Projector Sound Measurement

Transcription

1 Projector Sound Measurement Author: Colin Novak, Ph.D., P.Eng. Noise Vibration Harshness and Sound Quality (NVH-SQ) January 2010 University of Windsor

2 Executive Summary Noise emissions for high powered projectors, and how they are quantified, are not widely understood. Several measurement procedures exist, but they do not adequately quantify these emissions. The fundamental theory and application of the relevant acoustic metrics are described using case results. Introduction High power projectors are designed with cooling systems to neutralize the heat generated by the lamps. One of the challenges with a projector cooling system is that the fans are a source of noise no matter how well they are designed. For the manufacturers of high power projectors, to reduce the noise emissions adds to performance and compliance concerns. However, the biggest challenge is that the methods for measuring and the numbers used to describe the acoustic emissions are not widely understood by the industry or its customers. Through the presentation of the acoustic measurement results for three similar projector models, the different types of measurement metrics for quantifying noise and the relevant standards are presented in this paper. A significant aspect associated with noise emissions is missed if one only considers the problem to be a one-dimensional sound level issue. From a consumer s perspective, the perceived quality of the noise emitted takes precedence over what traditional acoustical analysis techniques of projector fan noise may imply. Here, the use of psychoacoustic or sound quality metrics may be a more applicable analysis method as it provides the quantification of these qualitative human impressions of noise. Quantifying Projector Noise Sound pressure level: Noise is the result of slight disturbances in the ambient atmospheric pressure measured in Pascal (Pa). The perceived magnitude in pressure variation is from 2 x 10-5 Pa up to 200 Pa. The order of magnitude is 10 7 and represents an inconveniently large order of values. The human ear does not respond to sound pressure amplitude in a linear manner; but it has been found that by using a logarithmic scale with units of decibels (db) it is easier to relate human response to sounds. Using this scale, 0 db represents the threshold of human hearing and 140 db in the region of irreversible physical damage to the inner ear. Table 1. illustrates several sound pressure level (SPL) values along with common examples relating to each level. Sound pressure level (db) Typical example 0 db Threshold of hearing 35 db Bedroom at night 40 db Library 58 db Conversational speech 66 db Business office 80 db Factory floor 110 db Stage front at rock concert 140 db 25m from jet takeoff Table 1. - Typical sound pressure levels for common human activities It is also important to appreciate the perceived change in loudness of a sound for a given change in sound pressure level. Table 2. demonstrates that a change in sound level less than 3 db is not perceptible by those with normal hearing abilities. Change in sound level (db) 1-3 Barely perceptible Change in perceived loudness 5 A noticeable difference 10 Twice (or half) as loud 15 Very large difference 20 Four times (or one quarter) as loud Table 2. - Perceived loudness for corresponding change in sound level. A change of 5 db is a noticeable difference while a 10 db increase in sound level is perceived as twice as loud. From this, it can be determined that it is important to always put measured sound level data into perspective when evaluating and comparing noise sources. 2

