# A Vietnamese sound engineer's $23 iPhone measurement app: 50k followers bought it up to #2 on the paid utilities chart · Mobile app

> Source: https://stringers.io/en/apps/ios_6811894469.html (Stringers / 什么值得做, generated 2026-11-08)
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> The creator is a Vietnamese audio-tuning teacher with 50k Facebook followers. Students and repeat clients bought after the post with the link. The selling point is "test phase on your phone without an external sound card," which is easy to demo, and the price is a fraction of the industry benchmark Smaart. A few sales a day were enough for top five on the Vietnamese paid utilities chart.

[Mobile App](https://stringers.io/en/apps/index.md) · iOS · written 11-6

P

ProPi System Analyzer · Anh Viet Do

# A Vietnamese sound engineer's $23 iPhone measurement app: 50k followers bought it up to #2 on the paid utilities chart

**What it is** An iPhone acoustic measurement tool for live sound engineers that measures spectrum, phase, and delay, and automatically adjusts phase and EQ. It's a one-time purchase of about $23.

**Why it took off** The author is a well-known sound teacher in Vietnam's audio scene with 50k+ Facebook followers. After he posted with a link, students and longtime clients bought it. The pitch, "measure phase on your phone without an external sound card," is easy to demo, and the price is a fraction of the industry benchmark Smaart. On Vietnam's paid utilities chart, a few sales a day is enough to reach the top five.

**C** **How you could follow** Don't copy this app. Partner with a craftsperson who has a community, turn a step that normally requires a computer or hardware into a one-time-purchase mobile version, launch it to their followers and student groups, and find local creators in neighboring countries to act as resellers.

C grade: it grew because of the author's own followers and students, which followers can't replicate. The first wave is probably a few hundred to a thousand copies, and ratings have stopped climbing after 11-04. Vietnam also has desktop alternatives that can easily move to mobile. The tactics are worth learning, but copying them isn't worth it. Circulating in small circles

Data (updated daily) · Launched 2026-10-19 · US store rating 0 · 11-08 best Top Paid overall rank #41 (Vietnam) · 11-08 charted in 10 stores

iOS · Top Paid overall rank · By store

Vietnam Sri Lanka Indonesia

Data 2026-10-28 → 2026-11-08 (12 days, 12 observations). Lower rank is higher; gaps mean it wasn't charted that day

iOS · Best category rank · US store

Data 2026-10-30 → 2026-11-05 (7 days, 3 observations). Best rank across each day's category charts

iOS · Number of stores charted (top 200 on any chart)

Data 2026-10-28 → 2026-11-08 (12 days, 12 observations)

ProPi System Analyzer is an iPhone acoustic measurement app made by Do Anh Viet, a blogger in Vietnam's sound engineering circles. It was listed in Oct 19 and sold as a one-time purchase for 599,000 Vietnamese dong (about $23). Within half a month it hit #2 on Vietnam's paid utilities chart and #3 in Indonesia, and by 11-03 it had 34 ratings, 33 of them from Vietnam. It looks like a niche professional tool that broke out on its own. In reality, a teacher with 50k+ followers moved the desktop software he had been selling for years onto the phone, and students and longtime clients came to buy it. Below: what it sells, what got it on the charts, and what you can learn from it.

### "Measure phase without buying a separate sound card"

At 3:20 AM on Oct 23, Do Anh Viet posted a 1-minute video to his Facebook page (53.7K followers). The caption was one line: "Propi trên Iphone, đo phase mà ko cần Sound Card rời" (Propi on iPhone, measuring phase without an external sound card), with the Vietnam App Store link pasted below. As of 11-06, the video had 3,900 views.

The next day, on Oct 24, the Vietnam App Store received 10 five-star reviews. They were short: "Quá ok" and "Quá ngon" (very good), "Ngon, bổ, rẻ, made in Việt Nam" (good, useful, cheap, made in Vietnam), and one that said "Tư vấn hỗ trợ nhiệt tình" (enthusiastic support and advice). The people praising the support were the ones he had personally served.

