AN3340 STMICROELECTRONICS | Alldatasheet
Document overview
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- PDF pages: 24
Technical content
Datasheet sections
- 1 Introduction
- 2 Overview
- 2.1 What is a loud speaker
- 2.2 Why compensate a loud speaker
- 2.3 How to compensate a loud speaker
- 3 Mechanical structure of a loud speaker
- 4 Electrical model of a loudspeaker
- 5 Frequency response of a loudspeaker
- 6 Loudspeaker compensation
- 6.1 General procedure
- 6.2 Speaker Tune tool
- 6.2.1 Speaker Tune tool in practice
- 6.2.2 An example and verification
- 7 Conclusions
February 2011 Doc ID 18453 Rev 1 1/24 AN3340 Application note Loudspeaker characterization and compensation
1 Introduction
The application note describes how to measure and analyze the performance of a loudspeaker and how to compensate the frequency response using the ST Speaker Tune software, a tool available in the APWorkbench. Scope Although the application note focusses on compensation using Sound Terminal® devices, the principles can be applied to a variety of other DSPs.
2 Overview
2.1 What is a loud speaker
A loudspeaker, or often called a speaker drive, is an electro-acoustic transducer. It is able to convert an electrical signal coming from an audio power amplifier into the movement of a diaphragm. Hence the diaphragm of the speaker moves in accordance with the variations of the electrical signal to cause pressure waves in the air which the listener detects as sound. A lot of speaker models and designs are available on the market. The simplest and most diffused solution consists of a single diaphragm which is able to reproduce a large portion of the audio frequency range. This sort of device is named a “full-range speaker”. The frequency response of a full-range speaker and its cost are driving a lot of TV manufacturers to select this kind solution in a considerable part of their products, mainly in low and mid level markets. The audio performance of a full-range speaker can be improved using a multi-way speaker system where two or more speakers are connected via a filter so that each speaker reproduces only a specific portion of the audio frequency range. A very diffused example of this solution is represented by a two-way speaker system where a large diameter speaker (woofer) reproduces the low frequencies while another speaker manages the mid and high frequencies. A further step is made using three speakers: a woofer for the low frequencies, a mid-range speaker for the middle frequencies and a tweeter speaker to reproduce only the high frequencies. Of course for, a multi-way solution each speaker must be appropriately driven with the correct portion of the audio band. For a three-way loud-speaker system, for instance, a low- pass, a high-pass and a band-pass filter must be designed and implemented to properly drive each loud speaker. These filters, called crossover filters, can be implemented with passive or active components. A passive crossover filter is made using only inductors and capacitors and it is connected between the output of the power amplifier and the loud-speaker system. It is generally placed in the speaker cabinet. In the active solution the crossover filter is implemented using active components, such as operational amplifiers or DSPs (in a digital system) and it is implemented just following the pre-amplifier. In this solution each speaker is driven using a dedicated power amplifier. It can be a very expensive solution.
2.2 Why compensate a loud speaker
In the flat-screen TV market the incessant tendency to reduce the thickness of the TV set forces the speaker manufactures to design new components with very small dimensions. The result of this process impacts negatively on the quality of the sound reproduction because the frequency response of this sort of full-range speaker is very narrow and the sensitivity is quite low. In high quality flat-screen TV sets the loud-speaker system is often implemented with a 2.1-channel active solution. Here, two channels are dedicated to reproduce the mid and high frequencies (for each left and right channel) while the lowest portion of the audio band, usually below 200 Hz, is reproduced in mono using a single large speaker (a subwoofer) enclosed in a suitable box. Each speaker is driven with a dedicated power amplifier (five
amplifiers in total) and the crossover filters are implemented using the DSP already present in the audio chain. Although this solution should provide good performance, the audio quality which the viewer/listener perceives is not so good because the dimensions of the speakers are very small. Some clever and original solutions have been developed in the past, for example, speakers with two or more coils placed side by side, but the effort was directed only to solve the problem of the mechanical dimensions, with the sound performance being neglected. So it seems that the simplest way to improve the overall audio performance of a loudspeaker system is to tailor the audio signal sent to the speaker in an attempt to compensate for the deficient speaker response.
2.3 How to compensate a loud speaker
In order to improve the overall audio performance we need to match the audio signal to the speaker. A simple and effective way to achieve this is to use the equalization circuits often incorporated in audio amplifiers. In this way the signal can be modified to compensate for the poor loudspeaker response. With the ST Sound Terminal ® devices this action is very simple to perform. In the DSP section of these devices is a very flexible equalizer that can be usefully programmed to compensate the speaker frequency characteristics. This procedure can be performed manually, for each filter adjusting the frequency, the gain and the quality factor. However, this procedure needs time and the final result depends on the operator ability, aptitude and experience. Another possibility is to use the automatic feature now available in the APWorkbench software: the ST Speaker Tune. The final result is an equalization curve which reflects the inverse speaker frequency response. Thus, the overall frequency response in the final application, in a flat-screen TV, for instance, will be flatter than the uncompensated system and at the same time the frequency bandwidth will be enlarged. But the most important effect is the viewer/listener experience who perceives a positive improvement. In the next chapters the speaker compensation process using ST Speaker Tune is explained and acoustic improvements are shown.
