Frequency Response and numbers

Discussion in 'Loudspeakers' started by ThA tRiXtA, Mar 3, 2004.

  1. ThA tRiXtA Full Audioholic

    ThA tRiXtA
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    <font color='#000000'>Could someone please explain to a home theatre newcomer exactly what is frequency response at +/- 3 dB?

    For example, my Misiion M74i tower speakers have a frequency response of 44 hZ at +/- 3dB

    And it also says low frequency response (-6 dB) ?

    I know this means that these speakers will produce those frequencies, but why don't they just say what they produce, without the dB rating? Either they reproduce 44 to 20000 hZ or not?

    Also, with that low frequency response rating, why is the speaker magically go to 38 hZ? Does each dB make it go down a hertz? It says minus 6 dB, and for some reason there is 6 hertz lower response... ?

    Thanks for your time

    Rob</font>
  2. ThA tRiXtA Full Audioholic

    ThA tRiXtA
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    <font color='#000000'>I know someone here must know of this stuff?

    Helllooooo anyone home? &nbsp;[​IMG]</font>
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  3. annunaki Moderator

    annunaki
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    <font color='#000000'>Rob,
    If your speakers are rated at 40hz-20,000hz +-3db, it means that your speaker will play realitively flat from 40hz to 20,000hz with no deviations greater than plus or minus 3db. A 3db change in volume is, for most people, just barely noticable with music. Now, if your speaker's specifications state that they will play down to 38 at -6db, it means that they will play down to 38hz but 6db quieter than the rest of the signal. It is just your speakers rolling off response due to limitations of the design (ported, sealed, driver size, ect.)

    For instance, I could say that a given pair of speakers have response from 20hz-22,000hz. Now if I don't state that that response is +-3db it does not accurately tell me how the speaker resonds. That 20hz to 20khz speaker could really be down-12db at 20 hz but only -3db down at 35hz. The same goes for the top end. The speaker could really be at +8db at 22khz and +3db at 19.5khz. If this were the case, I would have to state the response as 35hz-19.5khz +-3db. Above the 19.5khz and below the 35hz the response goes outside the -+3db window. If I still wanted to use the 20hz-22khz spec I would have to state it as 20hz-22khz &nbsp;-12db, +8db. Some manufacturers have even tighter specifications only limiting a deviation of -+1db! This is very difficult to acheive.</font>
  4. ThA tRiXtA Full Audioholic

    ThA tRiXtA
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    <font color='#000000'>wow thats great man, thank you very much!

    good explanation too!

    so is that good, +/- 3 dB 44 to 20 kHz? or is that pretty crappy?

    Also - when manufacturers state their specifications like that, are they usually truthfull?</font>
  5. JAB Audioholic Intern

    JAB
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    <font color='#000000'>Speaker manufacturers measure the frequency response of a speaker a in specially treated room. These rooms are designed to absorb all sound - in other words there is no reflected sound from any surfaces, resulting in a totally &quot;dead&quot; room. The speaker is placed on a stand and the frequency response is measured by a microphone set approximately 1 meter from the centre of the speaker. A sweep tone is then fed to the speaker and the response is recorded on a graph, the center being 0 db. The frequency response is stated in + or - db of this central point. Of course, the perfect speaker response would be + or - 0db (flat) from say 30-20,000 hz which is near impossible to acheive. If a modern speaker manufacturer cannot achieve a minimum deviation of +/- 3 db within a claimed frequency response of a particular model, it should be back to the drawing board!!

    As db (loudness) is a logarithmic measurement, every 3 db of change is a doubling (+) or decreasing (-) of the volume. Thus being down -3db at 40hz means the bass response is starting to descend in volume after this point. However, proper placement in a room can boost the lower bass response of a speaker that is only down -3db. You could probably expect around a 38hz bass response before the low end rolls off in volume. If a speaker is down -6db at 35hz or lower, you essentially have no bass response in the speaker below this point. I have never seen a manufacturer declare a -6db response in the bass region - what make and model are your speakers??

    As stated, a good modern speaker manufacturer aims for the + or - 3db range or better. Some manufacturers state even better specs but these can be misleading. There are critical areas where a given design should remain as flat as possible, and that is in the area where the ear is most sensitive (the upper bass and midrange). Where these deviations occur is what gives every speaker design it's own sound. Quite often in a speaker review you will read &quot; has a seroius hump in the upper bass area &quot; or &quot;a suck-out in the midrange&quot; or &quot;lacks air and extension in the upper frequencies,&quot; etc. These are examples of deviation from that perfect flat response in the critical hearing areas.

