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Loudspeaker parameters and loudness

Started by teemuk, February 15, 2008, 06:30:43 AM

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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...


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:


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. :)


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?
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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.


Ok since you offered to expound! As a total newbie I recently read that you should build your cab with a "guitar" speaker and not one from the home audio system. What's the difference? How can you tell when you have a speaker in your hand? I now have 2 8" and 2 4" 8 ohm home audio speakers that I wanted to build into speakers for my amp. Any reason not to do it?



Yes, excellent reason is you will probably tear the cones off the rest of the speaker assembly the first time you play. Don't despair because inexpensive musical instrument grade speakers are available ( from, and I hate to use this word, RadioShack) and others ( on the web, cut out the middleman). I'll do some more research and get back to you personally, (unfortunately I've been away from the repair industry for so many years I don't know whose in Business still). The point is a hi-fi speaker cone and coil cannot handle the power( unless you want to sound like a cat chasing a mosquito both ductaped in a shoebox with holes). On the other hand, hang on to the cabinet that you have. It's a good staring point. CrystalTech56.


One good place to look is www.colomar/Shavano/speaker_design.html. In fact, the entire site is great. And on that particular page, there is another page you should download as well! These used to be called "musical Instrumment speakers", but now are called "musical instrument and Pro Audio speakers", (see how out of touch I am? I'm old enough to remember seeing the Grateful dead play at M.I.T. on May 5,1970 the day after 4 students were killed at Kent State(no I wasn't I student at either place). The guy who did there sound for a while was named Bob Heil ( of Heil Talk Box fame). He has his own site, is still in business, and has a book still in publication,I think it's "Professional Guide to Concert Sound". The book isn't on the site, but I e-mailed him and he sent a phone number(I'll post it later, can't afford to buy any toys this week) to call to order. It really is a great book. Do an Amazon search on it(it is available if you can't afford new) and also Goole "Bob Heil" I'm reluctant to post any of it because of copyright infingement and Bob desreves the money.  CrystalTech56


The sensitivity of guitar speakers typically blows away that of other types of speakers at the expense of accuracy. If a home audio speaker can handle 100 watts from your home stereo, it can handle 100 watts from your guitar amp. It just won't be as loud, and might not sound "right". If you hook up a guitar speaker to a home stereo, the music will sound awful.

Some speakers are rated at the power where they cross a certain distortion threshold even though they're capable of handling much more power before taking on damage. I've seen this with car subwoofers.

Anyway, as long as the speakers can handle the power and the amp can handle the impedance, nothing terrible will happen if you hook them up.

J M Fahey

This is something I originally posted in an Orkut group, in Portuguese.
For those who understand it: http://www.orkut.com/Main#CommMsgs.aspx?cmm=144284&tid=5332929478380364898
For English speakers, here´s the translation:

