Although Rayleigh [9] completed the first edition of his 'The Theory of sound' without the benefit of electroacoustical instrumentation (only in the second edition did he mention 'Bell's Telephone' with no thought of using this device for measurement), modern acoustical measurements rely invariably on electroacoustics. Anyone intending to measure any of the descriptors described above would want to turn the acoustical signal into an electrical signal (if not a digital signal) as early in the measurement process as possible, and would use some of the electroacoustical instrumentation shown in Figure 1.

Measurement Microphones

The transducer which converts the acoustical signal to an electrical one is usually a condenser microphone. Given the requirement for a 120 dB dynamic range (12 orders of magnitude) this is one of the few suitable transduction mechanisms. The condenser microphone operates on the principle that the capacitance of two electrically charged plates varies with the separation between them. The charge may be generated by an external polarising voltage, or by the inclusion of an electret material into one of the plates. One of the plates is an extremely light diaphragm which moves in response to acoustic pressure variations and the resulting change in capacitance then produces the output voltage.

Because the introduction of the microphone into an acoustic field will change that field there are three different sensitivities defined for a measurement microphone [10,11]:

• The pressure sensitivity is defined as the voltage produced for a given sound pressure applied uniformly over the diaphragm of the microphone.
  
  
• The free-field sensitivity is defined as the voltage produced for a given sound pressure in a free progressive sound field that existed before the introduction of the microphone.
  
  
• The diffuse-field sensitivity is defined as the voltage produced for a given sound pressure in a diffuse field (i.e. one where the sound is equally likely to arrive from any direction) that existed before the introduction of the microphone.

In order for condenser microphones to have sufficient sensitivity they are typically 1 cm in diameter. Because the wavelength of sound is comparable with this at the higher audible frequencies there can be a significant difference (up to 15 dB) between these different sensitivities due to diffraction effects.

Few real acoustic fields will fall into one of these ideal cases but the procedural standards define which should be used for a particular measurement and the uncertainties have to allow for the possible differences.

Sound Level Meters

The most common form of instrumentation to use the signal from the microphone is a sound level meter [12]. There are two types of sound level meter: An exponential-averaging meter [13] is designed for the measurement of continuous sounds. It measures the root mean square value of the sound pressure. The signal from a microphone is fed to amplifier and weighting circuits which limit the frequency content to a prescribed range. The signal is then squared and passed through a single-pole low-pass filter providing a prescribed exponential time constant. This signal is then displayed in decibels as a sound pressure level.

An integrating-averaging sound level meter [14] is designed to measure sounds from specific events. It detects, frequency weights, and squares the signal in a similar way to an exponential-averaging meter. Then the squared signal is integrated. The logarithm of the integrated signal has the logarithm of time subtracted from it. The result is then displayed in decibels as an 'equivalent continuous sound pressure level' (Leq).

Instruments for the Measurement of Sound Intensity

Sound intensity is a measure of the magnitude and direction of the flow of sound energy. The unit is Wm-2. Although acousticians have attempted to measure sound intensity since the 1930s [15], the first reliable measurements of sound intensity did not occur until the late 1970s when advances in digital signal processing and the availability of digital instrumentation allowed commercial instruments to be produced. Most modern measurements of sound intensity are made using the simultaneous measurement of sound pressure with two closely spaced microphones. The sound intensity can be calculated from the mean pressure, the pressure difference (including phase information) and a knowledge of the specific acoustic impedance of the air. Sound intensity can be expressed in decibels relative to 10^-12 Wm^-2 when it is known as sound intensity level.

The measurement of sound intensity as a way of determining the sound power of a source offers distinct advantages over traditional methods which would require the source to be put in a specialised environment (either anechoic, hemi-anechoic, or reverberant room). But the instrumentation [16] and procedural standards are complicated and the method is not yet in widespread use.

Calibration Sources

Several devices exist which allow the routine calibration of measuring instrumentation.

A sound calibrator [17] contains a small, stable sound source which can be coupled to the microphone of a measuring instrument. The simplest device will generate a sound pressure at a single frequency and level, more complicated devices will generate a variety of levels and frequencies. They allow the calibration of an entire measurement system to be checked.

A reference sound source [18] generates a stable broad-band noise at a known sound power and allows systems that determine the sound power of machines to be checked.

Neither of these devices are absolute. They both require calibration themselves with microphones of known sensitivity.

Ear Simulators

Ear simulators [19] are standardised devices used in the calibration and characterisation of audiometers, earphones, hearing aids, telecommunications and audio equipment. There are a wide variety of designs of ear simulators standardised by different organisations. They range in complexity from a simple hollow cylinder with a microphone diaphragm occupying one end to a series of interconnected cavities, resistances and controlled leaks designed to simulate not only the acoustic impedance of the human ear but also designed to account for the way a user might apply an earphone to the ear.

Ear simulators for audiometric earphones allow audiometers to be calibrated in terms of 'hearing level', i.e. a decibel scale where 0 dB is intended to represent the threshold of hearing for 'otologically normal people'. This was achieved by testing the hearing thresholds of large groups of subjects and relating those thresholds to the sound pressures developed in the ear simulators. Consequently each audiometric ear simulator has a set of Reference Equivalent Threshold Sound Pressure Levels (RETSPLs) which allows the output of an earphone to be related to hearing level.

An ear simulator that simulates the bone conduction path is known as a mechanical coupler and a set of Reference Equivalent Threshold Force Levels (RETFLs) allow the output of bone vibrators to also be related to hearing level.

Compatibility Issues

Measurement microphones, sound calibrators and sound level meters are normally produced to standard patterns, in principle enabling a device from one manufacturer to be used with an associated device from a second manufacturer. However caution should be exercised with such practice. First, devices of a specific type are likely to have a different performance when used with associated devices from different manufacturers, e.g. a sound calibrator may produce different sound pressure levels on two generically similar microphones, but of different types. Second, the control of production tolerances may be different between manufacturers, resulting in subtle incompatibilities that can seriously affect performance or even cause damage to the devices, e.g. a microphone may fit well in its designated sound calibrator, but may be excessively tight in a sound calibrator from another manufacturer.

For the reasons given above, it is recommended that instrumentation from different manufacturers should only be combined where a manufacturer has stated specifically that this is acceptable. Ideally the manufacturer should also state the validity of any calibration and provide corrections where necessary to enable existing calibration data to be utilised for the specific combination of device types.

Precision Requirements >>