ROOM ACOUSTICS

Reflections

 

A reflection is a sound that has bounced off one or more surfaces in its path from a speaker to a listener. We all know that speakers, even directional ones, do not radiate sound on a laser line directly to our ears. Speakers fire sound out in an infinite number of directions. Reflections occur in every room, whether it is large or small. So, unless an audio system is outdoors, its sound will be affected by reflections. If an acoustic signal is riddled with reflections, we cannot hear the original, directed pure sound. 

 

Some of the sound radiated by a speaker does go straight from the speaker to our ears, but a lot of it bounces off of some surface (wall, ceiling, floor, console) first. Depending on the time of delay of the reflected audio signal, it will influence our perception of the sound. Long delays introduce the echoing effect, and short delays introduce fuzziness to the directed sound source. These are not desirable artefacts. When our ears combine all this reflected sound with the small amount of sound that comes straight from a speaker, the result is potential acoustic distortion. Some reflections at the right frequency range and with the right time delay provide an enhancement to the overall sound. To achieve this is difficult, and requires complex analysis of the sound field within the room. To adjust the reflective sound fields within a room, we introduce room acoustic panelling that influences the reflective nature of the walls, ceilings and floor. 

 

So you've just installed a swank new AV/Audio system in your prized room. It sounded better in the shop than in your house. Why? What most people do not realise is that room acoustics plays a significant role in the overall performance of the AV system in that room. 

 

Understanding the complex relationships of modal sound fields in a room is beyond the scope of most of us. This is a difficult field of research. However, through well-designed workflows, we at Support Technology have the experience and knowledge to analyse room sound field disparities, correct them, and enhance the final listening experience. This is not a black art, but is based on real science.

 

The plain truth is that acoustics largely determine the perceived sound quality of an audio monitoring system in a project room – by 50% or more in most cases. On the surface, the contribution of acoustics may not be as easy to understand as the contribution of some new piece of electronic wizardry, but the fact remains that the typical audio monitoring system in a project room can be improved more by the implementation of acoustic treatments than by the addition of any piece of electronic equipment.

Knowing that room acoustics is important wouldn’t do us much good if there were nothing we could do about it. Fortunately for us, acoustic problems in rooms are reasonably fixable! 

 

Before we can learn how to fix room acoustics, we must first understand a little bit about what makes acoustics tick. Acoustics can be thought of as the interface between a speaker and a listener. The elements of an acoustic interface are speakers, air, reflections, and listeners’ ears. 

 

Acoustic treatments, as we will discover in greater detail, interact with things called reflections, flutter echoes, reverberation, and standing waves, which, if untreated, reduce clarity and articulation, confuse sound localization, collapsed soundstages, and shift tonal balance. Therefore, a good acoustic treatment product will enhance clarity, articulation and localization; open soundstages; and restore an even tonal balance.  

 

When considering room acoustics, there are 6 major areas to consider:

 

  • Wall, floor and ceiling reflections

  • Wall, floor and ceiling absorption

  • Wall, floor and ceiling diffusion

  • Flutter Echoes

  • Reflection Decay Time

  • Standing wave formations

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Absorbers

 

All room surfaces absorb sound. The degree of absorption is based on room materials. For example, hard surfaces absorb little and reflect most acoustic energy. Softer surfaces, for example, plasterboard, will absorb some frequencies at different audio levels, and reflect the rest. One way to minimise the detrimental effects of reflections is to absorb them using acoustic treatments that are, remarkably, called absorbers. Absorbers come in many different forms. They operate like acoustic vacuum cleaners that suck in sound energy and convert it into heat energy through a resistive process. Little, if any, sound is reflected off an absorber.

 

The effectiveness of an absorber is determined by its thickness, which, contrary to popular opinion, mainly affects the range of sound absorbed, not how much sound is absorbed. For example, a 1” (25mm) think absorber absorbs sound over a range from 1,000 Hz to 20,000 Hz, a 2” (50mm) thick absorber from 500 Hz to 20,000 Hz, and a 4” (100mm) thick absorber from 250 Hz to 20,000 Hz.

 

Naturally, if we want to absorb as many reflections as possible, the thicker an absorber is, the better. Unfortunately, to absorb reflections over the entire range of audible sound, an absorber would have to be 64” (1,600mm) thick! In the world of studios, (or Home Theatres as illustrated below) 4” (100mm) thick absorbers provide the best compromise between range of absorption and practicality.

 

Diffusers

 

A diffusive surface will spray an incoming audio signal in many directions. The outgoing diffused audio signal amplitude will be reduced on the outgoing path. Again, by using acoustic treatments, we are able to diffuse reflective signals in a way that improves the overall sound field. Diffusers come in many different forms. Diffusers control reflections by breaking them up into many “little” reflections that bounce around a room randomly rather than combining with direct sound at our ears and causing acoustic distortion. Like absorbers, diffusers only perform their magic over a certain range of frequencies which is – you guessed it – determined by the depth of the diffuser (among other things).

