Bass Unit

wpe21.jpg (4233 bytes)The bassunit exist of a mono subwoofer. A bass speaker from Peerless is used for this purpose. The woofer has a long stroke voice coil specifically designed for the high demands of powerful subwoofers. The system impedance is 8 ohm, a value that is easily handled by modern amplifiers with sufficient damping. The woofer features a heavy voice coil securing a clean and dry bass response. The specific filter setting in combination with a high power 140Watt amplifier drives the unit to the mechanical limits in the very low bass end. Thermal overload is by far not reached since the voice coil is capable of handling 220Watts of electrical power. So, the subwoofer is limited by a maximal linear cone displacement instead of electrical (thermal) overload. photo of Subwoofer, click to see an enlarged view 
It must be noticed that the woofer is an 12" (31cm) woofer. Only with this kind of cone surfaces a high SPL at the lowest frequencies is physically possible. For normal home audio, the SPL is sufficient even when you like sometimes a bit more volume. For home audio the SPL is still impressive with 110dB at 170Watts.

The woofer is devoted to pure low frequency reproduction, therefore no requirements are set to the higher frequencies. The investment in a more expensive unit does not pay off since these units are generally better in performing at higher frequencies. Reproducing 200Hz from the same cone which is also producing a 50Hz bass adds additional requirements to the speaker. This speaker is selected on its parameters for low frequency reproduction.

The subwoofer hardware

Additonally, the subwoofer is equipped with a MCU controlling the acoustic parameters like cross over frequency, phase, and volume level. The indication is through a practical 3 digit LED display. The photo shows the prototype with the volume level set to -2.0dB.
The subwoofer board also features full remote controlled settings, this means each parameter can be easily set using a standard (RC5) remote controller. The advantage of the remote controller is, that two preset programs can be stored in the EEPROM of the subwoofer MCU and can be recalled from the remote controller.

led_bass.jpg (112278 bytes)

The enclosure

The speaker is built in a solid 22mm MDF box with numerous crossbars inside for box reinforcement to suppress resonances. The power amplifier is built in a separate enclosure inside the bassbox. The box is a closed box type. Combined with the correct electronic filter, this provides a very well defined bass with high resolution and deep bass. The closed box is modelled as a second order high pass sytem according to:



Bass reflex type enclosures can be modelled as third order systems and are therefore not chosen. They provide higher efficiency, generally at the expense of bass sound quality. Roll off of the frequencies at the low end is much steeper because of the increasing group velocity at this point. The rapid changes in phase is the cause of this effect ultimately resulting in a less attractive impulse of the system or Q-factor). Here, the best acoustical enclosure type is chosen. The electronic filter is able to correct for SPL at low frequencies without loosing bass quality which is otherwise often compromised when acoustically solved.
The Q factor is set 0.65, an ideal value for a "punchy" bass that will never sound "booming" or resonant. The high resolution of the speaker enables a good reproduction of a bass guitar and drums played at the same time. The drum is clearly audible and present while the bass guitar does not produce standing waves in the listening room.

Speaker measurements

The speaker impedance curve is measured using the small signal method. Filter parameters largely influence the total response curve as do the speaker parameters. From the impedance curve the most important speaker parameters can be derived such as fs, Q, Rdc and RLC-parameters.
A series resistance of 1 kohm is used in combination with the speaker impedance (which is low compared to the resistance). The voltage across the speaker terminals is measured and with Ohms law the impedance at each applied frequency (pure sine wave) is calculated.


 

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