Loudspeaker Basics

It helps to know loudspeaker basics, and at Chane we believe an informed reader is a smart shopper. The following points about loudspeakers aren't widely discussed, but they are intuitive and easy to understand. We'll recommend examining real-world speaker parameters and characteristics, and not just specifications that may not relate well to sound. We won't recommend a particular product, but we have prepared a general tutorial that should guide the shopper. See Sound Shoppingfor more.

Power Handling

The least informative loudspeaker specification may be its power rating. Expressed as a wattage figure, power ratings state approximately how much electrical current may flow across the speaker's load without harming it. Large speakers with larger or more drivers tend to have higher power capacity then smaller, simpler systems.

Since so many other things influence a speaker's maximum output, even when stated precisely, power ratings only give us an approximate sense of a speaker's loudest output before damage.

Tip: Speaker power ratings may not indicate a speaker's physical, thermal, and electrical limits and they do not represent sound quality.

Impedance

Electrical circuits resist or impede electrical current flow. Indicated with the Ohm unit, a loudspeaker's impedance magnitude is a complex, frequency-dependent electrical load for the audio amplifier, through which its current flows.

With a standard amplifier a lower speaker impedance increases current flow, raising loudness. A higher impedance reduces it, reducing loudness. While the industry standard for speakers has been approximately eight Ohms, some loudspeakers can present loads of four or fewer Ohms, initially increasing the amplifier and loudspeaker's loudness. This initial higher loudness is just a function of the amplifier's volume control setting, and all other factors being equal the eight Ohm speaker will play just as loudly at a higher setting as the amplifier reaches its power limit.

However, very low impedance speakers may be difficult for some amplifiers to drive, potentially impairing quality of sound.

A speaker's impedance is not directly related to how it sounds but how its amplifier reacts to it. Amplifier and speaker combinations are endless, each one different whether subtly or plainly audible. As with combined driver area and net internal cabinet volume, discussed below, it's also important to compare loudspeaker impedance before assessing their sensitivity ratings.

Tip: Higher impedance speakers are usually easier to drive. Lower impedance speakers may be louder at the same volume setting. Amplifiers matter, and the most successful combinations of the two may take experience and consultation to coordinate.

Efficiency

A loudspeaker's efficiency is the percentage of electrical power it converts into sound pressure. Sometimes stated as the speaker's output ("SPL", or Sound Pressure Level) measured at one meter distance (1M), efficiency is an exact specification referencing one electrical Watt of power consumed. Although consumer speakers vary significantly by size and acoustical class, they average less then one percent conversion efficiency, which is perfectly normal.

Sensitivity

Somewhat related to efficiency, sensitivity tells us how loud a speaker will play with a standard amplifier set to deliver a fixed audio signal voltage, such as 2.83v. Since this is one Watt across 8 ohms but two Watts across 4 ohms, sensitivity figures are therefore a somewhat more arbitrary specification not always including a strict speaker parameter.

Efficiency considers one electrical watt consumed by a speaker, but sensitivity also describes how an amplifier reacts to a speaker at a fixed setting. Sensitivity may not explicitly factor speaker impedance and therefore the total electrical power consumed.

Tip: Speaker efficiency relates speaker loudness to a unit of power. Speaker sensitivity involves the amplifier's level setting as well as it's behavior driving a speaker of a particular impedance.

Acoustical Size

Not explicitly stated by manufacturers, this abstract indicator refers to loudspeaker size and class. Acoustical size is roughly to speakers what weight and capacity are to automobiles or square footage to real estate. The shopper may not be aware of these foundational values in speakers, but they have much to do with how well a loudspeaker serves the user.

The initial acoustical size of any loudspeaker is set by the total area of its active (electrically driven) acoustical diaphragms - the vibrating "drivers" - in the cabinet, with an emphasis on the bass driver(s), which are the speaker's acoustical foundation. For most loudspeakers, this size roughly establishes the speaker's maximum loudness, relative distortion levels, and its ratio of bass extension to sensitivity to amplifier power.

For example, a 6.5" woofer's bass response will vary somewhat between speaker models, but like all drivers, its diaphragm area is closely tied by physics to the "response axis" that all 6.5" drivers fall upon. The mass of the acoustical diaphragm and the strength of the magnetic motor may vary, but all relate back to loudness, distortion, and bass response in important ways.