On Nov 1 he posted a "Propi ios" demo with an explanation: "Use the phone's microphone to measure the sound coming out of the phone's own speaker, no sound card needed." That video got 2,100 views and 12 comments.

### What it is

A measurement tool for live sound, wedding, and karaoke engineers: real-time spectrum (RTA), phase and amplitude, impulse response, group delay, SPL, delay alignment, subwoofer array alignment, and crossover tuning, plus the desktop version's signature automatic phase and EQ adjustment. The interface is English-only, it requires iOS 17, and the developer has only this one app.

Prices vary by store: Vietnam 599,000 VND, Indonesia 399,000 IDR, Mexico 399 MXN, France 22.99 €. No in-app purchases or subscriptions (App Store, 11-06).

Behind it is a desktop software product he has built for years. Propi's Win/Mac versions have kept getting updates, with new releases still shipping 2026 years on. His official site nexda.com.vn has been running since 2016. Early on he also built Pro2.G, which he billed as "Vietnam's first complete sound measurement software." Desktop licenses reportedly sell for around $199 (unverified). The iPhone version costs a fraction of that, and for people who already trust him, buying it is an easy decision.

### How big a business is #2 on the chart?

Not big. The bar for Vietnam's paid utilities chart is low: on 11-05, the app ranked #2 had zero ratings, and the #4 app had only 15 (Vietnam App Store). A few sales a day is enough to reach the top five.

ProPi's ratings growth (global total, App Store):

| Date | 10-28 | 10-30 | 11-02 | 11-03 | 11-04 to 11-06 |
|---|---|---|---|---|---|
| Score | 18 | 24 | 31 | 34 | 34, flat for three days |

On Vietnam's overall paid chart it fell from #10-28 on 6 to #11-04 on 89, then climbed back to #11-06 on 16. It also appears on the paid utilities charts in Mexico, France, Malaysia, and Taiwan, but those stores have zero ratings, so it likely squeezed in with just a handful of sales.

Working backward from the 3% to 10% rating rate typical of paid utilities apps, Vietnam's 33 ratings correspond to roughly 300 to 1,000 copies, implying iOS gross revenue of about $7k to $23k. This is essentially a one-time launch wave, and it is already slowing.

### Where users come from: people are the channel

1. **The author's own fans and past customers (confirmed).** Facebook, 53.7k followers. The page bio reads "Electronics - Audio, Propi software" and lists WhatsApp and Zalo numbers. YouTube channel @doanhviet-propi has 1,450 subscribers and 22 videos, all Propi tutorials. The first iOS buyers were these people, and the review spikes line up with his posting times.

2. **An Indonesian audio blogger and reseller (confirmed).** On TikTok, @misfoyana (about 9,600 followers) posted "bro, I found an analysis app called PROPI..." which got 10-23 9,458 views. He later made an auto crossover review and a "PROPI vs PA TRACE" comparison (11-03,3,114 views). His caption ends with "Want to buy analysis software like Smaart or PA TRACE, PROPI, VOS4? DM me," so he's selling too, not just reviewing. In early November, Do Anh Viet posted "Propi đi indonesia tối nay!!" (Propi goes to Indonesia tonight), so he seems to have gone there himself. Indonesia's paid utilities chart ranking at #2 to #6 likely came from this.

3. **Philippine authorized reseller (confirmed).** The Facebook page "Propi App PH" posted a launch announcement 10-26 listing features like Auto EQ, Auto FIR, Auto Phase, Auto XOver, RT60, and posted a comparison video against Smaart. It sells perpetual licenses locally and offers local support.

Not seen: Meta and TikTok ads, media coverage, X discussion (30 posts in the last 0 days).

### Why this one

- **Buyers trust this teacher.** Vietnam's live-sound engineers are a close circle, and many learned their tuning methods from him. To them, this is simply "the teacher's tool, now on phones."

- **A selling point you can demo in ten seconds.** Established iOS tools like AudioTools and Transfer Function need an external sound card to measure dual-channel phase. ProPi demos it using the phone's own speaker and mic.

