Power

Users quite naturally associate power with volume: "The more power, the louder your speakers". But this popular opinion is totally wrong! A 100 W speaker will not always be louder or better than "just" a 50 W speaker. Speaker's power indicates its mechanical reliability rather than its volume capacity. 50 W or 100 W do not mean the volume, which a speaker is capable to produce. Dynamic loudspeakers have low efficiency and convert only 2-3% of the electric signal power applied into sound vibrations (fortunately, the resulting sound volume is sufficient). A value specified by a manufacturer in loudspeaker specs just shows that a signal of this power will not damage this acoustic set (due to critical overheating and turn-to-turn short circuit, diaphragm rupture, surround damage, etc).

Thus, power of an acoustic set is a technical parameter, which does not have to do with volume of an acoustic set directly, although it depends on it to some degree. Nominal power values of loudspeakers may differ from those of the amplifier and the acoustic system. They are specified for optimal integration of components. For example, an amplifier of much less or much greater power may damage a loudspeaker, when set to maximum volume in both cases: in the first case — due to a high level of distortions, in the second case — due to non-nominal operating mode of the loudspeakers.

Power can be measured in various ways and in various test conditions. There exist conventional standards for such measurements. Let's review some of them, the most frequently used in product specs of western products:

RMS (Root Mean Squared). Power is measured by applying a 1000 Hz sinusoid signal to reach a certain level of non-linear distortions. Product specs usually mention this: 15 W (RMS). This value indicates that an acoustic set can work for a long time without damaging its dynamic speakers with a 15 W signal. RMS power values (Watts) are higher in PC speakers than in Hi-Fi speakers, because they are measured at very high harmonic distortions, often up to 10%. It's practically impossible to listen to music at such a distortion level due to strong wheezes and foreign sounds in a loudspeaker and its enclosure.

PMPO (Peak Music Power Output). In this case power is measured by applying a short sinusoid signal (less than one second) below 250 Hz (usually 100 Hz). A level of non-linear distortions is not taken into account. For example, power of a speaker equals 500 W (PMPO). It means that loudspeakers of the acoustic set were not damaged by a short LF signal. People call PMPO units "Chinese Watts", because power values reach thousands of Watts, when measured by this procedure! Just imagine — active speakers for a computer consume 10 W*A and their PMPO is 1500 W.

Along with western standards, there exist Soviet ones for various power types. They are specified by GOST 16122-87 and GOST 23262-88, which are still in force. These standards determine such notions as nominal, maximum noise, maximum sinusoid, maximum long-term, and maximum short-term powers. Some of them are published in specifications of Soviet (and Post Soviet) equipment. These standards are certainly not used in world practice, so we shall not dwell on them.

Let's draw our conclusions: RMS power (W) at 1% THD or less is the most important value in practice. But even product comparisons based on this parameter are very rough and may have nothing to do with reality - volume is characterized by a sound level after all. That's why "power of an acoustic system" parameter is useless.

Sensitivity

Sensitivity is one of parameters, specified by a manufacturer in acoustic specs. This value characterizes sound pressure intensity one meter away from a speaker, generated by a 1000 Hz 1 W signal. Sensitivity is measured in decibels (dB) relative to the audibility threshold (a zero sound level equals 2*10^-5 Pa). Sometimes you can come across the following value - SPL (Sound Pressure Level). It's expressed in dB/W*m or dB/W^1/2*m for short. You should understand that sensitivity is not a linear proportionality factor between a sound pressure level, signal power, and distance to a sound source. Many companies specify sensitivity of dynamic loudspeakers, measured in non-standard conditions.

Sensitivity is more important for designing custom acoustic systems. If you don't quite understand what this parameter means, you may not pay much attention to sensitivity, when you choose speakers for your PC (it's not specified very often at that).

Frequency response

Frequency response in the general case is a curve that shows the difference in input and output signal amplitudes across the entire frequency range. Frequency response is measured by applying a sinusoid signal of constant amplitude and changing frequency. A point on the curve, where the frequency equals 1000 Hz, is traditionally used to plot the 0 dB level on the ordinate. In an ideal case, frequency response is represented by a straight line. But acoustic systems do not have such characteristics in real life. You should pay much attention to non-uniformity, when you analyze the curve. The higher non-uniformity, the more timbre distortions.

Western manufacturers prefer to specify a frequency range, which is actually a squeezing of frequency response: it provides only borderline frequencies and non-uniformity. For example: 50 Hz - 16 kHz (+/-3 dB). It means that a given acoustic set provides true sounding from 50 Hz to 16 kHz. Non-uniformity grows much below 50 Hz and above 15 kHz, the frequency response "rolls off".

