Newbie questions... dB ratings, etc.

[user32i4723847289]

Let me first start by saying that this stuff really interests me, but I don't understand the technical jargon at all, and would like to. Could anyone here help me understand the meaning of a couple things? I understand the Hz rating is in reference to the sound waves / second... lower Hz means more "bass" like sound, and we cannot hear below approx 20Hz. Correct me if I'm wrong on this.

Now here's what's really bothering me. I see all these dB ratings for speakers... 94, 96, 102, 106... etc. I am at the understanding that dB means decibel, and the higher the rating, the louder it is. Should this really concern someone such as myself who never cranks their speakers past the halfway point?

In relation to dB again, take this link for example:

Polk Audio RTi12

It now talks about the Lower (-3dB) Limit for 30Hz and 26KHz... what does this mean? The speakers are rated at 18Hz but they're down to -3dB at 30Hz, would this not imply it's inaudible at -3dB and 30Hz, or is the decibel scale wacky?

Basically I think I just need an overview of dB

Sorry for the n00bish questions, just tryin to figure this stuff out, and what to look for when purchasing new speakers.

Thanks!

[bdc]

Well, I can't give you a full explanation of 'how decibels work' since I don't understand it all too well myself, but I'll answer you because I don't think anyone else is gonna reply to you anytime soon. I'm pretty sure when they say down 3dB at 30hz they mean that the sound coming from the speaker at 30hz is 3 decibels lower than everything else.

As for the sensitivity ratings, those tell you how many decibels a speaker will put out on 1 watt of electrical power measured from 1 meter away. A Klipschorn, for example, has a sensitivity rating of 104 dB/1W/1M. While a Klipsch Forte has a rating of 98/dB/1W/1M. If I remember correctly, a 10 dB increase in sound output will sound twice as loud. Klipsch has a sensitivity chart that tells you about how many watts are needed to drive speakers of varying sensitivity ratings. Now remember, I am not too familiar with this subject and I wouldn't be surprised if I've made a few errors. I'll let someone else do the overview since they will be much more knowledgable (and accurate).

[dantfmly]

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On 4/30/2004 3:22:23 AM bdc wrote:

Well, I can't give you a full explanation of 'how decibels work' since I don't understand it all too well myself, but I'll answer you because I don't think anyone else is gonna reply to you anytime soon. I'm pretty sure when they say down 3dB at 30hz they mean that the sound coming from the speaker at 30hz is 3 decibels lower than everything else.

As for the sensitivity ratings, those tell you how many decibels a speaker will put out on 1 watt of electrical power measured from 1 meter away. A Klipschorn, for example, has a sensitivity rating of 104 dB/1W/1M. While a Klipsch Forte has a rating of 98/dB/1W/1M. If I remember correctly, a 10 dB increase in sound output will sound twice as loud. Klipsch has a

sensitivity chart

that tells you about how many watts are needed to drive speakers of varying sensitivity ratings. Now remember, I am not too familiar with this subject and I wouldn't be surprised if I've made a few errors. I'll let someone else do the overview since they will be much more knowledgable (and accurate).

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that is not entirely correct. that -3db means it is that much lower then the average db level. which at one wat at one meter is 90db on the example he gave. so at 30 hz it would be 87db at one watt at one meter. the over all freq resp. is what that speaker will do + or - any amount of db. the 10 db thing is correct, but you must remember that it takes double power just to go up 3db. so this speaker at 2 watts will be 93 db at 4 watts it will be 96db 8 watts it will be 99 db. here is the polk chart (all ratings are 1 meter from the speaker):

1wt= 90db

2wt= 93db

4wt= 96db

8wt= 99db

16wt= 102 db

32wt= 105 db

64wt= 108 db

128wt= 111 db

where as my klipsch klf-20's are:

1wt= 100db

2wt= 103db

4wt= 106db

8wt= 109db

16wt= 112db

32wt= 115db

64wt= 118db

128wt= 121db

that is the advantage of klipsch speaker. As you see mine are twice as loud as those other speakers with samr amount of power applied. and to do the same volume i need considerbly less power. when my speakers are at 100db, i am using 1 watt. where as with the other speakers a person would be required to be using over 8 watts. so i need 1/8 of the power to run my speakers versus the polk speaker at a given volume.