- **Priced against the industry benchmark.** The industry-standard Smaart costs several hundred dollars. ProPi sells for $23. Reviews keep repeating "đáng tiền" (worth the money).

- **Pride in being "made in Vietnam."** Several review titles read "Hàng việt nam" or "made in Việt Nam."

### Who it's competing with

| Product | Where | Scale | Compared with ProPi |
|---|---|---|---|
| AudioTools(Studio Six Digital,2009) | iOS | 5,090 ratings in the US store, 93 in the Vietnam store | Entry price is about the same as ProPi, but modules are extra in-app purchases and dual-channel needs a sound card |
| Transfer Function (same developer) | iOS | VND 1,499,000 in the Vietnam store, 0 ratings | Requires a two-channel sound card, and the price is 2.5× ProPi's |
| Smaart | Desktop | About $595–995 | Industry standard, expensive |
| RemiX Audio Pro / Phase Align | Vietnamese desktop software | YouTube tutorials get thousands of views each | Sells auto phase and auto EQ too, markets itself as a "Vietnamese Smaart," and has no iOS version yet |
| VOS4, REW, Open Sound Meter | Desktop, free | Lots of Vietnamese tutorials | Free, but no phone version and no one-on-one teaching |
| PA TRACE | Desktop | Not verified | A similar tool the Indonesian blogger compared against ProPi; phone can control it remotely |

Phone measurement apps are everywhere, and there are several Vietnamese desktop "Smaart alternatives." ProPi's only clear lead right now is that it reached iPhone first. If RemiX ships an iOS version, all ProPi has left is its author's community. Bad reviews have already appeared: no full-screen on iPad, and laggy remote control.

### What you can take from this

- **In niche professional circles, well-known practitioners turn desktop tools or methods into one-time-purchase iPhone apps.** Tuning, auto repair, electrical work, instrument repair, and AC installation all fit this pattern. Price at roughly 10% of the desktop version or the industry benchmark, and post the link yourself on launch day.

- **Pick the selling point that "the phone can now do it alone, which used to need hardware."** That's something you can show in a ten-second video.

- **Going global without ads: one reseller-blogger per country.** Indonesia and the Philippines both worked this way. The reseller earns a commission on licenses and naturally makes review videos.

### If it were you

**Verdict:** This product took off because its author did. Followers can't take his 50k fans or his students. Its early sales weren't large, and its rating hasn't moved in three days. Building a Vietnamese audio-measurement app to copy it isn't worth it. The approach is what's worth learning.

**Who it suits:** Practitioners already inside a specialist craft community with thousands to tens of thousands of followers, or developers who can partner with such people: you provide the tech, they provide the audience.

**Who it doesn't suit:** People with no community who want to sell a measurement app through store search. Plenty of these tools already exist, and several Vietnam-store listings have 0 ratings.

**How to build it:** Start with someone already teaching a craft, selling courses, or offering services. Turn the step they use every day, which currently needs a computer or hardware, into a version that works on a phone. Sell it as a one-time purchase at about a tenth of the industry benchmark's price. An English interface can sell too, but support has to be in the local language.

**How to get users:** Launch by relying entirely on the partner's Facebook, YouTube, and student groups, with store links in the posts. Small-country paid utilities charts are easy to crack: a few dozen sales can push you into the top five, and you can screenshot that and share it again. In neighboring countries, find a local blogger to act as reseller and pay them a license commission.

**When to move:** Within the same community, before local competitors (here, RemiX and the VOS4 type) reach phones. Measure the window in months, not years.

**Signs you've misread it:** If ProPi's ratings climb outside Vietnam a month from now, say dozens each in Indonesia and the Philippines, then "blogger-reseller" can be replicated country by country and the approach is worth pursuing seriously. If ratings stay stuck around 34, it was a one-off fan launch, and the ceiling is the author's follower count.