What are the consequences? A reduction of LF level implies the loss of bass saturation. A rise in the LF range results in booming sound. HF roll-offs will make the sound dull, indistinct. HF boosts lead to irritating hissing sounds. Non-uniformity of frequency response in multimedia speakers is usually higher than in so-called Hi-Fi acoustic sets. You should be very critical concerning all ads about 20-20000 Hz frequency response of speakers (theoretical limit). Manufacturers often don't publish non-uniformity of the frequency response, which may reach inconceivable values.

As manufacturers of PC speakers often "forget" to specify non-uniformity of frequency response, you should be very careful, when you see speakers that are said to provide 20 Hz - 20000 Hz. It's very likely not to provide more or less uniform characteristic in 100 Hz - 10000 Hz. It's impossible to compare a frequency ranges with different non-uniformity.

Non-linear distortions, THD

THD - Total Harmonic Distortion. An acoustic system is a complex electroacoustic device with a non-linear amplification function. That's why having passed the audio section, a signal should have non-linear distortions. Harmonic distortions are the most obvious and simplest to measure.

Harmonic distortions are generated, when extra harmonics are added to the original signal due to input/output non-linearity. These parasitic harmonics change timbre and result in audio losses. Non-linear distortions are measured by applying a 1000 Hz sinusoid signal. A special filter is used to detect added harmonics and determine their power in an audio signal.

THD is a nondimensional value. It's expressed in percentage or in decibels. Here is the conversion formula: [dB] = 20 log ([%]/100). The higher THD, the worse sound (usually).

THD of a speaker depends much on the power of an applied signal. So it makes no sense to draw preliminary conclusions or to compare speakers only by their THD, without listening to their performance. Besides, manufacturers don't publish this parameter for working positions of the volume control (it's usually 30..50%).

Impedance

A loudspeaker possesses certain resistance to DC, which depends on wire thickness, length, and material in a coil (aka reactive impedance). When a musical signal is applied (AC), impedance of a speaker will change according to signal frequency.

Impedance is full resistance to alternating current, measured at 1000 Hz. Acoustic impedance usually equals 4, 6 or 8 Ohm.

On the whole, acoustic impedance will not tell you anything about audio quality of a given product. Manufacturers specify this parameter just to take impedance into account when you connect speakers to an amplifier. If impedance of a speaker is lower than the recommended load of an amplifier, there may appear distortions or the short circuit protection may snap into action. If it's higher, the speaker will be much quieter than in case of the recommended impedance.

Enclosure (cabinet), Topography

One of important factors affecting audio quality of an acoustic set is topography of a speaker. Manufacturers of acoustic sets usually face a problem of a topography choice. There are more than a dozen of them.

Topographies are divided into acoustically unloaded and acoustically loaded ones. The first means a topography where diaphragm vibrations are limited only by surround rigidity. In case of the second type, diaphragm vibrations are also limited by air elasticity and acoustic impedance to emission. Speaker topographies are also divided into single-action and double-action systems. A single-action system generates sound only by a single side of the diaphragm (emissions from the other side are neutralized by the speaker topography). A double-action system uses both sides of a diaphragm to generate sounds.

As a topography has little effect on HF and MF speakers, we shall describe popular topographies of a subwoofer.

"Closed box" topography is widely used. It belongs to the group of loaded topographies. It's a closed enclosure with a diaphragm of a speaker on the front panel. Pros: Good frequency response and impulse response. Cons: Low efficiency, requires a powerful amplifier, high THD.

But instead of fighting audio waves generated by vibrations of the back side of a diaphragm, they can be used. The most popular double-action topography is bass reflex. It's a tube of a certain length and cross-section, built into an enclosure. Length and cross-section of bass reflex are calculated so that at a certain frequency it creates sound wave vibrations in phase with vibrations generated by the front side of the diaphragm.

Subwoofers widely use compound enclosures. Unlike the previous type, diaphragm of a speaker is not installed on the front panel here, it's inside an enclosure on a partition. A speaker itself does not take part in generating low frequencies directly. Its diaphragm just excites LF vibrations to be amplified in a bass-reflex tube acting as a resonance chamber. Advantage of this design is high efficiency of a subwoofer and its small dimensions. The problem is in deteriorated phase and impulse characteristics, such speakers sound tiring.

An optimal choice is speakers of an average size in wooden enclosures (closed box or bass reflex). When you choose a subwoofer, you should pay attention not to its loudness (even cheap models usually excel at it), but to how correct it reproduces the entire low frequency range. From the point of audio quality, the worse speakers have thin-wall enclosures and small dimensions.

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