3 Mechanical structure of a loud speaker
Figure 1. Cross section of a speaker connected to the diaphragm, to move axially through a cylindrical magnetic gap. drive signal applied, that is, the output of the power amplifier. voice coil in the neutral position when no signal is applied. range of travel of the coil. coaxial speaker, as shown in Figure 2.
4 Electrical model of a loudspeaker
decouple front and rear air-pressure waves. Figure 4. Simple electrical model of a loudspeaker – Rr represents the suspension losses. only with specific mechanical tests. reading the modulus and phase of the current. equations can be written and solved for the five unknown parameters. A intelligent selection of the five frequencies can simplify the calculation.
5 Frequency response of a loudspeaker
measurement needs a dedicated environment and specialized tools. pressure that the cone produces when a signal is applied to the voice coil. anechoic chamber, to eliminate reflections and background noise. An accepted approach is based on the MLS procedure using Audio Precision equipment. This allows the test to be performed without an anechoic chamber6. However, even with this approach, the performance of the microphone must be known. Figure 6. Speaker frequency response
Loudspeaker compensation AN3340 10/24 Doc ID 18453 Rev 1
6 Loudspeaker compensation
It should now be clear that using an amplifier with a flat frequency response in the band 20 Hz to 20 kHz does not guarantee a flat overall response for the listener. The global performance is limited by the poor frequency response of the speakers. To compensate for a non-ideal speaker response we need to modify the response of the amplifier. This can be achieved using a set of independent tunable filters to modify the amplifier response either over a bandwidth as large as possible or over a defined frequency range. The filters could be analog filters in an analog system and could be digital filters in a digital system. Audio amplifiers already having an embedded DSP , such as the one in the Sound Terminal family of amplifiers, have the advantage that the processor can also be programmed as filters to tailor the amplifier response.
6.1 General procedure
For Sound Terminal® devices the DSP can be programmed manually by adding and modifying, step by step, the parameters of the filters. This manual procedure has, of course, its drawbacks: z it is time-consuming to repeatedly modify each filter in turn and measure the frequency response. z the result is user depended. The experience of the user to select the types of filters (such as peak, low pass, high shelf) and their parameters (such as cut-off frequency, gain, quality factor) plays a fundamental role.
6.2 Speaker Tune tool
The ST Speaker Tune tool available in the APWorkbench software allows very fast and reliable frequency compensation of the whole audio chain and in detail the speaker frequency response. The final result is unique because it is the result of an automatic procedure based on an algorithm that manages the filter parameters in line with defined and optimized procedures. Speaker Tune is a very user friendly tool. It does not need special settings or specific knowledge of filters. The only information that the tool needs is the frequency response of the uncompensated loudspeaker. The speaker frequency response can be imported into the tool in two different ways z tabulated in Microsoft® Excel format z graphically. In the tabulated method, an example is given in Figure 23 on page 22, the first column contains the frequencies and the second the sound level measurements. Maximum data length is 4096 samples and maximum number of header lines is 3. The frequency response can be measured using professional equipment and with an anechoic chamber or using an MLS test (with microphone and equipment from Audio Precision). The only stipulation is that the frequency response data must be expressed in dB or dBSPL.
panel and manually add filters to simulate the speaker curve. care and a good microphone (with flat response if possible).
6.2.1 Speaker Tune tool in practice
The following steps and accompanying figures will help you to get started.
- Upload the frequency response in Exel format by pushing the button indicated with the
red arrow in the tab named Equalization Editor in Figure 7. Figure 7. APW - import data - equalization editor
- Open the dedicated Speaker Tune menu by clicking “Panels” on the main menu then
shows the ST Speaker Tune menu. Figure 12. ST Speaker Tune window z Frequency range (red arrow). where the maximum frequency is around 15 kHz. distortion and increase power dissipation. the portion ot the frequency range where the result is going to be most effective. The compensation can be optimized in terms of peak or area. single PEQ; the effect of the filter is optimized but the execution time increases. is shorter than with the recursive action but the accuracy of final result is lower too.
6.2.2 An example and verification
LCD flat-screen TV is described. in Figure 17. The resonant frequency is around 232 Hz and the peak impedance is 12 Ω. Figure 17. Flat-screen-TV speaker impedance range above 10kHz. Indeed, above 5 kHz there is significant attenuation. Figure 18. Flat-screen-TV speaker frequency response This frequency response data must now be imported into the APW Speaker Tune section.
7 Conclusions
This application note has described the basic methodology and the ST Speaker Tune tool in order to assist the user to achieve good speaker compensation results. A speaker used in a LCD flat-screen TV set has been used as a practical example to analyze the frequency performance before and after speaker compensation. It has also been shown how the number of biquads and the frequency range affect the compensation.
Revision history
Table 1. Document revision history 11-Feb-2011 1 Initial release.