[iOS store page ↗](https://apps.apple.com/app/id6811894469)

**Research notes: competitor table, acquisition evidence, full research report**

Competitors

| Name | Where | Launched | Scale | How it differs from this one |
|---|---|---|---|---|
| AudioTools(Studio Six Digital) | iOS | 2009 | 5090 ratings in the US store, 93 in the Vietnam store; from $19.99, modules are extra in-app purchases | An established suite. Dual-channel needs an external sound card and extra in-app purchases. ProPi measures phase with the phone's mic, includes auto phase/EQ, and has Vietnamese-language support |
| Transfer Function(Studio Six Digital) | iOS | 2014 | VND 1,499,000 in the Vietnam store, 0 ratings | Requires a two-channel sound card, and the price is 2.5× ProPi's |
| SignalScope X | iOS | 2018 | US: 23 ratings | Leans toward acoustic and vibration testing, subscription-based, not aimed at live PA tuning |
| Smaart(Rational Acoustics) | Other | Many years (industry standard, desktop) | About $595–995 | Industry benchmark, high price. ProPi positions itself as the affordable alternative; the iOS version is about $23 |
| RemiX Audio Pro / Phase Align | Other | 2024–2026 (Vietnam desktop) | YouTube tutorials get thousands of views each | A Vietnamese auto phase/EQ tool with similar features, no iOS version |
| VOS4 Analyzer / REW / Open Sound Meter | Open source | Years | Free, lots of Vietnamese tutorials | Free but desktop only, no phone version, no one-on-one teaching |
| MeasureMate / SoundEngg / Soniclyze and other RTA apps | iOS | 2019–2026 | Vietnam-store ratings are all 0 | Single RTA/SPL tool, no auto alignment, no community |

Where users come from

- **Verified** **The founder's Facebook community (53.7k followers) and YouTube tutorial channel** : the FB posts "Propi trên iPhone, đo phase mà ko cần Sound Card rời" and "Propi ios" demo videos include the App Store link directly. YouTube @doanhviet-propi has 1.45K subscribers.

- **Verified** **Desktop Propi long-time users and students migrating** : the desktop version has been updated for 2026 years (1.0.6.4, 1.0.7.6). iOS reviews include familiar phrasing like "customer support was very helpful" and "Hàng việt nam."

- Inferred **Community mobilized to leave good reviews** : 10-24 brought 10 short five-star reviews in one day, with generic content.

- **Verified** **Indonesian TikTok/IG audio creators posting reviews on their own** : @misfoyana (9.6K followers) posted the PROPI video with 10-23 9,458 views, an 10-26 Auto X-Over review, and 11-03 PROPI vs PA TRACE.

- **Verified** **Philippine authorized reseller Propi App PH** : the FB page 10-26 posted a welcome announcement and a comparison video against Smaart.

- **Verified** **Exposure from low-barrier paid utilities charts** : in Vietnam's paid Utilities chart, 11-05 ranked #2 with Volra, which has 0 ratings. ProPi reached the top 10 to 50 of category charts in Mexico and France with only 0 ratings.

## Why ProPi System Analyzer (iOS, id6811894469) got traction

> In one line: Do Anh Viet (Đỗ Anh Việt), a KOL in Vietnam's audio tuning circle and indie developer, turned Propi, the desktop measurement software he'd sold for over a year, into an iPhone app. It's a one-time purchase at VND 599,000 (about $23). The pitch: "no external sound card, measure phase with just your phone." He has 53k followers on Facebook. Within a week of launch, those followers pushed it into the top three of Vietnam's paid utilities chart. An Indonesian audio creator posted reviews on their own, and a Philippine authorized reseller came on board, which added chart positions across Southeast Asia. It took off **because of the founder's own community, plus the very low bar of paid utilities charts** . It wasn't organic store traffic, and it wasn't paid acquisition.

### 1. Data (source: our appstore library app_day / review, iTunes lookup, 2026-11-06)

- Listed 2026-10-19, version 1.3, English-only interface, requires iOS 17, and this is the only app under the developer account (sellerUrl = nexda.com.vn).

- Pricing varies by store: VND 599,000 in Vietnam, IDR 399,000 in Indonesia, MXN 399 in Mexico, €22.99 in France. No in-app purchases, no subscription.

