From another Jewish web site:
Q: A cappella albums – singing without instrumental accompaniment – are becoming more and more common, especially during The Three Weeks and Sefirah when we are noheig not to listen to music. Is it halachically permissible to listen to a cappella music during these times of the year?
A Cappella
There are basically three types of a cappella.
One is where the musical sounds originate from human voices but the natural properties are digitally modified with computer software to attain quality of sounds that are not humanly possible, thus making it sound more like regular music. Such a cappella is halachically not viewed as being any different from regular music.
There are other forms of a cappella which sound very similar to regular music, although no digital modification is done to the voices. These types of a cappella should also not be listened to during Sefirah and The Three Weeks, as will be explained shortly.
The third type of a cappella is where regular songs are sung by an individual or choir. There is nothing halachically objectionable about listening to such a cappella during Sefirah and The Three Weeks.
To properly understand this topic, it would be helpful to briefly relate some technical information provided by experts in the music industry as to how a cappella music is created.
Digitally Modified A Cappella
Every sound is made up of many different sound waves, each at there own frequency. The individual frequencies and the velocity of each sound wave give each sound its unique tonal properties.
A second modification made is to the pitch of the notes. A bass guitar can play notes almost twice as low (two octaves lower) as a human voice can go. Therefore, in order to simulate the bass notes, some album producers lower the pitch of the notes beyond the capabilities of the human voice. This process of transposing the notes down an octave or more would also change the status of these notes from vocals to computer-generated sounds, and would be prohibited during Sefirah and The Three Weeks.
A third modification made is to the timing of the notes. The rhythmic structure of all music can be charted on a grid. The most common breakdown would be charted in eights. That means that each rhythmic hit would take place at exactly one interval of eights. It is not humanly possible for a musician to play 100% on the grid. This slight imperfection is what gives live music its human feel, as opposed to machine music which sounds much more rigid. It is even harder for a person to create a rhythm with his mouth, and keep it perfectly on grid. Many albums take the rhythmic parts and digitally place them exactly where they belong on the grid. This process is called quantization. Though this does not change the sound of the voice, when used in combination with any of the above processes it would give the sounds more of a status of music.
Unmodified A Cappella
If so, where does the entire issue of not listening to live or recorded music during Sefirah and The Three Weeks come from?
It appears that although there was no specific minhag not to listen to music, there was a minhag to abstain from things that bring about an excessive amount of enjoyment. Furthermore, technically, due to the churban Beis Hamikdosh, music should be forbidden all year round. However, there are certain kulos which we rely upon. It is during Sefirah and The Three Weeks that we have accepted upon ourselves not to rely on these kulos and practice aveilus in this regard. Music has this power more than most things that people do for enjoyment. Music can take someone out of this world, so to speak, and make him forget, at least temporarily, all his worries and problems. Thus, whether the music is live or recorded, it produces this effect, which is contrary to the minhag. A cappella that sounds very similar to music is also included in this minhag and should not be listened to.
Some rabbonim feel that once it sounds like music and is being played from an electronic device, that, too, renders it a musical device which is forbidden.
Choirs
A cappella music that was not modified at all, and sounds like a group of people singing, would be permissible.
It is interesting to note that the word a cappella literally means, a – in the style of, capella – a chapel. In a chapel, they have only a choir singing with no musical instruments (other than a pipe organ which blends together very well with the voices). The harmony of the choir is meant to produce the musical affect. This is where the concept of a cappella stems from.
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Harav Shlomo Miller, Rosh Kollel of Kollel Avreichim of Toronto and head of the Bais Horaah of Lakewood, has also stated his opinion that listening to a capella music during Sefirah is forbidden.