Working with Decibels

Transcription

1 Chapter 4 Working with Decibels Power Decibels Without a Calculator When working with decibels for power signals, you can use a rule of thumb called the 3 and 10 Rule to help you solve some level conversions without the aid of the calculator. This rule states that every time you double a power level there is a 3 db increase. Conversely, cutting a power level in half will result in a 3 db decrease. Multiply a power level by 10 and you will get a 10 db increase. Dividing a power level by 10 will result in a 10 db decrease. We can prove the 3 and 10 Rule mathematically based on the definition of power difference decibels (or power decibels for short), denoted Power. Power where is the original power and is the power we are comparing to the original power. First we show that each time you double the power, you increase the power by about 3 db: Now we show that each time you multiply the power by 10, you increase the power by about 10 db: Combining the two observations, we have the following:, Power Working with Decibels Page 1

2 Using these basic rules, you can do many simple level conversions in your head without a calculator. Consider the following questions: Question 1: How many db greater than 4 watts is 64 watts? We can solve this problem using the 3 part of the 3 and 10 rule :. Thus, Having shown that we know that. Thus, Answer: 64 watts is about 12 db greater than 4 watts. Question 2: How many db greater than 10 watts is 1000 watts? We can solve this problem using the 10 part of the 3 and 10 rule : Thus,. Having shown that we know that. Thus, Answer: 1000 watts is about 20 db greater than 10 watts. Question 3: How many db greater than 1 watt is 80 watts? We can solve this problem using both parts of the 3 and 10 rule : Working with Decibels Page 2

3 Observe that. Thus, Having shown that we know that Thus, If you prefer, you can do this calculation in your head stepwise this way: Answer: 80 watts is 19 db greater than 1 watt. Question 4: How many db less than 1 watt is 1 milliwatt (0.001 watts)? We can solve this problem using the 10 part of the 3 and 10 rule :. Thus, Having shown that we know that Thus, Answer: 1 milliwatt is 30 db less than 1 watt. Question 5: How many db less than 1 watt is 4 milliwatts (0.004 watts)? Observe that. Having shown that we know that Thus, Working with Decibels Page 3

4 Answer: 4 milliwatts is 24 db less than 1 watt. With a Calculator If you have a calculator, you can perform more complicated conversions using the formula for power decibels. P0 is the power level you started with and P1 is the power level you end up with. Power Question 1: How many db greater than 63 watts is 700 watts? Power Answer: 700 watts is db greater than 63 watts. Question 2: How many db greater than 3,000 watts is 10,000 watts? Power Answer: 10,000 watts is 5.23 db greater than 3,000 watts. Question 3: You want to purchase a new power amplifier for the sound system in your car. You want an 800 watt amplifier but can only afford a 350 watt amplifier. How many db of sound are you going to sacrifice using the smaller amplifier? Power Answer: The 350 watt amplifier is only about 3.59 db quieter than the 800 watt amplifier. 3 db is smallest difference in level that average listeners can perceive. In other words, the difference in level between these two amplifiers will be barely perceptible to the average listener. Power Ratios Sometimes you don't have two power levels to compare, but you know the db difference you d like to achieve. In these situations you need to figure out the power ratio for the db Working with Decibels Page 4

5 difference you need and then multiply that ratio by a known power level. The following equation can be used for this purpose: Question 1: You currently have a 300 watt amplifier in your sound system. You would like to purchase an amplifier that is 13 db louder. How many watts do you need? Answer: You will need a 6000 watt amplifier (rounded up from 5986) to get another 13 db out of your sound system. In this situation, it might be less expensive to get a more sensitive loudspeaker or figure out a way to send your 300 watt amplifier a line-level voltage signal that is 13 db higher. Question 2: If a loudspeaker can generate 60 dbspl at 1 meter with 2 watts of power, how much power would be required to get 98 dbspl at 1 meter from the loudspeaker? Answer: The power amplifier would need to put 2000 watts into the loudspeaker in order to generate 98 dbspl at 1 meter. Energy Decibels Without A Calculator Any time you work with voltage levels (dbu and dbv) or pressure levels (dbspl), you use the formulas for energy difference decibels (or energy decibels for short, and sometimes called voltage decibels, since sound pressure is often measured in volts). With energy decibels, you can do some conversions in your head just like you do with power decibels. This time the rule of thumb is called the 6 and 20 Rule. Doubling a voltage will result in a 6 db increase. Multiplying a voltage by 10 will result in a 20 db increase. Distance changes Working with Decibels Page 5