Two 6.5" woofers or one 8" woofer or four 5.25" woofers or any other combination of driver sizes and quantities best define that class of loudspeaker. Any comparison the educated shopper makes between any two loudspeaker models should therefore keep this in mind - it's more important than price, specification, or appearance.

Tip: More bass driver area generally means more audible impact and acoustical dynamics.

Internal Air Volume

Very closely related to the acoustical size of the loudspeaker, this rarely stated specification is the internal airspace in the cabinet devoted to the bass drivers. Just like the difference between a large cello and a small violin, more internal volume (for a given proper bass tuning) simply means some combination of deeper bass and better efficiency. It does not mean higher maximum loudspeaker output because that's determined by total active driver area, above, as well as maximum cone excursion.

Within the limits of the combined bass driver area, and regardless of specification, the larger speaker has an advantage over the smaller speaker with more bass or more per-Watt loudness or a predictable ratio of both.

Tip: Larger speakers convert more amplifier power into sound and play deeper bass.

Speaker Loudness

Sound pressure or volume is measured in Decibels, a unit of acoustical loudness. Although speakers are quite inefficient at converting electrical power to sound pressure, even one electrical watt will develop an average of nearly ninety Decibels from a single speaker, the loudness level of heavy traffic. Speaker loudness is not usually specified in Decibels unless an amplifier power or voltage specification is also included for reference.

Average home loudspeakers spend most of their time dissipating less than one electrical Watt, making power ratings in the dozens or hundreds of Watts fairly irrelevant. Exceptions may include large subwoofers in large cinema spaces, where speakers may expend momentary peaks of hundreds of Watts. Professional speakers in very large venues may dissipate thousands of watts for short periods, and even home speakers operating at reference level in large rooms may consume tens of watts.

Tip: Speakers are not usually rated by sound pressure, but loudness can be approximately predicted by factoring in the parameters above.

Speaker measurements

A speaker's acoustical output and electrical behavior are expressed in a variety of ways, the more basic of which can be measured. (The entire spectrum of loudspeaker behaviors is a topic too broad and complex for analysis by nearly any single source.)

Conventional static measurements do not adequately predict the way a loudspeaker will sound in a space, when connected to a particular amplifier, or when playing a certain performance at an average loudness level. However, to some degree simple measured data may help validate the fundamental soundness of a particular design. Remember too that while we strive for both, not all good-sounding speakers measure well, and not all well-measuring speakers sound good.

Amplitude response. Also called frequency response, a speaker's amplitude response is its acoustical loudness at each frequency across an entire operating spectrum, from bass to treble, typically expressed as a function of a fixed amplifier input level measured at a fixed distance and axis from the speaker. This output will be presented as sound loudness versus frequency, or typically, Decibel units within a common 20-20kHz frequency range.

The amplitude response has been reputed to be a primary criteria for loudspeaker fidelity or quality, however it merely it indicates how uniformly loud across its frequency range a loudspeaker is when measured at one point in space, at one angle to the speaker, and at one fixed level of signal input. As a discrete speaker analysis, however, amplitude response has little to tell a consumer about how a speaker sounds.

A uniformly neutral, "flat" amplitude response is theoretically desirable - both for obvious reasons and to a degree to infer if the speaker's design is fairly conventional - but such a response has little bearing on the broad audible characteristics of all the speaker types, classes, their intended environments, and their personalities or design schools. The practical manifestations of speaker output are more varied than any single response snapshot can illustrate.

Off-axis amplitude response. A variation on the simple amplitude response, the off-axis response is a speaker's loudness uniformity measured at other locations in space. These responses tell the engineer if the speaker has a uniformly neutral or diminishing loudness response at each frequency as the microphone moves around the speaker, recording responses at fixed angular increments to it. Since typical speakers cannot function as perfect omni-directional acoustical radiators, the off-axis responses indicate if the speaker is capable of maintaining uniform acoustical behavior at the relatively shallow angles from the speaker encountered in normal use. Correlation between this data and perceived sound is therefore subject to interpretation.

