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21 Aug 2011

Choosing a Subwoofer Featured


Subwoofer Specifications: One must know the facts when choosing a good subwoofer. Imagine your sound system is the house and the subwoofers are the foundation. Choose the wrong subs and the whole house could collapse.

At very low frequencies the subjective ability of pitch perception changes from one of pitch to a sensation of pressure between 12 Hz and 19 Hz, with the lower limit for musical pitch perception being approximately 60 Hz. It is also very important to understand that, for a desired sound pressure level, the threshold of human hearing at low frequencies determines the lower limit for sounds that are “audible.” The threshold of audibility for low frequency sounds increases with decreasing frequencies, i.e., from 50 Hz down to 20 Hz hearing sensitivity decreases 18 dB per octave and continues decreasing at about 12 dB per octave below 20 Hz. In other words you must play a 20 Hz tone approximately 18 dB louder than a 50 Hz tone in order to (a) discern the two tones and (b) for both tones to sound equally loud. This occurs as a result of the intrinsic behavior of the human auditory system. (For more detailed information read-up on research by Fletcher and Munson or better yet, Robinson and Dadson.) Environmental sounds often occur at high sound pressure levels (SPL) at very low frequencies.

The most important criteria related to subwoofer performance are: amplitude response (aka, frequency response,) the room's response, the maximum peak excursion capability of the subwoofer drivers, distortion and noise, and crossover frequency.

For most well-designed loudspeaker systems incorporating one or more subwoofers the crossover frequency for the subwoofer should be limited to no higher than 80 to 90 Hz.

Relative to frequency and SPL a subwoofer's second harmonic distortion should be no higher than 3%, the third harmonic should be less than 1% and the remaining higher harmonics should be less than 0.3%.

The maximum peak excursion capability can easily be exceeded (often resulting in very high distortion, i.e., nonlinear behavior, or even driver damage) if you're trying to accurately reproduce very low frequency sounds, for example environmental sounds, of any significant duration. The best method to over come this limitation is to use multiple, large volume displacement drivers in one or more subwoofer enclosures driven by very high powered amplifiers; not very practical or economical for the most people. Fortunately room gain helps control the subwoofer drivers' maximum peak excursion by substantially boosting low frequency output below 20 Hz, up to as much as 15 dB at 5 Hz due to boundary effects of the room.

The room's response can be very unpredictable as most dwellings do not contain rooms that were purposely designed, or are large enough, for the accurate reproduction of extremely low frequency sound with minimum distortion. Small rooms are more problematic in that they tend to emphasize and deemphasize certain low frequencies. Also, room resonances can cause a flat amplitude response for a subwoofer to vary by as much as ± 10dB. The only practical option to control low frequency room interaction in small rooms is to experiment with the placement of the subwoofer(s) and/or use a high performance equalizer or digital room correction; equivalently effective alternative solutions will be much more costly.

Regarding a subwoofer's amplitude (or frequency) response: to determine the cutoff frequency for accurate sound reproduction, with the goal being little or no loss in fidelity for certain music, instruments, or sound effects that fall within the subwoofer's frequency range, you will need a subwoofer that has a response that extends down to approximately 16 Hz (or as low as 12 Hz in rare cases) at a maximum peak SPL of 110 dB and approximately 10 to 12 Hz at a maximum peak SPL of 120 dB.

Last modified on Sunday, 21 August 2011 18:48


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