 

There is a rhyme and reason to using a blend of absorption and diffusion in a project room (or Home Theatre). A general rule of thumb is to implement a blend of 50% absorption and 50% diffusion. If too much absorption is applied, the resulting sonic character of a room is too “dry” and “dead.” On the other hand, too much diffusion can spray an overabundance of little reflections around a room and confuse soundstaging. Normally, we would add diffuser panels to the rear wall of your listening area.

 

Flutter Echoes

 

In addition to low, mid, and high frequency single point reflections, we must control pesky things called slap or flutter echoes. Flutter echoes occur when sound bounces back and forth between two large, flat, parallel surfaces. In rooms, we call these surfaces walls. Like reflections, which are close relatives, flutter echoes reduce clarity and articulation, confuse sound localization, collapse soundstages, shift tonal balance, and lead to bright sound with a characteristic “zingy” quality. Absorbers and Diffusers are very efficient over the range of sound where flutter echoes develop, so the Absorbers and Diffusers can be effectively employed to control flutters echoes.

 

Reflection Decay Time

 

Reflection decay time is another acoustic phenomenon that we must control in a project room. After a period of time, the reflections in a room that are not absorbed combine to create an ambiguous wash of decaying sound. The time that is required for this wash of sound to decay to a certain level is called the reflection decay time of a room.

 

Reflection decay time is very important. If the time window is too long, clarity and articulation will be reduced, sound localization will be confused, and stereo separation will suffer. Extensive research has been done to determine the proper level and time window for reflection decay time. This research shows that most people prefer a time window of about 0.2 to 0.4 seconds in a room the size of a typical project studio.

 

In large rooms, reflection decay time is called reverberation time, which is a statistically random sound field with no particular time or direction component. Small rooms are not big enough to exhibit true reverberation because the reflections die out before they reach their fully random modal character.

 

Reflection decay time is largely determined by the percentage of surface area in a room that is covered with absorptive material. Rooms with little or no absorption will have time windows that are too long. Again, using the correct combination of acoustic treatments, we can prevent the bounce-back effect having any significant consequence on our sound field.

 

Standing Waves

 

A standing wave formation is the consequence of two audio waves travelling in opposite directions and interfering with each other. Standing waves are the result of reflective audio waves hitting incoming waves to generate potentially null and reinforced sound points in space. The wavefronts typically will have the same amplitude and frequency and will interact across the whole audio frequency spectrum. The effects of standing waves are more consequential at the lower order frequencies. The consequence of this is a distorted audio sound field. A resonating wave is louder than waves that are not resonating, and also takes longer to decay. If we consider that standing waves occur between all three pairs of wall surfaces in our project rooms, we can understand why standing waves are so detrimental to sound quality!

 

For most project rooms, the sound waves that resonate in the length, width, and height dimensions are all bass sound waves from 30Hz to 150Hz. The increased volume and longer decay times of the resonating bass sound waves totally destroy any chance of bass sounding clean, tight, and chest-pounding like it does in large venues, commercial cinemas, and outdoor concerts. Furthermore, standing waves are not uniform across a room. Certain places in a room will experience much louder bass than others. We can only hope that our listening position is located in the sound field where standing wave reinforcement is minimal.

 

It is easy to see that we must do something to eliminate these bass standing waves. How do we go about it? Fortunately, we have a whole arsenal of ways to treat standing waves. Some ways are acoustic, some are electrical, and some are structural. For example, during the design phase of a project room, we can adjust the dimensions of the room so that the bass sound waves that resonate are all different. 

 

In addition, we can place loudspeakers, subwoofers, and listening positions so that their interaction with standing waves is reasonably limited. We can also use electronic equalization to reduce the volume of the resonating waves. Various room treatments are available to eliminate standing waves. Our product of choice to diffuse standing waves is a BaseTrap. BaseTraps are available in various shapes and sizes and can be incorporated into the majority of AV/Audio rooms. 

 

Unlike Absorbers and Diffusers, BaseTraps do not need to be placed on walls at reflection points in order to function properly. Due to the nature of standing waves, BaseTraps work most effectively when they are positioned in the corners of a room, either sitting on the floor, or hanging just below the ceiling. The BaseTrap’s unique shape accommodates easy and aesthetically-pleasing corner placement. 

 

Room Acoustics Summary

 

Reflections, flutter echoes, reflection decay time, and standing waves are all acoustic phenomena that ruin the sound in our project rooms. A blend of Absorbers, Diffusers and BaseTraps can be used to control reflections, kill flutter echoes, and lower the reflection decay time so that it lies within acceptable tolerances. Additionally, through the use of electronic hardware, we can adjust acoustic phenomena in a way that alleviates the poor auditory performance of the room.

 

Support Technology is HAA (Home Acoustics Alliance) Level III certified. We are the only Brisbane-based certified audio room calibrators. This means that we have the skill sets to fine-tune your audio space, to optimise your listening experience. We achieve this by using sophisticated computer-based software that analyses your room characteristics and presents a report outlining a methodology to correct room distortions. Solutions will generally encompass the modification of your room space with various acoustic treatments and potentially through corrective DSP audio corrections through hardware. You will be amazed at what we are able to do!

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