Acoustics: A guide to online resources
May 12, 2008
The single biggest factor impacting the performance of your home audio speakers or studio monitors, is the acoustics of the room or studio they are placed in. Recognizing this, over the last few years there has been a tremendous growth in the number of companies offering acoustic treatment products, along with providing extensive technical information. Below is a short overview of the biggest factors impacting home theater and studio acoustics, along with links to companies that offer products and technical resources which may assist you in addressing these issues.
Low Frequencies | Room Modes / Standing Waves:
In small acoustics spaces (generally defined as rooms of less than 12,000 cubic feet in volume, with a more common room volume being approximately 3000 Cubic Feet, such as 16’W X 9’H X 21’L) the frequency response of a speaker system in the region below 100Hz is dominated by the modal response of the room. Room modes, also known as standing waves, occur in all rooms at frequencies where the wavelength of sound is an integer fraction (i.e. 1/1, 1/2, 1/3, 1/4, etc.) of the distance between two walls, or the distance between the ceiling and floor (this is a slightly over simplified explanation). This means that invariably, some frequencies are reinforced and some frequencies are canceled, resulting in large peaks and dips in the frequency response at the listening position and low frequency ‘ringing’, as the room resonates at certain low frequency frequencies.
There are several ways to help minimize the severity of room modes and improve the consistency of your speaker / monitoring system.
- Bass-Management / subwoofers:
Bass management (which is used in 90% + of all home theatre / audio systems and most premium car audio systems) uses filters to extract low frequency information from the main channels and then reroutes that information, along with the LFE channel in a 5.1 system, to a single mono subwoofer (sometimes multiple subs are used for greater SPL or to improve in-room low frequency performance). The advantages are overwhelming and include smaller main speakers that are easier to place, better LF response, reduced inter-modulation distortion and more repeatable LF response from room to room (small or large).
As a general rule, bass–management should be used in any professional recording studio that is creating content for direct consumer consumption (stereo, surround, music, broadcast, radio, DVD etc.) and that is why it is incorporated into all Blue Sky monitoring systems (design and manufactured by ADL). As mentioned above, most home theater systems, all home THX certified audio systems and many premium car audio systems already use subwoofers and the requisite bass-management electronics.
For a more in depth overview of this concept, please follow this link. - Broadband absorption and “bass traps”:
One of the simplest ways to address low frequency standing wave problems is to install broadband absorbers. These are most commonly made of home insulation products, such as rigid fiberglass or Rockwool and have a material density of around three pounds per cubic foot. These types of acoustic treatments are usually framed or placed in a ‘fabric bag” and then mounted onto the walls of the room / studio. For the greatest effectiveness at low frequencies, it is typically desirable to mount them across a corner, or with an air gap between the wall and the broadband absorption panel.Â
The second, more narrowly focused “bass traps”, are those that use a helmoltz resonator or a membrane type of absorber. Because of the way these are constructed they tend to be more expensive, when compared to standard broadband absorption, but they can be more effective at lower frequencies.Â
Below is a short list of links to companies which offer complete broadband absorbers and narrow band ‘bass traps’ for purchase and / or kits to help end users fabricate their own broadband absorbers:
Auralex Acoustics: http://www.auralex.com/
GIK Acoustics: http://www.gikacoustics.com
MSR Inc: http://www.msr-inc.com
Ready Acoustics: http://www.readyacoustics.com/
RealTraps: http://www.realtraps.com
RPG Diffuser Systems: http://www.rpginc.com/ - Automated Room Correction / Room Equalization:
Room EQ has become increasingly popular, with regard to possibly addressing room related acoustic problems. Especially within the latest high-end consumer receivers, automated room correction has become a relatively prevalent feature. Unfortunately the effectiveness of these products is still heavily debated. Our general view is that they can be helpful, but that they are most effective when used in rooms with proper acoustic treatment and they certainly are not a substitute for proper acoustic treatment. The reason for this is usually related to the inability of automated systems to accurately measure the room and speaker interaction when there is not enough acoustic treatment in the room. The microphone and processing systems get overwhelmed by the reflections and the result is less than optimal equalization. Additionally, because many of these systems can only optimize for a relatively small listening area, they are often not ideal for optimizing or resolving general room acoustic problems. Lastly, there is only a limited amount of EQ available and it is often not sufficient alone at addressing the peaks and dips in most rooms.Â
Below are a few related links on this topic:
Audyssey: http://www.audyssey.com/technology/index.html
Meridian: http://www.meridian-audio.com/w_paper/Room_Correction_scr.pdf
RealTraps: http://www.realtraps.com/art_audyssey.htm
Mid to High Frequency | Flutter Echoes & Discrete Reflections:
Reflections building up between two parallel surfaces, such as side walls, are called “Slap Echoes†or “Flutter Echoesâ€. If the side walls are too reflective a slap echo will develop. Slap echoes have a bright metallic character and can severely affect the sound quality. Discrete reflections are a dominant effect in small rooms / studios which typically exhibit low reverberation times. They give a distant and “smeary†character to the sound, which severely degrades the ability of a pair of speakers to convey a proper stereo image and can also greatly reduce dialog intelligibility. Sound reflections can occur from side wall, rear walls, furniture, video monitors, or any other large surface within the path of the speakers’ radiation coverage area.