6 work reciprocally. Doubling the distance a sound wave travels in the air will result in a 6 db decrease in dbspl. Multiplying by 10 the distance a sound wave travels in the air will result in a 20 db decrease in dbspl. We can prove the 6 and 20 Rule mathematically based on the definition of voltage (or energy) difference decibels, which we denote as Voltage. where is the original voltage and is the voltage we are comparing to the original voltage. Below we show that each time you double the voltage, you increase the energy by approximately 6 db: Now we show that each time you multiply the voltage by 10, you increase the energy by about 20 db: Combining the two observations, we have the following: We can also show the 6 and 20 Rule for distance differences, based on the equation below. Notice that the original distance,, is in the numerator and the new distance,, is in the denominator. where is the original distance and is the distance we are comparing to the original distance. First we show that each time you double the distance, you decrease the energy by approximately 6 db: Working with Decibels Page 6

7 Now we show that each time you increase the distance by a factor or 10, you decrease the energy by approximately 20 db: To prove the full 6 and 20 Rule, we do the following:, Question 1: How many db greater than 2 volts is 10 volts? Answer: 10 volts is 14 db greater than 2 volts Question 2: How many db greater than 1 volt is 100 volts? Answer: 100 volts is 40 db greater than 1 volt. Question 3: If a loudspeaker measures 100 db SPL at 3 feet, how loud will it be at 30 feet? Answer: This loudspeaker will produce 80 db SPL at 30 feet. Working with Decibels Page 7

8 With a Calculator Using a calculator, you can perform more complicated db conversions with voltage levels. When working with voltage levels, you multiply by 20 the base 10 log of the voltage you ended up with divided by the voltage you started with. When working with db loss over distance level you use the same formula, but you divide the distance you started with by the distance you ended up with. If you know the db difference and one of the voltage values, you can find the voltage ratio of that db difference and multiply that by the voltage you started with to find the voltage level for that db difference. Here are the equations again: Also note in the following questions that dbv and dbu are both used for voltage decibels, but with different reference voltages. The reference voltage for dbv is 1 V while the reference voltage for dbu is V. Question 1: How many db greater than 2 volts is 63 volts? Answer: 63 volts is 30 db greater than 2 volts. Question 2: Professional line level is +4 dbu. O dbu = volts. How many volts is professional line level? Answer: Professional line level is 1.23 volts Question 3: If a loudspeaker generates 113 dbspl at 1 meter, how loud will it be at 30 meters? Working with Decibels Page 8

9 Answer: The loudspeaker will generate 83.4 dbspl at 30 meters Working with Power and Voltage Decibels Sometimes there will be situations where you will need to perform calculations involving both energy decibels and power decibels. For example, you might need to choose an appropriate power amplifier that will allow your loudspeaker to achieve a certain SPL level at a known distance. So you re trying to figure out what difference in power will result in a certain value in voltage. Question: The sensitivity of a loudspeaker is 97 dbspl at 1 meter with 1 watt of power. How powerful does the amplifier need to be in order for the loudspeaker to generate 100 dbspl at 10 meters away? First, calculate the db loss for 10 meters. This is an important step because you don t really know what the loudspeaker is capable of at 10 meters, but you know what it can do at 1 meter. So if you need 100 dbspl at 10 meters, how loud does it need to be at 1 meter? You can do this one in your head since we re dealing with a multiple of 10 (a loss of 20 db). If the loudspeaker needs to be 100 db SPL at 10 meters, it will need to be 20 db louder at 1 meter since you will lose 20 db by traveling that distance through the air. We know the loudspeaker can do 97 dbspl with only 1 watt. Before we can figure out how many watts is needed for 120 dbspl, we need to figure out the difference between these two values. In other words, we need 23 db more than the 97 dbspl that we get with only 1 watt. Now let s find the power ratio of 23 db. Working with Decibels Page 9

