  • Blog
  • How To Make An Audiobook: Publishing on ACX and Audiobook Marketing

How To Make An Audiobook: Publishing on ACX and Audiobook Marketing

Question: Which of the following book formats has the highest year over year growth in the last three years?

  1. Audiobooks
  2. Paperback books
  3. eBooks

If chose A, you’re absolutely correct. Unless you’ve been living in a cave for the last few years, you have experienced the “advent of audio.”

Think about it—we listen to podcasts in our cars and at the gym, portable high-quality Bluetooth speakers are abundant and everyone is listening to audiobooks. As a matter of fact, 77 million Americans listened to an audiobook in 2016 and most audiobook consumers listen to 15 books a year on average!

Putting this together, the audiobook market is growing at a rate of 30% per year, which nearly quadruples the growth rate for eBooks. This is a wave you have to catch!

The big question self publishers are left with is how to make an audiobook? What steps are involved? How complicated is it to publish an audiobook?

The barrier to entry into Kindle publishing is low—write a book, get a cover, input some metadata, and you’re done. Audiobooks have a higher barrier to entry because you need high quality recorded audio, but the revenue stream can be huge. So how do you get started if you want to learn to create an audiobook?

In this article, you will learn:

  • Why you should make an audiobook
  • Audiobook formats and what they mean
  • The steps for making an audiobook
  • Where to sell and how to market your audiobook

Why Should Authors Make An Audiobook?

The answer to this is pretty simple: If you’re not creating audiobooks, you’re leaving money on the table and not reaching all your potential readers. With millions of audiobook listeners worldwide consuming audiobooks at a fairly rapid clip, the business case for audiobooks just makes sense.

The incredible aspect of this growth is that there seems to be no end in sight. Formats for listening to audio change (remember mix tapes or 8 track players?) but the combination of smartphones, MP3s, and busy lives have yielded fertile ground for audiobook growth. In fact, many readers now prefer audiobooks to any other format for book consumption.

Here are some impressive statistics on audiobook market growth and evolution:

 

Books That Do Not Make Good Audiobooks

It’s vital to note here that some types of books do NOT make good audiobooks. In fact, Audible even specifically recommends against recording any of these books types as audiobooks.

These book types include:

  • Reference books
  • Quotation books
  • Home and garden or interior design books (image heavy books)
  • Any type of picture book
  • Cookbooks
  • Travel guides

Books That Make Excellent Audiobooks

Conversely, several types of book genres typically sell very well on Audible. Individual results obviously vary, but typically these book genres sell well:

  • Self-help/spirituality
  • History/biography
  • Mystery/thriller
  • Science fiction/fantasy
  • Health and fitness
  • Romance
  • Business

What Are The Audiobook Formats?

Just like with Kindle eBooks, you can have multiple formats of the files for audiobooks; this can be quite confusing. With Kindle, you have MOBI files, ePub files, and more…and with audiobooks, you have MP3 files, M4B files, WAV files, and even AIFF files for CDs. Let’s explain what these files are and where they’re used.

1. WAV files

WAV files originated way back in 1991, and you may remember them from the early Microsoft days. Remember those “ping” error sounds under the “Sounds” section of your control panel? All those very short sounds were WAV files. That worked great for very short sounds because WAV files are HUGE. These days, they’re used as the uncompressed file that stores all the nuances of the recording that then is used by the audio engineer during file mastering.

It’s easiest to understand all this by knowing what happens when you record a file. Let’s say you record in GarageBand. That original file is a “.band” file, which then is used by the engineer to extract all the individual WAV file tracks for each book chapter. Those files are mastered, and then the mastered WAV files are exported as MP3 files (for the Audiobook Creation Exchange [ACX]) or M4B files.

How can POTATO CHIPS, CURTAINS & LIP BALM help you sell more books? #BookMarketClick To Tweet

2. MP3 files

As we just learned, MP3 files are the final file format for most audio, including ACX audio. They’re much smaller and more compressed files compared to WAV, so they’re portable. That’s huge—the MP3 format has allowed us to put thousands of audio files on our smartphones, so this file format has been foundational to the growth of audiobooks.

3. M4B files

M4B files are similar to MP3 files in that they are much smaller than WAV files, and M4B files are used by iTunes. So, when you download an audiobook from iTunes, you’re actually downloading M4B files–whereas it’s MP3 files with Audible. The main advantage to M4B files is that they can be bookmarked, whereas MP3 cannot; this process, though, is very difficult for a do-it-yourselfer.

