Ten key parameters of audio system performance include efficiency, frequency response, total harmonic distortion, intermodulation distortion, phase distortion, transient distortion, transient response, dynamic range, sound pressure level, and sound power level.
This FAQ reviews how each of these parameters is specified and measured and closes with a look at how safe sound levels are specified by the Occupational Safety and Health Administration (OSHA) in the US and by various standards agencies in the European Union (EU).
Efficiency
Efficiency measures the production of sound for a given amount of power. The efficiency of audio amplifiers varies widely. Less efficient amplifiers generate relatively more heat and produce less sound power. For example, a typical class A/B amplifier is about 50% efficient. That means an amplifier rated for 100 W of output power would also dissipate 100W of heat. Class A/B performance in intermediate between Class A and Class B. Class A amplifiers are often considered the gold standard for audio quality in many audiophile circles, but they can have efficiencies as low as 15% to 35%. Class B amplifiers have efficiencies over 75%, but they suffer from crosstalk and distortion. Class A/B amplifiers have good sound quality and efficiencies between Classes A and B. Class D amplifiers use switch-mode power technology to achieve efficiency levels up to 90% with very good sound quality. Not all Class D amplifiers are created equal, multi-level Class D designs can be much more efficient compared with conventional Class D implementations (Figure 1).
Frequency response and range
The frequency response refers to the frequencies over which the output level for an amplifier remains within a specified dB range or no more than a given number of dB from the output at 1 kHz. Frequency range is a related concept but applied to loudspeakers and without referring to a dB range. Power bandwidth is another related specification that indicates the usable frequency range at high power. It contrasts with a frequency response that is typically measured at low power levels, where issues like transformer saturation don’t come into play.
An amplifier with a flat frequency response does not change the intensity of the output across a given frequency range, usually specified from 20 Hz to 20 kHz. While that is intended to reflect human hearing, the majority of people don’t hear sounds above about 16 kHz. A well-designed amplifier can have a frequency response that varies a maximum of 0.2 dB from 20 Hz to 20 kHz.
Total harmonic distortion
Harmonic distortion refers to spurious sounds at double or triple the frequency of the primary tone. Total harmonic distortion (THD), sometimes referred to as distortion factor, is the ratio of the sum of the powers of all harmonic components to the power of the primary frequency.
So-called high-fidelity amplifiers are expected to have THD under 1%. Reducing distortion using negative feedback can achieve low distortion levels but is a controversial technique with audiophiles. In addition, loudspeakers produce more distortion (1% to 5% for moderate volume levels) than most amplifiers, so there’s a limited benefit to high-fidelity amplification. In addition, human ears have a nonlinear sensitivity to distortion and are much less sensitive to distortion at low frequencies and low volumes. Finally, distortion that creates even-order harmonics can be less noticeable compared with odd-order distortion.
Intermodulation distortion
Intermodulation distortion (IMD) is not harmonically related to the signal being amplified. It’s the result of false signals resulting from combinations of different frequency input signals and results from system nonlinearities. Like THD, IMD can be reduced using negative feedback.
Controlling IMD in loudspeakers is more complicated. Like THD, IMD in loudspeakers is greater than in amplifiers. In speakers, IMD is proportional to cone excursion and can be reduced by reducing a driver’s bandwidth. In a practical solution, this is achieved by splitting the signal into several frequency ranges with separate drivers and feeding the resulting signals through a crossover filter network into the speaker. The use of active crossover networks can further reduce IMD.
Phase distortion
Phase distortion measures the phase shift between input and output signals. The use of crossover filter networks to reduce IMD can result in increased phase distortion. In general, phase distortion can be difficult to eliminate, but its importance in audio systems is limited since the human ear is relatively insensitive to it unless it’s extreme. There is no simple standard test for measuring phase distortion, and it’s not included in audio specifications. It can be a problem for RF and other high-frequency applications.
Transient distortion & transient intermodulation distortion
Transient distortion occurs when the acoustic output level of a loudspeaker does not change as quickly as the input signal. The psychoacoustic performance of the ear tends to make it relatively insensitive to transient distortion.
