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Acoustical Treatment Primer: Absorption

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I remember years ago, when it was only the most adventurous of Enthusiasts who would delve into treating their listening room. The promise of better sound tempted many to head to the hardware store and buy some Owens Corning 703 then fashion their own panels (many still do). Of course, some used egg cartons and carpeting in those days as well. At the time, the most prevalent advice on adding treatment was called the LEDE system. LEDE is an acronym for “live-end dead-end”. Which translates into deadening the front of your room and leaving the back reflective. This was not a bad idea in the absence of better information.

Mirror Points

Sometime later, the benefits of absorbing the “mirror points” was discovered. This was a more complex concept as it required the use of a mirror and an assistant (or willing spouse) to move a mirror along the side walls until you could see each of the speakers. Each mirror point was then the target for placement of an absorptive panel. Most of the time the result was quite audible, producing a decided improvement in focus and detail. Diagram one shows the mirror (reflection) points as the sound travels like a billiard ball bouncing off the wall to your ear; these are called specular reflections. While audible, absorbing sound at all the mirror points or even just the side wall mirror points is not completely beneficial and causes other problems.

Diagram 1

Today, we now know that simply absorbing all reflections is not the best choice.  Some reflections should be absorbed yet others enhance envelopment and spaciousness.  I like to call it reflection management.  I’ll try to use our blog here to share some information about each treatments function but also on their best use in your room.

Treatment Types

The various enthusiast forums are replete with a variety of treatment strategies using any number of different designs.  It can be confusing and costly if the result is not satisfactory.  The most basic strategies can usually provide benefits, but it’s good to know a little about the science.  First let’s look at the various types of treatment.

  • Absorption: Absorbing sound impacting the panel.  It reduces or removes the energy of a reflection.
  • Diffusion: Diffusive panels scatter sound.  While they are not generally designed to reduce the energy in sound they effectively reduce the energy in a reflection by dividing it into many reflections to various directions.
  • Reflection: Yes, reflection is an important element in room design.  Many reflections are beneficial and should be preserved.
  • Bass traps:  This category includes everything from very thick panels to diaphragmatic absorbers. The commonality is that they are most effective absorbing low frequencies.
  • Hybrid Treatments:  These types combine the functions of the above types.  They can be very useful when some frequencies should be absorbed while others reflected or scattered.

I’ll discuss each type in new blogs.  Today, I’m writing about absorption.

The most common treatment is absorption.  The most common type is fiberglass, although there are other materials (acoustical foam, mineral fiber, polyester, cotton, and others).  These types of panels are referred to as kinetic fibrous absorbers.  This means they reduce sound energy by slowing down the speed of the vibrating air molecules.  The air molecules must bounce against the fibers in the panel thus converting their kinetic energy into heat due to the collision.  The panels heat up (ever so slightly) as they absorb sound energy.

Absorption Coefficient (AC)

In the early days, most used 1” (25mm) thick panels.  It was cheap and plentiful.  Soon it was pointed out that using 2” (50mm) produced better results and then 3” (75mm) and even thicker layers were used. The science behind this is based on understanding the acoustical absorption coefficient (AC).  It is defined as the ratio of energy absorbed by a material to the energy incident upon its surface.  So, a coefficient of 1.0 means all the sound hitting the panel front surface is absorbed.  One inch treatments did a good job of absorbing high frequencies but very little at lower frequencies.  That’s because the AC became lower and lower as the frequency became lower.  These 1” panels absorbed the lively high frequencies but virtually passed unaffected the lower ones.  This means that a 1” panel placed at a mirror point was doing half the job (technically less than half the job).  Diagram 2 shows a comparison of the various thicknesses of Owens Corning 703 fiberglass panels.  Notice how 1” panels are operating at well below the desired AC of 1.0 at the all-important range below 1000 Hz.

Diagram 2

It’s interesting to note that the chart seems to show the thicker panels absorbing more than 100% of the sound.  This because the measurement procedure only counts the front surface area in the calculation of the AC.  The sides of the panel are not considered in the calculation of the surface area yet they do absorb sound.  It’s also important to point out that the measurement of AC includes the amount of random incidence sound absorbed by the panel.  This is good for calculating how many panels are required for a concert hall or church but it tells us much less about how the panel handles sound coming from a specific direction; a specular reflection.

New Data

Dr. Floyd Toole in his book “Sound Reproduction | Loudspeakers and Rooms” (Chapter 21.3.2) discusses the acoustical implications of typical fiberglass panels.  He provides the information in part to support his caveat that a good speaker’s first lateral reflections should not necessarily be absorbed; a topic we’ll discuss.  The data reveals that these absorbers do not uniformly absorb all frequencies despite the apparent uniformity typical AC plots show.  He also points out that regarding specular first reflections from the speaker to the listener, the absorption of typical fibrous panels depends on the angle the sound wave is incident to the panel.  Some noteworthy conclusions are:

  • AC graphs are insufficient because they are based on random incidence measurements and not specular ones. They have different absorption levels for various frequencies when the incident sound is at an angle versus at random angles.  They also lack the resolution to show the true impact on frequency response by the reflection.  Since the reflected sound has a clear impact on the measured listening position frequency response, this is important to know.  Finally, AC measurements do not include data on higher frequencies (up to 20K Hz) which show a lower absorption due to reflection of specular reflections at these higher frequencies.
  • Panels of at least 3” (75mm) or thicker are required to adequately absorb frequencies above the transition frequency. The transition frequency being the point above which specular reflections dominate our design and calibration strategies; usually between 300 Hz and 500 Hz in the typical small room.
  • The fabric used to cover the treatment has little effect on lower frequencies but can reduce the absorption at higher frequencies by making the panel reflective. This points out one of the values of using a fabric which has similar properties to speaker grill cloth which is designed to be acoustically transparent (at least it tries to be transparent).  It also points to the popularity of the ubiquitous Guilford of Maine FR701 fabric, which, while still reflective at higher frequencies provides a more “grill cloth-like” texture.