10 Now we can take this power ratio for 23 db and multiply it by the number of watts we already know about. In this case, that s 1 watt. Answer: You will need a 200 watt amplifier in order to generate 100 dbspl at 10 meters with this loudspeaker. Working with Decibel Reference Levels Technically, decibels are merely a measurement of the difference between two values. As discussed in Chapter 4, there are several reference levels that have been agreed upon in the professional sound community. When working with sound, you may need to convert a decibel value from its scale in one reference level into its position relative to a different reference level. The most common conversion is between dbu and dbv. Professional grade equipment uses dbu in its specifications while consumer grade equipment used dbv. However, consumer and professional grade equipment are often interconnected in sound systems. Let s begin by simply defining some known reference levels. Question 1: Nominal operating line level for consumer grade equipment is 10 dbv. How many volts does 10 dbv represent? First, find the Voltage Ratio of 10 db: Now multiply this voltage ratio by the reference level for 0 dbv (1 volt): Answer: 10 dbv represents an actual voltage level of volts. This means that unity gain for consumer equipment is when the signal is at volts. Question 2: Nominal operating line level for professional grade equipment is +4 dbu. How many volts does +4 dbu represent? First, find the voltage ratio of +4 db: Working with Decibels Page 10

11 Now multiply this voltage ratio by the reverence level for 0 dbu (0.775 volts): Answer: +4 dbu represents an actual voltage level of 1.23 volts. This means that unity gain for professional equipment is when the signal is at 1.23 volts. As you can see, there is a difference in nominal operating voltage level for consumer and professional grade equipment. This means that if you connect a consumer grade device to a professional grade device, you ll need to compensate for the difference in level using a preamplifier in order to maintain an optimized gain structure. Consider the situation where you re connecting two CD players to a mixing console. One CD player is a professional grade model, and the other is a consumer grade model. These two CD players will be generating their analog output signal at different levels. You can compensate for this using the preamplifiers in the mixing console. Let s see how much you ll need to compensate: Question 3: What s the db difference between professional line level and consumer line level? We know from the previous two questions that professional line level is 1.23 volts and consumer line level is volts. Let s just figure out the db difference between these two voltage levels: We can t really hear changes less than 1 db, and it would be difficult to dial in a fraction of a db on an analog preamp. So it would be appropriate to simply round this down to 12 db. Answer: The db difference between professional and consumer line level is 12 db. In other words, you ll have to turn up the preamplifier on the consumer CD player by 12 db to get the level in the mixing console to match the professional CD player. Working with Decibels Page 11

12 Practice Problems 1. If you want to replace a 5 watt power amplifier with an amplifier that was 7 db more powerful, how many watts will you need? 2. When looking to purchase a new power amplifier, you find two power amplifiers with similar features. The more expensive amplifier is 900 watts, and the less expensive amplifier is only 200 watts. How many more db will your money buy with the 900 watt amplifier? 3. An EAW UB12se model loudspeaker has a sensitivity of 89 dbspl at 1watt/1meter. Its peak dbspl is specified at dbspl at 1 meter. What is the peak power handling of this loudspeaker? 4. For this same UB12se loudspeaker, how many dbspl will you get at 1 meter with a 50 watt power amplifier? 5. What power would be 18 db greater than 3 watts? 6. What voltage would be 14 db greater than 19 volts? 7. 0 dbu = volts. 0 dbv = 1 volt. What is the db difference between the two? 8. If a loudspeaker can produce 102 dbspl at 5 feet, how loud will it be at 25 feet? 9. For this same loudspeaker, how loud will it be if you then move 7 feet closer from 25 feet? 10. If a loudspeaker s sensitivity is 98 db 1w/1m, how loud will it be at 23 feet away with 300 watts of power? 11. If a loudspeaker s sensitivity is 105 db 1w/1m, how much power will be needed to generate 98 dbspl at 35 feet? Working with Decibels Page 12