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Thread: Audio Equipment and Frequency Response

  1. #1 Audio Equipment and Frequency Response 
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    On average, people can hear sounds of frequencies as high as 15kHz. Why, then, do audio-equipment manufacturers design equipment that can presumably produce sounds with frequencies much higher than 15kHz?

    One obvious reason is that the highest-frequency sounds that are audible for some people are sounds that have frequencies that approach 20kHz.

    But, some manufacturers claim frequency-response for some of their equipment that far exceeds 20kHz. For instance, Polk Audio claims that their LSiM707 speakers have an overall frequency response of 20Hz to 40kHz.

    The only reason I can think of is aside from the reason I just mentioned is that the manufacturers wish to impress consumers with big numbers.

    Are there other reasons to design equipment with ultra-high frequency responses?

    Jagella


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  3. #2  
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    Hey, maybe your cats and dogs are into music too.

    Maybe markrting hype, but supposedly young people hear up to 22 kHz, and possibly even some of the higher harmonics.


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  4. #3  
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    Quote Originally Posted by MigL
    Hey, maybe your cats and dogs are into music too.

    Maybe markrting hype, but supposedly young people hear up to 22 kHz, and possibly even some of the higher harmonics.
    If the frequency response range greatly exceeds the audible range then it is quite likely that sound reproduction within the audible range is relatively free of distortion.

    Even digitally to reproduce signals accurately within a frequency range of you must sample at at least . See "Shannon sampling theorem".

    My dog sometimes watches TV. I have not noticed him wagging his tail to the rhythm of the stereo.
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  5. #4  
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    Quote Originally Posted by DrRocket
    If the frequency response range greatly exceeds the audible range then it is quite likely that sound reproduction within the audible range is relatively free of distortion.
    A sensible answer! That's more like it.

    I was thinking along the same lines. If a tweeter is designed to handle frequencies up to 15kHz, then a 19kHz signal might cause a forced vibration. Is a forced vibration one of the culprits in speaker distortion?

    Jagella
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  6. #5  
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    All sound emitted by a speaker is "forced vibration" That's kind of the point.
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    Quote Originally Posted by MeteorWayne
    All sound emitted by a speaker is "forced vibration" That's kind of the point.
    OK, I see what you mean, but doesn't the speaker cone have a "natural frequency" that it may vibrate at freely? The more the forced vibration varies from that natural frequency, the more likely it will distort the sound the speaker emits. A speaker that can handle ultrasonic sounds may have a natural frequency closer to the signal frequency than a speaker that cannot emit ultrasonic sound. The ultrasonic speaker may then handle high frequencies that are in the audible range better than the other speaker.

    Jagella
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  8. #7  
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    Quote Originally Posted by Jagella
    Quote Originally Posted by MeteorWayne
    All sound emitted by a speaker is "forced vibration" That's kind of the point.
    OK, I see what you mean, but doesn't the speaker cone have a "natural frequency" that it may vibrate at freely? The more the forced vibration varies from that natural frequency, the more likely it will distort the sound the speaker emits. A speaker that can handle ultrasonic sounds may have a natural frequency closer to the signal frequency than a speaker that cannot emit ultrasonic sound. The ultrasonic speaker may then handle high frequencies that are in the audible range better than the other speaker.

    Jagella
    No.

    The lighter the cone the higher the natural frequency, but the easier it is to drive the cone to match the intended acoustic signal.

    You don't want the natural frequency to be in the audible range, because that resonance would be a distortion on top of the intended driven signal. You also want the signal to be reasonably damped, just not so damped that response to the command signal suffers.

    Idealy the speaker responds to the command signal and nothing else. That is, of course, impossible since there is such a thing as inertia. So what you want to do is minimize inertia, consistent with enough size and power to reproduce low frequency sounds at desirable amplitude-- which is why there are woofers and tweeters.
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    True DrR, but we're not talking about digital sampling in which case it would be valid. We're talking about the measured frequency response of a speaker. If it is flat up to the cut-off frequency, why would you overdesign it to be flat to twice the cut-off frequency ? You won't get a flatter response up to the cutoff frequency, but will cost you much more in materials.
    And because of the steep analogue filtering used in speakers, feeding a 40kHz to a speaker limited to 20kHz will yield very little if any signal, so no distortion. And most sources ( CD, DVD,radio, blu-ray,etc ) also have very steep digital filtering to block frequencies and higher order harmonics beyond 22 kHz
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  10. #9  
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    Quote Originally Posted by MigL
    True DrR, but we're not talking about digital sampling in which case it would be valid. We're talking about the measured frequency response of a speaker. If it is flat up to the cut-off frequency, why would you overdesign it to be flat to twice the cut-off frequency ? You won't get a flatter response up to the cutoff frequency, but will cost you much more in materials.
    And because of the steep analogue filtering used in speakers, feeding a 40kHz to a speaker limited to 20kHz will yield very little if any signal, so no distortion. And most sources ( CD, DVD,radio, blu-ray,etc ) also have very steep digital filtering to block frequencies and higher order harmonics beyond 22 kHz
    Agreed, if you are talking about the entire speaker system, including the filtering. But if you are just talking about the loudspeaker itself, then the response won't be truly flat and the higher the frequency capability the flatter the response will be over the range of interest, particularly at the high end where things tend to roll off.

    There is obviously a practical and economic limit to how far you want to go.

    It is also worth remembering that high frequency capability is reflected in ability to track rapid changes in the time domain -- step function response -- and not just reproduction of high-pitched sinusoids. That transient capability is important too.

    It also depends on the listener. My wife can easily tell the difference between really good speakers or headphones and mediocre ones. She also hears my dog whistle. They all sound the same to me -- I can't even hear high-pitched alarm clocks.
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  11. #10  
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    That just means you're old Doc.
    I'm on the bad side of 50 myself now ( but still immature), seems like just yesterday I was in my 20s.
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  12. #11  
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    Quote Originally Posted by MigL
    That just means you're old Doc.
    I'm on the bad side of 50 myself now ( but still immature), seems like just yesterday I was in my 20s.
    Old and have been exposed to too many loud noises. Hoping to be exposed to many more.

    What is worse is that I just found out that Matt Dillon (James Arness) died last week. What is the world coming to ?
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  13. #12  
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    Quote Originally Posted by DrRocket
    The lighter the cone the higher the natural frequency...
    Is the higher natural frequency a result of the relatively low mass of the cone which has less inertia than a heavier cone? In other words, are the relative masses of speaker cones (woofers, tweeters, etc.) analogous to the strings in a piano? Longer and heavier strings vibrate at natural frequencies that are lower than the natural frequencies of the shorter strings.

    I suppose the analogy breaks down when we consider that piano strings are meant to vibrate at their natural frequencies whereas speaker cones are forced to vibrate at the frequencies of the incoming signals.

    Quote Originally Posted by DrRocket
    ...but the easier it is to drive the cone to match the intended acoustic signal.
    The ease of driving the cone does not equate with sound quality but with efficiency. That is, an efficient speaker can produce a sound at a given loudness with less power (wattage) than a less efficient speaker. The less efficient speaker may have much better sound, though.

    Quote Originally Posted by DrRocket
    You don't want the natural frequency to be in the audible range, because that resonance would be a distortion on top of the intended driven signal.
    And that is the reason for manufacturers making tweeters that can emit ultrasonic sounds? I haven't noticed many woofers having frequency-response specs below 20 Hz enabling them to produce infrasonic sounds. Now, Polk Audio offers two models of subwoofers that claim frequency responses down to infrasonic levels. I suppose it's a lot tougher to make speakers that can emit such low frequencies.

    Quote Originally Posted by DrRocket
    You also want the signal to be reasonably damped, just not so damped that response to the command signal suffers.
    What is a “damped” signal?

    Anyway, I was wondering how a speaker can produce vibrations in the speaker cone which is often made of paper. I know that the electricity in the speaker wires alternates which can affect the magnetic field in a magnet—but that magnetic field cannot cause paper to vibrate, at least not directly. After a little research, I found that speakers include two magnets; one magnet is a stationary, permanent magnet, and the other magnet is an electromagnet connected to the speaker wires and attached to the cone. The wires alternate the magnetic field in the electromagnet causing its polarity to switch. The change in polarity causes the electromagnet to move toward and away from the permanent magnet in sync with the alternating, incoming signal. The vibrating electromagnet forces the cone to vibrate, and we have Led Zeppelin in our living rooms.

    Ingenious, isn't it?

    Thanks for the great response! I like your avatar—I'm a fan of Richard Feynman, too. I plan to read Quantum Man soon.

    Jagella
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