3 The auditory field in Figure 1. illustrates the threshold limits of the human auditory system. The solid line denotes, as a lower limit, the threshold for a pure tone to be just audible. then the level measured in a reflective room. For example, the sound pressure level of a source located in the corner of a room can be as much as 9 db greater than the same source located in an open area. Because of this, sound measurements should always be conducted within a controlled environment. The most common of these are anechoic (no reflecting surface) or semi-anechoic (one reflecting surface) rooms. These rooms have wedged surfaces designed to absorb acoustic waves such that only the radiating acoustic sound waves of the source are measured. In the case of a semi-anechoic room the reflected energy from the one reflecting surface is easily determined. Figure 1. - Range of the human auditory system illustrating typical sounds with frequency and the nonlinear nature of the frequency response in the human ear. The upper dashed line represents the threshold of pain. If the limit of damage risk is exceeded for a longer time, permanent hearing loss may occur. Normal speech and music levels are in the shaded areas, while higher levels require electronic amplification. It is important to note that the human hearing system is extremely nonlinear. This means that for sounds that are very low or very high in frequency it is much more difficult for the human ear to detect them. For example, a 20 Hz tone must have an sound pressure level of 51 db to be barely perceptible as opposed to a 1000 Hz tone which can just be heard at 0 db. A similar characteristic exists for the upper frequency range of human hearing at approximately 18,000 Hz. Sound power level: One of the inherent disadvantages of quantifying the acoustic emissions of projector noise using sound pressure level is that the levels can vary with changes in distance between the source and the point of measurement. The preferred approach is to measure and report the sound power level. The sound power represents the total acoustic power generated by a source and has the advantage that it is an intrinsic property of the noise source alone and is not influenced by the environment in which the source is located or the space in which it is operated. The sound power level is measured either by using a sound intensity analyser or by measuring the sound pressure levels all around the source in an anechoic room and calculating the sound power level. The advantage of using a sound intensity probe that complies with the measurement standard ISO 9614 is that measurements can be conducted in most sound fields which removes the need for requiring a special room. The disadvantage is that the equipment can be very expensive and the measurements may take several hours to perform. The alternative is to use the standard microphone method as specified by ISO 3745 wherein the required tests are performed in a semi-anechoic room where the floor is reflective and the walls and ceiling absorb all radiating acoustic sound waves. A-weighting correction: To account for the nonlinearity of human hearing, a correction factor called the A-weighting scale is often applied to sound pressure level measurements. If used, the units of the sound level are instead given as dba. A-weighted sound level measurements are preferred over linear sound pressure level measurements for projector noise, as it better represents the perceived sound level that is detected by a person. When conducting projector noise measurements, the sound field in which the projector is located must be given careful consideration as the measured sound pressure level (db) or the sound level (dba) will be influenced by the environment in which the source is located. This means that the level measured in an environment free of reflecting surfaces will be very different 3

4 In this standard, the microphones are usually located on an imaginary hemi-spherical surface at a distance of at least 1 meter from the noise source as illustrated in Figure 2. Figure 2. - Hemi-spherical microphone placement for measuring and calculating projector sound power in a semi-anechoic room Once the sound pressure levels are measured and averaged together, the sound power level of the projector can be calculated using Equation 1. Equation 1: L W(Power) =L P(SPL) +20log(r)+11±correction_factors This approach is very fast and very repeatable with little chance for error. As such, it is the preferred method for quantifying the physical noise emissions of projector products. Irrespective of the measurement method used, sound power ratings are typically approximately 11 db greater than sound pressure levels. This is due to a constant value within the relationship expression which relates the sound pressure level to sound power level. Japan Business Machine Makers Association: Another procedure often followed for qualifying the noise emissions of digital projectors is the Guidelines for LCD Projector Measuring Procedure and Measuring Conditions established by the Japan Business Machine Makers Association (JBMMA) - Data Projector Committee. The general procedure here is to measure and logarithmically average the A-weighted sound pressure level at four locations the front, rear and both sides in both idle and operating modes of the projector in a room simulating semi-anechoic conditions. The procedure also details the height and location for each of the microphones. The caveat of this simple procedure is that the results do not fully reveal all the acoustic characteristics of a projector including tonality and frequency spectra of the noise. Unfortunately, the intent of the measurement procedure can also be circumvented to show satisfactory results for a poor performing product if the projector design is such that the cooling fan intake and exhaust ports are located away from the measurement locations as sounds from such sources can be highly directional in nature. Sound Quality In the evaluation of the acoustic comfort of a sound, fundamental quantities such as the acoustic sound pressure and sound power level do not adequately represent the actual hearing sensations perceived by the human cognitive system. The science of psychoacoustics involves the quantitative evaluation of these subjective sensations using sound quality metrics. The application of sound quality metrics provides a means to visualize the complicated relationship that exists between the physical and perceptual acoustic values. The most fundamental of the sound quality metrics is loudness which is an ISO standardized metric that describes the human perception of how loud a source is perceived as opposed to a simply reported sound pressure level. The human body, head and outer ear act as spatial and spectral filters on an acoustic signal. The inner ear also imparts nonlinear characteristics on a signal that are not reflected by a simple sound pressure level measurement. The application of the loudness metric in a measurement includes the effect of temporal processing and audiological masking effects of sounds across the frequency range. As a result, a reported loudness value (having units of sones) for projector noise emissions is a much better indication of how an end user will perceive the acoustic desirability or harshness of the product. There are other sound quality indicators that exist which are good metrics for the subjective evaluation of projector noise. Acoustic sharpness is an indicator of the annoyance of the high frequency component of noise which can be a significant characteristic of fan generated noise. Modulation metrics, such as fluctuation strength and roughness, are commonly used for applications when several cooling fans interact with each other and produce pulsating or beating sounds. Such sounds are very annoying but this acoustic unpleasantness is not adequately represented by simple sound pressure or sound power level measurements. 4

