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Loudspeaker parameters and loudness

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teemuk:
Since there seems to be so much confusion about the factors that determine speaker’s loudness let’s straighten out few parameters:

Speaker wattage = only determines the amount of power the speaker can handle before it gets damaged by overheating the voice coil or exceeding the cone excursion limits. Even though some manufacturers (especially in the auto-HiFi industry) like to market speakers as if their wattage would equal loudness it doesn’t, so be careful with vague terms such as “peak watts”. The average wattage - often mistakenly referred to as “RMS” - is the only interchangeable and comparable parameter. (RMS rating for watts is not the same thing as average power figure derived from sinusoidal wave that has an amplitude of x V RMS. Read more from here: http://www.hifi-writer.com/he/misc/rmspower.htm) The “scientifically valid” peak wattage rating is two times higher than the average wattage rating. Other stranger peak watt ratings may be as 10 or 100 times higher but they mean nothing. Since wattage essentially determines how much your speaker can tolerate power you must be able to compare the figure with the output power of your power amp. At this point imaginary and bloated ratings are only annoying and confusing, thus causing a serious risk of destroying the speaker with too much output power. Do also note that “American” wattage = peak rating while “British” wattage = average rating. (I don’t know if they use this rule anymore).

Cone size = only determines the approximate efficiency to couple certain frequencies. Large cones can move more air during a single cycle (thus provide a higher sound pressure) but they generally have too much mass that slows them down and prevents them from producing high frequencies. A smaller cone that moves faster (and ultimately moves the same volume of air in numerous cycles) can be equally loud (or louder) at the higher frequencies. However, it can’t couple those low frequencies efficiently enough. (In both examples we assumed the amount of “cone travel” is equal). Cone travel means the “distance” of excursion and is essentially the other parameter defining the amount of moved air volume. Naturally, if the cone can travel further it can move more air than a cone with equal area but a smaller travel. Any amplifier can drive any speaker with any cone area/size. It’s up to speaker’s efficiency how much travel the cone has at the amount of power fed to the speaker by the amplifier. If the speaker is inefficient it will produce less sound pressure with a certain amount of power input than a more efficient speaker would.

Magnet size = Larger magnets can improve the efficiency of the speaker’s electrical “motor” - so the magnet size is often sort of comparable to speaker’s quality and efficiency. Yet, this is not an absolute rule as the strenght of the magnetic force is only a single parameter in speaker design. A speaker with a big magnet may as well be very inefficient. Do note that AlNiCo, Ceramic and Neodymium materials have different magnetic properties so the magnet sizes are only comparable if magnets are made out of same material. Some manufacturers (especially in the auto-HiFi industry) glue metal pieces to the magnets to make them look bigger and more powerful. Again, about the only rating that can tell anything meaningful about loudness is the SPL.


Be happy to continue the list or discuss…

n9voc:
Query, Master Teemuk-
I have heard that speakers perform "best" (maximum sound pressure level) when driven in the range of 70% to 90% of their maximum rated average power levels.  I don't know much about speakers, can you address this thought?  thanks! :tu:

teemuk:
There are things that make the claim sound sensible: At about those power levels the speaker will be pushing quite a lot of sound pressure since the cone is pretty much using its whole travel. Many people interpret the sheer loudness as “good sound” (unless it hurts ears or something). At the extremes of cone travel other interesting things also begin to happen: The cone slightly deforms and therefore creates additional harmonics to the signal (distortion), the speaker’s internal structure may also begin to limit the magnetic force directed to voice coil. This is a protective measure that tries to prevent the cone from exceeding its maximum excursion - it also compresses the signal and creates more additional harmonics. I believe these are the properties many people prefer when they talk about “overdriving the speakers”.


And please, it's just Teemu or teemuk. :)

darwindeathcat:
Teemuk,  I noticed that you didn't comment about a speakers rated SPL level... I have read at various sources that an SPL of 95-100 is seriously desirous for amplifier speakers. I seem to remember that the SPL (sound pressure level) is a measurement of air pressure created by the speaker movementtaken at one watt at one cm from the cone??? is this right? anyway, what is the minimum SPL one should consider? for example, I have some speakers with an SPL of 86. Are these not powerful enough for use with a guitar amp?

teemuk:
SPL is measured with 1W of power but from a distance of 1 meter (not cm). As a general rule, doubling the distance to the driver results to a 6 dB drop. Thus 2 meters away the SPL has dropped 6 dB, four meters away 12 dB, eight meters away 18 dB and so on.

High SPL values are desirable mainly because they allow extracting more loudness with less power. To gain a 10 dB increase in SPL (doubled loudness) the amplifier must put out ten times more output power. Thus a 10W amplifier equipped with a speaker that has a SPL rating of 92dB is equally loud as a 100W amp with a speaker that has a SPL rating of only 82 dB. It is generally easier and cheaper to use efficient speakers than to increase the output power. The measure tells nothing about the speaker's tone, though.

We must remember that the quoted SPL is just a nominal measure – sort of like the nominal impedance of a speaker. In reality SPL varies according to frequency, and in the case of guitar speakers it usually does this quite a lot. The nominal SPL level can provide a rough guide about the expected loudness level but it certainly does not tell everything. Consider this rather extreme example: One can take a piezo horn with some impressive SPL rating and it is totally useless for bass. Bass speakers may be totally worthless for high and upper mid-range frequencies. One also can’t be sure how the manufacturer has derived the SPL rating without looking at the frequency response (e.g. is the SPL an average or a peak value). The graph of frequency response tells more than the nominal SPL rating. It will also allow you to estimate how well the speaker could stand out from the mix and how it will interact with your amplifier.

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