The main point here is if you’re using ACX to publish your audiobook, the MP3 files that you upload to ACX will go to BOTH iTunes and Audible and you don’t have to do any conversion to M4B files to get your audiobook on iTunes.

If for some reason you want to record your audiobook and then only publish it in M4B format, there are programs available to export the WAV files as M4B once they’ve been mastered as WAV files. One such program to make an M4B audiobook can be found here.

  

Ways To Create An Audiobook 

OK, so if the audiobook market is in the middle of a growth explosion, why aren’t more people making an audiobook?

The first reason is that most people think that it’s incredibly difficult to do. The second is that they are scared off by a perceived high cost.

However, recording your own audiobook—though a fair amount of work—can be done even by novices, and the production cost is not as astronomical as you might think.

Let’s cover these the two methods for how to make an audiobook: Pay for Production vs. Recording it Yourself…and discuss the main pros and cons of each.

 

OPTION 1: Pay For Production

Paying to have a voice artist record your audiobook is the route most people take to get their book recorded.

With this process, you:

1. find a narrator,

2. they record the book and have the audio files mastered by an audio engineer,

3. and you get the finished files with all the rights to the audio.

The vast majority of audiobooks are published on the Audiobook Creation Exchange (ACX)—more on this later—and that’s one place to find narrators.

Many companies (including my company Gutenberg Reloaded) also specialize in audiobook production. The advantage to a private company is that they typically will do any book length (even very short books around 5,000 words) and are open to paying for production contracts. Conversely, they typically employ fewer voice actors than the thousands you could find on ACX. ACX does offer more voice artists, but many of them are unwilling to do shorter audiobooks and many insist on a royalty share model where they receive 7 years of royalty payments from your audiobook.

Cost-wise, a good rule of thumb is that a high-quality audiobook can generally be recorded at the rate of about $225 per 10,000 words. As with anything, costs can vary widely and undoubtedly you can find cheaper narrators out there (and much more expensive ones as well), but $225 per 10,000 words is a good benchmark.

 

OPTION 2: Record Your Own Audiobook + Publish on ACX

This is a route many authors choose, especially if they only have one or two books that they will be recording. With the proper recording equipment and setup, it is possible for anyone to record their own audiobooks, but several key factors must be considered before jumping into this type of project.

 

1) You MUST have quality equipment.
I’ve compiled a list of recommended equipment that we use and we’ve recorded about 250 audiobooks for authors worldwide–so we know our stuff! You can get that PDF here.

 

2) You have to be familiar with the recording specifications that ACX insists upon (if you’re recording for ACX)
This means you’ll have to find a great audio engineer to master your files. We’ve had numerous authors approach us about mastering the audio they’ve self-recorded. In most instances, we can master the audio, but sometimes we’ve had whole books recorded with substandard audio or the wrong specifications for ACX, and the author has had to record the entire book over again. The lesson here is to be very familiar with the specifications, and if you’re unsure about the setup, be sure to contact your audio engineer before you start recording!

 

3) You have to know that this project will take a significant amount of time.
Voice acting, like any skill, takes time and practice to master. Most people can speak at the rate of 8,000 to 9,000 words per hour, and its hard work to speak for an hour unless you’re a voice actor! Voice actors can just go into their studio, sit down, and knock out hours of recording on end. Like any technical skill, if you’re doing it for the first time, it’s going to take a LONG time. Ask me how I know this? I actually recorded the audio for my book Recording Audiobooks myself! If you want to hear a sample, head on over to my book’s mini-site and have a listen. Great results, but a lot of work.

 

It's All About Quality

The most important thing to remember with either the pay for production or the DIY option is that you’re going to need audio files that are high quality! ACX puts all audio files through a two-step quality control check to ensure that the audio sounds great for their listeners. We have a perfect track record—every single book we’ve ever recorded has been accepted by ACX—but if you have poor quality files, whether those are from an ACX narrator or your own recorded files, you stand a chance of having your entire book rejected by ACX.

With that, if you have recorded your own files and want us to review them for you to ensure that they can be mastered to ACX specifications, simply contact us at contact@gutenbergreloaded.com and we’re happy to review your files for free!

