Speaker Isolation
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Image Credit: IsoAcoustics
To isolate or not to isolate that is the question? This is yet another of those acoustics and audiophile topics that can give rise to some heated discussions. Well after years in the recording industry, a significant amount of trail and error and some basic physics I can assure you speaker isolation is not only audible but it really is essential in order to extract the best performance from both your room and speakers. Many would ask why? Well here goes.
There are three facts of speaker life:
- Like it or not speaker cabinets vibrate, it DOES NOT matter how well made or solid they are.
- Sound travels MUCH faster through solid objects like wood, concrete, steel and stone than in the air.
- To every action there is an equal and opposite reaction….Newtons third law.
So what has the above got to do with speaker isolation and why do we need it? There are two effects that we need to look at:
- Structure born vibration
- Sound Flanking
Vibration & Sound Flanking
All structures vibrate when sound strikes them or travels through them. In some cases this vibration is put to good use as in acoustic sound absorbers where the sound energy is absorbed, turned into heat and little re-radiated. However, most structures do not absorb much of the sound energy that strikes them and their high densities cause the sound to travel much faster within them when compared to air.
- Typical speeds of sound:
- Air: 1100 ft/s
- Rubber: 6000 ft/s
- Wood: 11,000-16,000 ft/s
- Concrete: 12,100 ft/s
- Brick: 13,700 ft/s
- Glass: 14,900 ft/s
- Steel: 19,400 ft/s
When these internal vibrations hit the surface of the material or a discontinuity they will be re-radiated and turned back into sound energy.
An often forgotten acoustic effect is flanking. This effect occurs when a sound enters a physical structure like a wall, is conducted by it, being radiated from the structure as it travels through it. For speakers the sound vibrations from the cabinet enter the structure to which it is mounted in or on and can then be heard in another distant space or arrives at the MLP BEFORE the direct sound, an effect sometimes referred to as ‘early sound‘. As seen above sound in solid materials travels MUCH faster than through air. In concrete or wood it travels almost TEN times faster. So for every foot a speakers direct sound travels through air, it travels ten feet in a typical structure. This can result in sound being heard from a radiating surface BEFORE the direct sound arrives at the MLP. Clearly this is to be prevented.
Flanking or Early Sound Path
We see that despite the physical sound path through the wall being more than twice that of the direct sound this flanking sound arrives far earlier than the direct sound and can impact the spatial qualities of the sound image.
While floor, wall and ceiling vibrations can be beneficial to sound absorption, particularly low frequency energy. If they are not properly damped and controlled they can give rise to sound energy being radiated not only through walls etc. but also back into the room. This reradiated energy can give rise to various acoustic issues like poor decay and ‘color’ the sound, particularly if the vibrating surface is not highly damped. (See my white paper (pages 23-37) on my four sub alignment to read about the effects of ceiling and wall vibrations.)
Speaker Isolation
Flanking and ‘early sound’ can be significantly reduced by isolating the speaker cabinets from whatever they are mounted on or in. This isolation also reduces or prevents the structure from sympathetic vibrations from the low frequencies generated by LF units and in particular high power subs. The overall effect is to produce less structure vibration and a tighter and cleaner bass together with an improved sound stage (my personnel experience). It is important to note here that the method of isolation is important. Remember Newtons third law “to every action there is an equal and opposite reaction“. So the speakers resilient mount MUST NOT let the speaker move, or it will impact the effective cone excursions as the cabinet tries to move backwards and forwards in opposition to the cones movement. However, it must provide a sufficiently resilient mounting to prevent cabinet borne vibrations from entering the supporting structure. This is especially true for high power subs and larger speakers.
DIY Isolators
Making effective isolators is very simple and requires little practical skills and financial outlay. See the following diagrams.
Homemade Speaker Isolation Pad Construction
When making mounts the materials need to provide two primary functions:
- Resilience and
- Isolation
They need to be stiff enough to prevent the speakers movement due to the air pressures generated by the cone (only really applicable to subs or large speakers) and absorptive enough to prevent the transmission of vibration. The above mounts use high density MDF as the base support and then 1/8″ rubber and 1/8″ felt to dampen and decouple any vibrations. In the case of the sub additional resilient isolation is added in the form of high density foam blocks. All these materials having very different resonant frequencies. In the case of the sub support, the spaces between the rigid 1″ Sanding Blocks (from Home Depot) are filled with rigid 1″ fiberglass to dampen any resonances and coupling effects. While the very rough surface of the blocks provides excellent grip at numerous tiny points.
The above methods of mounting have been used by me in both professional and home rooms and I have always found them to be very effective.
Commercial Products
If you feel that DIY is not in your future, there are many speaker isolation products/options available, some of which are not too expensive. If your speaker arrangement requires the speakers to be tilted up or down make sure that the selected solution offers the required angle. I show a selection of commercial options below:
Click on the image to see the current Amazon pricing and any options.
Auralex:
- MOPADs: Approx. $39.99 a pair
- ProPADs: Approx. $274.99 a pair
Adam Hall SPADECO2: Approx. $21.40 a pair (lowest cost)
Primacoustic RX7 Recoil Stabilizer: Approx. $110.00
Ultimate Support MS-80: Approx. $159.99
IsoAcoustics:
- Iso-200Sub Subwoofer Isolation: Approx. $59.99 each
- GAIA I: Approx. $299.99 per four
SVS SoundPath Sub Isolator System: Approx $49.00 for four
Other Acoustic Considerations
While we are discussing how to improve your sound a couple of other issues that you should consider are reviewed below.
Absorber/Diffuser Distribution
Using a basic formula like Sabins formulae (see this post) you can calculate the amount of absorption you need to achieve the room decay time you are looking for. Although a good starting point this is only approximate. There are other much more detailed calculators on line that allow more accurate assessment of a rooms absorption requirements. Remember, you can never have too much bass absorption, and I mean never. So the skies the limit on how much LF absorption you can get into your room. When it comes to mid band and HF absorption you need to be more careful as to how much and how it is distributed. You are always wanting to create a diffuse sound field so do not put all your absorption at one point. Make smaller panels and spread them around the room, keeping things generally symmetrical about the MLP. They will be much more effective. I do not personally recommend porous, Helmholtz or diffusers panels smaller than 18″x18″ and membrane panels smaller than 24″x24″ or 18″x36″.
Edge Diffraction
An often overlooked acoustic issue is edge diffraction. When a sound travels around an abrupt corner as opposed to a curve it ‘sprays’ out in all directions creating a secondary point of radiation and multiple paths. This occurs at speaker and grill abrupt edges and any supporting woodwork abrupt edges. The major reason why my Genelec 8030s and 8040s have rounded speaker case corners, while my Genelec 1038’s have square corners and are ideally meant to be flush mounted in a monitor wall – see my speaker mounting here.
Edge Diffraction Effects
These effects may be significantly reduced by rounding the edges of all woodwork immediately around the speaker, including grill structures, mounting speakers flush with the rooms structure and placing large amounts of absorption immediately adjacent to each side of the speaker. In my case there was a significant reduction in many reflection levels during the rooms initial time delay (ITD) gap by applying these techniques, all of which improved imaging and tonal balance.
If you want to learn more about the basics of room acoustics and speakers read the following three part posts:
