Measurements provide defi nite quantities which describe and rate sounds.
Sound measurements also permit precise, scientifi c analysis of annoying sounds. The measurements give us an objective means of comparing annoy- ing sounds under different conditions.
Sound levels are measured using a microphone attached to some form of electrical signal conditioning and analysis equipment. There are many types of microphone with different operational means of converting fl uctu- ating pressure (or pressure difference) into an electrical signal. Two basic types of microphone are condenser and electro-dynamic. The condenser microphone is favoured for vehicle noise investigations because of its fre- quency response. Electro-dynamic microphones start to lose sensitivity around 80Hz and are down considerably at 50Hz. Condenser micro- phones, on the other hand, remain linear below 50Hz and some are linear even down to 20Hz. Condenser microphones are also more sensitive than electro-dynamic types because of their use of pre-amplifi ers.
The pre-amplifi er converts the microphone’s high output impedance to low impedance suitable for feeding into the input of accessory equipment.
This impedance conversion next to the microphone serves to minimize the pickup of noise in the signal cable to the accessory equipment.
4.4.1 Condenser microphones
The operating principle of a condenser microphone is to use a diaphragm as the moving electrode of a parallel plate air capacitor. It features a tensioned metal (nickel) diaphragm supported close to a rigid metal back-plate. The microphone’s output voltage signal appears on a gold-plated terminal mounted on the back-plate which is isolated from the microphone casing (or cartridge) by an insulator. The cartridge’s internal cavity is exposed to atmo- spheric pressure by a small vent and the construction of the microphone is completed with the addition of the distinctive diaphragm protective grid.
The diaphragm and back-plate form the parallel plates of a simple air capacitor which is polarized by a charge on the back-plate. When the dia- phragm vibrates in a sound fi eld the capacitance varies and an output voltage is generated. The voltage signal replicates the sound-fi eld pressure variations as long as the charge on the microphone back-plate is kept fi xed.
The sensitivity of the condenser microphone is discussed below.
Sensitivity
As might be expected, the larger the electrodes within the microphone, the greater is the voltage produced by a given defl ection of the diaphragm, and the greater is the sensitivity of the microphone.
Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com ETH Zuerich (307-97-768) Sunday, August 28, 2011 12:01:07 AM IP Address: 129.132.208.2
Precision condenser microphones usually come in four sizes, denoted by their external diameters:
• One inch (25.4mm)
• Half inch (12.7mm)
• Quarter inch (6.35mm)
• Eighth inch (3.175mm).
Most of them are omnidirectional, i.e. sensitive to sound arriving from all directions. The two smallest microphones have the best omni- directional characteristics at audio frequencies. They respond equally to all frequencies arriving from all directions because their physical presence in the sound fi eld does not have a big infl uence on incoming sound waves. The larger (one-inch) microphones, as a direct result of their size, are not sensitive to frequencies above 5kHz which approach from the sides and rear of the microphone. The omnidirectionality of the one-inch microphone can be improved by fi tting a nose cone or the special windscreen.
The open-circuit sensitivity is usually quoted for direct comparison between microphones. This is the voltage (mV) produced per pascal of pressure at 250Hz with 200V polarization (except for a few microphones requiring 28V or 0V polarization). The open-circuit sensitivity is the sen- sitivity of the capsule on its own before it is electrically loaded by being attached to a pre-amplifi er. The sensitivity of the microphone varies with temperature and atmospheric pressure.
Frequency response
There are three basic types of microphone:
• Free-fi eld-response
• Pressure-response
• Random-response.
Free-fi eld-response microphones are used for measuring sound coming mainly from one direction. Their frequency response curve is designed to compensate for the pressure build-up at the diaphragm caused by interfer- ence and diffraction effects. Measured sound-pressure levels are assumed equal to those that would exist in the sound fi eld if the microphone were not present. This means that the physical size of the microphone is small compared with the wavelength of the impinging sound, its presence does not affect the local sound fi eld and it measures the true fl uctuating pressure.
The response of the microphone in these conditions is known as the free- fi eld response. At higher frequencies – near the resonant frequency of the microphone – the impinging sound is refl ected and diffracted by the pres- ence of the microphone and, as a result, the pressure at the diaphragm is
Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com ETH Zuerich (307-97-768) Sunday, August 28, 2011 12:01:07 AM IP Address: 129.132.208.2
increased from its free-fi eld value. The response of the microphone to this artifi cially higher pressure is the pressure response.
It is also possible to construct microphones that respond linearly with frequency (up to a limiting frequency) to the actual pressure at the dia- phragm. These are known as pressure-response microphones. Pressure- response microphones do not compensate for the pressure build-up at the microphone diaphragm – they measure the actual sound-pressure level at the diaphragm. Uses include measuring sound pressure levels at a surface or in a closed cavity. Pressure-response microphones can be used as free- fi eld microphones if they are oriented at right angles to the direction of sound propagation, but their effective frequency range is then reduced.
When making measurements within small cavities or couplers or when the microphone diaphragm is mounted fl ush with a hard surface, it is the pres- sure response that is of interest. When making measurements in a diffuse fi eld it is the random incidence response that is of interest and a random incidence microphone should be used.
The frequency response of any device that operates over a range of fre- quencies is that range of outputs that are within 3dB of a 0dB reference line as shown in Fig. 4.6. This −3dB point usually occurs at between 1 and 3Hz at low frequencies, above which the open-circuit sensitivity of a preci- sion condenser microphone remains constant with changing frequency.
However, the sensitivity does become frequency-dependent at higher fre- quencies. The change in sensitivity with frequency is known as the fre- quency response of the microphone. At higher frequencies the microphone’s
dB
+5
0
–5
Free-field response
Pressure response
Random incidence response
Hz
50 200 1000 5000
4.6 Frequency responses of free-fi eld, pressure- and random-response microphones (courtesy of Brüel & Kjær Sound & Vibration A/S, Measuring Sound, 1988, p. 22).
Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com ETH Zuerich (307-97-768) Sunday, August 28, 2011 12:01:07 AM IP Address: 129.132.208.2
frequency response curve tails off after the diaphragm resonance. The high frequency cut-off is the frequency at which the frequency response curve falls 2dB below the 0dB reference line.
Dynamic range
The lower limit of the dynamic range of the condenser microphone and pre-amplifi er combination is determined by the levels of internal (electri- cal) noise. The upper limit is determined by distortion, i.e. the unacceptable change to the wave shape of the sound waves being sensed by the micro- phone. Typical dynamic ranges are:
• One-inch microphone: 12–150dB(A)
• Half-inch microphone: 25–155dB(A)
• Quarter-inch microphone: 40–170dB(A)
• Eighth-inch microphone: 55–175dB(A).
Signifi cant levels of distortion can be expected with any size of microphone at very high sound levels (140dB and above, i.e. 200Pa and above).
Pre-amplifi ers
Pre-amplifi ers are designed to have input impedance of around 10–50GΩ. The input capacitance is usually around 0.2pF, which is rather small com- pared with the capacitance of the polarized microphone capsule (around 3–65pF at 250Hz, with the smallest value for eighth-inch microphones and the largest for one-inch microphones). The high input impedance produces a reasonable voltage level from the charge output of the microphone capsule. The noise fl oor of the pre-amplifi er is dependent on the capaci- tance load imposed by the microphone capsule. In general, larger capsules with the highest capacitance yield the lowest noise.
Pre-amplifi ers are designed to have low output impedance (around 25–100 Ω) in order to preserve high frequency response (usually fl at in the range of 1–200kHz). The capacitive output load given by the microphone cable and the input impedance of the next device in the signal chain also determine the frequency response. For this reason, microphone cables are restricted in length to a few metres. Typical pre-amplifi ers have a gain of 0dB, refl ecting their role in impedance conversion rather than voltage amplifi cation in the usual sense.
Power supplies
The stabilized polarization voltage (200V or 28V) is provided by the microphone power supply: a large battery-operated box (or mains via an
Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com ETH Zuerich (307-97-768) Sunday, August 28, 2011 12:01:07 AM IP Address: 129.132.208.2
adaptor) used within a few metres from the pre-amplifi er. Low output impedance permits the use of longer cables (usually of a BNC-connected coaxial type) between the power supply and the next device in the signal chain.
Time and frequency weightings
Sound pressure data are commonly fi tted with three time weightings:
• Fast – having an exponential time constant of 125ms corresponding approximately to the integration time of the ear
• Slow – having an exponential time constant of 1s to allow for the average level to be estimated by the ear with greater precision
• Peak – having an exponential time constant of below 100 μs to respond as quickly as possible to the true peak level of transient sounds.
They may also be fi tted with one further time weighting:
• Impulse – having a 35ms exponential rise but a much longer decay time, which is thought to mirror the ear’s response to impulsive sound.
Sound pressure data are also fi tted with the following three frequency weightings:
• A-weighting approximately follows the inverted shape of the equal loudness contour passing through 40dB at 1kHz.
• B-weighting approximately follows the inverted shape of the equal loudness contour passing through 70dB at 1kHz.
• C-weighting approximately follows the inverted shape of the equal loudness contour passing through 100dB at 1kHz.