Summary

Tying this all together tells us important things about a loudspeaker:

1. We cannot make exact comparisons between speakers in two strongly different classes - Speaker A's 4.5" woofer and Speaker B's 6.5" woofer put them into different categories.
2. The smaller acoustical system sacrifices bass extension or efficiency or both. Larger loudspeakers generally have deeper bass, greater one-Watt loudness, or a ratio of both.
3. Larger speakers generally play louder with lower distortion and increase power handling, but do not necessarily lower the speaker's bass cutoff frequency. Typically, ganging multiple bass drivers together in a single speaker makes it more efficient.
4. Lower impedance speakers have higher initial loudness but not higher real efficiency for their response and/or size class.
5. If we're interested in sound quality, there may be no predictive data we can use to select a good-sounding speaker without hearing it. Data is relatively narrowly defined, and while potentially highly accurate and essential to various design elements, cannot encompass the human reaction to myriad aspects of reproduced audio. Measured data can, if not used carefully, even create a bias where an assumption about speaker sound replaces the speaker's real sound.

 

The universe of loudspeaker types, styles, design philosophies, characteristics, and ultimately, nature of sound is so vast that the subject has filled small libraries. (One of the best treatises on speakers and speaker design philosophy is Lynn Olson's The Art of Speaker Design at Nutshell HiFi.) However, comparing any two or more loudspeakers effectively means understanding how to make fair comparisons. From them you can then take the general acoustical classification you've arrived at and use it to shop among comparable products.

Is audio a science?

Practical audio engineering is a series of trade-offs, all ideally gauged and selected by the design team for value against genuinely neutral, authentic sound. As seen above, objective analysis exists and is widely incorporated by speaker designers and sound engineers. However, among the audio sciences are diverse beliefs as to what sounds a particular way, if it's important, and how to achieve it. The science exists but interpreting and applying it is subject to many artful variables.

For an example of this variability, among the less cataloged but still widespread findings about sound is the prominent phenomenon currently enjoying renewed interest called SSI or sound stage & imaging. Highly valued by audiophiles for decades but not usually present in the audio arts except empirically, SSI is the experience that when well optimized, well-tuned loudspeakers connected to suitably transparent-sounding electronics and recordings will recreate a convincing 3D illusion of the sonic landscape of the original performance and recording venue. Such an experience acoustically replaces the visible room and equipment with a virtual sonic landscape, transporting the listener back to the original performance. No signal processing is used, and the effect occurs from two speakers operated in simple stereo configuration.

Another useful descriptive term is PRAT, or pace, rhythm and timing. PRAT was coined to describe the sound of speakers with a superior sense of musical release, or getting out of the way of the recording, or speed, or other subjective terms aimed at natural musical pace and ease. While at first PRAT seems out of place - a passive speaker cannot explicitly change the electrical timing of an applied signal - it may be a useful descriptor of a speaker's apparent acoustical timing.

Audiophiles seek out these acoustically authentic experiences at great effort and many times great cost. Chane finds them effective tests of good sound. Chane has found that a speaker projecting these illusions is suitable for any type of musical reproduction, whether simple stereo music or complex multi-channel home theater playback.

Being based in subtle nuances of design and engineering, there is currently no direct, correlative metric to accurately specify what causes these effects and there may be none soon. The listener is again encouraged to experience it, a familiar recommendation for any audiophile. The phenomenon is another example of a valid, rooted, and common effect valued by many listeners for which there is little correlation in technical loudspeaker measurement sets.

Audio may also delve into areas dictated more by the sociology of its science than by its own real phenomenon, goals, and effects. Schools of belief arise within various audio tributaries and may apply variably to authentic sound. Again the shopper should identify the basics about loudspeakers and then listen to as many types and models as possible. There is virtually no touchstone in the audio sciences from which to select any one retail product for sound without listening.

Progress continues, however, and audio is therefore an expanding art informed essentially and very importantly by applied science. From time to time new findings also aid the creative, empirical audio arts enjoyed by enthusiasts and experimenters around the world, and eventually some may evolve into a new and accepted science.

Start with the loudspeaker basics, arrive at an acoustical size class, weigh your requirements for loudness, frequency response, physical size, and approximate price range, and then compare as many models as you can. See our article Sound Shopping for more technical explanations.