 |
 |
 |
 |
 |
 |
 |
|
The following article is based on a published White Paper Report written by Dr. Peter D'Antonio President/CEO of RPG Inc. - the worlds leading authority on diffusion technology. | |||||||||
ROOM REFLECTIONS CAUSE ACOUSTIC DISTORTION | |||||||||
 | |||||||||
The audio product created in a studio needs to sound similar in a wide range of listening environments. Thus the audio product must be transferable to these different environments. The engineer must be aware of the acoustic signature of the room in which the audio product is created so that the room’s acoustic signature is not embedded into the audio product. Since we have no control over the acoustical design of the environments into which the audio product will be transferred to, every effort should be made to provide good acoustics in the creation environment so that the necessary image and signal balances can be executed correctly so that countless hours creating the product are not wasted. The recording industry is conscious of electronic distortion, but acoustic distortion is often overlooked in the pursuit of more electronic equipment. Experience the difference When the sound from a loudspeaker encounters the boundaries of a room, a very complex series of reflections occur. It is very difficult to isolate the direct sound alone, because these reflections interact with it and among themselves to produce a wide range of effects, which we call acoustical distortion. If proper acoustic design is not utilized, a room will introduce sonic distortion which prevents the Engineer from hearing all of the detail information the loudspeakers and electronics are capable of delivering. The Acoustic Distortion introduced by the room can be so influential that it dominates the overall sonic impression. The causes of acoustic distortion are:-
When one stops and realises that the speaker / room interface is your acoustical microscope, it seems prudent to strive for the ultimate sonic resolution.
ACOUSTIC DISTORTION Acoustic distortion affects three psychoacoustic perceptions, namely:-
All three forms of distortion occur across the entire frequency spectrum, but room modes and the speaker/boundary interactions predominate at low frequencies. | |||||||||
|
Electronic Equivalent | |||||||||
| |||||||||
|
Imagine secretly inserting a parametric equaliser and digital delay unit into the signal path prior to a critical mix with the circuit set to boost:- 71Hz, 142Hz and 213Hz by 10dB The engineer would mix the sound and subconsciously attempt to adjust for these effects. Imagine the engineers surprise when they were informed of the trick and they played back the mix with the equaliser and delay switched off !! This is NOT a practical joke because the ROOM is playing tricks on the engineer-
| |||||||||
| |||||||||
The first harmonic, however, is at a maximum. It can be inferred from this plot that, in the center of the room all even harmonics are absent and all odd harmonics are at a maximum.
| |||||||||
| |||||||||
|
As the frequency increases, room modes are still present, but their number and density increase and are not perceived as a problem. This form of modal acoustic distortion is best addressed with low frequency absorption. One can easily simulate the effect of room modes by maximizing a parametric equalizer at only specific frequencies associated with the room's dimensions. If the room did not introduce any modification to what is being heard Fig2 would be a flat line! | |||||||||
Speaker-Boundary Interference Response | |||||||||
| |||||||||
|
Room modes develop as reflected sound interferes with itself. This type of acoustic distortion is due to the coherent interference between the direct sound from a loudspeaker and the reflections from the room, in particular the corners immediately surrounding it. | |||||||||
| |||||||||
The low frequency rise illustrates why one can add more bass by moving the speakers into the corners of a room. | |||||||||
COMB FILTERING | |||||||||
| |||||||||
|
Another form of acoustic distortion introduced by room reflections is comb filtering. It is due to interference between the direct sound and a reflected sound. In Professional Control Rooms, we are primarily concerned with the interaction between the direct sound and the first-order (i.e. single-bounce) reflections. Reflections cause a time delays, because the reflected path length between the Engineer and source is longer than the direct sound path. Thus when the direct sound is combined with the reflected sound, we experience notching and peaks referred to as comb filtering. The location of the first notch is given by the speed of sound divided by twice the total path length difference. The spacing between subsequent notches is twice this frequency. For example, a 1ms delay corresponds to a path length difference of 1.13 feet. The first notch occurs at 500Hz, with subsequent notches 1000Hz apart. The audible effect of comb filtering is easy to experience using a delay line and produces various effects such as chorusing or flanging, depending on the length of the delay. Shorter delays have wider bandwidth notches and thus remove more power than longer delays. This is why microsecond and millisecond delays are so audible. The effect of a reflection is illustrated in Fig5 and Fig6. | |||||||||
| |||||||||
|
| |||||||||
|
IMAGING | |||||||||
| |||||||||
|
Imaging refers to our ability to perceive and accurately locate the instruments, voices and effects which comprise the soundstage. The factors affecting localization and the acoustic soundstage are reflections from the room's boundary surfaces which cause comb filtering. These reflections cause frequency response notches and peaks which fool the ear/brain (auditory system) and degrade our ability to experience the sonic images as they were intended to be perceived. Imaging is optimized by an Imaging Acoustic Treatment which absorbs the room's first order reflections over a wide range of frequencies. | |||||||||
|
SPATIAL IMPRESSION | |||||||||
| |||||||||
|
The spatial impression refers to the intangible sensation of openness, envelopment or spaciousness and the feeling that we are part of or immersed in the audio/video event we are experiencing. Good Control Rooms allow us to faithfully recreate an experience. The sense of envelopment is related to the amount of sound reaching the mix position from the sides of the room. The use of sound diffusing surfaces on the rear wall help to naturally create an impression of "passive surround sound".
Thus Diffusive Acoustic Treatment is used to create envelopment, widen the sweet spot, add warmth and naturalness to the sound,
and uniformly distribute the sound throughout the room.
| |||||||||
|
FREQUENCY RESPONSE | |||||||||
| |||||||||
|
The perceived bass response in a room is controlled by:-
Since most of today's speakers are of high quality and we may not have any control over the rooms dimensions, we can focus our attention on proper speaker placement and acoustical treatment. Proper speaker placement can minimize exciting the room's resonant frequencies and the interference between the speaker and reflected sounds from the nearby corner. This can result in severe cancellations if the speaker is located an equal distance from all boundaries. After locating the speakers (and engineer) properly, Bass Absorption should be utilised to reduce the room resonances and minimize the speaker/boundary interference. | |||||||||
|
DAMP the MODES | |||||||||
| |||||||||
|
The magnitude of the room mode effects can be reduced by increasing the damping factor of all the modes by suitable use of low frequency absorbers. The most common are acoustical foam or other porous absorbers which absorb sound by converting sound energy into heat.. The efficiency of a porous absorber is highest when the sound is travelling at its highest velocity. This point is reached at ¼ of the wavelength and thus varies with frequency. At low frequencies like 100Hz, this distance is about 2.5’ from the wall. Unfortunately at the wall surface where the porous absorber is usually placed, the particle velocity is zero. This is where the soundwaves change direction on reflection and hence the velocity is zero. Since porous absorbers rely on particle velocity, they have minimal efficiency at low frequencies. The point to be taken away is that:-
| |||||||||
|
ACOUSTIC TREATMENTS | |||||||||
| |||||||||
Absorption coefficients of many absorbers are measured and quoted for Random Incidence Sound Reflections.
The reflections in a studio, especially from the side-walls, are mostly at grazing angles.
Under these circumstances many materials can appear reflective to the higher frequencies and hence their performance will be below that expected.
| |||||||||
|
| |||||||||
Modal Coupling- the acoustical coupling between the speakers and listener with the room’s modal pressure variations or room modes.
Pound for Pound the acoustical treatment in a room will
Fig2. The (1,0,0), (2,0,0), and (3,0,0) modes are identified.