[JohnA]

Vex,

Let's start with deci-bels, or 1/10th of a bel; named after Alexander G. Bell, I believe. One decibel is the smallest difference the average person can detect. Three dB is small, but easily noticible. A dB is a ratio of one sound to another, one voltage to another, or one power level to another.

It is also a log base 10 scale, i.e. a change of 10 is twice as loud. This happens because it is how your ears are made.

In the electrical realm, it takes 10 times the power to make 10 dB (coincidence) and twice the power is 3 dB. This is how log scales work.

Imagine a straight, horizontal line. A perfect speaker would look like this if its response was graphed. It would be straight from at least 20 Hz to 20,000 Hz. No speaker can do this and it is rare to actually acheive flat +/-3 dB.

In your example, the speaker is -3 dB at 30 Hz. That means the response graph is beginning to sag as frequency goes down. The maker is telling you that the sag is really just beginning at 30 Hz. Remember 3 dB is small and the speaker is probably not +/-3 db anywhere, in reality.

A proaudio speaker is commonly spec'd at -3 dB at 50 Hz and -10 dB at 43 Hz. The sag begins at 50 and drops like a rock.

The same applies on the top at high frequencies.

Efficiency (sensitivity) is the rating of a speaker's response to power input. The higher the better. High efficiency generally means the speaker will have low distortion at low to medium loudness. This is because the efficiency allows low cone movement and low mechanical stress at any given output. High efficiency allows you to use smaller power amps, too.

High efficiency is not without trade-offs, though. As a rule, a speaker has to be larger, or made with horns, thus huge, to get that efficiency.

[Colin]

Very nice, Mr John Albright, remember Vex, that decibels are a loudness measure.

A frequency response is a measure of the loudness at each frequency. It would be best to see this plotted as a chart, yet manufacturers prefer to summarize it in a bland statistic. The chart would show the decibels measured at each frequency. It would show if the frequency response was a rocky road. The statistics only summarize the frequency response: 30-20kHz, within 3 dB. Generally the deeper and the higher, the better.

Minus three decibels is a NOT a whole lot. Except if the next frequency is up three decibels, then the difference is a whole lot 6 decibels, which is clearly inaccurate and noticeably audible yet how this inaccuracy sounds or worse, is described is quite subjective and inaccurate. If the bass was down 6dB, you would certainly know it; but how you describe is something else. You might say it was weak, lean, notched, flabby, etc. Generally the flatter, the better. The heavy Dunlevy loudspeakers were quite flat, within an incredible 1.5 dB; so they were very accurate, though some people described them as colorless or bland.

The depth of the Polk loudspeakers is 18Hz, which is quite good for a loudspeaker of that price range, but you might NOT be able to hear a lot of that low bass, because it begins to roll off at 30Hz (down 3 dB). The high end is also quite good, although many people say that we need nothing above 13, or 16kHz.

The efficiency rating is actually sensitivity. The Polk loudspeaker will make 90dB with one watt, measured at one meter (about three feet) or dB/w/m. This is above average sensitivity for most tweaking audiophile loudspeakers. Triple that power for the actual requirements of your listening spot in a real room. The loudspeaker actually uses more like 3 watts to make a single note at 90dB at your seat (and it depends on the note, bass notes will require more energy). It also means that you need about 34-watts to reproduce fast musical peaks about 15dB higher than average 90dB levels.

Here again, an impedance chart would tell you a more than a mere statistic. It would tell WHERE the loudspeaker was efficient or NOT. The chart shows the ohms at each frequency response. Any loudspeaker nominally rated at 8-ohms, whether sensitive or NOT, can have sharp impedance curves and low dips (4 or 2-ohms). These make the loudspeaker harder for the amplifier to drive. A high sensitivity loudspeaker (90dB/w/m or better) might or might NOT be an easy load for the amplifier to drive.

The crossover is also described as a loudness curve. It is down 12dB (a lot) from the average level. The average level is usually 1kHz, the middle of the ears very sensitive range (1 to 4 kHz). Beginning at 120Hz the crossover slopes down, (so the woofer doesnt interfere with the mid-range driver).