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A soprano and a bass are singing a duet. While the soprano sings an A-sharp at 932 Hz, the bass sings an A-sharp but three octaves lower. In this concert hall, the density of air is 1.20 kg/m3and its bulk modulus is1.42×105Pa. In order for their notes to have the same sound intensity level, what must be (a) the ratio of the pressure amplitude of the bass to that of the soprano and (b) the ratio of the displacement amplitude of the bass to that of the soprano? (c) What displacement amplitude (in m and in nm) does the soprano produce to sing her A-sharp at 72.0 dB?

Short Answer

Expert verified

A) The ratio of pressure amplitude of the bass to that of the soprano is 1.00.B) The ratio of displacement amplitude of the bass to the soprano is 8.00.C) The displacement amplitude produced by the soprano to sing A-sharp at 72.0 dB is 47.3 nm

Step by step solution

01

STEP 1 Calculate the ratio of pressure amplitude of the soprano from the relation between the intensity to pressure

Formula is lS=vp2maxs2Bwhere, p2maxsis the pressure amplitude of the soprano. B is the bulk modulus of the air, lSis the intensity of the soprano. is the speed of sound in air.

The intensity of the bass is role="math" localid="1668079470446" lb=124π2fb2Ab2ρBand the intensity of the soprano is ls=124π2fs2As2ρBwhere, lbis the intensity of the bass, B is the bulk modulus of the air. p is the density of the air. fb. is the frequency of the bass, fs is the frequency of soprano and is the intensity of soprano

02

Calculate the ratio of pressure amplitude of the soprano

Rewrite the above relation in terms of p2maxswhich is given aspmaxs=2Blsv

Formula to calculate the pressure amplitude of the bass ispmaxb=2Blbv where, pmaxbis the pressure amplitude of the basslb is the intensity of the bass

It is given that the intensity of the both the bass and soprano is same. Divide equation (Il) by the equation (l) to findrole="math" localid="1668080143577" pmaxb/pmaxs

role="math" localid="1668080153255" pmaxbpmaxs=2Blsv2Blsv=lbls

Substitute I for lb and I for ls in the above equation to findpmaxb/pmaxs

pmaxbpmaxs=ll=1.00

Therefore, the ratio of pressure amplitude of the bass to that of the soprano is 1.00.

03

Calculate the ratio of displacement amplitude of the bass to the soprano

The intensity of the bass is lb=124π2fb2Ab2ρB (Ill)and the intensity of the soprano is ls=124π2fs2As2ρB(IV)

It is given that the intensity of the both the bass and soprano is same so that.

Substitute equation (Ill) and (IV) in the equation (V) to findAbAs

124π2fb2Ab2ρB=124π2fs2As2ρBfb2Ab2=fs2As2Ab2As2=fs2fb2AbAs=fsfb

Formula to calculate the frequency of the bass is, Substitute 932 Hz for fs in the above equation to find fb

fb=fs8=116.5Hz

Thus, the frequency of the bass is 116.5Hz

Substitute 116.5 Hz for fb and 932 Hz for fs to find Ab/As

AbAs=932Hz116.5Hz=8.00

Therefore, the ratio of displacement amplitude of the bass to the soprano is 8.00.

04

Calculate the displacement amplitude of the soprano

Formula is given byA=12πf2lρB and the intensity of the level of sound is Where,β=10logll0is the intensity level of the sound,l0is the intensity level of the reference sound.

Substitute 72.0 dB forβ and 1×10-12W/m2in the above equation to find I

72.0dB=(10dB)log11×1012W/m2log11×1012W/m2=72.0dB(10dB)I=10721×1012W/m2=1.585×1015W/m2

Thus, the intensity of the soprano is 1.585×10-15W/m2

A=12π(932Hz)21.585015W/m21.20kg/m31.42×105Pa=4.73×108m=47.3nm

Therefore, the displacement amplitude produced by the soprano to sing A-sharp at 72.0 dB is 47.3 nm

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