Acoustics Answers

a. Using only two components, draw the electrical circuit diagram of a simple low-pass filter and explain its operation in general terms. There is no need to include precise component values.

b. Draw roughly the frequency response of a low-pass filter with a cut-off frequency of 250Hz.

A small concert hall measures 25m x 20m x 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 880 Hz and the velocity of the wave in the string is 1400ms-1 then how long is the string?

a. 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 metres from the source (assume inverse square law applies)
b. If the point source in part a is fixed to the middle of a large wall so that it can only radiate sound in a hemispherical pattern, explain how this would alter the situation.

If eight violinists in a string section each separately generate an SPL of 65dB then calculate the SPL when all eight violins play together in unison.

a. A sine wave sweep is played through a speaker in a large room. An SPL meter shows a significant rise in the level of the sine wave at 43Hz, 86Hz and 129Hz. Assuming that this is caused by standing waves between one set of parallel walls, calculate the distance between the walls.

b. From the information given above state the next frequency that is likely to cause a rise in level of the meter. What will the wavelength of this frequency be?

Draw (approximately) the response patterns of cardioid and omni-directional microphones. Include an indication of the frequency of your polar response.

Describe briefly the operations of a dynamic and condenser microphone, with diagrams if necessary.

The frequency of human hearing is typically stated as being between 20Hz to 20kHz. Calculate the wavelengths of these two frequencies in air.

Sketch the line spectrum of a simple sine wave, sawtooth wave and a square wave. You only need to show approximate amplitudes for harmonics - but give an indication of the harmonics you would expect to see and their relative amplitude.