Wren – a CHP-70 Bass Reflex
In all my previous posts on the CHP-70, it was with a 1.7 liters sealed box. What I wanted to find out then was how the CHP-70 fared as a midrange in a very compact 1.7 liters sealed box. For this project, I’m testing out the CHP-70 as a full range driver. This is the first time I will be hearing this 4″ in a bass reflex. I have no idea how it’ll sound like. To help me in designing the bass reflex, I extracted the Thiele & Small Parameters with DATS.
Fig 1 – CHP-70 Thiele & Small Parameters
Once I have the figures, I did some simulations on the bass reflex alignment. After a few models, I eventually decided on a 7 liters box with a +3dB peak at 100Hz. This will compensate for the bass loss as the woofer rolls off.
Fig 2 – CHP-70 in 7 liters bass reflex
Fig 2 is what the modeling looks like when I tuned the box to 85Hz. This is done with a 2″ PCV tube cut to 3-1/2′ length. The -3dB or F3 is at 70Hz. For a small full range driver, this is perfectly acceptable. Anything lower than this, one will have to resort to a subwoofer.
Fig 3 – CHP-70 Bass Reflex Impedance
The impedance sweep in Fig 3 shows the port tuning spot on at 85Hz. It’s remarkable how accurate some of these simulators are. As long as the data entered are accurate, the results will be accurate.
Fig 4 – CHP-70 RAW Response
Now that the box is settled, it’s time to listen to the CHP-70. Fig 4 is the RAW response of the CHP-70. This is what you will hear when you use the CHP-70. For me, it was impossible to listen to. My ears nearly bled from the excessive treble.
It is not the fault of the manufacturer. In fact, the CHP70 is actually quite an impressive driver. The problem comes from the methodology.
Almost all manufacturers use an IEC baffle in their measurements. That in itself is not the problem. The problem arises when the same driver is installed in a typical box. Take my 7 liters box as an example. My baffle width is 8″ and that will create a baffle step which shows up as a rise of +5dB at 1.5kHz upwards. This is the cause of the excessive treble.
Fig 5 – CHP70 with EQ
I understand there are some full range purist would be aghast at using any passive components. Well, with the CHP-70, they will have to live with what it is. Eventually, they will detest it and blame it on the driver.
The uneven response can easily be resolved. The Blue plot in Fig 5 is what I did. No problem with the music now. All it took was one component to get it flat. At this point, I should point out that below 400Hz is in Nearfield. This is a good approximation of the speaker response without the room interactions.
Fig 6 – Wren Frequency Response
The Blue plot in Fig 6 is the final Wren frequency response. The Red plot is from the mouth of the tuning port. That is where my +3dB peak is emitting from.
I’m unable to sum the port with the nearfield with this software. For that I’ll need to switch over to my LMS. But that’s not really necessary. Just draw a line from the Blue to the Red and roughly, that is the summed response.
How does the Wren Sound like
I have no issues listening to it. Most important is the treble is not harsh. It doesn’t have the sweetness of a tweeter but that is expected of a full range. Some may argue that it’s only good up to 5kHz. That is true but there’s very little information in the upper highs anyway. In practical terms, there won’t be the “zing” in the highs. That’s the sweetness I was referring to.
Another argument is it doesn’t extend down to 40Hz. Like I said before, add a subwoofer. Don’t expect a 4″ driver to do 40Hz up to 20kHz. But can I really accept an F3 of 70Hz.
Definitely. That’s because I have the upper bass, the 100Hz region. If I didn’t tune the Wren the way I did, there would be no upper bass and the speaker will sound very “lean”. That will lead to listener fatigue.
Can the Wren be improved further. Yes, add a tweeter. That is my next project, a 2-way with the CHP-70.
April 26, 2020Projects