5 Case Study In order to illustrate the measurement approaches described above, a study of the acoustics of three projectors was performed by the Noise Vibration Harshness and Sound Quality (NVH-SQ) research group located at the University of Windsor. Figure 3. - Christie HD10K-M (left) and Panasonic PT-DW1000U (right) in the semi-anechoic room during testing. For this investigation, acoustical measurements were conducted on the Christie HD6K-M, Christie HD10K-M and Panasonic PT-DW10000U projectors (Figure 3.). The A-weighted sound power level for each projector was measured under different power operating conditions. Sound pressure level measurements were conducted in a semi-anechoic environment for each of the four sides of the projectors to determine the averaged A-weighted sound pressure levels as per the JBMIA guideline. At the same time, the sound quality metric of loudness was also measured. The acoustical results of the various tests are presented in Table 3. The Panasonic projector had a significant 10 db higher A-weighted sound power level compared to the Christie HD6K-M projector and a 5 db higher sound power level compared to the Christie HD10K-M projector. Projector Christie HD6K-M (full power, dual lamp operation) Christie HD10K-M (full power, dual lamp operation) Panasonic PT-DW10000U (full power) Average sound level (dba) Maximum loudness (sones) Sound power level (dba) Table 3. - Noise measurement results for projector models tested. more powerful Christie HD10K-M for the operating conditions evaluated. It should be noted that the Christie HD10K-M and Panasonic PT-DW10000U are the most comparable models as they both claim similar performance specifications in term of light output. However, these two projectors showed little difference in measured sound pressure levels but the loudness and the sound power of the Christie projectors was appreciably less than the Panasonic model. The other important observation is that the average sound levels of the Christie HD10K-M and Panasonic machines were virtually identical (a 0.2 db difference is imperceptible to even the most well trained human ear as shown in Table 2.) yet the maximum loudness and sound power level of the Christie HD10K-M was lower. In summary, it was found that while few differences were found with the measured sound pressure levels of the tested projectors, appreciable differences were realized between the measured sound power and loudness levels with the Christie models performing better than the Panasonic projector. This is significant as it demonstrates that the reporting of sound levels may have little significance in the overall quantification of projector noise and that either the sound power or loudness metrics provide more effective characterisitics of the overall sound emissions. Conclusion Noise emissions for high powered projectors and how it is quantified is not widely understood by both the industry and customers alike. An attempt is given above to demystify the fundamental concepts of noise and measurement procedures. Sound power level and sound quality metrics should be paramount for the quantification of projector noise as they provide the less ambiguous results and better correlation with the perceptual qualities for an end user. This was demonstrated by the measurement results for the three projectors for which the measured average sound level for the Panasonic model demonstrated adequate results but was shown not to be the best performer from a sound quality or sound power perspective. It was also found that the measured maximum loudness for the Panasonic PT-DW10000U projector was appreciably higher than the Christie HD6K-M projector, as well as the 5