Taking the RAAL-requisite SR1a Headphones To Another Level

On April 30, 2020 I published a review of the RAAL-requisite SR1a headphones. The review included statements such as

• "The RAAL-requisite SR1a headphones are unequivocally the most sensational audio product I've ever heard."
• "Honestly, no product has ever captured my attention, caused me to listen to more music, or garnered my enthusiasm like the SR1a Earfield headphone monitors."
• "I frequently sit across the room from a $100,000+ audio system, looking at it, while listening through these headphones."
• "These headphones literally change the game."

It's now Early June, 2021 and I stand by everything I said in that review. However, I "discovered" a way to make the SR1a headphones even better, more sensational, and even more accurate. The best part is that this improvement is available for a relatively cheap price compared to the headphones, requires no new hardware purchases, and can be enjoyed by many listeners immediately. 

I placed the word discovered in quotes above because I literally didn't discover anything in the traditional sense, but I discovered for myself that it was possible to make the SR1a even better and I believe this will be a fantastic discovery for the vast majority of SR1a owners. 

The journey to improve upon the best headphones available started one afternoon with an email to Mitch Barnett of Accurate Sound. @mitchco created the convolution filters for my main system's DSP room correction and has done so for many people around the world through his Accurate Sound services. I emailed Mitch to ask if he could measure the SR1a headphones, and create correction filters for them. These wouldn't be room correction filters, rather filters that make the frequency response of the SR1a even more accurate. No headphones are perfect, but the pursuit of perfection marches on with advances in technology. In this case, advanced measurements and extremely powerful digital signal processing.

Mitch replied right away saying that he was well situated to measure the SR1a and create filters to make them even better. He's measured headphones for quite a while, long before my interest in this area. I told Mitch that I had no idea how the SR1a would measure and I hoped he could work his usual magic. Then I got to thinking, why doesn't Mitch measure "every" headphone and offer a headphone convolution filter store like the App Store? More on that later. 

I contacted RAAL-requisite who enthusiastically sent Mitch a pair of SR1a headphones and we were off to the races. 

Given that Mitch is the expert in this area of audio, I asked him to provide some details about his work measuring the SR1a and creating the convolution filters.

Here's Mitch ...

Headphones are hard to measure and even harder to eq properly. When looking around at the various websites that measure headphones, usually with artificial ears that contain mics, there is quite a bit of measurement variability between headphones and even of the same headphone.

Those measurements are typically used as the basis for equalizing headphones to flatten the frequency response or by applying a target frequency response like the Harman target for example. I wrote about the Harman target curve here on Audiophile Style, which was peer reviewed by Sean Olive. Again, looking around at various websites that provide eq profiles for headphones, we see even more variability for any given headphone.

To further complicate matters, everyone’s Head-Related Transfer Function (HRTF) are different. Finally, our ears frequency response varies with sound pressure level (SPL), which is explained by the equal-loudness contour.

With all of this variability, which measurement approach/technique best represents the actual headphone response? When applying eq does it “sound right?”

What is meant by sounding right is that all frequencies sound even, with no frequencies “sticking out” or recessed. The mix sounds evenly balanced, not only across the frequency spectrum, but where the instruments and vocals reside in 3D space. An uneven frequency response will alter the depth of field presentation of the recording. So not only is the tonal balance inaccurate so is the depth of field of where instruments and vocals “sit” in the mix.

For example, if you have a frequency response peak in the 2 kHz range, which also happens to be where our hearing is the most sensitive, then vocals and guitar solos will be pushed forward in the mix, sounding bright and “in your face.” While some headphones are “voiced” with their signature frequency responses, it is nice to have the option of obtaining a flat or neutral or accurate response to our ears.

After years of research and experimentation with measuring headphones in different ways, one can measure headphones with minimal HRTF interaction. This results in a flatter frequency response and more representative of the actual headphone frequency response.

The measurement approach I am using does not use “artificial ears.” My approach results in a frequency response that is more flat than other types of measurement approaches. Therefore, if eq is applied using this measurement profile, then the resulting sound should be considered flat, independent of who is wearing the headphones.

