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Question about Tom Nousaine's subwoofer freq. response measurements
When reading Tom's frequency response measurements done in magazines,
the frequency response specification is not always given in (what I've always thought was) the typical +/- 3 dB fashion. Here are some examples from his subwoofer tests, as pulled from the Web: 48-115Hz +/-2.1 dB 25-117Hz +/-3.5 dB 28-117Hz +/-2.4 dB 25-82Hz +/-3.2 dB 27-90Hz +/-1.7 dB Does anyone have an explanation for why the tolerances are different for each speaker? It seems to me that if the speakers were measured and the results all shown as x-y Hz +/- 3 dB, the numbers would be more meaningful. As it is, it's not clear which of the above really has the flattest graph in any particular frequency band. Thanks for any insight. |
#2
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Without the context it is hard to tell. One guess is that these represent
the freq range provided by the manufacture as the bottom and high based on roll off and the low pass xover. Using that range the +/- db figure is then an empirical observation. Another possibility is that these are the results measured at a given spl level, where the low will often vary as a function of that level. In any case he should soon read and reply here. When reading Tom's frequency response measurements done in magazines, the frequency response specification is not always given in (what I've always thought was) the typical +/- 3 dB fashion. Here are some examples from his subwoofer tests, as pulled from the Web: 48-115Hz +/-2.1 dB 25-117Hz +/-3.5 dB 28-117Hz +/-2.4 dB 25-82Hz +/-3.2 dB 27-90Hz +/-1.7 dB Does anyone have an explanation for why the tolerances are different for each speaker? It seems to me that if the speakers were measured and the results all shown as x-y Hz +/- 3 dB, the numbers would be more meaningful. As it is, it's not clear which of the above really has the flattest graph in any particular frequency band. Thanks for any insight. |
#3
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tk wrote:
When reading Tom's frequency response measurements done in magazines, the frequency response specification is not always given in (what I've always thought was) the typical +/- 3 dB fashion. Here are some examples from his subwoofer tests, as pulled from the Web: 48-115Hz +/-2.1 dB 25-117Hz +/-3.5 dB 28-117Hz +/-2.4 dB 25-82Hz +/-3.2 dB 27-90Hz +/-1.7 dB Does anyone have an explanation for why the tolerances are different for each speaker? It seems to me that if the speakers were measured and the results all shown as x-y Hz +/- 3 dB, the numbers would be more meaningful. As it is, it's not clear which of the above really has the flattest graph in any particular frequency band. Thanks for any insight. Seems abundantly clear to me! The only thing we don't know is what happens beyond the end frequencies. Gary Eickmeier Gary Eickmeier |
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When reading Tom's frequency response measurements done in magazines,
the frequency response specification is not always given in (what I've always thought was) the typical +/- 3 dB fashion. Here are some examples from his subwoofer tests, as pulled from the Web: 48-115Hz +/-2.1 dB 25-117Hz +/-3.5 dB 28-117Hz +/-2.4 dB 25-82Hz +/-3.2 dB 27-90Hz +/-1.7 dB Does anyone have an explanation for why the tolerances are different for each speaker? It seems to me that if the speakers were measured and the results all shown as x-y Hz +/- 3 dB, the numbers would be more meaningful. As it is, it's not clear which of the above really has the flattest graph in any particular frequency band. Thanks for any insight. +/- 3db is actually quite generous. Today, a good speaker can easily reach +/- 2db or better. |
#6
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(tk) wrote:
When reading Tom's frequency response measurements done in magazines, the frequency response specification is not always given in (what I've always thought was) the typical +/- 3 dB fashion. Here are some examples from his subwoofer tests, as pulled from the Web: 48-115Hz +/-2.1 dB 25-117Hz +/-3.5 dB 28-117Hz +/-2.4 dB 25-82Hz +/-3.2 dB 27-90Hz +/-1.7 dB Does anyone have an explanation for why the tolerances are different for each speaker? It seems to me that if the speakers were measured and the results all shown as x-y Hz +/- 3 dB, the numbers would be more meaningful. As it is, it's not clear which of the above really has the flattest graph in any particular frequency band. Thanks for any insight. These numbers are the tolerance of the speakers response relative to the half power points (-3 dB.) For example, 27-90Hz +/-1.7 dB, tells us that the half power points were 27 Hz on the low end and 90Hz on the high end. However it's possible to have wider variations in between. For example if this woofer had a 6-dB peak at 40 Hz and a 9-dB notch at 50 Hz the tolerance would be +/- 7.5 dB. Another way of specifying this would be (27-90 Hz (+6/-9 dB) but the managing editor sometimes saves a few spaces of copy with our current copy style. |
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#8
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"tk" wrote in message
