Acoustics Answers

An acoustic screen 90cm square stands upright on the floor in a recording booth. The screen is being used to try and reduce the level of sounds reaching a microphone at the opposite end of the booth. Explain in general terms why the screen is very effective at reducing the level of a shaker played behind the screen, but has little effect on the level of a bass guitar amplifier that is placed on the floor behind the screen

An amplifier produces a signal gain such that an input of 1.5volts rms produces an output of 6.0 volts rms. Express the gain of this amplifier in decibels. (Calculate to the nearest decibel)

Calculate the length of an open-ended tube that would produce a fundamental frequency of 491Hz

A small concert hall measures 25m × 20m × 4m. The total absorption for the surfaces in this space is 200 absorption units at 500Hz. Calculate the RT60 reverberation time from these values. The figure you have calculated is for an empty room. Describe in general terms how this figure would be affected by a seated audience in the concert hall.

If a thin string is held in tension between two fixed points and has a fundamental frequency of 880Hz and the velocity of the wave in the string is 1400ms-1, then how long is the string?

If a point source radiates sound equally in all directions and at 4 metres from the source the sound intensity is 92dB. What will the SIL be at 8.7 metres from the source (assume inverse square law applies)

Describe, with diagrams if necessary, the operation of a bass reflex speaker
cabinet.

A student from Film Studies is attempting to record a nature documentary, but despite having a good quality cardioid condenser microphone, is having great problems recording bird-song clearly. Explain why this might be (with diagrams if necessary), and suggest alternative techniques or equipment
that may help.

A particular spring usually measures 5cm long with no weight attached. A
mass of 100g is suspended from the spring, increasing its length to 9cm.
Calculate the string constant K, and then work out the frequency that the
mass/spring system will oscillate at if set in motion.