 

How To Make Your Own Audiobook

Let’s say that you want to take the plunge and record your own audiobook. Below I’ll give you the major steps involved and some initial guidance, but I have a special offer just for Kindlepreneur fans that covers all the details of how to make an audiobook, from A to Z.

Get The Free PDF of my book Recording Audiobooks for the full details of How To Record An Audiobook. Simply click the button below!
Get Your Free Book Here!

Now, let’s jump in.

Step 1: Obtain the RIGHT equipment

When you record an audiobook, you simply CANNOT start recording with your iPhone or your computer’s built-in microphone. Why? Well, most computers (and iPhones) use very low-quality recording and audio processing equipment simply due to the fact that you don’t need high-quality audio to do simple phone calls and run iTunes. Recording audio? It’s much different, and you’re going to need to spend some money to get the following items:

  • Good mic. My current favorite is a Blue Yeti but be SURE you have it on a solid table and in a quiet room since it picks up background noise easily. Expect to spend $50-$100
  • Pop screen. Minimizes plosives (the “p” sound in “pop” or “pickle”). Expect to spend $5
  • GarageBand or Audacity; you’ll actually record in these programs and they’re free (Audacity) or already installed on your computer (GarageBand)

 

Step 2: Equipment & Space Setup

Set up your equipment in a space that is suitable for recording (read: QUIET) and make sure all your equipment functions correctly. You’re going to need to dampen any hard surfaces, focusing on covering areas both right in front of you and behind you. Essentially, the more soft coverings (blankets, duvets, curtains) you can place around your recording space, the better. Also, be sure to keep pets out of the room, turn off HVAC vents, and stop the washing machine, dryer, and dishwasher before you start recording. My wife is a voice artist for our company, and we have experienced all of these little interruptions during her recording sessions.

Here’s a great little video of the actual recording setup I used in my dining room to record my audiobook version of Recording Audiobooks.

 

Step 3: Record your audio. Simple, not easy, but here are a couple key tips:

  • Potato chips. Eat a couple before you start recording to keep your lips moist.
  • Shea butter lip balm. Trust me.
  • Keep water to sip handy.
  • If you’re recording over several days, re-read the last few paragraphs you recorded the day before. Then, listen to your recording of that same section too. Doing this will help set the tone for your recording, and will help ensure your audio sounds more uniform.
  • If possible, record at the same time of day. Your voice changes throughout the day, so if you can record about the same time each day, that will also keep your audio more uniform.

 

Step 4: Send your audio to an audio engineer for editing.

This is something we can help you with and you can also find good engineers on sites like Upwork or Fiverr.

 

Step 5: Modify your cover for ACX standards

ACX requires a different size cover (square rather than Kindle rectangle) and you cannot just stretch out your cover to square size. This is pretty easy—numerous Fiverr contractors can do this for $5.

 

Step 6: Create your ACX account

Be sure you enter your bank account and tax information for royalty payments! See the “All About ACX” section below for a guide to do this, including a video.

 

Step 7: Upload Your Audiobook & Cover

You’ll upload your book description, choose a category for your audiobook, upload your audio and cover, and submit to ACX for their review.

Audiobook Markets: Where To Publish Your Audiobook

As with seemingly most of the world economy these days (even grocery stores!), Amazon is the 800-pound gorilla in the book publishing room. Audible is owned by Amazon, and ACX is owned by Audible. ACX is to Audible audiobooks as Kindle Direct Publishing is to Kindle eBooks.

ACX hosts the audio files, book cover, and metadata for the audiobook just like Kindle Direct Publishing does for Kindle eBooks.

Interestingly, ACX also distributes to iTunes. When I found out this fact years ago, I was as amazed as you are now, but this is one example of Apple-Amazon collaboration, which is pretty amazing for these stiff competitors.

There are other options out there for audiobook publication, but none as huge as ACX. It’s worth mentioning the other options here to make sure we cover all options for audiobook publication, but it's also worth noting that you’re going to have to choose a non-exclusive publication deal with ACX to go on any of these platforms. This means you will receive significantly lower ACX royalties.

The 3 big players are:

  1. Overdrive
  2. Audiobooks.com
  3. Downpour

Overdrive distributes to libraries, which could be a good opportunity to get your audiobook distributed even more widely. However, Overdrive is very selective in choosing which authors to accept, and typically you’re going to need at least a dozen or more audiobooks recorded to even get your foot in the door.