With respect to eq, I prefer Finite Impulse Response (FIR) filters with 65,536 samples in length over any other type of filter. Rather than using a handful of parametric eq’s (PEQ) that don’t adequately smooth the response, I prefer the high-resolution approach of making a large number of small adjustments in the frequency domain, and if required, an independent phase adjustment can be applied in the time domain. Note that headphones are typically minimum phase devices.

Using this new measurement technique, here is the frequency response of the RAAL Requisite SR1a of the left channel. It shows both frequency and phase response in one chart:

I also included the cursor which is the straight line at 75 dB SPL as a guide to help show the frequency response differences. 1/12 octave smoothing has been applied.

This chart shows accurate measured frequency response up to 3 kHz and then a drop in level, but flat with some ripples. I am still fine tuning the measurement technique as the drop in level is not quite right, but the response is correct.

The SR1a’s ribbon measures very well out of the box. The SR1a’s sound pretty much what we see in the chart. The SR1a’s are -3dB at 30 Hz and -10 dB at 20 Hz. A bit of a bass bump from 50 to 100 Hz and a bit of a peak at 2.2 kHz. Then we see an overall drop in level and a bit up and down in response as we get into ear canal frequency range.

The idea behind this measurement approach is to minimize individual HRTF, so when eq is applied, everyone “hears” the same relative result. This is the benefit.

Here are 3 stereo measurements of the SR1a as I varied the headphone position. There is a consistent response:

We do see the ears’ canal part of the HRTF really having its way as a major cancellation in the 10 to 11 kHz range for one ear. Finer tuning of the measurement technique is required.

But of course, we do not eq that deep notch anyway. In fact, we are just interested in the “envelope” of the frequency response which characterizes the SR1a’s tonal response. This is where the art of applying eq meets science.

This is the level of accuracy and precision we are able to achieve using a high resolution FIR correction filter:

The blue trace is the raw measurement and green is the corrected response. Here we bring down the two peaks above the green curve, in addition to smoothing out the response. Literally, it’s a ruler flat frequency response like an amp or DAC, except for my measurement gain level discrepancy above 2.2 kHz. That’s a work in progress. But as a relative level, we can see further smoothing of the corrected response in that region and beyond as compared to the raw response.

Let’s look at the corrected left channel with phase (right channel is identical):

Amazingly ruler flat frequency response for any headphone. Virtually perfect minimum phase response.

We can also see an improvement in the timing or step response. Raw measure:

Corrected:

Virtually textbook perfect and fits the definition of accurate sound. You won’t see phase or step response measurements with other headphone measurement techniques/approaches as the dummy head and artificial ears really mess up the timing response.

With this new approach, I feel confident that “what you see is what you get” relative to a flat or neutral response. I am still working out the HF cancellations to eliminate the HRTF component of the measurement so we are left with the actual frequency response of the headphones. Therefore, when eq is applied it “sounds right” to everyone.

This measurement approach and FIR filter designer is encapsulated in prototype headphone DSP software product I am developing and hope to offer as a commercial release. In the meantime, I can eq headphones in the lab and offer the resultant high resolution FIR filter eq to who desire it.

For the SR1a, I made 3 correction filters: one neutral or flat, another with a bit of bass extension, and the third filter with both bass and treble extension.

Back to you Chris, let me know how it sounds.

Proof Is In The Listening

The goal of improving upon my absolute favorite headphones was set. Mitch measured the SR1a and created three filters to accomplish this goal. 

The final results, without any hesitation, elevate the SR1a headphones beyond stratospheric levels and into a vacuum where only the SR1a exists. I know of nothing on Earth that sounds like the SR1a when used with these convolution filters, and I've never heard a headphone come close to sounding this good. The speed of the true ribbon transducer is undeniable and absolutely stunning, while the frequency response is now as close to picture perfect a possible. 

My system for these listening sessions consists of a Constellation Audio Inspiration integrated amplifier feeding the RAAL-requisite interface box via AudioQuest Robin Hood SILVER speaker cables. I can't say enough about these speaker cables as they of course sound fantastic, but they're also extremely easy to work with and maneuver behind my desk. These are the opposite of typical stiff and heavy garden hose cables, and I love them for it. Sources for this system include Roon, Audirvana Studio, and HQPlayer running on my CAPS Twenty PC, with DACs from Berkeley Audio Design (Reference Series 3) and Denafrips (Terminator II). Audio endpoints include a Sonore signature Rendu SE Optical and CAPS 20.1. 

