most home theater systems
could not reproduce that decibel level cleanly, if at all. In addition exposure to that volume
would be damaging to your hearing.
The other benefit of having good acoustic isolation is that it helps you do what’s known as
“suspend disbelief”. If you are enjoying a movie and really involved in it, external noises can
distract you and cause you to really “pull out” of your involvement with the story. You are
just not as immersed in it anymore. For great sound isolation solutions, see the DIY
Soundproofing Guide.
pretty lackluster if you don’t pay attention to basic acoustics. This article will just touch upon
the basics, as entire texts have been written upon the subject of room acoustics. There are
two areas we are concerned with respect to acoustics in a home theater. The two areas are
interior acoustics, or how sound is affected inside the theater and noise transmission.
For a comprehensive guide to sound proofing to make sure the noise from your home
theater stays inside, and noise from outside doesn't ruin the show, The DIY Soundproofing
Guide is an excellent reference. It is comprehensive and can save you from spending
money on sound treatment methods that don't work very well, or worse, making a very
costly mistake.
How sound behaves from when it leaves the speaker to when it reaches your ear can have
a greater effect on the sound than the speaker itself. It is very important design the theater
to minimize negative room interaction with the audio. Arguably the two most important things
to consider are standing waves and what’s known as the “first reflection”.
Standing waves are caused by the relationship of a room dimension, be it height, width, or
length, and a certain frequency. These are known as axial modes because they coincide
with the main axis of the room. Standing waves occur mostly in the bass frequencies in most
normal sized listening rooms. A standing wave is caused by sound bouncing back & forth
between two parallel surfaces. If that frequency’s wavelength, ½ or ¼ wavelength
corresponds to one of the room’s dimensions, a standing wave will be created. This
manifests itself as a cancellation or accentuation of that frequency depending upon your
position in the room. Standing waves are a function of the room’s dimensions and cannot
be eliminated with equalization or room treatments.
Equalization can be used to take energy out of the room at certain frequencies to help
compensate for peaks, but areas of cancellation (nulls) cannot be fixed by adding energy.
Parametric EQs are the best for this application as they can be tuned to the specific
frequency where the problem lies. In addition, the Q, or width, of the frequency band being
adjusted can be altered to change only the specific frequency range desired.
The effects of standing waves can be minimized by careful selection of room dimensions,
speaker position and seating placement. Use room dimensions that are not even multiples
of each other. Obviously this is much easier if you are building the room from scratch. For
example, a room that was 10’ wide and 20’ long would cause more problems than a room
that was 13’ wide and 17’ long. A good ratio of height/width/length is 1:1.3:1.7 there are
many other good ratios. Another good one is 1.2:1.3:1.9. The worst ratio would be 1:1:1, a
perfect cubical room. The problem arises because standing waves can be created in any of
the three dimensions, and a cubical room would have standing wave problems in all 3
dimensions at the same frequency.
Seating is best placed in neither an area where many frequencies are being cancelled or
boosted, where the frequency response is the smoothest. Sitting in a null is a big no-no.
You will have a big dip in the frequency response at the seating location and you will be
unable to correct it.
Speaker placement to minimize standing waves mainly concerns the subwoofer(s) unless
you have full range main speakers. Placing the sub in a corner will energize the maximum
number of room modes and give the smoothest frequency response. It will also add the
most room gain and can over bass the room, making it sound boomy. If this is a problem,
simply move the sub along the wall farther into the room. In most cases avoid placing the
sub directly in the center of the wall, as this could place it in a null and eliminate certain
frequencies. As you try to compensate for this deficiency by increasing the level of the sub
it can become overdriven and distort.
First reflection refers to the sound that is reflected off an adjacent surface before reaching
the listener’s ear. The sound takes two paths to the listener’s ear, the direct path from the
speaker to the ear, and the reflected path from the speaker to the ear. Because the
reflected path is longer, the sound arrives at the listener’s ear a small fraction of a second
later than the direct sound. This causes, among other things, a loss in dialogue
intelligibility. The normal result to this loss is that most listeners will increase the volume.
The increase will exacerbate the problem.
Absorptive acoustic treatment on the wall surface area that creates the first reflection at the
listening area will reduce the first reflection. 1” thick panels will perform this function but 2”
thick panels will work better. A little trick for determining where to place the panels is as
follows. Sit at the listening position. Have someone move a small (4” x 6” or so) mirror along
the wall until the speaker is visible in the mirror. That is the location for the center of the
acoustic panel.
Noise transmission is also very important. You don’t want to disturb other members of the
household or condo complex with noise from explosions, sirens and car crashes. It is also
important to keep the noise floor inside the theater as low as possible. Keeping the noise
floor low will allow you to reproduce the greatest dynamic range.
For example, if the ambient noise inside the theater is reduced to say, 40b and the dynamic
range of a particular soundtrack is 70db you can hear the quietest sounds by setting the
volume to where the softest sounds are reproduced at a volume of just over 40db. If your
theater is capable, you can reproduce the loudest portions of the soundtrack and it will be
just over 110db (pretty loud). If the noise floor in the theater was 55db, you would need to
set the volume such that the softest portions of the soundtrack were louder than 55db. The
loudest parts would now reach 125db. That is extremely loud. Most home theater systems
could not reproduce that decibel level cleanly, if at all. In addition exposure to that volume
would be damaging to your hearing.
The other benefit of having good acoustic isolation is that it helps you do what’s known as
“suspend disbelief”. If you are enjoying a movie and really involved in it, external noises can
distract you and cause you to really “pull out” of your involvement with the story. You are
just not as immersed in it anymore. For great sound isolation solutions, see the DIY
Soundproofing Guide.
Great Home Theater Acoustics Resources
Copyright 2004-8 1 touch movie.com
Home theater and Automation Guide
All rights reserved.
Home theater and Automation Guide
All rights reserved.
Basic Home Theater Acoustics
1 Touch Movie.com