Audiobooks.com is in second place behind ACX in terms of audiobooks sales, but ACX is way out in the lead!

Downpour is another platform that’s right behind Audiobooks.com in sales. Both of these platforms, however, are going to require you to negotiate a deal to be published on them, while on ACX you can simply upload your book and start selling audiobooks.

All About ACX

Let’s dive deeper into ACX, as currently, they’re the best place to publish your audiobook.

Setting up your ACX account is fairly simple—you need your tax information and your Amazon login to get started and you’re taken to this signup page below to input your data. Very simple and easy to do.

Once you’ve done all this, you’ll input your tax information and your bank account information, and you’re ready to start uploading your audiobook!

This process I’ve laid out in this video below to make it easy for you. You’ll need your audiobook files, your square book cover, and about 5 minutes to get this task completed.

Now, you do have one major decision to make with ACX before submitting your audiobook for review, and that is: do you want an exclusive or non-exclusive publication deal with ACX?

Exclusive vs Non-Exclusive

Similar to deciding whether or not to go exclusive on KDP with your Kindle book, you have the same decision to make with your audiobook. Here are the main pros and cons of each:

Exclusive – Publish on ACX Only

Pro: Royalty share will be higher 40% vs 25% with non-exclusive publication. Can start selling fast and don't need to negotiate a publication deal.
Con: Exclusive to ACX and cannot publish your audiobook on competing platforms.

Non-Exclusive – Publish in Multiple Places

Pro: Publish audiobook on your own website and other markets. Usually, a good choice if you have a large email list and wide distribution base that would love to buy your audiobook.
Con: Lower royalties on ACX – 25% vs 40%

 

Here’s a chart that helps explain each choice:

And remember, per ACX terms you’re stuck with your choice for 7 years, so choose wisely!

How Much Can You Make Selling Audiobooks?

One question that always seems to come up is royalty payments, and how much you can expect to make off audiobooks. This is pretty confusing, so let me first explain how books are priced on ACX.

With ACX, they choose the price for you based on the runtime of your audiobook as follows:

  • Under 1 hour: $3.95 to $7.00
  • 1-3 hours: $7 to $10
  • 3-5 hours: $10 to $20
  • 5-10 hours: $15 to $25
  • 10-20 hours: $20 to $30
  • Over 20 hours: $25 to $35

Clearly, this is somewhat subjective, as there’s overlap between the pricing breakdown above, but this is right off of ACX’s site and this is how they set prices.

Royalties then are based on how the audiobook is sold. Here’s an example of what you’ll see on your monthly PDF of your sales data. The example below is for a short audiobook that sells for $3.95 on Audible:

Doesn’t make a lot of sense, does it?

Let’s break this down. ACX royalties are based on whether or not the customer is an Audible Listener (i.e. has an Audible membership) and whether they buy it on iTunes or Audible.

 

“ALC” sales are ala carte sales by people without an Audible Listener membership or by iTunes sales.

“AL” sales are audiobook sales where an Audible Listener buys your audiobook with one of their member credits.

“ALOP” sales are sales where an Audible Listener purchases your audiobook but NOT using one of their member credits.

 

You can see from the example above that listener credit royalties are much lower than the other categories. But, don’t forget the ACX Bounty payment!

Make More Money With ACX Bounty

The ACX Bounty is a $50 payment made to you when someone signs up for an Audible Listener membership and they buy your audiobook as their first purchase. This can add up fast!

Many months my Bounty payments have outstripped my royalty payments, and this is the key method by which ACX makes up for their relatively low royalty payments (20% to 40% vs. up to 70% on Kindle).

In summary, producing an audiobook via ACX remains the best option today especially for startup authors, but there are other options out there once you start publishing more audiobooks.

Audiobook Marketing – How To Market Your Audiobook

Once you’ve created your audiobook and its live for sale on Audible and iTunes, you’ll have a unique opportunity to market your audiobook. About 7-10 days after your book is live, you’ll get an email from ACX with 25 free promotional codes that you can distribute to anyone you like. This is a superb opportunity as the perceived value of an audiobook is high (and it should be since the shortest audiobook sells for $3.95!).

