Homework help please!

[analogtapemania]

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.

[Ron E]

1.
20 * log10(20/60) = -9.5dB (-6dB for each doubling of distance)
73+9.5 = 82.5 dB if measured at 20 feet.

2.
Depends on the crest factor of the music. If you are treating the music as a sine wave, or talking RMS power, then 3dB of headroom means you are using half power - which is 500 W RMS or twice what the woofer is rated for.

IF you take into account crest factor and never go more than 500W even on peaks, and the crest factor is more than 3dB, you will be within the thermal power handling of the driver, but possibly not the mechanical limits depending on the signal.

3.
You got it.
delta dB = 10* log10(0.25/1) = -6dB

[janneman]

Originally posted by analogtapemania
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.

If you need to take the exam, didn't they actually taught this then?

Jan Didden