Sound intensity level & decibels

State what is meant by the threshold of hearing for a person with good hearing.

In a hearing test, a person is found to have good hearing. Figure 4 shows the equal loudness curve produced in the test.

The eardrum of the person being tested has a cross-sectional area of 60 mm².

Calculate the power that is incident on the eardrum when the frequency is 50 Hz.

Sketch, on Figure 4, an equal loudness curve that passes through 100 dB at 1 kHz.

A stadium is full of spectators. The peak sound-intensity level at the centre of the stadium is 110 dB.

On another occasion the number of spectators in the stadium is reduced by 60%.

Estimate the peak sound-intensity level at the centre of the stadium.

You should assume that on both occasions:

• the sound intensity produced by each spectator is the same

• the spectators are distributed evenly around the stadium.

Describe the changes to a person’s hearing that may result from prolonged exposure to sound at 110 dB.

Define sound intensity.

The intensity level, in dB, of a sound is I.

What is the intensity level of a sound with double the intensity?

Tick () one box.

The amplitude of the pressure wave at the oval window of an ear is 20 times greater than at the tympanic membrane.

area of oval window = 5.9 × 10⁻⁶ m² area of tympanic membrane = 7.2 × 10⁻⁵ m²

Sketch an equal loudness curve on Figure 3 showing the normal response of a healthy ear. Annotate the frequency axis with an appropriate scale.

Describe the procedure used to gather the data for an equal loudness curve.

Calculate the intensity of a sound that produces an intensity level of 30 dB.

A point source of sound has a power of 17 W.

Calculate, in dB, the intensity level at a distance of 12 m from the source.

The frequency of a sound is increased from 3.0 kHz to 8.0 kHz with no change in intensity.

One change in the sound perceived by a person with normal hearing is an increase in pitch.

Explain one other change to the sound perceived by the person as the frequency is increased from 3.0 kHz.

Three customers, P, Q and R, are sitting in a café listening to music from a loudspeaker. Customer P is 11 m from the loudspeaker. At the position of customer P, the sound

Customer P moves to a distance of 7.0 m from the loudspeaker.

Calculate the sound intensity at the new position of customer P. Assume that the loudspeaker is a point source.

The sound intensity level is 65 dB at the position of customer Q and 42 dB at the position of customer R.

Customer Q perceives the loudness of the sound differently to customer R.

Discuss whether the use of intensity level or intensity is more appropriate to compare the perceived loudness.

Customers P, Q and R move to the same distance from the loudspeaker.

Customer P is 80 years old and has hearing loss due to her age. Customer Q is 35 years old and has hearing loss due to working in an extremely noisy environment. Customer R is 35 years old and has no hearing loss.

The hearing defects of P and Q affect their perception of the music being played.

Describe how their perceptions are different from that of R.

A source of constant output power is used to generate a sound which is measured using a sound meter.

The dashed line in Figure 3 shows the intensity level curve over a range of frequencies with the meter set to the dB setting.

A student sketches a curve, over the same frequency range, which he thinks would be obtained when the meter is changed to the dBA setting. The curve drawn by the student is shown as the solid line in Figure 3.

Discuss whether the dBA curve drawn is correct.

Mesh barriers are set up to keep pedestrians at a safe distance from a noisy drill. The maximum noise level which pedestrians should be subjected to is 110 dB The drill emits sound with a power of 7.8 W and acts as a point source. The mesh barriers are set up a distance of 2.0 m from the drill.

Discuss whether this will keep pedestrians at a safe distance from the sound source.