As such, you need to maximize this opportunity to distribute these codes in order to do things like:

  • Build your email list
  • Gather reviews for your audiobook (obviously full disclosure is needed by the reviewer as well as compliance with Amazon’s Terms of Service, but this is still a great way to get reviews)
  • Reward loyal readers and customers

Now, you should know that marketing an audiobook is still somewhat in its infancy. And, unlike Kindle Direct Publishing, ACX really has no built-in promotional methods like the Kindle Countdown Deal or free promotional days for your Kindle eBook.

So essentially, you’re going to have to work harder to promote your audiobook. But with far fewer audiobooks published compared to Kindle eBooks, if you wisely work the marketing end, you can get ahead of the relatively smaller competitive field.

A fundamental strategy for marketing your book is to gather reviews as noted above. How can authors get reviews?

Podcast Episode – Steps to Getting Book Reviews
                                          

Plus, below is a list of websites (both free and paid) where you can post your audiobook to help promote it and ask for feedback.

Free Audiobook Promotion Sites

Goodreads Audiobooks Group → Great place to get solid reviews of your audiobook and audiophiles tend to gather here.

 

LinkedIn Audiobook Listeners Group → A private LinkedIn group but worth asking permission to join.

 

Reddit Audiobooks Thread → On Reddit forums you can post your book but be aware that Reddit can get nasty (I’ve seen it happen) so you must ensure your book is excellent before posting! That said, I’ve had good results with Reddit for my audiobooks as this is a well-traveled subreddit.

 

Facebook Group for Audiobook Giveaways → Although Facebook group marketing kind of sucks for eBooks, this specific group currently has over 2,000 members and you can post your promo codes here. With this group, though, you can’t post more than three total giveaways for any given book.

 

Facebook Group Everything Audiobook → This group is smaller but growing fast, and it’s another place you can post promotional codes for your audiobook.

 

Facebook Group for Audiobook Promos → One more place to post your free promotional codes on Facebook; it’s small at only 99 members.

 

Audiobook Jukebox  → Audio Jukebox hosts a number of audiobooks on their site, and the best part is you don’t have to give up your free Audible download codes in exchange for a review! You simply fill out the form on their “solid gold reviewer program” page and your book is put up on their site for review. They even have an option to submit multiple books at once to make it easier for you.

 

Christian Audio → If you have a Christian-themed audiobook, you can simply post it as a suggested title for them to host on their site by including the book title, author’s name, and a quick reason why they should include your book.

Asking For Reviews From Professional Reviewers

Another strategy for gathering reviews is to ask professional reviewers for one, and fortunately, there are a fair number of websites where you can find these reviewers.

Here’s a list of sites to find reviewers:

Audio Book Reviewer

AudioFile

Eargasms Audiobook Reviews

Audiobook Jungle

Audiothing

Don't miss out on the explosive audiobook market. Make your own audiobook without spending a ton of money #SelfPubClick To Tweet

How To Make An Audiobook Summary

If you’re serious about book publishing, you owe it to yourself to ensure you’re reaching as many readers as possible, and the numbers don’t lie: The audiobook market is expanding rapidly with no real end in sight.

There are a number of great options to produce an audiobook ranging from cheap (really just the cost of a good microphone) to expensive if you hire the $1,000/hour ACX narrators.

With that, there are very good middle ground options for self-published authors that allow you to produce an audiobook for a reasonable price. That relatively low cost combined with the burgeoning market makes a solid business case for producing audiobooks from your eBook or paperback content.

Since we’ve been producing audiobooks for years with nearly 250 audiobooks produced and over 5,000 audiobooks sold, we’re happy to help you with any questions you may have. Simply reach out to us at contact@gutenbergreloaded.com and you can find more information on our site at http://www.gutenbergreloaded.com.

About the Author

George Smolinski is an international #1 bestselling author and is the founder of Gutenberg Reloaded, a publishing company dedicated to the needs of today’s self-published authors. When he’s not helping authors, he busies himself with his wife and four boys, extreme sports, and he’s a practicing sports medicine physician.

FAQs

How much does it cost to make an audiobook?

The cost of an audiobook depends on many things such as if you are going to record the audio yourself or pay a professional as well as how long the recording will be.

What are different ways to create an audiobook?

Audiobooks can be created in two ways. You can record your own audiobook or you can pay a professional to do. ACX has different ways to pay a professional such as a royalty share or a one-time fee.