Listening to the Tsuyoshi Yamamoto Trio's album Midnight Sugar (Impex limited edition remaster), one can hear the improvements from these convolution filters immediately. Isoo Fukui's bass, opening the track, is deeper, richer, and can be heard much better with the convolution filters enabled. This continues throughout the track as his bass provides a foundation for, and can be heard beneath, Yamamoto's brilliant piano playing. The piano on this track is also even better than before because the SR1a is reproducing it closer to what's on the recording. What makes this whole experience amazing is that I can't hear any negative side effects of the convolution filters. In the past I've heard transient edges rounded a bit with some filters, but this is not the case at all with these filters and the SR1a. Yamamoto's piano is smooth yet sharp when the hammers stike a chord with authority. The true ribbon tweeter has lightning fast speed that only improved when fed the audio signal routed through these convolution filters. 

Moving to other Three Blind Mice albums (or any album for that matter), one can repeat the experience I previously described, all day long. On the Terumasa Hino Quintet's Live! album, the track Stella By Starlight is made even better with these convolution filters. The bass is extended, the highs are clearer, but the over all tone and impact of Hino's trumpet remains striking. Yes, it's an audiophile cliche to say the bass is extended and the highs are clearer, etc... but this is backed up by both listening sessions and measurements. I don't know what else to say it, if I can't speak the truth. This really is a breakthrough. I want filters for all my headphones and I want convolution engines on all my sources. It's time for more manufacturers and developers to step into this century, and enable the latest tools that bring our favorite music to us in a whole new light.

Using these filters created by Mitch, I've listened to everything from Rage Against The Machine to Dua Lipa to The Black Keys to Crosby, Stills, Nash, & Young's 50th anniversary version of Deja Vu. The experience is all the same. The music sounds better (subjectively) because I'm hearing it more accurately (objectively). The filter I use most is the one with extended low and high frequencies, but the neutral filter is a close second. I'd say I split my time about 60/40 with these two, and don't use the extended low frequency filter at all. This is the beauty of DSP, I can enable/disable it whenever I want and select different filters for different music, as I wish. I'm sure some may listen to the low frequency filter only, and that's totally cool with me. There isn't a right or wrong filter, it's all personal choice. 

Using the RAAL-requisite SR1a with convolution filters is an experience I hope all music lovers can enjoy sooner rather than later. A stunning pair of headphones is now even better, through the use of objective measurements and masterfully created convolution filters. If there was previously  one common item that some people took issue with, with the SR1a, it was brightness and the ability of these headphones to rip one's head off at times. Now, those concerns should be completely gone. The convolution filters absolutely alleviate any sense of brightness, and deliver the speed and accuracy without any downside. 

How To Get This

The filters created by Mitch for the SR1a will be available to purchase through the Accurate Sound website. The very reasonable price* Mitch quoted me is a steal, for the increase in performance achieved. If you own the SR1a, you need these filters. It's as simple as that. 

*I'll let Mitch finalize everything before I release any further details. 

What about those who own other headphones? This is where it gets very interesting and the future looks very bright. Accurate Sound can measure almost any type of headphone once, and have the filters available for all consumers with those headphones. I know I sound like the master of the obvious, but I'll just restate that once a headphone is measured, everyone with that headphone can benefit from the filter. There is no room or listening space to measure as there is with our main audio systems. 

I look forward to seeing more headphones measured and more filters created by Mitch at Accurate Sound. I highly recommend manufacturers get in touch with him and fans of specific headphones contact manufacturers to have them work with Mitch. This will expedite the number of filters available for purchase by all of us. It's a win-win, for minimal cost. Who can argue with that? (That's a rhetorical question).

Product Information:

• RAAL-requisite SR1a Earfield Monitor Headphones 
• Accurate Sound Convolution Filters

Associated Music:

• Audiophile Style 50 (Qobuz)
• Computer Audiophile PJ4CA Playlist (Tidal)

Associated Equipment: