Some Audio Related Questions

[ChrPie]

Here are some questions that I need help with, I'm a music performance major, but have to take a physics of sound class to graduate, please help!

I have three questions:
1. Your city ordinance states that a motor vehicle is in violation if it produces a sound level of 80 dBA at 20 feet from the vehicle. You're standing 60 feet away with your SPL meter which reads 73 dBA. Is the vehicle in violation of the ordinance? by how many dB?

-I think this might have to do with inverse square law so I think it would be yes, but I have no idea by how many dB.

2. You have a 1000 watt power amp. If you intend to maintain 3dB of headroom what is the spl level for a speaker system that is rated 94dBspl for a 1 watt input at 1 meter? at a distance of 10 meters? If the power handling spec for the speaker is 250 watts rms is the speaker likely to blow?

-I have no idea on this one.

3. A speaker system has a sensitivity spec such that it produces 96dBspl at 1 meter with 1 watt input. What will be its output at 1 meter with an input of .25 watts

-Would it be 90dBSPL?

Hopefully someone can help me out, I've tried to get my dad to help me, but he had no idea either.

 

[anathema]

Hi there, as a scientist with a background in acoustics and sound, I'd be happy to help you with these questions!

1. Based on the information given, the vehicle is not in violation of the ordinance. This is because the sound level at 60 feet is below the limit of 80 dBA at 20 feet. To calculate the exact difference in dB, we can use the inverse square law, which states that sound intensity decreases by 6 dB for every doubling of distance. Since 60 feet is three times the distance of 20 feet, the sound level decreases by 6 dB three times, resulting in a difference of 18 dB. Therefore, the sound level at 60 feet would be 73 dBA - 18 dB = 55 dBA, which is below the limit of 80 dBA.

2. To calculate the SPL level for the speaker system, we can use the formula SPL = 20*log(Po/Pref), where Po is the output power and Pref is the reference power (1 watt in this case). For a 1000 watt power amp, the output power would be 1000 watts. Plugging this into the formula, we get SPL = 20*log(1000/1) = 60 dB. This is the SPL level at 1 meter.

To calculate the SPL level at 10 meters, we can use the inverse square law again. Since we know the SPL level at 1 meter, we can calculate the difference in dB at 10 meters by using the formula 20*log(d2/d1), where d1 is the initial distance and d2 is the new distance. In this case, d1 = 1 meter and d2 = 10 meters, so the difference would be 20*log(10/1) = 20 dB. Therefore, the SPL level at 10 meters would be 60 dB - 20 dB = 40 dB.

As for the power handling of the speaker, we can use the formula Pmax = 2*Pnom, where Pmax is the maximum power handling and Pnom is the nominal power handling. In this case, Pnom = 250 watts, so Pmax = 2*250 watts = 500 watts. Since the output power of the amp is 1000 watts, the speaker is not likely to blow.

3. Your calculation is correct, the output at 1 meter with

 

[ogb p]

I can provide some answers and explanations to your questions.

1. Based on the information provided, the vehicle is not in violation of the ordinance. This is because the sound level at 60 feet is lower than the limit of 80 dBA at 20 feet. The difference in sound level is 7 dBA, which is less than the limit. The inverse square law does apply, but in this case, it works in favor of the vehicle as the sound level decreases with distance.

2. To calculate the SPL level for a 1 watt input at 1 meter, you can use the formula SPL = 20log(Po/Pr), where Po is the output power and Pr is the reference power (1 watt in this case). So for a 1000 watt power amp, the SPL level would be 120 dBSPL. At a distance of 10 meters, the sound level would decrease by 20 dB due to the inverse square law, resulting in an SPL level of 100 dBSPL. As for the speaker handling, it is not likely to blow as long as the power input does not exceed its RMS rating of 250 watts.

3. Your calculation for the output at 1 meter with an input of 0.25 watts is correct. The output would be 90 dBSPL.

I hope this helps clarify some of the concepts and calculations involved in these questions. Physics of sound can be challenging, but with practice and understanding of the principles, you will be able to tackle these types of questions with confidence. Don't hesitate to seek help from your professor or classmates if you need further clarification. Good luck with your studies!