Category: (((—Acoustics Engineering—))))

The sound intensity level LI is given by (3.24) where I is the component of the sound intensity in a given direction and Iref = 10−12 W/m2 is the reference sound intensity.

The sound power level of a source, LW, is given by (3.23) where W is the sound power of a source and Wref = 10−12 W is the reference sound power. Some typical sound power levels are given in Figure 3.5.

The sound pressure level Lp is given by (3.22) where pref is the reference pressure, pref = 20 μPa = 0.00002 N/m2 (= 0.0002 μbar) for air. This reference pressure was originally chosen to correspond to the quietest sound (at 1000 Hz) that the average young person can hear. The sound pressure level is often abbreviated as SPL. Figure 3.4 shows some sound pressure levels of typical sounds.

The range of sound pressure magnitudes and sound powers of sources experienced in practice is very large. Thus, logarithmic rather than linear measures are often used for sound pressure and sound power. The most common measure of sound is the decibel. Decibels are also used to measure vibration, which can have a similar large range of…

Again in the case of the oscillating piston, we will consider the sound power radiated by the piston into the tube. The sound power radiated by the piston, W, is (3.18) But from Eqs. (3.10) and (3.14) the power is (3.19) and close to the piston, the rms particle velocity, urms, must be equal to the rms piston velocity. From Eq. (3.19),…

Consider the case again of the oscillating piston in Figure 3.1. We shall consider the sound energy that is produced by the oscillating piston, as it flows along the tube from the piston source. We observe that the wavefront and the sound energy travel along the tube with velocity c metres/second. Thus after one second, a column of…

We see that for the one‐dimensional propagation considered, the sound wave disturbances travel with a constant wave speed c, although there is no net, time‐averaged movement of the air particles. The air particles oscillate back and forth in the direction of wave propagation (x‐axis) with velocity u. We may show that for any plane wave traveling in…

As the piston vibrates, the gas immediately next to the piston must have the same velocity as the piston. A small element of fluid is known as a particle, and its velocity, which can be positive or negative, is known as the particle velocity. For waves traveling away from the piston in the positive x‐direction, it can be…

With sound waves in a fluid such as air, the sound pressure at any point is the difference between the total pressure and normal atmospheric pressure. The sound pressure fluctuates with time and can be positive or negative with respect to the normal atmospheric pressure. Sound varies in magnitude and frequency and it is normally…

The propagation of sound may be illustrated by considering gas in a tube with rigid walls and having a rigid piston at one end. The tube is assumed to be infinitely long in the direction away from the piston. We shall assume that the piston is vibrating with simple harmonic motion at the left‐hand side…