How to "read " speaker curves.
I´m posting two widely different speakers , a "Premium" Guitar Speaker, the famous Celestion Vintage 30, and a "good" PA/DJ woofer, the Brazilian made (but easily available in the USA) Selenium 10PW3.
Both have big magnets (Celestion over 150mm , Selenium 147 mm) the same voice coil size (44mm, 1 3/4"), the same voice coil wire (round copper), same base material (Kapton), same adhesives (high temperature epoxy), etc. , yet they sound *very* different.
I´m posting their frequency curves.
They have "too much" information, and can be confusing for us guitar lovers.
To begin with, the classic HiFi range, 20Hz to 20kHz is "too much" for us.
We should be concerned with SPL: sound pressure levels (in this case, measured at 1 meter, with 1 Watt applied to the speaker) over 90dB, which is the minimum efficiency needed to "battle" successfully a drummer, who makes a lot of noise, doesn´t have a "volume control", and sets the minimum sound level we must achieve.
Any lower, will probably work (even be too much) in a bedroom, but not on stage or even the rehearsal room.
Besides, the SPL scale (the vertical scale) is not Linear but Logarithmic. Don´t worry too much about the Math, it simply means: a +3dB (deciBels) difference equals twice the measured sound volume.
Some useful values: +3 dB: twice the volume ; +6dB: 4 times as much; +10dB: 10 times as much.
I had some extra work to re-dimension the graphs, which usually are printed on a different scale, perhaps to make difficult their direct comparison.
I could not superimpose them, which is the best comparison method. I usually scale and print them on transparent paper but don´t know how to make it on a computer screen.
I did draw two green "boxes" showing the part of the graph that concerns us.
The left and right sides of the "boxes" show frequencies of 80Hz (Hertz or cycles-per-second), about the lowest guitar frequency, and 6KHz (6 Kilo-Hertz or 6000 "Cycles"), the practical highest harmonics of importance and anyway the practical higher limit of Guitar Speakers.
Top and bottom sides of the "boxes": 100dB (a very desirable efficiency) end 90dB (the absolute minimum acceptable for live work)
Now we can see that the V30 has no less than 97 dB from 150Hz to 350Hz (lower midrange) and practically 100dB from 450 Hz to almost 5000 Hz, with the added "gift" of almost 6dB extra (106 dB max.) from 1500 to 4500 Hz (brightness, attack).
An impressive speaker indeed, worth every extra cent it costs. As another extra, we should consider the exceptionally flat midrange response from 500 to 1200 Hz, the famous "V30 midrange". It´s unusual among guitar speakers, almost *all* of them have a response "dip" in that area anywhere from -3dB up to -9dB which usually produces "buzzy" sound, especially from SS gear.
Now on to the 10PW3.
The response is clearly less efficient and very irregular.
For example: from 120Hz and 2400 Hz (practically all the range of guitar strings) the response goes up and down, between 92 and 94 dB, with a small (98dB) peak at 1800Hz, a dip at 2400 Hz and a final 3800Hz peak.
All of that means a muffled, low efficiency speaker, which in most of the "guitar range" gives almost -6dB compared to the other speaker.
In practical means: a 5 or 6W Champ or AX84 tube amp, using a V30 or a good Eminence of similar specs will easily match a 30 to 50W SS amp driving a poor speaker.
That´s some of  the basis of the so-called "Tube superiority over SS". Truth is, usually Tube amps (expensive) have an expensive speaker; SS amps, perceived as "cheap" often use "cheap" speakers. The justly famous Lab Series often used very efficient EV speakers, or the best Eminences available. Same goes for JC120s and their impressive Pioneer made speakers.
What point am I trying to stress?: we SSGuitar fans, *must* use good speakers to make justice to our work.
Don´t need to break the bank: There are many reasonable priced Eminences and of course, the excellent Italian made Jensens to fill the bill, as well as good used speakers salvaged from dead amps or bought cheaply on EBay.
Try to design your own "green box" on any speaker curve you are interested in, to know how it will sound, even before actually hearing it.
Have fun.


Wow JM thats an excellent tip using your "Greenbox" to compare spkrs.
Especially using the V30 as a reference point really shows how those little dips and peaks will affect the sound.
I am curious you mentioned the Italian made Jensens.
I own some Electric Lightnings and love em.
Have you any experience with the Sica brand spkrs made in same factory?
The specs on them are impressive 100.3 spl 100watt rms
But the "Greenbox " on them is very erratic but seems close to the V30.
They are very inexpensive compared to thier siblings from the same factory.
Also do you know of any suppliers of the Sica line?
I can only find one in USA but no longer stocking the Sica guitar spkrs only PA and bass ones.

J M Fahey

Hi Brymus.
Sica s and indeed Most Italian speakers, are among the best in the world, period.
Particularly the RCF ones are second to none .
In fact, people trying to build modern reproductions of JBL cabinets, including Dual Showman ones, use Spanish Beymas and Italian RCF.
They were excellent copies of old JBLs and now that the "original" lowered its quality, the "second up winner" became the best, simply because the old champion gave up.
I guess that was the reason why Jensens are made at the Sica factory.
I also guess that they can't build two very similar products, either for contractual reasons or simply financial ones.
I'd say: just get the Jensens.

J M Fahey

Very interesting and very sobering.
Thanks por posting.