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|Sound & Hearing
The acoustic suspension principle revolutionised the loudspeaker industry in the late fifties. Based on a mathematical model of woofer behavior in tandem with the air in a speaker cabinet, it provides a way to design a small speaker system (as opposed to the refrigerator sized monsters previously used to get halfway decent bass) with effective, linear low frequency response.
An acoustic suspension, or "sealed box," design, usually employs a woofer with very soft, compliant suspension components, and uses the air sealed inside the cabinet as a spring to return the cone to its "rest" position and control the system resonance. The restoring force of this trapped air is linear in nature, and the use of soft suspension materials reduces any non-linearities due to them. Other common features of woofers designed properly for sealed systems are very long throw voice coils, small diameter, and fairly large diameter cross section surrounds.
In the ideal sized box (for a given woofer), the frequency response will be flat to a fairly low point, often not much above the woofer's "free air" resonance, and then decrease smoothly at lower tones. A slightly smaller box will raise this roll off point a bit, and cause a small increase in the output above that frequency, whereas an oversized box will give a lower final roll off with less output in the octave or so above it.
It is not unusual for speaker using this design to have useful output down to 30 to 40 Hz from a box of only one to two cubic feet or so, which is very easy to place in the listening room.
The limiting factors on bass response will be:
- The free air resonance of the woofer - the system resonance will always be higher, and output below this frequency will be attenuated.
- The linear excursion of the woofer - excursion requirements for bass notes can be quite high with small diameter drivers and so they need a large amount of freedom of movement and a long voice coil.
- The cabinet size and efficiency (loudness for a given power level) desired. These are direct trade offs with the low frequency extension and become one of the main compromises in the design process.
- Breakup modes of the woofer cone at high excursion and low frequency. These are apparent as harmonics of the desired tone and muddy the response if their level is too high.
The principal advantages of a well designed acoustic suspension system will be:
- Small cabinet size with tight, clean bass response.
- "Reasonable" efficiency
- The small diameter woofer makes a two way system, with its attendant simplicity, very acceptable.
- Well controlled response below the system resonance (less excursion, less output, no added distortion).
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