- Ratings: 10-28 total, 11-03 went from 34 to 18, 11-04 to 11-06 no new ratings for three days running. Of the 34 global ratings, Vietnam has 33, Indonesia 1, and every other country 0. Average: 4.70.

- 24 reviews, all in Vietnamese plus 1 in Indonesian. The earliest is dated 10-22; **10-24 brought 10 in a single day** , 10-27 brought 4 more, and 10-30 brought 4. Most are short lines like "Quá ok", "Ngon, bổ, rẻ made in Việt Nam", "Tư vấn hỗ trợ nhiệt tình" (the support team was enthusiastic), the kind of thing buyers who know the developer and were served by him write. Negative reviews: 2. Complaints: no full-screen on iPad, laggy remote control.

- On the Vietnam Top Paid Utilities chart it held at #2 to #6, and its overall Top Paid rank slid from #6 to #89 (10-28 to 11-04), then 11-06 back to #16. On the Indonesia Top Paid Utilities chart it sits steady at #2 to #6, and overall Top Paid at #31 to #69. Its category ranks in the Mexico and France stores jump around between 8 and 120, but neither store has a single rating.

- **Benchmark for making the chart** : on the same day (11-05), the #2 Vietnam Top Paid Utilities app was Volra with 0 ratings; #4 was Panic Lookup Pro with 15 ratings. The Vietnam paid-tools chart reaches the top five with just a few sales a day. So "#2" means there were sales, not that volume is high.

- Rough volume estimate (unverified): Vietnam has 33 ratings. Working back from the 3–10% rating rate typical for paid tools, that's roughly 300–1000 units. At $23 each, iOS gross revenue is about $0.7k–$23k. Most of it is probably one-off launch-wave sales.

### 2. Competitors

| Name | Where | Launched | Scale | How it differs from ProPi |
|---|---|---|---|---|
| AudioTools(Studio Six Digital) | iOS | 2009 | US store 5090 ratings, US store $19.99; Vietnam store 599,000 VND, 93 ratings | An established iOS measurement suite. Modules like transfer-function measurement are sold separately as in-app purchases and need an external sound card. ProPi's one-time price matches the rival's entry price, but it bundles Auto EQ, Auto Phase and Vietnamese-language support. |
| Transfer Function (same developer) | iOS | 2014 | VND 1,499,000 in the Vietnam store, 0 ratings | Dual-channel FFT phase measurement requires an external two-channel sound card. ProPi's pitch is "measure phase with just your phone's mic." |
| SignalScope X(Faber Acoustical) | iOS/Mac | 2018 | US store 23 ratings. Free download, features by subscription. | Leans toward acoustic and vibration testing, not live PA tuning. |
| MeasureMate, SoundEngg, Soniclyze, livetoolkit | iOS | 2019–2026 | Vietnam-store ratings are all 0 | These show up in ProPi's "customers also bought" list. Each is a single RTA/SPL tool with no automatic phase or EQ correction and no community. |
| Smaart(Rational Acoustics) | Desktop | Industry standard | About $595–995 | The industry benchmark. Expensive, English-only. ProPi's desktop version is about $199 (recall card, unverified), the iOS version about $23. It's positioned as a Smaart alternative. |
| REW / Open Sound Meter / VOS4 Analyzer | Desktop (free, open source or vendor-provided) | Years | VOS4 has plenty of free tutorials in Vietnam's audio community. | Free, but no mobile version and no one-on-one tuning lessons. |
| RemiX Audio Pro / Phase Align (RemiX Audio, Vietnam) | Desktop | 2024–2026 | YouTube tutorials get thousands of views each | A Vietnamese competitor, marketed as a "Vietnamese RITA/Smaart alternative," selling Auto Phase / Auto EQ. I haven't found an iOS version, which is the one area where ProPi clearly leads right now. |
| PA TRACE | Desktop (Indonesian creators compare it against Propi) | Not verified | Not verified | A similar auto phase and EQ analysis tool. There are PROPI vs PA TRACE comparison videos on TikTok in Indonesia. |

Takeaway: phone measurement apps are everywhere, and several Vietnamese desktop "Smaart alternatives" already exist. ProPi is the first to bring a Vietnamese-made desktop auto phase / auto EQ tool to iPhone and sell it openly on the App Store.

