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AUDIO FREQUENCY RESPONSE TUNING USING A MECHANICAL ADJUSTABLE RESONATOR

IP.com Disclosure Number: IPCOM000009410D
Original Publication Date: 1999-Sep-01
Included in the Prior Art Database: 2002-Aug-21
Document File: 4 page(s) / 186K

Publishing Venue

Motorola

Related People

Stefan Lichterfeld: AUTHOR [+2]

Abstract

This paper describes improvements on speech intelligibility and on electrical efficiency on audio systems using a mechanical design solution.

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Developments Technical 0 M MOTOROLA

AUDIO FREQUENCY RESPONSE TUNING USING A MECHANICAL ADJUSTABLE RESONATOR

by Stefan Lichterfeld and Wolf-Dietmar Pollert

  l This paper describes improvements on speech intelligibility and on electrical efficiency on audio systems using a mechanical design solution.

BACKGROUND:

  In communication systems a received signal is converted to clear and loud Sound Pressure Level. Dependent on the speakers-and listeners demands, the SPL should be adjustable over tone frequency to get best intelligibility or smooth sound, if required.

  This usually is done by electronic filtering to compensate a non-linear SPL conversion. Current communication equipment usually does not have the ability of individual voice response matching.

  The acoustical resonator tuning directly works on the loudspeaker outlet (grill) and can be imple- mented in every electronic design, because no extra circuitry needed.

  One may understand this technique as mechani- cal adjusted voice improvement by optimising the conversion from electrical PA output signal to SPL.

ADVANTAGES OF THE MECHANICAL ADJUSTED VOICE IMPROVEMENTI

Improved voice intelligibility by decreasing SPL at noise-frequencies > 3.5 kHz.

l Possibility of matching to individual language metalities.

DESCRIPTION OF THE MATCHING SYSTEM ON A TWO WAY PORTABLE RADIO:

  The loudspeaker grill of a portable radio includes a mechanical shifter which varies the effec- tive grill-opening-area.This shifter defines two dif- ferent audio frequency responses, which are matched to the individual requirements of the user (Figures 1 to 3).

  Referring to (Figure 2.) Ll and Cl are defining the fundamental resonance Fr of the loudspeaker (spring-mass equivalence). The graph shows Fr = 300 Hz.

  Rl,R2,C3 are describing the parasitics of the speaker.

  C2 is constant and corresponds to an acoustical volume of 1 cm3 between the loudspeakers cone and the grill-outlet.

  L2 corresponds to the accelerated loaded mass and is dependent on the openings of the grill.

  L2 varies with numbers and area of grill open- ings and affects the resonant frequency and there- fore the upper cut-off frequency.

  C2 and L2 must be designed that magnitude increases by resonance at upper speech frequencies. The sound pressure level is increased by up to 4 dB just in that frequency range where most speech intelligibility is available. (1...