Audio Engineering Society Singapore Section

On Friday, 26 November 99, at 7.00 pm, 5 AES members and 7 guests gathered to attend the AES Singapore Section seminar at Touch Community Theatre, 3615 Jalan Bukit Merah, Singapore 15946. After some light refreshments at 7.30pm the Chairman Jibby Jacob called the meeting to order and invited the speaker Ms K A Poornima a doctoral student specialising in Electroacoustics at the National University of  Singapore (NUS) to present topic. 

Poornima began her presentation with a background on sound systems. She defined the many types of transducers available currently. She decided to focus on the most widely used transducer i.e. the electrodynamic one. Historically the parameters for an electrodynamic transducer was first defined in 1925 by Rice & Kellogg and commented that the advances in loudspeaker technology has not generally kept pace with the advances in the other components of the audio chain. Moreover the loudspeaker tended to have the lowest efficiency in the sound reproduction chain, in spite of it's extraordinary capability in handling high power. 

The speaker Ms K A Poornima presenting the problems of heating in the voice coil of the loudspeaker - photograph by Robert Soo.

The low efficiency of the loudspeaker is due to the fact that most of the electrical energy is converted to heat and only a fraction of this changes into acoustic energy. Though the speaker specifications claim power ratings up to 1500 watts, users are made to believe, that it is quite indestructible. In this case horn loaded loudspeakers are used but are disadvantaged by their big size, increased price, and higher acoustical distortions. In her opinion digital transducers exist but none of the alternatives are practically viable in terms of efficiency, size and cost compared to the conventional loudspeaker system. Moreover the final stage of the audio chain is still analog in that electrical signals has to be converted to sound waves for the ear to interpret.

Moving on, Poornima declared that how loud a speaker sounds depends on how much air is moved by the cone without the voice coil heating. How much air can be moved further depends on the surface area of the cone and the maximum excursion the voice coil can make. 

Failure of loudspeakers can be classified as physical or thermal failure. Physical failure occurs when the voice coil undergoes excessive average excursion as in worst case it hit's the backplate causing acute acoustical distortion. This could be due to a large transient signal or poor enclosure design. Some of the clues in an impaired loudspeaker are the damaged cone, the physically split surround, the shattered dust cap, the misplaced voice coil or the shifted magnetic poles. 

Thermal failure occurs when the voice coil assembly overheats, physically deforms and then fail . In the worst cast the voice coil burns out. With an increase in signal input, the voice coil temperature & it's resistance increases thus reducing the sensitivity & the efficiency of the loudspeaker. Thus power compression is dependent on these factors , increase in voice coil resistance, increase in electrical quality factors, reduction in the sensitivity and motor strength. This can result in the adverse effect of physical deformity of the loudspeaker

Increase in the voice coil temperature causes other side effects like mechanical stress to the paper, cloth and other parts of the loudspeaker. Sometimes the glue bonds and the adhesives softens resulting in loose connections or in worse case detaching from the speaker assembly when high power is applied. The main heating element in the loudspeaker is the voice coil. The heat transfer from the coil to the surrounding parts are usually through the conduction. 

Thermo-plastic materials have been offered as a better solution to conduct the heat away efficiently. In addition Ferro fluids applied to the magnetic gap of the loudspeaker is another practice to cool the voice coil. It is not without disadvantages which include high viscosity and at low frequencies, the fluid tends to leak out of the gaps. Good damping techniques must be applied to reduce the voice coil temperature and compensate for increase in coil resistance.
 

Several compensation techniques have been developed to keep voice coils at room temperature under high power inputs. Knowledge of the actual voice coil temperature under working conditions is required to compensate for this power compression and to prevent speaker damage. 

Another technique developed was a cost effective moving magnet stationary coil assembly which brings down the temperature and thus reduces the power compression to a minimum. The main advantage of this system noted Poornima is that it eliminates the air gap between the voice coil and the magnetic structure and further it also helps to dissipate the heat quickly. Poornima noted that the voice coil temperature cannot be measured directly but by it's direct co-relation to the resistance of the voice coil. 

Here Poornima proposed a thermal model and a PC controlled data acquisition system to predict the voice coil temperature at different instants of time through computer simulation. She then touched on the digital techniques to compensate for the increase in voice coil resistance in real time.

She concluded her presentation with the remarks that in spite of all the techniques used, a commercial loudspeaker, if it is not suitably matched with other source components in the audio chain offering high quality broad frequency signals, it would be a waste in terms of these developments of the loudspeaker.

Ms Poornima (left) receiving an AES plaque of appreciation from Jibby Jacob, Chairman of the AES Singapore Section - photograph by Robert Soo.

Attendees then asked some questions pertaining to the materials currently used in speaker construction and Poornima explained that a wide variety of materials are being researched and tested, each one having it's own pros and cons.  At 9.30pm the Chairman then closed the well participated session and presented a plaque of appreciation to the speaker. 

AES Singapore Section would like to thank AES member Mr Robert Soo for extending the Touch Community Theatre for this meeting.

Copyright 1999 AES Singapore Section