### 3. Where users came from (growth tactics)

1. **Founder's personal channel, confirmed** : Do Anh Viet's Facebook page has 53.7k followers (bio: "audio electronics, Propi software," with WhatsApp / Zalo contact). After the iOS launch he posted "Propi trên iPhone, đo phase mà ko cần Sound Card rời" (Propi on iPhone, measure phase without an external sound card) and a "Propi ios" demo video (35 reactions, 12 comments), with the Vietnam App Store link in the post. His YouTube channel @doanhviet-propi has 1.45K subscribers and 22 videos, all Propi how-to tutorials. (Sources: facebook.com/doanhviet3388, youtube.com/@doanhviet-propi, Jina search)

2. **Reviews look like community mobilization, most likely** : 10-24 got 10 short five-star reviews in a single day, plus comments like "the support team was enthusiastic." The timing lines up with his iOS post, which suggests students and longtime customers downloaded and reviewed together. We read it as normal guided reviewing. I found no evidence of review farming.

3. **Desktop users migrating, confirmed** : Propi's desktop version (Win/Mac) has been updated for 2026 years. Facebook posts show versions like 1.0.6.4 and 1.0.7.6. Desktop users are a natural first wave of iOS buyers. Years earlier he also built Pro2.G, billed as "Vietnam's first complete audio measurement software." The nexda.com.vn site has run since 2016 and still sells merchandise like opening and wedding music.

4. **Indonesian creator organic review, verified** : On TikTok, @misfoyana (9,570 followers, an Indonesian audio creator) posted "Bro, gua nemu software analyzer bernama PROPI…" (Bro, I found an analyzer software called PROPI…) with 9,458 views 10-23. They then posted an "Auto Crossover" review with 3,007 views 10-26, and a "PROPI vs PA TRACE" video with 3,110 views 11-03. Similar content also ran as Instagram Reels. This is likely how it reached the #2 to #6 spot on Indonesia's paid utilities chart. (Source: TikHub)

5. **Overseas authorized reseller, confirmed** : The Facebook page "Propi App PH" posted a welcome message on 10-26, listing features like Auto EQ, Auto FIR, Auto Phase, Auto XOver, RT60, and posted a comparison video against Smaart. It sells perpetual licenses in the Philippines and provides local support.

6. **Not found** : Meta or TikTok ads, X discussion (x-radar: 0 posts in the last 30 days), media coverage, TrustMRR. The Mexico and France chart spots have no ratings, so probably a handful of sales got them onto a low-bar category chart. Not promotion.

### 4. Why it took off

- **People are the channel** : In Vietnam, live sound, wedding, and karaoke tuning is a tight circle. Buyers aren't buying the app for itself. They trust Do Anh Viet and the tuning methods he teaches. For this group, the iOS version is just "the teacher's tool, now on phone."

- **A demo-able selling point** : "Measure phase with your phone, no sound card needed" (using the phone's own speaker and mic in an internal loopback) is a ten-second demo. Competitors Transfer Function and AudioTools both need an external sound card for dual-channel measurement.

- **Price anchor** : Smaart costs hundreds of dollars. Desktop Propi is about $199, and the iOS version about $23 one-time. For a live tuner, paying that to "try it on my phone" is an easy call. Reviews repeat "đáng tiền" (worth the money) and "ngon bổ rẻ" (good, useful, cheap).

- **"Made in Vietnam" pride** : Review titles include "Hàng việt nam" and "made in Việt Nam," and people on Facebook share it as "Vietnamese using Vietnamese products."

- **Low bar for paid-tools charts amplifies visibility** : Vietnam's paid Utilities top five needs only a few sales a day, so one community launch brought ten days at the top and even chart spots in other countries. The chart position itself may not bring many new users. Ratings stopped growing after 11-04.