You can help limit the impact of flutter echoes & discrete reflections in three basic ways; absorption, diffusion and speaker placement / aiming:
- Absorption:
For most home audio and studio applications, absorption should probably be your primary method for addressing this type of acoustic problem, especially for the first reflections off of the side walls, ceilings and floor. As explained above, broadband absorption can also help to improve the low frequency characteristics of the room and is also a relatively inexpensive method of acoustic treatment.Â
Below is a short list of links to companies which offer complete broadband absorbers for purchase and / or kits to help end users fabricate their own broadband absorbers:
Auralex Acoustics: http://www.auralex.com/
GIK Acoustics: http://www.gikacoustics.com
MSR Inc: http://www.msr-inc.com/
Ready Acoustics: http://www.readyacoustics.com/
RealTraps: http://www.realtraps.com
RPG Diffuser Systems: http://www.rpginc.com/ - Diffusion:
The second method which can be employed to address flutter echoes and reflections is diffusion. Diffusers work by scattering reflections around the room or studio randomly, rather than combining with direct sound and causing spectral or time based distortion. Diffusion is especially useful in rooms where acoustic absorption is already used to a great extent and additional absorption is either not practical, or is making the room sound unnaturally “dead”. Although you can use items such as bookshelves filled with books to help defuse the reflections, it is typically much more effective to use a dedicated product specifically manufactured for this purpose.
Below is a short list of links to companies which offer acoustic diffusers:
Auralex Acoustics: http://www.auralex.com/
GIK Acoustics: http://www.gikacoustics.com
MSR Inc: http://www.msr-inc.com/
RealTraps: http://www.realtraps.com
RPG Diffuser Systems: http://www.rpginc.com/
- Speaker Placement:
Although speaker placement will not solve all mid to high frequency acoustic reflection problems, it can help to limit their severity. In recording studios one particularly useful solution, is to put your front monitors on speaker stands, instead of placing them on the recording console bridge. This will greatly reduce the reflections off of the console, which are typically the source of the first reflection in a recording studio. Also, aiming the speakers at the listening area and away from reflective surfaces, can help reduce the severity of the reflections.
For more information about Blue Sky’s “The Stand”, please follow this link.
Reverberation Time (RT):
In the room sizes we are talking about, reverberation times are rarely too long to impact sound quality. However, tonal smoothness of the reverberation time can affect sonic performance. The ideal room should exhibit a completely smooth decay of sound, in which no special sonic event or peculiarity stands out. Also, rooms with too little reverberation because of excessive absorption, can have sonic problems of their own (see the diffusion section above). In most cases, living rooms, home theatres and smaller control rooms, with a regular amount of furniture don’t suffer from reverberation time problems.
Additional Educational Resources:
Blue Sky International: http:www.abluesky.com
GIK Acoustics: http://www.gikacoustics.com/education.html
RealTraps: http://www.realtraps.com/articles.htm
Design Services and Acoustical Consultants:
Charles Salter Associates: http://www.cmsalter.com/
Keith Yates Design Group: http://www.keithyates.com
Munro Acoustics: http://www.munro.co.uk/
Performance Media Industries Ltd: http://www.pmiltd.com/
Russ Berger Design Group: http://www.rbdg.com/
If you have any comments, questions or suggestions regarding this topic, please leave them below, or contact ADL directly.
