Acoustic simulation of mobile phone coupled to artificial ear

Abstract

Artificial ear is being used to evaluate the acoustic response and the sound quality of the mobile telephony devices, which simulates the practical listening condition of the outer ears. In this paper, a method to estimate the coupled acoustic response of the device with an artificial ear is studied to be effectively used in the design. To this end, an equivalent circuit model of the total receiver system including all accessory elements is established. Acoustic impedance of artificial ear, which is essential in the equivalent model, is directly measured by using three microphones arranged in tandem on the duct wall connected to the artificial ear. Input impedances of two artificial ears, Type 3.3 and 3.4, which are currently employed as the standard devices, are measured. The measured data is incorporated into the model to predict the acoustic response. To validate the proposed model using the measured impedance, the measured acoustic responses of two simulation systems including mobile phone and artificial ear are compared with the predicted ones. A reasonably good agreement between measurement and prediction is observed, and their difference is less than 4.5 dB at the narrow communication band for a mobile phone (f < 3.4 kHz). It is also found that the input impedance of Type 3.3 ear is more robust to the change of measuring condition than Type 3.4 ear.

Introduction

In the acoustical design of a handset type audio-visual (AV) telephony device, an artificial ear is used to simulate the acoustic performance in the actual hand-held usage condition in contact with an ear. Artificial ears, Type 3.3 and 3.4, are currently used as the standard test rigs in telecommunication industries. The acoustic response of such telephony device is measured at the drum reference point (DRP) and converted to the response at the ear reference point (ERP) by using the calibration table specified in the ITU-T P.57 [1]. The measured acoustic response of the sample is to be compared with the desired preset performance. The foregoing experimental procedure is repeated in a trial-and-error way until the desired response is reached by modifying design parameters.

There have been several virtual design approaches for the compact acoustic system of the mobile phone. Automatic design method [2] was proposed to improve the degraded sensitivity caused by the leakage of the coupler, i.e., so-called pseudo ear, according to the IEC-60318 standard. They tried to optimize the design parameter and structure of the derived electric circuit model by using the genetic algorithm. The same method was applied to the automatic design of an artificial ear Type 3.2 to fit the receiver response in the global system for mobile (GSM) specification [3]. However, the comparison of the automatic design method with the experimental ones was not shown for both cases.

To understand the performance of simulators as representing the average human ear, ITU-T Study Group 12 suggested the measurement of impedance of artificial ear Type 3.3 and 3.4 [4]. A device, equipped with a 1/2-in. microphone as a constant velocity source and a probe microphone as a pressure sensor, was employed for the measurement of acoustic impedance at a point in the close near-field of the artificial ear.

In this work, to avoid empirical conditioning of design parameters of the sound reproduction system, the electric circuit model based on the electro–mechano-acoustical analogy is derived by employing all the detailed system components [5], [6]. Impedance of an artificial ear, Type 3.3 or 3.4, is directly measured by using the multiple microphone technique within a straight duct, which is excited by an external sound source [8]. It is shown that the suggested model with the measured impedance of an artificial ear is in good agreement with the measured result for a test condition in the frequency range of 300–3400 Hz, which corresponds to the narrow communication band defined in the telephone industry.