Loudspeaker Industry Sourcebook

LIS 2018

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Page 130 of 179

2018 Loudspeaker Industry Sourcebook 129 INDUSTRY FEATURES objective measurements on high-end audio devices. The measurement applications that follow are viewed from two angles. T h e f i r s t v i e w d e s c r i b e s t h e t y p i c a l measurements on the various devices along the signal chain, from the signal source to the amplifier over the cables to the loudspeakers and finally the room in which the system is installed. The main concern is to provide a very linear, low-noise and low-distortion signal through the entire signal chain. T he s e cond view cove r s the dis tinc tive measurements at different life-cycle stages of the various devices, from R&D to incoming QC, end-of-line (EOL) QC, and installation of the system as well as service and maintenance. R&D The R&D phase builds the foundation of a high- quality product. Any shortcomings not solved or managed at this early stage of the product development will be passed through to all the later stages. This applies to the complete signal chain covered by the product. Electrical Signal Path Any unwanted noise before or within the amplifier will be irreversibly audible at the loudspeakers. This means it is essential to keep the signal clean from the very beginning of the signal chain. Distortion—In most systems, distortion is an unwanted effect and is avoided at all costs as it affects the perception of sound quality. It's important to realize that distortion is mainly a result of the system reaching an upper limit of operation. Thus, the testing shall be executed close to these limits. The distortion can be quantified as total harmonic distortion (THD) or as individual harmonic distortion values. Some tube amplifiers produce a significant amount of even-numbered harmonic components (k2, k4, and k6). Advocates argue that this creates a more pleasant sound. Hum & Noise—The presence of noise or hum can cover fine details of sound. So, providing a low-noise circuit design is one of the most important tasks in the R&D phase. Noise can be measured with or without the presence of a test signal. In either case, it is important to realize that amplifier stages amplify both, the signal and the noise. Therefore, a high signal-to- noise ratio (SNR) is desired. When using balanced analog audio channels, it is important to measure the Common Mode Rejection Ratio (CMRR) as this determines the ability to cancel out cable- inducted noise or hum. Noise and especially hum can also be introduced from the system power supply. Therefore, it is recommended to perform these measurements with different mains-power conditions and settings. Bandwidth—The bandwidth is measured with a frequency response measurement. The measured level has to be within the e xpec ted limit s throughout the specified frequency bandwidth. Modern sound systems cover bandwidths up to ultra-sonic frequencies. Line arit y—A ll the previou sly me ntione d m e a s u r e m e n t s c a n b e i n f l u e n c e d b y t h e amplitude of the input signal, as well as the power of the output signal. An appropriate way to verify this system behavior is to run an amplitude sweep signal at the system input. The amplitude shall range from zero to the maximum allowed input signal. The output signal is measured with level and distortion and shall be within the expected limits through the input signal range. Channel Balance and Separation—The spatial awareness of the listener is based on having more than one sound source (e.g., stereo). It is vital that all channels behave as uniformly a s p o s sib l e. A ll t h e p r evio u s l y m e ntio n e d measurement results shall be compared for all channels of the system. Furthermore, the influence of one channel on other channels (e.g., caused by unwanted electro-magnetic induction or capacitive coupling) must be checked. This is done with a Crosstalk measurement. Acoustic Signal Path In the R&D phase, the developer has to contend with loudspeaker drivers as well as loudspeaker systems. Besides the acoustical parameters, the electrical parameters are of interest. However, b e f o r e a n y m e a s u r e m e n t r e s u l t s c a n b e assumed to be reliable, new bass and midrange loudspeakers need to be operated for a certain time in order to have the suspension material reach its final characteristics. Woofer and Midrange Driver Conditioning— Several burn-in procedures for loudspeaker drivers are known. The proposed test signals range from noise to music. Practical experience at Art Déco Acoustics showed that a low-frequency sine wave signal delivered excellent results. The loudspeakers' suspension is displaced in a very controlled and predictable way, while not producing bothersome loud noise.

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