Prompted by a recent mention of loudspeaker types as used by Hacker in the Sovereign II, and in particular a comment that the Sovereign III
might sound better. I've expressed my subjective thoughts about radio sound quality plenty of times, but I've been meaning to do some basic objective measurement to support these views for a while now...
This was just a quick 15 minutes in-between jobs, so I won't pretend that these measurements are in any way authoritative or rigorous. However, they do agree with my subjective impressions, and illustrate some interesting differences between sets. The sets tested were just the first few I could easily lay my hands on without venturing up into the attic or routing through boxes, etc. I can add to this in the future though.
The first measurement was the loudspeaker+cabinet only (power amp straight into the loudspeaker terminals). Then I did an "end to end" test - this involves turning the test stimulus into FM, then receiving that signal on the FM tuner. To provide FM pre-emphasis, I used my
Radiometer SMG40 prior to my FM generator. This result was close enough to the "raw" response that I decided to omit the speaker-only tests subsequently.
The measurement was made using
Omnimic, which consists of a calibrated microphone, software, and a CD containing the test stimulus. This is something I've used for many years, and have good levels of confidence in it. I chose to use a quasi-nearfield measurement at 30cm to get a reasonable compromise - usually you'd measure at 1m, but the room reflections distort the results too much unless you use gating, which removes the LF information. The grilles prevented a true nearfield measurement, and the slots, etc, behind the grilles would also cause problems.
I'll start with the RP72.
My objection with these sets has always been the upper midrange - there is a "honk" which can't be removed with the treble control. It doesn't seem to bother plenty of folk though, so perhaps it's just me?
The black trace is the raw loudspeaker. The red trace is the end-to-end result, with the tone controls in their central position. The blue trace is after I've adjusted the tone controls to get the best subjective result.
The first surprise is just how quickly the HF drops after 5kHz. Listening "raw", it sounds like an AM radio! The fall at LF - below 400Hz - is to be expected (Google "Baffle step correction").
The red trace shows some lift centred at 4kHz. I haven't measured the electrical frequency response at the loudspeaker terminals, but would expect that to be apparent there. Otherwise, things agree closely.
After adjustment, the best I could get was the blue trace. I've turned up the treble to try to get some HF, but that lifts the 4kHz peak by large amount - as you'd expect. At 10kHz, things are 3dB down compared to 1kHz, and the 4kHz peak is about 10dB up! Note also that the LF is still pretty poor after adjustment, although there is at least some energy below 100Hz.
OK, this isn't meant to be a hi-fi speaker, so perhaps we have unrealistic expectations? Well, let's look at some more...