Basic Observations

So, what is a DIY Enthusiast to do?  Glue 4” (100mm) panels on the wall?  Use no panels?  Everyone knows that the sound is better with panels… right?

Let’s look at some basic observations:

  • Absorbing the first lateral reflections does indeed improve focus (stereo imaging) and detail by increasing the amount of direct sound with relation to the reflected or indirect sound. If that’s your preference use a thick absorber to reduce frequency response distortion.
  • Absorbing the first reflections also reduces the apparent soundstage width and spaciousness of the sound field. If you prefer a wider, and more three-dimensional sound stage, don’t absorb first lateral reflections. This where Dr Toole’s assertion that these reflections should not necessarily be absorbed originates.  A good speaker (one with excellent off-axis response) will create reflections that enhance envelopment and spaciousness.  Since the reflections are less frequency response distorted (due to absence of panels) the listening position frequency response will be more like the speakers engineered response.
  • If you are a “near-field” listener (sitting very close to the speakers), panels will have less effect because the primary sonic source you are hearing is the direct sound of the speaker which minimizes the reflections effect on the sound at the listening position. This scenario, is not uncommon for many hard-core two-channel audiophiles.
  • If you have a theater and more than one seat (especially if you have more than one row), it’s not practical to hog the one good seat (the sweet spot for near-field listening). You’ll need to have the seats deeper in the room to provide a satisfactory experience to all.  Reflections begin to take on a much larger role. See my article on the sweet triangle in this blog.  Over-absorbing reflections can also reduce the width of the sweet triangle.  In the case of a surround sound system, over absorbing can create a scenario where speakers do not create a cohesive surround field.

Unfortunately, you see the use of absorbing panels is not a cut-and-dried situation.  The proper use depends on the speaker you own, your listening habits, and, of course, your room.

The Room

One topic which seems to pop up in this kind of discussion is that of the RT60 of the room.  The reverberation time, at a particular frequency, is defined as the time taken for sound to decay by 60 dB; this is often abbreviated to RT60.  The significance of RT60 is that the reverberation time is proportional to the reverberant sound level in the room.  Basically, its how we compare the reverberant versus the direct sound levels.  It was impractical to measure the reverberant sound level directly in the early days (still is) so a series of empirical values of RT were collected over time representing the desired RT60 for different size spaces and different uses.  Larger performance spaces have long RT60’s, school class room auditoriums tend have lower targets.  In an HAA calibration we primarily use our RT measurement to tell if we are over-treating the room.   An especially small room can become over treated with just a handful of absorptive treatments.  The truth is that for most home listening rooms, calculating RT60 is not a useful tool.  We use RT measurements as a warning sign to stop putting absorptive panels on the wall (or to remove some in a finished install).  This is true for a variety of reasons:

  • The fundamental basis for acoustical calculation in large room acoustics depends on a “free space” model. This why random incidence measurements of AC for treatments are so useful for commercial acoustics.  The reverberant sound field in a large space is largely uniform and diffusive; randomized.  Most home theaters are anything but a free space (no pun intended).  Early reflections are considerably louder in a small room because of the short distance they travel to our ears.  The sound field is not uniform, highlighting our interest in specular reflections over calculating the aggregate effect of all reflections as in a concert all or auditorium.  The fundamental requirement for an accurate RT60 is a largely uniform reverberant sound field.  That’s why in the HAA we use the term RT (reverberation time) to let everyone know we know the RT60 is not a true RT60 even though we use the same procedure to measure it.
  • Our rooms are small. The recommended RT times for small rooms, as given by a variety of organizations, is between 200 ms and 500 ms (500 milliseconds = ½ second).  Most residential rooms already fit that specification.  There is little need to reduce the RT unless the room is exceptionally large or lacks the typical absorptive elements found in a home (furniture, drapes, carpeting, moose heads, etc.).
  • Large room acousticians use RT60 as a calculation tool in the design phase of a project. Using formulas like Sabines, Fitroy’s, Eyring, and others, they can accurately calculation how much surface area needs to be treated to hit a target RT60.  These formulas break down as the space becomes smaller.  They are not a useful tool to use in a small room.  In the THX-HAA HT3 workshop we demonstrate how placing treatments in various places has a greater effect on the RT than changing the number of panels in the room.  Using a formula to calculate how many panels you need usually over-treats a room and it doesn’t tell you where to put the panels.

Other Factors

Ok, so it looks like absorptive acoustical treatment is of limited value in a small room… but it sounds better when we use it!!!  The reality is that treating a small room is necessary in a high performance installation but it doesn’t depend on absorbing all or even most reflections. It depends on understanding the complex structure of the sound field and the nature of each reflection point.  We use absorption (along with other treatments) to craft a seamless and spacious sound stage while preserving high quality focus (imaging) and detail.  You can read my post about focus and envelopment here.  Other factors include the utility of using the right kind of absorber to control low frequency effects like SBIR (speaker boundary interference response), compensating for poor speaker off axis response, crafting the sound stage by using a distributed pattern of absorption, increasing the diffusiveness of the sound field, and others.

Perhaps the most tangible thing this discussion points out is that treating reflections with absorption depends on the reflection; its source and its direction.  We need a strategy based upon specular reflections referencing the personal preference of the client.  Panels in the correct position are good, but panels in the wrong place are bad.  Above all, over treating a room with absorption is a definite no-no.  A good treatment strategy is possible in a small room but I’m afraid this post is already too long and I need to promise I’ll continue this series in my next blog article.  Let me know your questions in the forum and I’ll tailor the articles to your needs.

 

 

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