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Microphone placement and SPL are irrelevant. To measure transient response (impulse response) - one must work or use testing gear designed to function in the time domain rather than frequency domain. A unique series of frequency impulses (maximum length sequence) is sent to the unit being tested. The microphone then picks up this sequence and a series of calculations is performed determining the deviations in the original sequence compared to the return sequence. Once the impulse response is calculated, nearly every other loudspeaker measurement can be determined from this data, including frequency response, by applying an FFT to the impulse response data.
https://en.wikipedia.org/wiki/Maximum_length_sequence
It is not something the typical hobbyist will be able to measure, which is likely why it is so rarely discussed regarding subwoofer performance. Additionally, it does take the right equipment and user knowledge to perform correctly. Besides Rythmik, I am not even sure if any of the other ID sub manufactures have access to this critical performance data, or even care about it. It is something we take very seriously (#1 priority in the design of many of our loudspeakers) and it is vital to Rythmik as well - transient accuracy is greatly enhanced by Direct Servo.
Not quite sure what you are referring to when you mentioned "rule it out". I have evaluated every type of subwoofer out there - from models designed to fill large theaters and concert halls - to those designed for ruler flat response and as good as possible transient accuracy. Like speakers, different subs for different purposes - no point in comparing apples to oranges.
That remark was not directed at you or others members within this thread... if I dropped the "you" perhaps it would not sounded as a challenge...like you said it's about trade offs and I know unequivocally that all subs do not sound the same and fwiw....I use a different house curve for music vs. home theater for that reason....albeit with the same sub.
Speakers 5.1.2: TitanTowers v2 & STC(RAAL v2), MA CP-WT&CT260
Sub: Funk Audio 18.0 SantosRW
Source: Denon X3800H, Oppo BDP 103D, UBK-90 4K & LG B9 65"
Office 2.0: Philharmonic True Mini(coming-soon), Fosi TB10D via Wiim mini.
Thanks Dave, really well explained. The link to maximum length sequence was perfect to get me reading more on this.
Do you design a driver with an impulse response as a target or is it a result of the rest of the design process the determines this?
There are many factors in driver design that influence its impulse response, optimize one characteristic and you end up changing other parameters. It is all a compromise.
While I won't get into any of our design secrets, one simple aspect to improve impulse response (all other things being equal) is to decrease moving mass. However, in a woofer, decreasing moving mass also increases efficiency and decreases bass extension.
I always try to achieve what I feel is the right balance of all driver characteristics to match the particular design. For example, in designing the final version of the Sierra-2 woofer, we sacrificed bass extension in order to go with a very low mass cone to improve impulse response to better match the near perfect impulse response of our Sierra-2 ribbon tweeter. We were lucky because we were able to go with a larger diameter cone in the same space which helped us get back some of the lost extension due to the higher efficiency of the larger cone. Many other optimizations to this woofer were implemented to improve impulse response - even in the crossover design, but I won't reveal this info.
It is for these reasons why I spend so much time on driver design instead of simply using off-the-shelf components. I'm not saying off-the-shelf is bad (it would certainly make things easier and less costly for us) - but this is how we do things as I am obsessed with total performance, not just good performance in one or two characteristics.
Dave, please do continue to be obsessed! We like the results
Sierra RAAL V2 Towers, Axiom EP 500 Sub, Morrow SP7 Grand Reference Speaker Cables, Phillips CD880 XLR Balanced Variable Line Output to Orchard Audio Strakrimson 375w/per ch Stereo Ultra GaNFET Amp..... (Dedicated 2.1 Acoustical Music Listening Room)!