Transient Intermodulation distortion (TIM) relates to amplifier performance and can be measured by putting a burst of a fixed frequency into an amplifier and comparing that with the output. TIM can be more noticeable to human hearing than other types of distortion. Sound quality is more related to the types of distortion that are present rather than the THD of an audio amplifier. So, while THD is important, high-performance audio amplifiers will also specify TIM.
Transient response
An amplifier or loudspeaker may have low distortion for steady-state signals but not respond quickly enough to transients. In amplifiers, a poor transient response can result from inadequate high-frequency performance or to too much negative feedback. Transient response in loudspeakers results from the mass and resonances of drivers and enclosures. Delays added by crossover filters or misaligned loudspeaker drivers add to transient response problems. Transient response can be difficult to quantify and is not generally included in audio specifications.
Dynamic range
Dynamic range is the ratio of maximum to minimum loudness that can be handled by an amplifier or loudspeaker. It’s measured in dB and is the ratio between the noise floor of the amplifier or loudspeaker with no input signal and the maximum signal that can be handled at a specified distortion level.
Sound pressure
Sound pressure, also called acoustic pressure is the local deviation from the ambient atmospheric pressure caused by a sound wave. It’s measured in Pascals (Pa) using a microphone. It is also be expressed in dB using a reference level of 20 x 10-6 Pa. The power of a sound wave goes up with the square of the pressure. In the US OSHA defines safe levels of sound pressure based on time of exposure. Daily limits set in OSHA standards include 90 dB of noise for 8 hours or 105 dB for one hour. Sound intensity can be confused with sound pressure. They aren’t the same. Sound intensity is equal to the mathematical product of sound pressure and sound particle (air molecule) velocity. Sound pressure is a scalar quantity and sound intensity is a vector, so sound intensity is a vector quantity.
Sound power
Sound power, also called acoustic power, is the rate at which sound energy is emitted or received per unit of time. The W is the SI unit to measure sound power. Sound power is sometimes referred to as sound flux or acoustic flux. There are numerous regulations designed to control noise emissions from a variety of sources. Some of the regulations specify the method for measurement in dB. Some standards include labeling requirements for sonic emissions.
Noisy machines
Noise limits have been set for various types of machines. For example, in the EU, outdoor machines are limited in the amount of noise they can produce. Other types of equipment like consumer appliances may not be required to meet maximum limits but must be labeled with guaranteed maximum sound power levels. There’s a wide array of noise limit standards in Europe; examples include:
- EC/2000/14 directive on noise emission by equipment for outdoor use
- ISO 15744: hand-held nonelectric power tools
- ISO 7779/ECMA-74: information technology and telecommunications equipment
- ISO 9296/ECMA-109: declared noise emission values of computer and business equipment.
- IEC 60704: household electrical appliances
- DIN 45635 standards
- ISO 6393: earth-moving machinery measured in stationary conditions.
- ISO 6395: earth-moving machinery measured in dynamic conditions.
Sound pressure-based sound power measurements are often used when performing certification measurements. A set of ISO standards governs these requirements and detail measurement procedures. Sound pressure-based standards include 2000/14/EC, ISO 3744, ISO 3745, ISO 6394, ISO 6395, and Air-Conditioning, Heating, and Refrigeration Institute (ARI) 260. There are also ISO standards that govern sound intensity-based measurement procedures. Sound intensity is measured in sound power per area (W/m3). ISO standards for intensity-based sound power include ISO 9614-1 and ISO 9614-2.
Summary
Key parameters for audio system performance begin with efficiency, including frequency response and range, several forms of signal distortion, sound pressure, and sound power. In addition to quantifying the performance of audio systems, these parameters are used to set safe sound levels for a variety of equipment types, operating environments, and applications in the US and EU.
References
Audio system measurements, Wikipedia
Amplifier efficiency, PS Audio
European Commission Energy Efficiency Label Reform, Power Integrations
MERUS class D audio solutions Cooler, smaller and lighter amplifiers, Infineon
Noise level certification, how to select the right standard?, Siemens
Sound Pressure, Sound Power, and Sound Intensity: What’s the difference?, Siemens