### 5. Differentiation, and whether that's why it broke out

Differentiation has three layers. First, features: "phone's built-in mic for phase measurement" plus auto phase, EQ and crossover. Second, distribution: the founder teaches and consults personally, and has overseas resellers. Third, pricing: one-time purchase, a fraction of Smaart. What actually made it take off is **the second layer, the founder's private network and service** . The feature angle gives a good demo, but Vietnamese peers like RemiX Audio and VOS4 are similar on features and just aren't on iOS yet. The day they ship iOS too, ProPi is left with only its community.

### 6. How visible it is

Word is spreading in small circles: Vietnamese audio tuning groups on Facebook, a ten-thousand-follower Indonesian TikTok audio creator, a Philippine reseller page. Revenue is not public, no media coverage, 0 X posts discussing it, and nothing on TrustMRR.

### 7. Copycats

There is no iOS version of the rivals yet. On desktop, Vietnam already has RemiX Audio Pro / Phase Align and VOS4 and similar tools. They are peers in the same wave of "Vietnamese Smaart alternatives," not copies of it.

### 8. What to borrow

- Niche professional communities (tuning, auto repair, electricians, instrument repair) include practitioners with followings who turn a desktop tool or method into a one-time-purchase iOS version. Price it around 10–20% of the desktop or industry-benchmark price, mobilize your own community at launch, and you can crack the top five of a small country's paid-tools chart.

- Pick one feature that "the phone can do natively now, which used to need hardware," and show it in a ten-second video.

- Overseas, copy it country by country with "authorized reseller + local creator reviews" (the Philippines, Indonesia), without paying for ads.

- The ceiling is also clear: a launch built on a private network stops growing after the first wave (ratings haven't moved since 11-04).

### 9. Still unclear

- How many units were actually sold: Apple doesn't disclose it, and the rating rate is only an estimate.

- Who is buying in the Mexico, France and Japan chart spots (local resellers? overseas Vietnamese?): not found.

- Who makes "PA TRACE" and how big it is: not verified.

- The $199 desktop price and the 10.5% discount come from a recall card, and I couldn't reconfirm them on the web this time.

- No evidence of rewards for reviews or friends being recruited to leave them. This is inferred only from the timing clustering and generic wording.

### Sources

- App Store / iTunes lookup: [itunes.apple.com/lookup?id=6811894469&country=vn](https://itunes.apple.com/lookup?id=6811894469&country=vn) ; customers also bought: [apps.apple.com/ml/app/6811894469?see-all=customers-also-boug…](https://apps.apple.com/ml/app/6811894469?see-all=customers-also-bought-apps)

- Our database: appstore.app_day, review (2026-10-28 to 11-06)

- Founder's FB: [www.facebook.com/doanhviet3388/](https://www.facebook.com/doanhviet3388/) ; iOS demo post: [www.facebook.com/doanhviet3388/videos/propi-ios/165796149611…](https://www.facebook.com/doanhviet3388/videos/propi-ios/1657961496117874/)

- YouTube:[www.youtube.com/@doanhviet-propi](https://www.youtube.com/@doanhviet-propi)

- Website: [nexda.com.vn/](https://nexda.com.vn/)

- Philippine reseller: [www.facebook.com/propiappph/posts/122122375323299102/](https://www.facebook.com/propiappph/posts/122122375323299102/)

- Indonesian creator: TikTok @misfoyana (TikHub search, three videos: 10-23, 10-26, 11-03)

- Competitors: AudioTools id325307477, Transfer Function id824037130, SignalScope X id1304775893 (iTunes lookup); RemiX Audio [remixaudio.vn/](https://remixaudio.vn/) ; VOS4 [newsound.vn/san-pham/vos4-analyzer/](https://newsound.vn/san-pham/vos4-analyzer/) ; Smaart alternative price comparison [www.youtube.com/watch?v=6pMDc3FHqJw](https://www.youtube.com/watch?v=6pMDc3FHqJw)

This article was written by AI from public data and the data we collect daily. Numbers are sourced, and inferences are labeled.