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I m p u l s e   R e s p o n s e   F i l e   " L o u d n e s s "

A little known fact - that becomes very important when trying to audition IR's due to the "louder sounds better" phenomenon - is that in addition to how we individually percieve and are sensitive to certain frequencies on an indirect level, reverberation information and decay tail length in an impulse response can also directly affect how “loud” most IR’s “play” in IR loaders, dependent upon the loader's build. Unless an IR loader has an inbuilt output normalizing function, the amount of captured and presented reflection information can inadvertently change the perceived output volume significantly even though the files are all peak normalized to the same value. The level of said captured reverberation energy in a given IR can vary due to several factors:

Open or Closed Back Cabinet

With an open back cabinet, some energy escapes and is not reflected back to be captured by the microphone directly, of which a portion of that is then lost to the room the cabinet is in. With a closed back cabinet, the energy "bounces around" inside the cabinet and what is not lost to absorption can mostly only escape through the front baffle speaker cutout and paper cone of the driver - the least dense part of the internal structure - and thus into the path of the microphone.

Thickness of the Grill Cloth

Thicker grill cloth – especially basketweave – depending on how tight the weave, keeps the reflections trapped more so than thinner (or no) cloth, as well as the effect it has on the frequency response (soaking up direct high frequency energy). Thusly, a thick "cane" style grill covering with a tight weave knit will absorb significant high frequency energy as well as reflecting/absorbing the sound "bouncing around" the inside of the cabinet, preventing quite as much from escaping into the path of the microphone.

Volume the Stimulus was Driven At

While not a factor for OwnHammer IR’s, which drive speakers at their mechanically and tonally ideal output levels (courtesy of the knowledge acquired in our early-days development with dynamic convolution), if an IR was generated at extremely low volumes, the bulk of the volumetric heavy lifting is done by the outboard gear after the mic and/or the digital normalization process when compiling the IR, resulting in less reverberant energy to be available to be captured and rendered.

Length of Time the Stimulus was Driven

Similar to the above entry, additional properties of the stimulus come into play. The length of time a stimulus file lasts determines how much time the cabinet can be resonated, and the volumetric amount of return reverberant energy that can be created and transduced. Since certain frequencies will resonate more or less, this also allows less time for the cabinet to be heavily stimulated in those areas, like they would in real life application in playing prolonged musical source material. Again, when a specific IR creation/distribution source consistently has low amounts of reverberant decay, and overall have a sort of "flat" and "splatty" sound, the likelihood increases that short "blip" sweeps were used, and also likely at lower levels to compensate for a less than ideal capture environment. On the contrary, while other factors like the room are surely at play, if files from a given IR creation/distribution source consistently return high levels of reverberant decay, it can likely be deduced that a longer stimulus file was implemented, though not guaranteed.

Size of the Room and Amount of Reflections

This is less a factor for close mic’ing compared to the escaping reflections/resonance from the inside of the cabinet, which is the PRIMARY "room" being captured in a close mic'd IR. As such, room reflections are a less pervasive part of the overall captured sound in a close mic’d impulse response file, counter to much and common misconception and misinformation, and counter to the relationship of these reflections in distance mic'ing. This is why shortening the IR past a certain point homogenizes the sound – it takes away the character of the CABINET more than what it does to the small amount of actual room information, to where the sound moves towards that of a speaker suspended in air with additional frequency/phase response alteration that the resonance chamber it is attached to (as well as the effect of volumetric sympathetic resonance created by additionally attached drivers) is providing.

In Summary

The rule of thumb is: the more reverberation that is captured, the quieter the file plays without output normalization in the IR loader; and vice versa. To this end, an open back 1x12 IR will “play” louder than a closed back 4x12 IR. A truncated 20 ms file will “play” louder than a full length and well populated 500 ms file. An IR that is only frequency response with zero reverberation (aka an EQ) will be DRASTICALLY louder. The varying amount of reverberant energy in an IR also can also change how it “feels” under the fingers. The less that is there, the more dry and unforgiving the feel. The more that is there, despite being linear gives a spongy feeling like sag, or 2nd/3rd order harmonic distortion. Too much, however, results in a sound that is sluggish and murky, so a balance must be had for optimal results.

This phenomenon is dependent upon the data inside each unique IR, is not consistent but does follow common patterns, and should be predicted and accommodated for so as to not fall victim to “loudest is best”. This may be useful when comparing and auditioning different files, or in wondering why output levels may need to be adjusted when switching between IR’s that have all been normalized to universal standards. For loading platforms with normalizing functions on the playback output, this is much less of a factor, if not a near non-existent one.


Impulse Response Libraries
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