Applied Acoustics test - 23/10/2015

The SPL at 15 m from a point source is 80 dB. At which distance you find an SPL=74 dB ?

(one answer only)

The SPL at 15 m from a line source is 80 dB. At which distance you find an SPL=74 dB ?

(one answer only)

The effect of wind increases the SPL downwind and attenuates it upwind for this reason :

(multiple answers allowed)

¨  The speed of wind sums with speed of sound downstream, and subtracts from it upstream

¨  The vertical gradient of speed causes the sound rays to curve downward downstream, and upward upstream

¨  The wind cools the ground, causing a vertical temperature gradient, which curves the sound rays

¨  The wind “carries” the sound, exactly the same phenomenon of convective heat transfer

¨  It is a psychoacoustics effect, in reality the SPL is the same upstream and downwind

What’s the cause of the proximity effect over a cardioid microphone

(multiple answers allowed)

¨  The air emitted from the mouth causes noise due to turbulence on the microphone grid

¨  Sound pressure increases as 1/r, particle velocity increases as 1/r², so at short distance and low frequency the velocity level becomes much larger than the pressure level

¨  It depends on the fact that a cardioid microphone is partially sensitive to sound pressure, and partially sensitive to particle velocity (half and half)

¨  It is due to the fact that when kr<<1 sound pressure and particle velocity get out of phase

¨  It is due to the fact that when kr<<1 the acoustic impedance tends to zero

¨  It is due to the fact that when kr<<1 the acoustic impedance tends to infinity

A point source placed over a reflective plane radiates in free field with a power of 1+F/10 W. Compute the value of SPL at a distance of 10+D meters.

Lp = Lw - 11 -20*log10(d) + 3 dB

(write number and measurement unit)

A photo shows that the avreage distance between centers of vehicles running along a road is 80+10*E m. If the sound power level of a single vehicle is 100 dB(A), what's the SPL at a distance of 50+F meters?

Lp=Lwp-10*log10(a)-6-10*log10(d)

(write number and measurement unit)

At 15m from a point source, the long-term averaged spectrum of noise is as follows: 80+F dB at 125 Hz, 75+E dB at 250 Hz, 80+D dB at 500 Hz, 82+C dB at 1 kHz, 84+B dB at 2 kHz, 80+A dB at 4 kHz. Compute the total SPL in dB(A)

(write number and measurement unit)

In the case of previous exercise, re-compute the total SPL in dB(A) after inserting a noise barrier at 5m from the source with an effective height of 3+F/5 m