    There has been examples of excellent flat response from a speaker in these treated rooms, but they sound like crap in the real world - your listening room with all it's frequency altering furnishings and surfaces.

    Designing a great sounding speaker is a real art, part science, part alchemy, and measuring devices are not the end all. It can take years of critical listening evaluation by dedicated ears to perfect a design.

    Al</font>
    JAB,
  6. ThA tRiXtA Full Audioholic

    ThA tRiXtA
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    <font color='#000000'>Hey

    My speakers are the Mission M74i towers.

    They aren't rated at  +/- 6 dB all the way through the frequency response, it is +/- 3 dB 44 to 20 kHz, and I guess it loses 6 dB going down to 38 hZ.

    thanks for your help man [​IMG]</font>
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  7. annunaki Moderator

    annunaki
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    <font color='#000000'>Just a little clarification. A -+3db change is JUST NOTICABLE. A -+10db change is perceived as DOUBLE and /or TWICE AS LOUD/QUIET. It takes a doubling of power (e.g. 100 watts to 200 watts) to experience a 3db increase on paper. In the real world, due to a speaker's inefficency at turning wattage to motion, a doubling of power usually translates into a 1.5db-2.5db increase in volume.

    The response on you speakers is fine. They will not go down to the depths of the bass range, but that is what a good subwoofer is for. In your listening room you may actually get pretty good bass response below 40hz depending upon placement and room acoustics.

    The acoustical testing room that manufacturers use, that JAB referred to, is called an anechoic chamber. Some manufacturers state their specs as (e.g.) 30hz-21khz +-3db anechoic. Remember, specs will only get you so far. Let your ears be the judge.</font>
  8. ThA tRiXtA Full Audioholic

    ThA tRiXtA
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    <font color='#000000'>if it takes that much power to push the speaker just to double the volume from 3 dB to 6 dB if an amp is only rated for 100 w per channel?

    Thanks for your reply Annunaki</font>
  9. annunaki Moderator

    annunaki
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    <font color='#000000'>Trixta, your speakers stated response from the manufacturer looks some thing like this:

    Freq. response: 44hz-20khz +-3db &nbsp; -6db at 38hz

    Correct?

    If that is the case they are showing you how steeply the response rolls off below 44hz.

    My PSBs do it very similar. 38hz-21khz +-3db. -10db at 33hz (LF cutoff)</font>
  10. annunaki Moderator

    annunaki
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    <font color='#000000'>Trixta,
    Say your speakers, for example, at 50 watts rms unclipped power reach 105db. In order to reach (on paper) 108db you would have to bump the unclipped power to 100 watts.

    Now If we are already at 105 db at 50 watts unclipped and we want the system to sound TWICE as loud. We would now have to reach 115db and increase our unclipped power from 50 watts to about 667 watts!!!

    Granted all the numbers I used above are just examples. In the real world you could probably be higher than 105db at 50 watts. If a speakers efficency at 1w/1m is 91 db and you have 50 watts rms unclipped you would be looking at about 107 db on paper at 1 meter from the speaker anechoic. Real world you are looking at slightly less volume at roughly 104.5db anechoicly. Room efficency could bring this up considerably. However, this is plenty loud.
    Sorry if I am confusing you. I am already starting to go too far below the surface. If it sounds good and is loud enough to suit your needs, that is all that matters! We need to enjoy our systems, not over analyze them. [​IMG] &nbsp; [​IMG]</font>
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  11. ThA tRiXtA Full Audioholic

    ThA tRiXtA
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    <font color='#000000'>Hey no this is great stuff, your not confusing me, I am just hungry for knowledge.

    I know I am just a beginner, and I concur with what you say when you mention &quot;If it sounds good and is loud enough to suit your needs, that is all that matters! We need to enjoy our systems, not over analyze them&quot; but something makes me want to know the what, why, how etc... I just like to know how this stuff works...

    I believe a fundamental knowledge of technology like this allows you to make mroe educated buying decisions and get the best product you can afford.

    I really appreciate your help and energy, Do you mind answering these questions?

    I know they are very amateur, but they are helping me out a great deal! &nbsp;[​IMG]</font>
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  12. annunaki Moderator

    annunaki
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    <font color='#000000'>No, I do not mind [​IMG] . Most people do not ask the right questions as to receive informed responses. [​IMG] Having knowledge, as to the how and why, definitely can help a buying decision. However, once in a while, you wish you never knew [​IMG] . If everyone actually wanted to learn about equipment they purchase, as well as some of the how and the why, the entry level market would have to improve. Not &nbsp;to mention marketing people would be out of jobs [​IMG] .</font>
    Last edited by a moderator: Oct 23, 2014

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