Can I make my own audiobook?

Yes! You can create your own audiobook for use on Amazon’s ACX or other platforms. In this post, we will tell you about specific equipment, setting up your recording space, recording your audio, editing, and ACX’s standards.

“Gain insight from Kindlepreneur on how you can optimize marketing for your books."
- Kindle Direct Publishing

Sell more books on Amazon

rankings-ebook-mockup
Free Download

Amazon Kindle Rankings E-Book

Learn how to rank your Kindle book #1 on Amazon with our collection of time-tested tips and tricks.

78 thoughts on “How To Make An Audiobook: Publishing on ACX and Audiobook Marketing

Comments
  1. Bridget

    Hi George. Wow! This is the best, most comprehensive info I’ve found on audio books so far. Thank you so much for this. I do have a couple more questions I’d like to ask you, if you have a moment…
    I’m a voice-over artist, with my own studio, and my dad has just published a small collection of Irish stories. I’d like to help him set up an audio book account, but I’ve also been looking to do more audio book readings for others…does your company hire voice-artists by any chance? I realize companies like voice123 are a commun collection for voice-artists…just curious if you have a branch of this too.

  2. Abi

    Hi
    Thanks for the tons of information. I have a series of pre school children’s books that will be published soon and available as an e book and paper back.

    Each story is no more than 500 words and I would love to one of the books as an audio freebie book but would like to hire a narrator and have the recording done for me. Can you point me in the right direction or is it a service you would provide.

    1. George

      Hi Abi,
      That’s a great idea and even short audiobooks like these have a market. Please reach out to us at contact@gutenbergreloaded.com and we can assist you.
      Thanks
      George

  3. Michele Davis

    Great, informative article. Thanks! I have written 8 books, mostly with Simon and Schuster. My last book, however, was self-published. It is in paperback and ebook formats. I’d like to do an audiobook. Is there any way to sell the audiobook on amazon and itunes and audible WITHOUT cutting in ACX at all? Even with my kindle book, I receive 70% royalty. If I do the audiobook myself or hire someone to do it with me and pay them upfront, is there any way to get a larger than 40% royalty?

    1. George Smolinski

      Michele, that’s a great question! Unfortunately ACX is an Amazon company so you have to use them to get your book on Amazon and Audible, but there might be a chance to get it on iTunes (though unlikely). They’re the 800 lb gorilla in the room…

Leave a comment

Your email address will not be published. Required fields are marked *

[name="email_address"]
[name="email_address"]
[name="fields[freebie]
[name="fields[freebie]
[name="email_address"]
[name="email_address"]
[name="fields[freebie]
[name="fields[freebie]
[name="email_address"]
[name="email_address"]
[name="fields[freebie]
[name="fields[freebie]
[name="email_address"]
[name="email_address"]
[name="fields[freebie]
[name="fields[freebie]
[name="email_address"]
[name="email_address"]
[name="fields[freebie]
[name="fields[freebie]
https://kindlepreneur.com/how-to-make-an-audiobook/
kindlepreneur.com
crawl=CC-MAIN-2021-10
200001
application/http; msgtype=response
sha1:6DZWLHY63GHKDBZD7VLE7LFKUMGGG3QZ
<urn:uuid:39bb137d-d1a9-4ad2-bbc7-458d5a492887>
2021-02-26T00:59:44Z
194.1.147.12
text/html
sha1:JXPWLMUXSRQKANBR3IQ46ZJD2MUFSUIE
<urn:uuid:d6413b61-7f21-47c8-b087-41777c081c1f>
https://kindlepreneur.com/how-to-make-an-audiobook/
response
<urn:uuid:badc9ef4-e8f6-4947-8164-1cf86cf0ec53>
isPartOf: CC-MAIN-2021-10
publisher: Common Crawl
description: Wide crawl of the web for February/March 2021
operator: Common Crawl Admin (info@commoncrawl.org)
hostname: ip-10-67-67-39.ec2.internal
software: Apache Nutch 1.18 (modified, https://github.com/commoncrawl/nutch/)
robots: checked via crawler-commons 1.2-SNAPSHOT (https://github.com/crawler-commons/crawler-commons)
format: WARC File Format 1.1
conformsTo: http://iipc.github.io/warc-specifications/specifications/warc-format/warc-1.1/