... Cool, an answer from the man himself! Thanks! (Nousaine) wrote in message ... (tk) wrote: 27-90Hz +/-1.7 dB These numbers are the tolerance of the speakers response relative to the half power points (-3 dB.) For example, 27-90Hz +/-1.7 dB, tells us that the half power points were 27 Hz on the low end and 90Hz on the high end. However it's possible to have wider variations in between. For example if this woofer had a 6-dB peak at 40 Hz and a 9-dB notch at 50 Hz the tolerance would be +/- 7.5 dB. Another way of specifying this would be (27-90 Hz (+6/-9 dB) but the managing editor sometimes saves a few spaces of copy with our current copy style. Thanks for the response! So the half power points, if I understand correctly, are the low and high frequencies beyond which (in either direction) the frequency reponse is always at least 3 dB lower than the 0 dB reference point, when fed a constant input amplitude. Is that right? I'm still not clear as to how to 0 dB reference point is established, though. In the case of the second speaker above, the +/- 1.7 dB tolerance indicates a total range of 3.4 dB between the end points. I assume that the 0 dB reference level is actually attained at some point between 27 Hz and 90 Hz. Thus: 1. To accomodate a 3.4 dB total range, the response must peak to 0.4 dB above reference at some point, or 2. Maybe at some point between 27 Hz and 90 Hz the response drops to 3.4 dB below reference before climbing up again, and so the max level is 0 dB, or 3. Maybe the peak level is between 0 and 0.4 dB above 0 dB reference, which agains leaves the question of how the 0 dB point is chosen. (I think the answer does have some interesting implications... Let's say that a given speaker has a +/- 3 dB range between the end points. If the answer is (1) above, then the frequency response never drops below that of the end points while within the range (but it does peak at +3 dB above "0 dB"). If the answer is (2) above, the response actually is lower (at 6 dB below reference) than that of the end points at some point(s) within the range, but never climbs above "0 dB". If the answer is (3), then we're in the dark about the nature of the frequency response within the end points, relative to the levels of the end points.) I'm sorry if this has been a very ignorant or pointless question... Am I missing something really obvious? Thanks for your patience! I had exactly the same reaction to Tom's explaination. It just doesn't seem like a very orthodox use of tolerance specs, and leads to the speculations you have made above. |
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(tk) wrote:
Thanks for the followup! (Nousaine) wrote in message ... (tk) wrote: You're working the analysis too hard :-) The transfer function measurement technique simply assigns the zero value to an average of output over a useful bandwidth. You could do trhe same thing by using a sound pressure value. Okay, I think I get the point; essentially in these measurements you are saying "this is the main useful bandwidth of the speaker, and the speaker has this degree of fluctuation within the bandwidth." At least, this is the impression I get. If life were to fall my way I'd simply publish a full set of graphs with all directivity data included. But I'm simply using the space the managing editor allows for presentation. I understand -- it would be cool for the web site to host enhanced versions of your articles. Well perhaps I might do just that someday. But again 'comparing speakers' on paper is pretty easy. Learning to implement and live with your choice may be more problematic. I routinely advise both home and car enthusiasts to optimize the equipment they already own before deciding that 'throwing money at the problem' is a useful alternative. There's more to speaker performance than frequency response at a given fixed level of output. That's why I use a separate technique to illustrate the linear dynamic capability of subwoofersand other speakers (Bass Limits.) That's another question I had -- how you decide what frequency to use for the "useful bass extension" or "bass limit" number, e.g. 75 dB at 20 Hz versus 83 dB at 25 Hz. The frequency 'falls out' of the experiment. I just keep moving upward in drive level and downward in frequency to find the lowest frequency where the subwoofer can generate a sound pressure (= 10% distortion) at an SPL that will be audible to listeners. Here's the procedure. Start at 62 Hz (In my estimation the beginning of the 'subwoofer' range .... IOW 100 Hz isn't a subwoofer frequency; arguably 62 Hz isn't either) and give the subwoofer input (with the volume control set to maximum and the crossover controlset to maximum bandwidth) a 6.5-cycle ramped tone burst (aka Linkwitz and Keele) at that preferred 1/3 octave frequency and keep increasing drive until distortion reaches 10% ( at 10% a woofer sill still sound 'clean' but will be leaving its linear operating range and distortion will be increasing exponentially) and then record the Sound Pressure Level. Now repeat this process for every 1/3 octave frequency at lower frequencies. Record the Bass Limit as the lowest frequency that a speaker will produce a clean SPL above the threshold of audibility. For subwoofers average the SPL figures over the 26-62 hz range (the heart of subwoofer action.) I also publish a Bass Uniformity figure (the 25-62 Hz number / maximum SPL at any frequency) to draw attention to those products that handicap the 25-62 Hz number with products that have a huge output at the upper-end of their bandwidth but with rapidly falling lower frequency capability. The other thing that I sometimes found confusing was the measurement of max SPL at 10% THD. (E.g. a speaker maxes out at 100 dB at 25 Hz with 10% THD but only at 85 dB at 20 Hz with 10% THD). I had thought this was indicative of frequency response, but it's apparent that that is quite a different issue. It can be. That's because frequency response measurements are made with a fixed (and fairly low) drive level. I've evaluated products that claimed an 8-Hz bandwidth and found that the 8 Hz could only be obtained at very low frequencies with close-mic measurements and small drive levels. At typical levels in a real room the actual bandlimit was closer to 40 Hz. Thanks for the feedback. It's very useful that someone has done so many measurements of different speakers under the same conditions, making it possible to compare performance in a meaningful way. Thank you. I'm so happy that folks are listening. |
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